Microsoft Encarta: Desember 2008

Kamis, 11 Desember 2008

All About Earth

Earth (planet)

INTRODUCTION

Earth

An oxygen-rich and protective atmosphere, moderate temperatures, abundant water, and a varied chemical composition enable Earth to support life, the only planet known to harbor life. The planet is composed of rock and metal, which are present in molten form beneath its surface. The Apollo 17 spacecraft took this snapshot in 1972 of the Arabian Peninsula, the African continent, and Antarctica (most of the white area near the bottom).

NASA/Science Source/Photo Researchers, Inc.

Earth (planet), one of nine planets in the solar system, the only planet known to harbor life, and the “home” of human beings. From space Earth resembles a big blue marble with swirling white clouds floating above blue oceans. About 71 percent of Earth’s surface is covered by water, which is essential to life. The rest is land, mostly in the form of continents that rise above the oceans.

Earth’s surface is surrounded by a layer of gases known as the atmosphere, which extends upward from the surface, slowly thinning out into space. Below the surface is a hot interior of rocky material and two core layers composed of the metals nickel and iron in solid and liquid form.

Unlike the other planets, Earth has a unique set of characteristics ideally suited to supporting life as we know it. It is neither too hot, like Mercury, the closest planet to the Sun, nor too cold, like distant Mars and the even more distant outer planets—Jupiter, Saturn, Uranus, Neptune, and tiny Pluto. Earth’s atmosphere includes just the right amount of gases that trap heat from the Sun, resulting in a moderate climate suitable for water to exist in liquid form. The atmosphere also helps block radiation from the Sun that would be harmful to life. Earth’s atmosphere distinguishes it from the planet Venus, which is otherwise much like Earth. Venus is about the same size and mass as Earth and is also neither too near nor too far from the Sun. But because Venus has too much heat-trapping carbon dioxide in its atmosphere, its surface is extremely hot—462°C (864°F)—hot enough to melt lead and too hot for life to exist.

Although Earth is the only planet known to have life, scientists do not rule out the possibility that life may once have existed on other planets or their moons, or may exist today in primitive form. Mars, for example, has many features that resemble river channels, indicating that liquid water once flowed on its surface. If so, life may also have evolved there, and evidence for it may one day be found in fossil form. Water still exists on Mars, but it is frozen in polar ice caps, in permafrost, and possibly in rocks below the surface.

Earth from the Moon

In the late 1960s, people saw for the first time what Earth looked like from space. This famous photo of Earth was taken by astronauts on the Apollo 8 mission as they orbited the Moon in 1968.

NASA

For thousands of years, human beings could only wonder about Earth and the other observable planets in the solar system. Many early ideas—for example, that the Earth was a sphere and that it traveled around the Sun—were based on brilliant reasoning. However, it was only with the development of the scientific method and scientific instruments, especially in the 18th and 19th centuries, that humans began to gather data that could be used to verify theories about Earth and the rest of the solar system. By studying fossils found in rock layers, for example, scientists realized that the Earth was much older than previously believed. And with the use of telescopes, new planets such as Uranus, Neptune, and Pluto were discovered.

In the second half of the 20th century, more advances in the study of Earth and the solar system occurred due to the development of rockets that could send spacecraft beyond Earth. Human beings were able to study and observe Earth from space with satellites equipped with scientific instruments. Astronauts landed on the Moon and gathered ancient rocks that revealed much about the early solar system. During this remarkable advancement in human history, humans also sent unmanned spacecraft to the other planets and their moons. Spacecraft have now visited all of the planets except Pluto. The study of other planets and moons has provided new insights about Earth, just as the study of the Sun and other stars like it has helped shape new theories about how Earth and the rest of the solar system formed.

As a result of this recent space exploration, we now know that Earth is one of the most geologically active of all the planets and moons in the solar system. Earth is constantly changing. Over long periods of time land is built up and worn away, oceans are formed and re-formed, and continents move around, break up, and merge.

Life itself contributes to changes on Earth, especially in the way living things can alter Earth’s atmosphere. For example, Earth at one time had the same amount of carbon dioxide in its atmosphere as Venus now has, but early forms of life helped remove this carbon dioxide over millions of years. These life forms also added oxygen to Earth’s atmosphere and made it possible for animal life to evolve on land.

A variety of scientific fields have broadened our knowledge about Earth, including biogeography, climatology, geology, geophysics, hydrology, meteorology, oceanography, and zoogeography. Collectively, these fields are known as Earth science. By studying Earth’s atmosphere, its surface, and its interior and by studying the Sun and the rest of the solar system, scientists have learned much about how Earth came into existence, how it changed, and why it continues to change.

EARTH, THE SOLAR SYSTEM, AND THE GALAXY

Solar System

Earth is the third planet from the Sun, after Mercury and Venus. The average distance between Earth and the Sun is 150 million km (93 million mi). Earth and all the other planets in the solar system revolve, or orbit, around the Sun due to the force of gravitation. The Earth travels at a velocity of about 107,000 km/h (about 67,000 mph) as it orbits the Sun. All but one of the planets orbit the Sun in the same plane—that is, if an imaginary line were extended from the center of the Sun to the outer regions of the solar system, the orbital paths of the planets would intersect that line. The exception is Pluto, which has an eccentric (unusual) orbit.

Earth’s orbital path is not quite a perfect circle but instead is slightly elliptical (oval-shaped). For example, at maximum distance Earth is about 152 million km (about 95 million mi) from the Sun; at minimum distance Earth is about 147 million km (about 91 million mi) from the Sun. If Earth orbited the Sun in a perfect circle, it would always be the same distance from the Sun.

The solar system, in turn, is part of the Milky Way Galaxy, a collection of billions of stars bound together by gravity. The Milky Way has armlike discs of stars that spiral out from its center. The solar system is located in one of these spiral arms, known as the Orion arm, which is about two-thirds of the way from the center of the Galaxy. In most parts of the Northern Hemisphere, this disc of stars is visible on a summer night as a dense band of light known as the Milky Way.

Milky Way Galaxy

Our own solar system exists within one of the spiral arms of the disk-shaped galaxy called the Milky Way. This false-color image looks toward the center of the Milky Way, located 30,000 light-years away. Bright star clusters are visible along with darker areas of dust and gas.

Morton-Milon/Science Source/Photo Researchers, Inc.

Earth is the fifth largest planet in the solar system. Its diameter, measured around the equator, is 12,756 km (7,926 mi). Earth is not a perfect sphere but is slightly flattened at the poles. Its polar diameter, measured from the North Pole to the South Pole, is somewhat less than the equatorial diameter because of this flattening. Although Earth is the largest of the four planets—Mercury, Venus, Earth, and Mars—that make up the inner solar system (the planets closest to the Sun), it is small compared with the giant planets of the outer solar system—Jupiter, Saturn, Uranus, and Neptune. For example, the largest planet, Jupiter, has a diameter at its equator of 143,000 km (89,000 mi), 11 times greater than that of Earth. A famous atmospheric feature on Jupiter, the Great Red Spot, is so large that three Earths would fit inside it.

Earth has one natural satellite, the Moon. The Moon orbits the Earth, completing one revolution in an elliptical path in 27 days 7 hr 43 min 11.5 sec. The Moon orbits the Earth because of the force of Earth’s gravity. However, the Moon also exerts a gravitational force on the Earth. Evidence for the Moon’s gravitational influence can be seen in the ocean tides. A popular theory suggests that the Moon split off from Earth more than 4 billion years ago when a large meteorite or small planet struck the Earth.

As Earth revolves around the Sun, it rotates, or spins, on its axis, an imaginary line that runs between the North and South poles. The period of one complete rotation is defined as a day and takes 23 hr 56 min 4.1 sec. The period of one revolution around the Sun is defined as a year, or 365.2422 solar days, or 365 days 5 hr 48 min 46 sec. Earth also moves along with the Milky Way Galaxy as the Galaxy rotates and moves through space. It takes more than 200 million years for the stars in the Milky Way to complete one revolution around the Galaxy’s center.

Earth’s axis of rotation is inclined (tilted) 23.5° relative to its plane of revolution around the Sun. This inclination of the axis creates the seasons and causes the height of the Sun in the sky at noon to increase and decrease as the seasons change. The Northern Hemisphere receives the most energy from the Sun when it is tilted toward the Sun. This orientation corresponds to summer in the Northern Hemisphere and winter in the Southern Hemisphere. The Southern Hemisphere receives maximum energy when it is tilted toward the Sun, corresponding to summer in the Southern Hemisphere and winter in the Northern Hemisphere. Fall and spring occur in between these orientations.

III

EARTH’S ATMOSPHERE

The atmosphere is a layer of different gases that extends from Earth’s surface to the exosphere, the outer limit of the atmosphere, about 9,600 km (6,000 mi) above the surface. Near Earth’s surface, the atmosphere consists almost entirely of nitrogen (78 percent) and oxygen (21 percent). The remaining 1 percent of atmospheric gases consists of argon (0.9 percent); carbon dioxide (0.03 percent); varying amounts of water vapor; and trace amounts of hydrogen, nitrous oxide, ozone, methane, carbon monoxide, helium, neon, krypton, and xenon.

Layers of the Atmosphere

Divisions of the Atmosphere

Without our atmosphere, there would be no life on Earth. A relatively thin envelope, the atmosphere consists of layers of gases that support life and provide protection from harmful radiation.

The layers of the atmosphere are the troposphere, the stratosphere, the mesosphere, the thermosphere, and the exosphere. The troposphere is the layer in which weather occurs and extends from the surface to about 16 km (about 10 mi) above sea level at the equator. Above the troposphere is the stratosphere, which has an upper boundary of about 50 km (about 30 mi) above sea level. The layer from 50 to 90 km (30 to 60 mi) is called the mesosphere. At an altitude of about 90 km, temperatures begin to rise. The layer that begins at this altitude is called the thermosphere because of the high temperatures that can be reached in this layer (about 1200°C, or about 2200°F). The region beyond the thermosphere is called the exosphere. The thermosphere and the exosphere overlap with another region of the atmosphere known as the ionosphere, a layer or layers of ionized air extending from almost 60 km (about 50 mi) above Earth’s surface to altitudes of 1,000 km (600 mi) and more.

Greenhouse Effect

Earth’s atmosphere and the way it interacts with the oceans and radiation from the Sun are responsible for the planet’s climate and weather. The atmosphere plays a key role in supporting life. Almost all life on Earth uses atmospheric oxygen for energy in a process known as cellular respiration, which is essential to life. The atmosphere also helps moderate Earth’s climate by trapping radiation from the Sun that is reflected from Earth’s surface. Water vapor, carbon dioxide, methane, and nitrous oxide in the atmosphere act as “greenhouse gases.” Like the glass in a greenhouse, they trap infrared, or heat, radiation from the Sun in the lower atmosphere and thereby help warm Earth’s surface. Without this greenhouse effect, heat radiation would escape into space, and Earth would be too cold to support most forms of life.

Other gases in the atmosphere are also essential to life. The trace amount of ozone found in Earth’s stratosphere blocks harmful ultraviolet radiation from the Sun. Without the ozone layer, life as we know it could not survive on land. Earth’s atmosphere is also an important part of a phenomenon known as the water cycle or the hydrologic cycle. See also Atmosphere.

The Atmosphere and the Water Cycle

Water Cycle

The water cycle simply means that Earth’s water is continually recycled between the oceans, the atmosphere, and the land. All of the water that exists on Earth today has been used and reused for billions of years. Very little water has been created or lost during this period of time. Water is constantly moving on Earth’s surface and changing back and forth between ice, liquid water, and water vapor.

The water cycle begins when the Sun heats the water in the oceans and causes it to evaporate and enter the atmosphere as water vapor. Some of this water vapor falls as precipitation directly back into the oceans, completing a short cycle. Some of the water vapor, however, reaches land, where it may fall as snow or rain. Melted snow or rain enters rivers or lakes on the land. Due to the force of gravity, the water in the rivers eventually empties back into the oceans. Melted snow or rain also may enter the ground. Groundwater may be stored for hundreds or thousands of years, but it will eventually reach the surface as springs or small pools known as seeps. Even snow that forms glacial ice or becomes part of the polar caps and is kept out of the cycle for thousands of years eventually melts or is warmed by the Sun and turned into water vapor, entering the atmosphere and falling again as precipitation. All water that falls on land eventually returns to the ocean, completing the water cycle.

EARTH’S SURFACE

Earth’s surface is the outermost layer of the planet. It includes the hydrosphere, the crust, and the biosphere.

Hydrosphere

The hydrosphere consists of the bodies of water that cover 71 percent of Earth’s surface. The largest of these are the oceans, which contain over 97 percent of all water on Earth. Glaciers and the polar ice caps contain just over 2 percent of Earth’s water in the form of solid ice. Only about 0.6 percent is under the surface as groundwater. Nevertheless, groundwater is 36 times more plentiful than water found in lakes, inland seas, rivers, and in the atmosphere as water vapor. Only 0.017 percent of all the water on Earth is found in lakes and rivers. And a mere 0.001 percent is found in the atmosphere as water vapor. Most of the water in glaciers, lakes, inland seas, rivers, and groundwater is fresh and can be used for drinking and agriculture. Dissolved salts compose about 3.5 percent of the water in the oceans, however, making it unsuitable for drinking or agriculture unless it is treated to remove the salts.

Crust

The crust consists of the continents, other land areas, and the basins, or floors, of the oceans. The dry land of Earth’s surface is called the continental crust. It is about 15 to 75 km (9 to 47 mi) thick. The oceanic crust is thinner than the continental crust. Its average thickness is 5 to 10 km (3 to 6 mi). The crust has a definite boundary called the Mohorovičić discontinuity, or simply the Moho. The boundary separates the crust from the underlying mantle, which is much thicker and is part of Earth’s interior.

Oceanic crust and continental crust differ in the type of rocks they contain. There are three main types of rocks: igneous, sedimentary, and metamorphic. Igneous rocks form when molten rock, called magma, cools and solidifies. Sedimentary rocks are usually created by the breakdown of igneous rocks. They tend to form in layers as small particles of other rocks or as the mineralized remains of dead animals and plants that have fused together over time. The remains of dead animals and plants occasionally become mineralized in sedimentary rock and are recognizable as fossils. Metamorphic rocks form when sedimentary or igneous rocks are altered by heat and pressure deep underground.

Oceanic crust consists of dark, dense igneous rocks, such as basalt and gabbro. Continental crust consists of lighter-colored, less dense igneous rocks, such as granite and diorite. Continental crust also includes metamorphic rocks and sedimentary rocks.

Biosphere

The biosphere includes all the areas of Earth capable of supporting life. The biosphere ranges from about 10 km (about 6 mi) into the atmosphere to the deepest ocean floor. For a long time, scientists believed that all life depended on energy from the Sun and consequently could only exist where sunlight penetrated. In the 1970s, however, scientists discovered various forms of life around hydrothermal vents on the floor of the Pacific Ocean where no sunlight penetrated. They learned that primitive bacteria formed the basis of this living community and that the bacteria derived their energy from a process called chemosynthesis that did not depend on sunlight. Some scientists believe that the biosphere may extend relatively deep into Earth’s crust. They have recovered what they believe are primitive bacteria from deeply drilled holes below the surface.

Changes to Earth’s Surface

Earth’s surface has been constantly changing ever since the planet formed. Most of these changes have been gradual, taking place over millions of years. Nevertheless, these gradual changes have resulted in radical modifications, involving the formation, erosion, and re-formation of mountain ranges, the movement of continents, the creation of huge supercontinents, and the breakup of supercontinents into smaller continents.

The weathering and erosion that result from the water cycle are among the principal factors responsible for changes to Earth’s surface. Another principal factor is the movement of Earth’s continents and seafloors and the buildup of mountain ranges due to a phenomenon known as plate tectonics. Heat is the basis for all of these changes. Heat in Earth’s interior is believed to be responsible for continental movement, mountain building, and the creation of new seafloor in ocean basins. Heat from the Sun is responsible for the evaporation of ocean water and the resulting precipitation that causes weathering and erosion. In effect, heat in Earth’s interior helps build up Earth’s surface while heat from the Sun helps wear down the surface.

Weathering

Weathering is the breakdown of rock at and near the surface of Earth. Most rocks originally formed in a hot, high-pressure environment below the surface where there was little exposure to water. Once the rocks reached Earth’s surface, however, they were subjected to temperature changes and exposed to water. When rocks are subjected to these kinds of surface conditions, the minerals they contain tend to change. These changes constitute the process of weathering. There are two types of weathering: physical weathering and chemical weathering.

Physical weathering involves a decrease in the size of rock material. Freezing and thawing of water in rock cavities, for example, splits rock into small pieces because water expands when it freezes.

Chemical weathering involves a chemical change in the composition of rock. For example, feldspar, a common mineral in granite and other rocks, reacts with water to form clay minerals, resulting in a new substance with totally different properties than the parent feldspar. Chemical weathering is of significance to humans because it creates the clay minerals that are important components of soil, the basis of agriculture. Chemical weathering also causes the release of dissolved forms of sodium, calcium, potassium, magnesium, and other chemical elements into surface water and groundwater. These elements are carried by surface water and groundwater to the sea and are the sources of dissolved salts in the sea.

Erosion

Glacial Erosion

Glaciers erode the Earth’s surface through processes such as abrasion, crushing, and fracturing of the material in the glacier’s path. Glaciers move by growing or shrinking, depending on the climate. Moving glaciers erode and transport large quantities of rocks, sand, and other particles along their path. The icy path shown here is a moraine formed by a glacier in Switzerland.

Paolo Koch/Photo Researchers, Inc.

Erosion is the process that removes loose and weathered rock and carries it to a new site. Water, wind, and glacial ice combined with the force of gravity can cause erosion.

Erosion by running water is by far the most common process of erosion. It takes place over a longer period of time than other forms of erosion. When water from rain or melted snow moves downhill, it can carry loose rock or soil with it. Erosion by running water forms the familiar gullies and V-shaped valleys that cut into most landscapes. The force of the running water removes loose particles formed by weathering. In the process, gullies and valleys are lengthened, widened, and deepened. Often, water overflows the banks of the gullies or river channels, resulting in floods. Each new flood carries more material away to increase the size of the valley. Meanwhile, weathering loosens more and more material so the process continues.

Erosion by glacial ice is less common, but it can cause the greatest landscape changes in the shortest amount of time. Glacial ice forms in a region where snow fails to melt in the spring and summer and instead builds up as ice. For major glaciers to form, this lack of snowmelt has to occur for a number of years in areas with high precipitation. As ice accumulates and thickens, it flows as a solid mass. As it flows, it has a tremendous capacity to erode soil and even solid rock. Ice is a major factor in shaping some landscapes, especially mountainous regions. Glacial ice provides much of the spectacular scenery in these regions. Features such as horns (sharp mountain peaks), arêtes (sharp ridges), glacially formed lakes, and U-shaped valleys are all the result of glacial erosion.

Wind is an important cause of erosion only in arid (dry) regions. Wind carries sand and dust, which can scour even solid rock.

Many factors determine the rate and kind of erosion that occurs in a given area. The climate of an area determines the distribution, amount, and kind of precipitation that the area receives and thus the type and rate of weathering. An area with an arid climate erodes differently than an area with a humid climate. The elevation of an area also plays a role by determining the potential energy of running water. The higher the elevation the more energetically water will flow due to the force of gravity. The type of bedrock in an area (sandstone, granite, or shale) can determine the shapes of valleys and slopes, and the depth of streams.

A landscape’s geologic age—that is, how long current conditions of weathering and erosion have affected the area—determines its overall appearance. Relatively young landscapes tend to be more rugged and angular in appearance. Older landscapes tend to have more rounded slopes and hills. The oldest landscapes tend to be low-lying with broad, open river valleys and low, rounded hills. The overall effect of the wearing down of an area is to level the land; the tendency is toward the reduction of all land surfaces to sea level.

Plate Tectonics

Opposing this tendency toward leveling is a force responsible for raising mountains and plateaus and for creating new landmasses. These changes to Earth’s surface occur in the outermost solid portion of Earth, known as the lithosphere. The lithosphere consists of the crust and another region known as the upper mantle and is approximately 65 to 100 km (40 to 60 mi) thick. Compared with the interior of the Earth, however, this region is relatively thin. The lithosphere is thinner in proportion to the whole Earth than the skin of an apple is to the whole apple.

Scientists believe that the lithosphere is broken into a series of plates, or segments. According to the theory of plate tectonics, these plates move around on Earth’s surface over long periods of time. Tectonics comes from the Greek word, tektonikos, which means “builder.”

According to the theory, the lithosphere is divided into large and small plates. The largest plates include the Pacific plate, the North American plate, the Eurasian plate, the Antarctic plate, the Indo-Australian plate, and the African plate. Smaller plates include the Cocos plate, the Nazca plate, the Philippine plate, and the Caribbean plate. Plate sizes vary a great deal. The Cocos plate is 2,000 km (1,000 mi) wide, while the Pacific plate is nearly 14,000 km (nearly 9,000 mi) wide.

These plates move in three different ways in relation to each other. They pull apart or move away from each other, they collide or move against each other, or they slide past each other as they move sideways. The movement of these plates helps explain many geological events, such as earthquakes and volcanic eruptions as well as mountain building and the formation of the oceans and continents.

D3a

When Plates Pull Apart

Magma Upwelling

Mid-ocean ridges occur along boundaries between plates of Earth’s outer shell where new seafloor is created as the plates spread apart. As plates move apart under the ocean, molten rock, or magma, wells up from deep below the surface of the seafloor. Some of the magma that ascends to the seafloor produces enormous volcanic eruptions. The rest solidifies on the edges of the plates as they spread apart, creating new rocky seafloor material.

Archive Photos

When the plates pull apart, two types of phenomena occur depending on whether the movement takes place in the oceans or on land. When plates pull apart on land, deep valleys known as rift valleys form. An example of a rift valley is the Great Rift Valley that extends from Syria in the Middle East to Mozambique in Africa. When plates pull apart in the oceans, long, sinuous chains of volcanic mountains called mid-ocean ridges form, and new seafloor is created at the site of these ridges. Rift valleys are also present along the crests of the mid-ocean ridges.

Most scientists believe that gravity and heat from the interior of the Earth cause the plates to move apart and to create new seafloor. According to this explanation, molten rock known as magma rises from Earth’s interior to form hot spots beneath the ocean floor. As two oceanic plates pull apart from each other in the middle of the oceans, a crack, or rupture, appears and forms the mid-ocean ridges. These ridges exist in all the world’s ocean basins and resemble the seams of a baseball. The molten rock rises through these cracks and creates new seafloor.

D3b

When Plates Collide

Converging Plates

The outer layer of the Earth, the lithosphere, is broken into about 20 pieces, called tectonic plates. These plates slowly slide around on the asthenosphere below, periodically colliding with each other.

When plates collide or push against each other, regions called convergent plate margins form. Along these margins, one plate is usually forced to dive below the other. As that plate dives, it triggers the melting of the surrounding lithosphere and a region just below it known as the asthenosphere. These pockets of molten crust rise behind the margin through the overlying plate, creating curved chains of volcanoes known as arcs. This process is called subduction.

If one plate consists of oceanic crust and the other consists of continental crust, the denser oceanic crust will dive below the continental crust. If both plates are oceanic crust, then either may be subducted. If both are continental crust, subduction can continue for a while but will eventually end because continental crust is not dense enough to be forced very far into the upper mantle.

Mount Everest

Mount Everest, the world’s highest mountain at 8,850 m (29,035 ft), is located in the Himalayas. The Himalayas form the highest mountain system in the world, with more than 30 peaks towering 7,600 m (25,000 ft) or more.

Keren Su/Tony Stone Images

The results of this subduction process are readily visible on a map showing that 80 percent of the world’s volcanoes rim the Pacific Ocean where plates are colliding against each other. The subduction zone created by the collision of two oceanic plates—the Pacific plate and the Philippine plate—can also create a trench. Such a trench resulted in the formation of the deepest point on Earth, the Mariana Trench, which is estimated to be 11,033 m (36,198 ft) below sea level.

On the other hand, when two continental plates collide, mountain building occurs. The collision of the Indo-Australian plate with the Eurasian plate has produced the Himalayan Mountains. This collision resulted in the highest point of Earth, Mount Everest, which is 8,850 m (29,035 ft) above sea level.

D3c

When Plates Slide Past Each Other

San Andreas Fault, California

The San Andreas Fault, unlike most faults that stay below the ocean, emerges from the Pacific Ocean and traverses hundreds of miles of land. It runs through California for about 1,000 km (about 600 mi) from Point Arena to the Imperial Valley. The fault marks the boundary between the North American and Pacific tectonic plates; earthquakes are caused by these plates sliding together.

Francois Gohier/Photo Researchers, Inc.

Finally, some of Earth’s plates neither collide nor pull apart but instead slide past each other. These regions are called transform margins. Few volcanoes occur in these areas because neither plate is forced down into Earth’s interior and little melting occurs. Earthquakes, however, are abundant as the two rigid plates slide past each other. The San Andreas Fault in California is a well-known example of a transform margin.

The movement of plates occurs at a slow pace, at an average rate of only 2.5 cm (1 in) per year. But over millions of years this gradual movement results in radical changes. Current plate movement is making the Pacific Ocean and Mediterranean Sea smaller, the Atlantic Ocean larger, and the Himalayan Mountains higher.

EARTH’S INTERIOR

Internal Structure of the Earth

Earth is made up of a series of layers that formed early in the planet’s history, as heavier material gravitated toward the center and lighter material floated to the surface. The dense, solid, inner core of iron is surrounded by a liquid, iron, outer core. The lower mantle consists of molten rock, which is surrounded by partially molten rock in the asthenosphere and solid rock in the upper mantle and crust. Between some of the layers, there are chemical or structural changes that form discontinuities. Lighter elements, such as silicon, aluminum, calcium, potassium, sodium, and oxygen, compose the outer crust.

The interior of Earth plays an important role in plate tectonics. Scientists believe it is also responsible for Earth’s magnetic field. This field is vital to life because it shields the planet’s surface from harmful cosmic rays and from a steady stream of energetic particles from the Sun known as the solar wind.

Composition of the Interior

Earth’s interior consists of the mantle and the core. The mantle and core make up by far the largest part of Earth’s mass. The distance from the base of the crust to the center of the core is about 6,400 km (about 4,000 mi).

Scientists have learned about Earth’s interior by studying rocks that formed in the interior and rose to the surface. The study of meteorites, which are believed to be made of the same material that formed the Earth and its interior, has also offered clues about Earth’s interior. Finally, seismic waves generated by earthquakes provide geophysicists with information about the composition of the interior. The sudden movement of rocks during an earthquake causes vibrations that transmit energy through the Earth in the form of waves. The way these waves travel through the interior of Earth reveals the nature of materials inside the planet.

The mantle consists of three parts: the lower part of the lithosphere, the region below it known as the asthenosphere, and the region below the asthenosphere called the lower mantle. The entire mantle extends from the base of the crust to a depth of about 2,900 km (about 1,800 mi). Scientists believe the asthenosphere is made up of mushy plastic-like rock with pockets of molten rock. The term asthenosphere is derived from Greek and means “weak layer.” The asthenosphere’s soft, plastic quality allows plates in the lithosphere above it to shift and slide on top of the asthenosphere. This shifting of the lithosphere’s plates is the source of most tectonic activity. The asthenosphere is also the source of the basaltic magma that makes up much of the oceanic crust and rises through volcanic vents on the ocean floor.

The mantle consists of mostly solid iron-magnesium silicate rock mixed with many other minor components including radioactive elements. However, even this solid rock can flow like a “sticky” liquid when it is subjected to enough heat and pressure.

The core is divided into two parts, the outer core and the inner core. The outer core is about 2,260 km (about 1,404 mi) thick. The outer core is a liquid region composed mostly of iron, with smaller amounts of nickel and sulfur in liquid form. The inner core is about 1,220 km (about 758 mi) thick. The inner core is solid and is composed of iron, nickel, and sulfur in solid form. Because the inner core is surrounded by a liquid region, it can rotate independently. Recent scientific studies indicate that the inner core may actually rotate faster than the rest of the planet, making one full extra spin over a period of 700 to 1,200 years. The inner core and the outer core also contain a small percentage of radioactive material. The existence of radioactive material is one of the sources of heat in Earth’s interior because as radioactive material decays, it gives off heat. Temperatures in the inner core may be as high as 6650°C (12,000°F).

The Core and Earth’s Magnetism

Earth’s Magnetic Field

Scientists believe that Earth’s liquid iron core is instrumental in creating a magnetic field that surrounds Earth and shields the planet from harmful cosmic rays and the Sun’s solar wind. The idea that Earth is like a giant magnet was first proposed in 1600 by English physician and natural philosopher William Gilbert. Gilbert proposed the idea to explain why the magnetized needle in a compass points north. According to Gilbert, Earth’s magnetic field creates a magnetic north pole and a magnetic south pole. The magnetic poles do not correspond to the geographic North and South poles, however. Moreover, the magnetic poles wander and are not always in the same place. The north magnetic pole is currently close to Ellef Ringnes Island in the Queen Elizabeth Islands near the boundary of Canada’s Northwest Territories with Nunavut. The south magnetic pole lies just off the coast of Wilkes Land, Antarctica.

Not only do the magnetic poles wander, but they also reverse their polarity—that is, the north magnetic pole becomes the south magnetic pole and vice versa. Magnetic reversals have occurred at least 170 times over the past 100 million years. The reversals occur on average about every 200,000 years and take place gradually over a period of several thousand years. Scientists still do not understand why these magnetic reversals occur but think they may be related to Earth’s rotation and changes in the flow of liquid iron in the outer core.

Aurora Borealis

The aurora borealis, commonly known as the northern lights, creates a spectacular light show near Fairbanks, Alaska. Auroras, most frequently seen in the far northern and far southern regions of the globe, are common sights in the Alaskan sky. Luminous displays visible to the naked eye only at night, auroras occur when charged particles from the Sun interact with gases in Earth’s atmosphere.

Jack Finch/Science Source/Photo Researchers, Inc.

Some scientists theorize that the flow of liquid iron in the outer core sets up electrical currents that produce Earth’s magnetic field. Known as the dynamo theory, this theory appears to be the best explanation yet for the origin of the magnetic field. Earth’s magnetic field operates in a region above Earth’s surface known as the magnetosphere. The magnetosphere is shaped somewhat like a teardrop with a long tail that trails away from the Earth due to the force of the solar wind.

Inside the magnetosphere are the Van Allen radiation belts, named for the American physicist James A. Van Allen who discovered them in 1958. The Van Allen belts are regions where charged particles from the Sun and from cosmic rays are trapped and sent into spiral paths along the lines of Earth’s magnetic field. The radiation belts thereby shield Earth’s surface from these highly energetic particles. Occasionally, however, due to extremely strong magnetic fields on the Sun’s surface, which are visible as sunspots, a brief burst of highly energetic particles streams along with the solar wind. Because Earth’s magnetic field lines converge and are closest to the surface at the poles, some of these energetic particles sneak through and interact with Earth’s atmosphere, creating the phenomenon known as an aurora.

EARTH’S PAST

Origin of Earth

Most scientists believe that the Earth, Sun, and all of the other planets and moons in the solar system formed about 4.6 billion years ago from a giant cloud of gas and dust known as the solar nebula. The gas and dust in this solar nebula originated in a star that ended its life in a violent explosion known as a supernova. The solar nebula consisted principally of hydrogen, the lightest element, but the nebula was also seeded with a smaller percentage of heavier elements, such as carbon and oxygen. All of the chemical elements we know were originally made in the star that became a supernova. Our bodies are made of these same chemical elements. Therefore, all of the elements in our solar system, including all of the elements in our bodies, originally came from this star-seeded solar nebula.

Due to the force of gravity tiny clumps of gas and dust began to form in the early solar nebula. As these clumps came together and grew larger, they caused the solar nebula to contract in on itself. The contraction caused the cloud of gas and dust to flatten in the shape of a disc. As the clumps continued to contract, they became very dense and hot. Eventually the atoms of hydrogen became so dense that they began to fuse in the innermost part of the cloud, and these nuclear reactions gave birth to the Sun. The fusion of hydrogen atoms in the Sun is the source of its energy.

Many scientists favor the planetesimal theory for how the Earth and other planets formed out of this solar nebula. This theory helps explain why the inner planets became rocky while the outer planets, except for Pluto, are made up mostly of gases. The theory also explains why all of the planets orbit the Sun in the same plane.

According to this theory, temperatures decreased with increasing distance from the center of the solar nebula. In the inner region, where Mercury, Venus, Earth, and Mars formed, temperatures were low enough that certain heavier elements, such as iron and the other heavy compounds that make up rock, could condense out—that is, could change from a gas to a solid or liquid. Due to the force of gravity, small clumps of this rocky material eventually came together with the dust in the original solar nebula to form protoplanets or planetesimals (small rocky bodies). These planetesimals collided, broke apart, and re-formed until they became the four inner rocky planets. The inner region, however, was still too hot for other light elements, such as hydrogen and helium, to be retained. These elements could only exist in the outermost part of the disc, where temperatures were lower. As a result two of the outer planets—Jupiter and Saturn—are mostly made of hydrogen and helium, which are also the dominant elements in the atmospheres of Uranus and Neptune.

The Early Earth

The Early Earth

Life originated on Earth about four billion years ago, when oceans dotted with volcanic islands covered most of Earth’s surface and continents were very small. The air was hot and contained almost no breathable oxygen. The Moon was much closer to Earth, and a day was less than 15 hours long. Meteorites fell more frequently, and there was more volcanic activity than there is today.

Within the planetesimal Earth, heavier matter sank to the center and lighter matter rose toward the surface. Most scientists believe that Earth was never truly molten and that this transfer of matter took place in the solid state. Much of the matter that went toward the center contained radioactive material, an important source of Earth’s internal heat. As heavier material moved inward, lighter material moved outward, the planet became layered, and the layers of the core and mantle were formed. This process is called differentiation.

Not long after they formed, more than 4 billion years ago, the Earth and the Moon underwent a period when they were bombarded by meteorites, the rocky debris left over from the formation of the solar system. The impact craters created during this period of heavy bombardment are still visible on the Moon’s surface, which is unchanged. Earth’s craters, however, were long ago erased by weathering, erosion, and mountain building. Because the Moon has no atmosphere, its surface has not been subjected to weathering or erosion. Thus, the evidence of meteorite bombardment remains.

Energy released from the meteorite impacts created extremely high temperatures on Earth that melted the outer part of the planet and created the crust. By 4 billion years ago, both the oceanic and continental crust had formed, and the oldest rocks were created. These rocks are known as the Acasta Gneiss and are found in Canada’s Northwest Territories. Due to the meteorite bombardment, the early Earth was too hot for liquid water to exist and so it was impossible for life to exist.

Geologic Time

Fossil-bearing Rocks

Sedimentary rocks, such as this fossil-bearing limestone, can help geologists determine geologic time. Because the bottom layers were deposited first, the oldest fossils are found in the bottom layers of sedimentary rocks. The accumulation of shells or shell fragments and other fossils in limestone provides geologists with a record of the evolution of the animals that used to live in the ancient oceans.

Carolina Biological Supply/Phototake NYC

Geologists divide the history of the Earth into three eons: the Archean Eon, which lasted from around 4 billion to 2.5 billion years ago; the Proterozoic Eon, which lasted from 2.5 billion to 543 million years ago; and the Phanerozoic Eon, which lasted from 543 million years ago to the present. Each eon is subdivided into different eras. For example, the Phanerozoic Eon includes the Paleozoic Era, the Mesozoic Era, and the Cenozoic Era. In turn, eras are further divided into periods. For example, the Paleozoic Era includes the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian Periods.

Geologic Time Scale

The Archean Eon is subdivided into four eras, the Eoarchean, the Paleoarchean, the Mesoarchean, and the Neoarchean. The beginning of the Archean is generally dated as the age of the oldest terrestrial rocks, which are about 4 billion years old. The Archean Eon ended 2.5 billion years ago when the Proterozoic Eon began. The Proterozoic Eon is subdivided into three eras: the Paleoproterozoic Era, the Mesoproterozoic Era, and the Neoproterozoic Era. The Proterozoic Eon lasted from 2.5 billion years ago to 543 million years ago when the Phanerozoic Eon began. The Phanerozoic Eon is subdivided into three eras: the Paleozoic Era from 543 million to 248 million years ago, the Mesozoic Era from 248 million to 65 million years ago, and the Cenozoic Era from 65 million years ago to the present.

Stratigraphic Column

Fossils preserved in rock strata provide scientists with clues to evolutionary history. This stratigraphic column is based on paleontological evidence and shows the order in which organisms appeared in the fossil-rich Paleozoic era. Each layer represents a particular time frame and shows a representative organism that flourished during that time. Although fossils are rarely found in the idealized and localized fashion shown here, they are often in more or less chronological order. Generally, the oldest fossils appear in lower layers, and the most recent fossils at the top, so that placement may be used as an aid in dating the specimens.

Geologists base these divisions on the study and dating of rock layers or strata, including the fossilized remains of plants and animals found in those layers. Until the late 1800s scientists could only determine the relative ages of rock strata. They knew that in general the top layers of rock were the youngest and formed most recently, while deeper layers of rock were older. The field of stratigraphy shed much light on the relative ages of rock layers.

The study of fossils also enabled geologists to determine the relative ages of different rock layers. The fossil record helped scientists determine how organisms evolved or when they became extinct. By studying rock layers around the world, geologists and paleontologists saw that the remains of certain animal and plant species occurred in the same layers, but were absent or altered in other layers. They soon developed a fossil index that also helped determine the relative ages of rock layers.

Beginning in the 1890s, scientists learned that radioactive elements in rock decay at a known rate. By studying this radioactive decay, they could determine an absolute age for rock layers. This type of dating, known as radiometric dating, confirmed the relative ages determined through stratigraphy and the fossil index and assigned absolute ages to the various strata. As a result scientists were able to assemble Earth’s geologic time scale from the Archean Eon to the present. See also Geologic Time.

Precambrian

Cyanobacteria

Cyanobacteria (formerly blue-green algae) are among the most ancient organisms on earth. These photosynthetic organisms can be single-celled, connected in afilamentous form as shown here, or arranged in simple colonies. Cyanobacteria are capable of enduring a wide variety of environmental conditions ranging from freshwater and marine habitats to snowfields and glaciers. They are capable of surviving and flourishing even at extremely high temperatures.

Peter Parks/Oxford Scientific Films

The Precambrian is a time span that includes the Archean and Proterozoic eons and began about 4 billion years ago. The Precambrian marks the first formation of continents, the oceans, the atmosphere, and life. The Precambrian represents the oldest chapter in Earth’s history that can still be studied. Very little remains of Earth from the period of 4.6 billion to about 4 billion years ago due to the melting of rock caused by the early period of meteorite bombardment. Rocks dating from the Precambrian, however, have been found in Africa, Antarctica, Australia, Brazil, Canada, and Scandinavia. Some zircon mineral grains deposited in Australian rock layers have been dated to 4.2 billion years.

The Precambrian is also the longest chapter in Earth’s history, spanning a period of about 3.5 billion years. During this timeframe, the atmosphere and the oceans formed from gases that escaped from the hot interior of the planet as a result of widespread volcanic eruptions. The early atmosphere consisted primarily of nitrogen, carbon dioxide, and water vapor. As Earth continued to cool, the water vapor condensed out and fell as precipitation to form the oceans. Some scientists believe that much of Earth’s water vapor originally came from comets containing frozen water that struck Earth during the period of meteorite bombardment.

By studying 2-billion-year-old rocks found in northwestern Canada, as well as 2.5-billion-year-old rocks in China, scientists have found evidence that plate tectonics began shaping Earth’s surface as early as the middle Precambrian. About a billion years ago, the Earth’s plates were centered around the South Pole and formed a supercontinent called Rodinia. Slowly, pieces of this supercontinent broke away from the central continent and traveled north, forming smaller continents.

Life originated during the Precambrian. The earliest fossil evidence of life consists of prokaryotes, one-celled organisms that lacked a nucleus and reproduced by dividing, a process known as asexual reproduction. Asexual division meant that a prokaryote’s hereditary material was copied unchanged. The first prokaryotes were bacteria known as archaebacteria. Scientists believe they came into existence perhaps as early as 3.8 billion years ago, but certainly by about 3.5 billion years ago, and were anaerobic—that is, they did not require oxygen to produce energy. Free oxygen barely existed in the atmosphere of the early Earth.

Archaebacteria were followed about 3.46 billion years ago by another type of prokaryote known as cyanobacteria or blue-green algae. These cyanobacteria gradually introduced oxygen in the atmosphere as a result of photosynthesis. In shallow tropical waters, cyanobacteria formed mats that grew into humps called stromatolites. Fossilized stromatolites have been found in rocks in the Pilbara region of western Australia that are more than 3.4 billion years old and in rocks of the Gunflint Chert region of northwest Lake Superior that are about 2.1 billion years old.

For billions of years, life existed only in the simple form of prokaryotes. Prokaryotes were followed by the relatively more advanced eukaryotes, organisms that have a nucleus in their cells and that reproduce by combining or sharing their heredity makeup rather than by simply dividing. Sexual reproduction marked a milestone in life on Earth because it created the possibility of hereditary variation and enabled organisms to adapt more easily to a changing environment. The very latest part of Precambrian time some 560 million to 545 million years ago saw the appearance of an intriguing group of fossil organisms known as the Ediacaran fauna. First discovered in the northern Flinders Range region of Australia in the mid-1940s and subsequently found in many locations throughout the world, these strange fossils appear to be the precursors of many of the fossil groups that were to explode in Earth's oceans in the Paleozoic Era. See also Evolution; Natural Selection.

Paleozoic Era

The Earliest Animals

The earliest known animals on Earth were a bizarre collection of life forms that emerged just prior to and during the Cambrian Period, some of which were exquisitely preserved in fossil beds in various parts of the world. Some of the more extraordinary creatures (depicted in this artist's conception) were the formidable predator Anomalocaris (foreground upper right) about to make a meal of Waptia, which it holds in its extended claws. Just below Anomalocaris and slightly to its left is Opabinia using its long, trunklike snout to grasp Burgessochaeta, a bristle worm. The fernlike objects (left and center) are actually animals, as are the primitive sponges (center foreground) that resemble a saguaro cactus. The depictions of these fernlike animals are based on a group of fossils known as the Ediacaran fossils and date from about 550 million years ago.

D.W. Miller

At the start of the Paleozoic Era about 543 million years ago, an enormous expansion in the diversity and complexity of life occurred. This event took place in the Cambrian Period and is called the Cambrian explosion. Nothing like it has happened since. Almost all of the major groups of animals we know today made their first appearance during the Cambrian explosion. Almost all of the different “body plans” found in animals today—that is, the way an animal’s body is designed, with heads, legs, rear ends, claws, tentacles, or antennae—also originated during this period.

Fishes first appeared during the Paleozoic Era, and multicellular plants began growing on the land. Other land animals, such as scorpions, insects, and amphibians, also originated during this time. Just as new forms of life were being created, however, other forms of life were going out of existence. Natural selection meant that some species were able to flourish, while others failed. In fact, mass extinctions of animal and plant species were commonplace.

Most of the early complex life forms of the Cambrian explosion lived in the sea. The creation of warm, shallow seas, along with the buildup of oxygen in the atmosphere, may have aided this explosion of life forms. The shallow seas were created by the breakup of the supercontinent Rodinia. During the Ordovician, Silurian, and Devonian periods, which followed the Cambrian Period and lasted from 490 million to 354 million years ago, some of the continental pieces that had broken off Rodinia collided. These collisions resulted in larger continental masses in equatorial regions and in the Northern Hemisphere. The collisions built a number of mountain ranges, including parts of the Appalachian Mountains in North America and the Caledonian Mountains of northern Europe.

Toward the close of the Paleozoic Era, two large continental masses, Gondwanaland to the south and Laurasia to the north, faced each other across the equator. Their slow but eventful collision during the Permian Period of the Paleozoic Era, which lasted from 290 million to 248 million years ago, assembled the supercontinent Pangaea and resulted in some of the grandest mountains in the history of Earth. These mountains included other parts of the Appalachians and the Ural Mountains of Asia. At the close of the Paleozoic Era, Pangaea represented over 90 percent of all the continental landmasses. Pangaea straddled the equator with a huge mouthlike opening that faced east. This opening was the Tethys Ocean, which closed as India moved northward creating the Himalayas. The last remnants of the Tethys Ocean can be seen in today’s Mediterranean Sea.

The Paleozoic came to an end with a major extinction event, when perhaps as many as 90 percent of all plant and animal species died out. The reason is not known for sure, but many scientists believe that huge volcanic outpourings of lavas in central Siberia, coupled with an asteroid impact, were joint contributing factors.

Mesozoic Era

Extent of Pleistocene Epoch Glaciation

During the Pleistocene epoch of the Quaternary Ice Age, glaciers (represented on map in white) covered much of the Earth’s northern hemisphere. Ice Ages consist of glacial periods and warmer interglacial periods. Although the Pleistocene, the Earth’s most recent glacial event, ended 10,000 years ago, many scientists believe that the Earth remains in an interglacial state of the Quaternary Ice Age.

The Cenozoic Era, beginning about 65 million years ago, is the period when mammals became the dominant form of life on land. Human beings first appeared in the later stages of the Cenozoic Era. In short, the modern world as we know it, with its characteristic geographical features and its animals and plants, came into being. All of the continents that we know today took shape during this era.

A single catastrophic event may have been responsible for this relatively abrupt change from the Age of Reptiles to the Age of Mammals. Most scientists now believe that a huge asteroid or comet struck the Earth at the end of the Mesozoic and the beginning of the Cenozoic eras, causing the extinction of many forms of life, including the dinosaurs. Evidence of this collision came with the discovery of a large impact crater off the coast of Mexico’s Yucatán Peninsula and the worldwide finding of iridium, a metallic element rare on Earth but abundant in meteorites, in rock layers dated from the end of the Cretaceous Period. The extinction of the dinosaurs opened the way for mammals to become the dominant land animals.

The Cenozoic Era is divided into the Tertiary and the Quaternary periods. The Tertiary Period lasted from about 65 million to about 1.8 million years ago. The Quaternary Period began about 1.8 million years ago and continues to the present day. These periods are further subdivided into epochs, such as the Pleistocene, from 1.8 million to 10,000 years ago, and the Holocene, from 10,000 years ago to the present.

Glaciers

Discovery Enterprises, LLC

Early in the Tertiary Period, Pangaea was completely disassembled, and the modern continents were all clearly outlined. India and other continental masses began colliding with southern Asia to form the Himalayas. Africa and a series of smaller microcontinents began colliding with southern Europe to form the Alps. The Tethys Ocean was nearly closed and began to resemble today’s Mediterranean Sea. As the Tethys continued to narrow, the Atlantic continued to open, becoming an ever-wider ocean. Iceland appeared as a new island in later Tertiary time, and its active volcanism today indicates that seafloor spreading is still causing the country to grow.

Late in the Tertiary Period, about 6 million years ago, humans began to evolve in Africa. These early humans began to migrate to other parts of the world between 2 million and 1.7 million years ago.

The Quaternary Period marks the onset of the great ice ages. Many times, perhaps at least once every 100,000 years on average, vast glaciers 3 km (2 mi) thick invaded much of North America, Europe, and parts of Asia. The glaciers eroded considerable amounts of material that stood in their paths, gouging out U-shaped valleys. Anatomically modern human beings, known as Homo sapiens, became the dominant form of life in the Quaternary Period. Most anthropologists (scientists who study human life and culture) believe that anatomically modern humans originated only recently in Earth’s 4.6-billion-year history, within the past 200,000 years. See also Human Evolution.

VII

EARTH’S FUTURE

With the rise of human civilization about 8,000 years ago and especially since the Industrial Revolution in the mid-1700s, human beings began to alter the surface, water, and atmosphere of Earth. In doing so, they have become active geological agents, not unlike other forces of change that influence the planet. As a result, Earth’s immediate future depends to a great extent on the behavior of humans. For example, the widespread use of fossil fuels is releasing carbon dioxide and other greenhouse gases into the atmosphere and threatens to warm the planet’s surface. This global warming could melt glaciers and the polar ice caps, which could flood coastlines around the world and many island nations. In effect, the carbon dioxide that was removed from Earth’s early atmosphere by the oceans and by primitive plant and animal life, and subsequently buried as fossilized remains in sedimentary rock, is being released back into the atmosphere and is threatening the existence of living things. See also Global Warming.

Even without human intervention, Earth will continue to change because it is geologically active. Many scientists believe that some of these changes can be predicted. For example, based on studies of the rate that the seafloor is spreading in the Red Sea, some geologists predict that in 200 million years the Red Sea will be the same size as the Atlantic Ocean is today. Other scientists predict that the continent of Asia will break apart millions of years from now, and as it does, Lake Baikal in Siberia will become a vast ocean, separating two landmasses that once made up the Asian continent.

In the far, far distant future, however, scientists believe that Earth will become an uninhabitable planet, scorched by the Sun. Knowing the rate at which nuclear fusion occurs in the Sun and knowing the Sun’s mass, astrophysicists (scientists who study stars) have calculated that the Sun will become brighter and hotter about 3 billion years from now, when it will be hot enough to boil Earth’s oceans away. Based on studies of how other Sun-like stars have evolved, scientists predict that the Sun will become a red giant, a star with a very large, hot atmosphere, about 7 billion years from now. As a red giant the Sun’s outer atmosphere will expand until it engulfs the planet Mercury. The Sun will then be 2,000 times brighter than it is now and so hot it will melt Earth’s rocks. Earth will end its existence as a burnt cinder. See also Sun.

Three billion years is the life span of millions of human generations, however. Perhaps by then, humans will have learned how to journey beyond the solar system to colonize other planets in the Milky Way Galaxy and find another place to call “home.”

Reviewed By:
Alan V. Morgan

Geology

INTRODUCTION

Cliffs of Normandy

Composed of the same form of limestone as the White Cliffs of Dover in England, the Cliffs of Normandy are a distinctive landmark on the French coastline. Geological evidence suggests that a land bridge connected the two land masses during the Cretaceous Era. The arches shown here are a result of the action of water on softer sections of the rock.

Alan Carr/Robert Harding Picture Library

Geology, study of the planet earth, its rocky exterior, its history, and the processes that act upon it. Geology is also referred to as earth science and geoscience. The word geology comes from the Greek geo, “earth,” and logia, “the study of.” Geologists seek to understand how the earth formed and evolved into what it is today, as well as what made the earth capable of supporting life. Geologists study the changes that the earth has undergone as its physical, chemical, and biological systems have interacted during its 4.5 billion year history.

Geology is an important way of understanding the world around us, and it enables scientists to predict how our planet will behave. Scientists and others use geology to understand how geological events and earth’s geological history affect people, for example, in terms of living with natural disasters and using the earth’s natural resources. As the human population grows, more and more people live in areas exposed to natural geologic hazards, such as floods, earthquakes, tsunamis, volcanoes, and landslides. Some geologists use their knowledge to try to understand these natural hazards and forecast potential geologic events, such as volcanic eruptions or earthquakes. They study the history of these events as recorded in rocks and try to determine when the next eruption or earthquake will occur. They also study the geologic record of climate change in order to help predict future changes. As human population grows, geologists’ ability to locate fossil and mineral resources, such as oil, coal, iron, and aluminum, becomes more important. Finding and maintaining a clean water supply, and disposing safely of waste products, requires understanding the earth’s systems through which they cycle.

The field of geology includes subfields that examine all of the earth's systems, from the deep interior core to the outer atmosphere, including the hydrosphere (the waters of the earth) and the biosphere (the living component of earth). Generally, these subfields are divided into the two major categories of physical and historical geology. Geologists also examine events such as asteroid impacts, mass extinctions, and ice ages. Geologic history shows that the processes that shaped the earth are still acting on it and that change is normal.

Many other scientific fields overlap extensively with geology, including oceanography, atmospheric sciences, physics, chemistry, botany, zoology, and microbiology. Geology is also used to study other planets and moons in our solar system. Specialized fields of extraterrestrial geology include lunar geology, the study of earth’s moon, and astrogeology, the study of other rocky bodies in the solar system and beyond. Scientific teams currently studying Mars and the moons of Jupiter include geologists.

GUIDING PRINCIPLES OF GEOLOGY

Geologists use three main principles, or concepts, to study earth and its history. The first concept, called plate tectonics, is the theory that the earth’s surface is made up of separate, rigid plates moving and floating over another, less rigid layer of rock. These plates are made up of the continents and the ocean floor as well as the rigid rock beneath them. The second guiding concept is that many processes that occur on the earth may be described in terms of recycling: the reuse of the same materials in cycles, or repeating series of events. The third principle is called uniformitarianism. Uniformitarianism states that the physical and chemical processes that have acted throughout geologic time are the same processes that are observable today. Because of this, geologists can use their knowledge of what is happening on the earth right now to help explain what happened in the past.

Plate Tectonics

Plate Tectonics

Plate tectonics is the unifying theory of geology. It was established in the 1960s, making it one of the most recent revolutions in all of science. The theory describes the lithosphere (the outer rocky layer of the earth) as a collection of rigid plates that move sideways above a less rigid layer called the asthenosphere. The asthenosphere is made up of rock that is under tremendous pressure, which softens it and allows it to move and circulate slowly. Plate tectonics is useful in the field of geology because it can be used to explain a variety of geologic processes, including volcanic activity, earthquakes, and mountain building. See also Earth.

Geologic Cycles

A second guiding principle of geology is the principle of recycling materials, or using materials many times. All processes in geology can be viewed as a series of mostly closed cycles, meaning the materials of the cycles are found on earth, and very few materials from outside our world are introduced into these cycles. The energy that drives geologic recycling comes from two sources: the sun and the earth's interior. Two examples of geologic cycles are the rock cycle and the water cycle.

The rock cycle begins as rocks are uplifted, or pushed up by tectonic forces. The exposed rocks erode as a result of surface processes, such as rain and wind. The eroded particles, or sediment, travel by wind or moving water until they are deposited, and the deposited material settles into layers. Additional sediment may bury these layers until heat and pressure metamorphose, or change, the underlying sediment to metamorphic rock. Additional sediment may compact the layers into sedimentary rocks. Rocks can also be subducted (sunk down into the lower layers of the earth) by plate tectonic processes. Buried and subducted rocks may also melt and recrystallize into igneous rocks (see Magma). Metamorphic, sedimentary, and igneous rocks may then be uplifted, starting the rock cycle again.

The water cycle is also known as the hydrologic cycle. Phases of the water cycle are storage, evaporation, precipitation, and runoff. Water is stored in glaciers, polar ice caps, lakes, rivers, oceans, and in the ground. Heat from the sun evaporates water from the earth’s surface and the water then condenses to form clouds. It falls back to the earth as precipitation, either as rain or snow, then runs into the oceans through rivers or underground and begins the cycle again.

Uniformitarianism

Uniformitarianism, or actualism, helps geologists use their knowledge of modern processes and events to reconstruct the past. The principle of uniformitarianism depends on the 'uniformity of laws,' which assumes that the laws of physics and chemistry have remained constant. To test uniformity of laws, geologists can examine preserved one-billion-year-old ripples that look very much like ripples on the beach today. If gravity had changed, water and sand would have interacted differently in the past, and the ripple evidence would be different. Also, minerals in three-billion-year-old rocks are the same as minerals forming in rocks today, confirming the uniformity of chemical laws. Uniformitarianism contrasts with, for example, the idea that past events such as floods or earthquakes were caused by divine intervention or supernatural causes. Catastrophism, which calls on major catastrophes to explain earth’s history, is also sometimes contrasted with uniformitarianism. However, uniformitarianism can include past catastrophes.

III

THE GEOLOGIC TIME SCALE

Geologic Time Scale

Geologists have created a geologic time scale to provide a common vocabulary for talking about past events. The practice of determining when past geologic events occurred is called geochronology. This practice began in the 1700s and has sometimes involved some personal and international disputes that led to differences in terminology. Today the geologic time scale is generally agreed upon and used by scientists around the world, dividing time into eons, eras, periods, and epochs. Every few years, the numerical time scale is refined based on new evidence, and geologists publish an update.

Geologists use several methods to determine geologic time. These methods include physical stratigraphy, or the placement of events in the order of their occurrence, and biostratigraphy, which uses fossils to determine geologic time. Another method geologists use is correlation, which allows geologists to determine whether rocks in different geographic locations are the same age. In radiometric dating, geologists use the rate of decay of certain radioactive elements in minerals to assign numerical ages to the rocks.

The process of determining geologic time includes several steps. Geologists first determine the relative age of rocks—which rocks are older and which are younger. They then may correlate rocks to determine which rocks are the same age. Next, they construct a geologic time scale. Finally, they determine the specific numerical ages of rocks by various dating methods and assign numbers to the time scale.

Relative Time

Geologists create a relative time scale using rock sequences and the fossils contained within these sequences. The scale they create is based on The Law of Superposition, which states that in a regular series of sedimentary rock strata, or layers, the oldest strata will be at the bottom, and the younger strata will be on top. Danish geologist Nicolaus Steno (also called Niels Stensen) used the idea of uniformity of physical processes. Steno noted that sediment was denser than liquid or air, so it settled until it reached another solid. The newer sediment on the top layer is younger than the layer it settled upon. Since this is what happens in the world today, it should also determine how rock layers formed in the past. Crosscutting relationships are also used to determine the relative age of rocks. For instance, if a thin intrusion of granite, called a dike, cuts through a layer of limestone, the granite must be younger than the limestone.

Biostratigraphy

Fossil-bearing Rocks

Carolina Biological Supply/Phototake NYC

In the field of biostratigraphy geologists study the placement of fossils to determine geologic time. British surveyor William Smith and French anatomist Georges Cuvier both reasoned that in a series of fossil-bearing rocks, the oldest fossils are at the bottom, with successively younger fossils above. They thus extended Steno's Law of Superposition and recognized that fossils could be used to determine geologic time. This principle is called fossil succession. Smith and Cuvier also noted that unique fossils were characteristic of different layers. Biostratigraphy is most useful for determining geologic time during the Phanerozoic Eon (Greek phaneros, “evident”; zoic, “life”), the time of visible and abundant fossil life that has lasted for about the past 570 million years. Although fossils exist that are as old as three billion years or more, they are not common. Few fossils exist that are useful for determining geologic age from time before about 1 billion years ago, so biostratigraphy is of limited use in older sedimentary rocks.

Correlation

Using correlation to determine which rocks are of equal age is important for reconstructing snapshots in geologic history. Correlation may use the physical characteristics of rocks or fossils to determine equivalent age. For example, the limestone at the top of one side of the Grand Canyon can be correlated to the opposite side of the canyon. Also, ash from a volcanic eruption can be correlated over long distances and wide areas. Fossils are the most useful tools for correlation. Since the work of Smith and Cuvier, biostratigraphers have noted that 'like fossils are of like age.” This is the principle of fossil correlation.

Radiometric Dating

Another fundamental goal of geochronology is to determine numerical ages of rocks and to assign numbers to the geologic time scale. The primary tool for this task is radiometric dating, in which the decay of radioactive elements is used to date rocks and minerals. Radiometric dating works best on igneous rocks (rocks that crystallized from molten material). It can also be used to date minerals in metamorphic rocks (rocks that formed when parent rock was submitted to intense heat and pressure and metamorphosed into another type of rock). It is of limited use, however, in sedimentary rocks formed by the compaction of layers of sediment. One of the great triumphs of geochronology is that numbers acquired by radiometric dating matched predictions based on superposition and other means of geologic age determination, confirming the assumption of uniformitarianism. Using dated rocks, geologists have been able to assign numbers to the geologic time scale. See also Dating Methods.

GEOLOGIC SPATIAL SCALES

Uluru

Uluru, also called Ayers Rock, is one of the largest monoliths, or rock masses, in the world. Located in Uluru-Kata Tjuta National Park in central Australia, the monolith measures about 3.6 km (2.2 mi) long and 348 m (1,142 ft) high. Rock paintings made thousands of years ago by Aboriginal artists cover the walls of many caves in Uluru.

Art Wolfe/Tony Stone Images

In order to understand geologic processes and to reconstruct the geologic past, geologists work at different spatial, or size, scales—scales that range from microscopic to planetary. In order to work at these spatial scales, they use a number of tools. At the microscopic level, traditional tools include the petrographic microscope, used to identify minerals and examine rock textures. Modern tools for examining rock chemistry and structure include complex scanning electron microscopes, microprobes that can obtain very small geologic or mineralogic samples, and mass spectrometers (instruments that measure the quantity of atoms, or groups of atoms, in a geologic sample). Geologists can also use lasers and particle accelerators for high-precision work, such as in argon-argon radiometric dating, the use of isotopes of the element argon to date geologic samples.

Some geologic features are very large, and geologists must create detailed maps to observe them completely. Geologists use maps to record basic information, to examine trends, and to understand processes and geologic history. For example, a map may record the locations of historical earthquakes, helping to identify faults. Geologic maps can help geologists understand the history of a mountain belt or locate new mineral deposits. On a planetary scale, geologists can map the earth’s surface using data from orbiting satellites. Geologists also make maps reconstructing a view of the earth at some time in the past; such maps are called paleogeographic maps. Geologists who study Mars map the planet’s surface features with the help of images and information from spacecraft probes sent to Mars.

Traditionally, maps have resulted from fieldwork. In the field, geologists locate exposures of rock, or rock outcrops, and features such as faults, folds, or other geologic structures on a base map or aerial photograph. Mapping has improved through the use of remote sensing techniques, such as radar and infrared mapping from aircraft and satellites, and this in turn has helped geologists better understand the earth. Geologists can now determine latitude and longitude positions on the earth by using the global positioning system of satellites (GPS). Map information can now be stored digitally, as in geographic information systems (GIS). Subsurface, or underground, mapping is becoming more common. This technique uses drilled cores and sound waves sent below the ground to map structures such as faults.

FIELDS OF GEOLOGY

Geologists have found it useful to divide geology into two main fields: physical geology, which examines the nature of the earth in its present state, and historical geology, which examines the changes the earth has undergone throughout time.

Physical Geology

Eruption of Mount Saint Helens

The volcano known as Mount Saint Helens, in the southwestern portion of Washington state in the United States, began to erupt on March 27, 1980, after a long period of dormancy. It continued to burble until its first large-scale eruption on May 18, 1980. This violent blast sent clouds of ash and other volcanic debris into the atmosphere and killed 57 people. With the eruption, the mountain’s elevation dropped from 2950 m (9677 ft) to 2550 m (8365 ft).

Krafft-Explorer/Science Source/Photo Researchers, Inc.

Physical geology can be subdivided into a number of disciplines according to the way geologists study the earth and which physical aspects they study. Fields such as geophysics, geochemistry, mineralogy and petrology, and structural geology apply the sciences of physics and chemistry to study aspects of the earth. Hydrology, geomorphology, and marine geology incorporate the study of water and its effects on weathering into geology, while environmental, economic, and engineering geology apply geologic knowledge and engineering principles to solve practical problems.

Geophysics

In the field of geophysics, geologists apply the concepts of physics to the study of the earth. Geophysics is such a broad field that scientists sometimes consider it a separate field from geology. The largest subdiscipline in geophysics is seismology, the study of the travel of seismic waves through the earth. Seismic waves are generated naturally by earthquakes, or they can be made artificially by explosions from bombs or air guns. Seismologists study earthquakes and construct models of the earth's interior using seismic techniques. Geophysics also includes the study of the physics of materials such as rocks, minerals, and ice within the fields of petrology, mineralogy, and glaciology. Geophysicists study the behavior of the planet’s oceans, atmosphere, and volcanoes. Specialists called volcanologists study the world’s volcanoes and try to predict eruptions by using seismology and other remote sensing techniques, such as satellite imagery. Monitoring active volcanoes is especially important in highly populated areas.

Geochemistry

Geochemistry is the application of chemistry to the study of the earth, its materials, and the cycling of chemicals through its systems. It is essential in numerical dating and in reconstructing past conditions on the earth. Geochemistry is important for tracing the transport of chemicals through the earth’s four component systems: the lithosphere (rocky exterior), the hydrosphere (waters of the earth), the atmosphere (air), and the biosphere (the system of living things). Biogeochemistry is an emerging field that examines the chemical interactions between living and nonliving systems—for example, microorganisms that act in soil formation. Geochemistry has important applications in environmental and economic geology as well as in the fields of mineralogy and petrology.

Mineralogy and Petrology

Rock Crystals in a Lunar Sample

A petrographic microscope is used to make an image of a thin sample of lunar rock. The different colors represent different mineral compositions.

Jan Hinsch/Science Source/Photo Researchers, Inc.

The fields of mineralogy (the study of minerals) and petrology (the study of rocks) are closely related because rocks are made of minerals. Mineralogists and petrologists study the origin, occurrence, structure, and history of rocks or minerals. They attempt to understand the physical, chemical, and less commonly, biological conditions under which geologic materials form. Mineralogy is important for understanding natural materials and is also used in the materials engineering field, such as in ceramics. Petrology focuses on two of the three rock types: igneous rocks—rocks made from molten material—and metamorphic rocks—those rocks that have been changed by high temperatures or pressures. The third rock type, sedimentary rocks, are the focus of sedimentary geology, commonly classified under historical geology.

Structural Geology

Structural geology deals with the form, arrangement, and internal structure of rocks, including their history of deformation, such as folding and faulting. Structural geology includes everything from field mapping to the study of microscopic deformation within rocks. Most geologic reconstructions require an understanding of structural geology. The term tectonics is commonly used for large-scale structural geology, such as the study of the history of a mountain belt, or plate tectonics (the study of the crustal plates). Neotectonics is the study of recent faulting and deformation; such studies can reconstruct the history of active faults, and the history can be used in hazard analysis and land-use planning.

Hydrology and Geomorphology

The earth's surface processes are the focus of hydrology and geomorphology. Hydrology is the study of water on the earth's surface, excluding the oceans. Hydrogeology is the study of groundwater (water under the ground) and the geologic processes of surface water. As water is necessary for life, hydrology and hydrogeology are important for economic and environmental reasons, such as maintaining a clean water supply. Geomorphology is the examination of the development of present landforms; geomorphologists attempt to understand the nature and origin of these landforms. They may work from the large scale of mountain belts to the small scale of rill marks (small grooves in sand). Geomorphologists commonly specialize in one of many areas, such as in glacial or periglacial (near glaciers), fluvial (river), hillslope, or coastal processes. Their work is important for a basic understanding of the active surface that humans live on, a surface that is subject to erosion, landslides, floods, and other processes that affect our daily lives.

Marine Geology

Geology specific to the ocean environment is called marine geology. Marine geologists may be specialists in a number of fields, including petrology, sedimentology, stratigraphy, paleontology, geochemistry, geophysics, and volcanology. They may take samples from the ocean while out at sea or make measurements through remote sensing techniques. Drilling platforms and drilling ships allow earth scientists to make more-detailed studies of the history of the oceans and the ocean floor. For example, in 1984 an international team of geoscientists from 20 nations formed the Ocean Drilling Program, an outgrowth of the earlier Deep Sea Drilling Program. This program is designed to set up drilling through the top sedimentary layer and the ocean crust in deep-sea sites around the world. This work has helped the field of paleoceanography (the reconstruction of the history of the oceans, including ancient ocean chemistry, temperature, circulation, and biology). See also Ocean and Oceanography.

Environmental, Economic, and Engineering Geology

Relief Map

Relief maps are three-dimensional models of the terrain in an area; on them, color and scale are used to indicate geographical features rather than simply to delineate political boundaries. Because of this feature, relief maps are extensively used in engineering and the military. This map shows portions of Alaska and northwestern Canada.

United States Geological Survey

The application of geologic knowledge to practical problems is the focus of the fields of environmental, economic, and engineering geology. Environmental geology involves the protection of human health and safety through understanding geological processes. For example, it is critically important to understand the geology of areas where people propose to store nuclear waste products. The study of geologic hazards, such as earthquakes and volcanic eruptions, can also be considered part of environmental geology. Economic geology is the use of geologic knowledge to find and recover materials that can be used profitably by humans, including fuels, ores, and building materials. Because these products are so diverse, economic geologists must be broadly trained; they commonly specialize in a particular aspect of economic geology, such as petroleum geology or mining geology. Engineering geology is the application of engineering principles to geologic problems. Two fields of engineering that use geology extensively are civil engineering and mining engineering. For example, the stability of a building or bridge requires an understanding of both the foundation material (rocks, soil) and the potential for earthquakes in the area. See also Engineering: Geological and Mining Engineering.

Historical Geology

Historical geology focuses on the study of the evolution of earth and its life through time. Historical geology includes many subfields. Stratigraphy and sedimentary geology are fields that investigate layered rocks and the environments in which they are found. Geochronology is the study of determining the age of rocks, while paleontology is the study of fossils. Other fields, such as paleoceanography, paleoseismology, paleoclimatology, and paleomagnetism, apply geologic knowledge of ancient conditions to learn more about the earth. The Greek prefix paleo is used to identify ancient conditions or periods in time, and commonly means “the reconstruction of the past.”

Stratigraphy

Stratigraphic Column

Fossils preserved in rock strata provide scientists with clues to evolutionary history. This stratigraphic column is based on paleontological evidence and shows the order in which organisms appeared in the fossil-rich Paleozoic era. Each layer represents a particular time frame and shows a representative organism which flourished during that time. Although fossils are rarely found in the idealized and localized fashion shown here, they are often in more or less chronological order. Generally, the oldest fossils appear in lower layers and the most recent fossils at the top, so that placement may be used as an aid in dating the specimens.

Stratigraphy is the study of the history of the earth's crust, particularly its stratified (layered) rocks. Stratigraphy is concerned with determining age relationships of rocks as well as their distribution in space and time. Rocks may be studied in an outcrop but commonly are studied from drilled cores (samples that have been collected by drilling into the earth). Most of the earth's surface is covered with sediment or layered rocks that record much of geologic history; this is what makes stratigraphy important. It is also important for many economic and environmental reasons. A large portion of the world's fossil fuels, such as oil, gas, and coal, are found in stratified rocks, and much of the world's groundwater is stored in sediments or stratified rocks.

Stratigraphy may be subdivided into a number of fields. Biostratigraphy is the use of fossils for age determination and correlation of rock layers; magnetostratigraphy is the use of magnetic properties in rocks for similar purposes. Newer fields in stratigraphy include chemostratigraphy, seismic stratigraphy, and sequence stratigraphy. Chemostratigraphy uses chemical properties of strata for age determination and correlation as well as for recognizing events in the geologic record. For example, oxygen isotopes (forms of oxygen that contain a different number of neutrons in the nuclei of atoms) may provide evidence of an ancient paleoclimate. Carbon isotopes may identify biologic events, such as extinctions. Rare chemical elements may be concentrated in a marker layer (a distinctive layer that can be correlated over long distances). Seismic stratigraphy is the subsurface study of stratified rocks using seismic reflection techniques. This field has revolutionized stratigraphic studies since the late 1970s and is now used extensively both on land and offshore. Seismic stratigraphy is used for economic reasons, such as finding oil, and for scientific studies. An offshoot of seismic stratigraphy is sequence stratigraphy, which helps geologists reconstruct sea level changes throughout time. The rocks used in sequence stratigraphy are bounded by, or surrounded by, surfaces of erosion called unconformities.

Sedimentology

Folded Rocks of the Dynamic Earth

The theory of plate motions explains how mountains are built by forces that shape the earth’s crust. Large pieces of crust on the surface of the earth move laterally. This creates huge compressional forces that may bend or even break rocks. These sedimentary rock layers show an anticlinal fold, in which the layers bend downward from the crest.

V. Englebert/Photo Researchers, Inc.

Sedimentology, or sedimentary geology, is the study of sediments and sedimentary rocks and the determination of their origin. Sedimentary geology is process oriented, focusing on how sediment was deposited. Sedimentologists are geologists who attempt to interpret past environments based on the observed characteristics, called facies, of sedimentary rocks. Facies analysis uses physical, chemical, and biological characteristics to reconstruct ancient environments. Facies analysis helps sedimentologists determine the features of the layers, such as their geometry, or layer shape; porosity, or how many pores the rocks in the layers have; and permeability, or how permeable the layers are to fluids. This type of analysis is important economically for understanding oil and gas reservoirs as well as groundwater supplies.

Geochronology

The determination of the age of rocks is called geochronology. The fundamental tool of geochronology is radiometric dating (the use of radioactive decay processes as recorded in earth materials to determine the numerical age of rocks). Most radiometric dating techniques are useful in dating igneous and metamorphic rocks and minerals. One type of non-radiometric dating, called strontium isotope dating, measures different forms of the element strontium in sedimentary materials to date the layers. Geologists also have ways to determine the ages of surfaces that have been exposed to the sun and to cosmic rays. These methods are called thermoluminescence dating and cosmogenic isotope dating. Geologists can count the annual layers recorded in tree rings, ice cores, and certain sediments such as those found in lakes, for very precise geochronology. However, this method is only useful for time periods up to tens of thousands of years. Some geoscientists are now using Milankovitch cycles (the record of change in materials caused by variations in the earth's orbit) as a geologic time clock. See also Dating Methods: Radiometric Dating.

Paleontology and Paleobiology

Paleontologist with Seismosaurus Bone

Paleontologists often spend hours to uncover a single bone, painstakingly removing the dirt and rock that surrounds it. Here, a paleontologist reattaches a rib bone of a seismosaurus before excavation continues.

Ray Nelson/Phototake NYC

Paleontology is the study of ancient or fossil life. Paleobiology is the application of biological principles to the study of ancient life on earth. These fields are fundamental to stratigraphy and are used to reconstruct the history of organisms' evolution and extinction throughout earth history. The oldest fossils are older than 3 billion years, although fossils do not become abundant and diverse until about 500 million years ago. Different fossil organisms are characteristic of different times, and at certain times in earth history, there have been mass extinctions (times when a large proportion of life disappears). Other organisms then replace the extinct forms. The study of fossils is one of the most useful tools for reconstructing geologic history because plants and animals are sensitive to environmental changes, such as changes in the climate, temperature, food sources, or sunlight. Their fossil record reflects the world that existed while they were alive. Paleontology is commonly divided into vertebrate paleontology (the study of organisms with backbones), invertebrate paleontology (the study of organisms without backbones), and micropaleontology (the study of microscopic fossil organisms). Many other subfields of paleontology exist as well. Paleobotanists study fossil plants, and palynologists study fossil pollen. Ichnology is the study of trace fossils—tracks, trails, and burrows left by organisms. Paleoecology attempts to reconstruct the behavior and relationships of ancient organisms.

Paleoceanography and Paleoclimatology

Paleoceanography (the study of ancient oceans) and paleoclimatology (the study of ancient climates) are two subfields that use fossils to help reconstruct ancient conditions. Scientists also study stable isotopes, or different forms, of oxygen to reconstruct ancient temperatures. They use carbon and other chemicals to reconstruct aspects of ancient oceanographic and climatic conditions. Detailed paleoclimatic studies have used cores from ice sheets in Antarctica and Greenland to reconstruct the last 200,000 years. Ocean cores, tree rings, and lake sediments are also useful in paleoclimatology. Geologists hope that by understanding past oceanographic and climatic changes, they can help predict future change.

HISTORY OF GEOLOGY

Geology originated as a modern scientific discipline in the 18th century, but humans have been collecting systematic knowledge of the earth since at least the Stone Age. In the Stone Age, people made stone tools and pottery, and had to know which materials were useful for these tasks. Between the 4th century and 1st century bc, ancient Greek and Roman philosophers began the task of keeping written records relating to geology. Throughout the medieval and Renaissance periods, people began to study mineralogy and made detailed geologic observations. The 18th and 19th centuries brought widespread study of geology, including the publication of Charles Lyell’s book Principles of Geology, and the National Surveys (expeditions that focused on the collection of geologic and other scientific data). The concept of geologic time was further developed during the 19th century as well. At the end of the 19th century and into the20th century, the field of geology expanded even more. During this time, geologists developed the theories of continental drift, plate tectonics, and seafloor spreading.

Ancient Greek and Roman Philosophers

In western science, the first written records of geological thought come from the Greeks and Romans. In the 1st century bc, for example, Roman architect Vitruvius wrote about building materials such as pozzolana, a volcanic ash that Romans used to make hydraulic cement, which hardened under water. Historian Pliny the Elder, in his encyclopedia, Naturalis Historia (Natural History), summarized Greek and Roman ideas about nature.

Science as an organized system of thought can trace its roots back to the Greek philosopher Aristotle. In the 4th century bc Aristotle developed a philosophical system that explained nature in a methodical way. His system proposed that the world is made of four elements (earth, air, fire, and water), with four qualities (cold, hot, dry, and wet), and four causes (material, efficient, formal, and final). According to Aristotle, elements could change into one another, and the earth was filled with water and air, which could rush about and cause earthquakes. Other philosophers of this era who wrote about earth materials and processes include Aristotle's student Theophrastus, the author of an essay on stones.

Chinese Civilizations

Chinese civilizations developed ideas about the earth and technologies for studying the earth. For example, in 132 AD the Chinese philosopher Chang Heng invented the earliest known seismoscope. This instrument had a circle of dragons holding balls in their mouths, surrounded by frogs at the base. The balls would drop into the mouths of frogs when an earthquake occurred. Depending on which ball was dropped, the direction of the earthquake could be determined.

Medieval and Renaissance Periods

The nature and origin of minerals and rocks interested many ancient writers, and mineralogy may have been the first systematic study to arise in the earth sciences. The Saxon chemist Georgius Agricola wrote De Re Metallica (On the Subject of Metals) following early work by both the Islam natural philosopher Avicenna and the German naturalist Albertus Magnus. De Re Metallica was published in 1556, a year after Agricola’s death. Many consider this book to be the foundation of mineralogy, mining, and metallurgy.

Medieval thought was strongly influenced by Aristotle, but science began to move in a new direction during the Renaissance Period. In the early 1600s, English natural philosopher Francis Bacon reasoned that detailed observations were required to make conclusions. Around this time French philosopher René Descartes argued for a new, rational system of thought. Most natural philosophers, or scientists, in this era studied many aspects of philosophy and science, not focusing on geology alone.

Studies of the earth during this time can be placed in three categories. The first, cosmology, proposed a structure of the earth and its place in the universe. As an example of a cosmology, in the early 1500s Polish astronomer Nicolaus Copernicus proposed that the earth was a satellite in a sun-centered system. The second category, cosmogony, concerned the origin of the earth and the solar system. The Saxon mathematician and natural philosopher Gottfried Wilhelm, Baron von Leibniz, in a cosmogony, described an initially molten earth, with a crust that cooled and broke up, forming mountains and valleys. The third category of study was in the tradition of Francis Bacon, and it involved detailed observations of rocks and related features. English scientist Robert Hooke and Danish anatomist and geologist Nicolaus Steno (Niels Stenson) both made observations in the 17th century of fossils and studied other geologic topics as well. In the 17th century, mineralogy also continued as an important field, both in theory and in practical matters, for example, with the work of German chemist J. J. Becher and Irish natural philosopher Robert Boyle.

Geology in the 18th and 19th Centuries

By the 18th century, geological study began to emerge as a separate field. Italian mining geologist Giovanni Arduino, Prussian chemist and mineralogist Johan Gottlob Lehmann, and Swedish chemist Torbern Bergman all developed ways to categorize the layers of rocks on the earth's surface. The German physician Georg Fuchsel defined the concept of a geologic formation—a distinctly mappable body of rocks. The German scientist Abraham Gottlob Werner called himself a geognost (a knower of the earth). He used these categorizations to develop a theory that the earth's layers had precipitated from a universal ocean. Werner's system was very influential, and his followers were known as Neptunists. This system suggested that even basalt and granite were precipitated from water. Others, such as English naturalists James Hutton and John Playfair, argued that basalt and granite were igneous rocks, solidified from molten materials, such as lava and magma. The group that held this belief became known as Volcanists or Plutonists.

By the early 19th century, many people were studying geologic topics, although the term geologist was not yet in general use. Scientists, such as Scottish geologist Charles Lyell, and French geologist Louis Constant Prevost, wanted to establish geology as a rational scientific field, like chemistry or physics. They found this goal to be a challenge in two important ways. First, some people wanted to reconcile geology with the account of creation in Genesis (a book of the Old Testament) or wanted to use supernatural explanations for geologic features. Second, others, such as French anatomist Georges Cuvier, used catastrophes to explain much of earth’s history. In response to these two challenges, Lyell proposed a strict form of uniformitarianism, which assumed not only uniformity of laws but also uniformity of rates and conditions. However, assuming the uniformity of rates and conditions was incorrect, because not all processes have had constant rates throughout time. Also, the earth has had different conditions throughout geologic time—that is, the earth as a rocky planet has evolved. Although Lyell was incorrect to assume uniformity of rates and conditions, his well reasoned and very influential three-volume book, Principles of Geology, was published and revised 11 times between 1830 and 1872. Many geologists consider this book to mark the beginning of geology as a professional field.

Although parts of their theories were rejected, Abraham Gottlob Werner and Georges Cuvier made important contributions to stratigraphy and historical geology. Werner's students and followers went about attempting to correlate rocks according to his system, developing the field of physical stratigraphy. Cuvier and his co-worker Alexandre Brongniart, along with English surveyor William Smith, established the principles of biostratigraphy, using fossils to establish the age of rocks and to correlate them from place to place. Later, with these established stratigraphies, geologists used fossils to reconstruct the history of life's evolution on earth.

Age of Geologic Exploration

In the late 18th and the 19th centuries, naturalists on voyages of exploration began to make important contributions to geology. Reports by German natural historian Alexander von Humboldt about his travels influenced the worlds of science and art. The English naturalist Charles Darwin, well known for his theory of evolution, began his scientific career on the voyage of the HMS Beagle, where he made many geological observations. American geologist James Dwight Dana sailed with the Wilkes Expedition throughout the Pacific and made observations of volcanic islands and coral reefs. In the 1870s, the HMS Challenger was launched as the first expedition specifically to study the oceans.

Expeditions on land also led to new geologic observations. Countries and states established geological surveys in order to collect information and map geologic resources. For example, in the 1860s and 1870s , , John Wesley Powell, and George Wheeler conducted four surveys of the American West. These surveys led to several new concepts in geology. American geologist described the Basin and Range Province and first recognized laccoliths (round igneous rock intrusions). Reports also came back of spectacular sites such as Yellowstone, Yosemite, and the Grand Canyon, which would later become national parks. Competition between these survey parties finally led the Congress of the United States to establish the U.S. Geological Survey in 1879.

Geologic Time

Determining the age of the earth became a renewed scholarly effort in the 19th century. Unlike the Greeks and most eastern philosophers, who considered the earth to be eternal, western philosophers believed that the planet had a definite beginning and must have a measurable age. One way to measure this age was to count generations in the Bible, as the Anglican Archbishop James Ussher did in the 1600s, coming up with a total of about 6000 years. In the 1700s, French natural scientist George Louis Leclerc (Comte de Buffon) tried to measure the age of the earth. He calculated the time it would take the planet to cool based on the cooling rates of iron balls and came up with 75,000 years. During the 18th century, James Hutton argued that processes such as erosion, occurring at observed rates, indicated an earth that was immeasurably old. By the early 19th century, geologists commonly spoke in terms of 'millions of years.' Even religious professors, such as English clergyman and geologist William Buckland, referred to this length of time.

Other means for calculating the age of the earth used in the 19th century included determining how long it would take the sea to become salty and calculating how long it would take for thick piles of sediment to accumulate. Irish physicist William Thomson (Lord Kelvin) returned to Buffon's method and calculated that the earth was no more than 100 million years old. Meanwhile, Charles Darwin and others argued that evolution proceeded slowly enough that it required at least hundreds of millions of years.

With the discovery of radioactivity in 1896 by French physicist Henri Becquerel, scientists, such as British physicist Ernest Rutherford and American radiochemist Bertram Boltwood, recognized that the ages of minerals and rocks could be determined by radiometric dating. By the early 20th century, Boltwood had dated some rocks to be more than 2 billion years old. During this time, English geologist Arthur Holmes began a long career of refining the dates on the geologic time scale, a practice that continues to this day.

Theory of Continental Drift

In 1910 American geologist Frank B. Taylor proposed that lateral (sideways) motion of continents caused mountain belts to form on their front edges. Building on this idea in 1912, German meteorologist Alfred Wegener proposed a theory that came to be known as Continental Drift: He proposed that the continents had moved and were once part of one, large supercontinent called Pangaea. Wegener was attempting to explain the origin of continents and oceans when he expanded upon Taylor’s idea. His evidence included the shapes of continents, the physics of ocean crust, the distribution of fossils, and paleoclimatology data.

Continental drift helped to explain a major geologic issue of the 19th century: the origin of mountains. Theories commonly called on the cooling and contracting of the earth to form mountain chains. The mountain-building theories of German geologist Leopold von Buch and French geologist Leonce Elie de Beaumont were catastrophic in nature. American geologists James Hall and James Dwight Dana proposed the geosynclinal theory of mountain building—a theory based on the downward bending of the earth’s crust (a geosyncline). Austrian geologist Eduard Suess developed a related theory. Hall, Dana, and Suess believed that continents and ocean basins were ancient, permanent features on earth and that mountain belts formed at their edges.

Most geologists did not accept the theory of continental drift in the 1920s and 1930s. British geologist Arthur Holmes supported continental drift and proposed that convection (a type of heat movement) inside the earth drove continental drift. Others who favored the idea included South African geologist Alex du Toit, who studied geologic evidence for the southern continents of Gondwanaland, part of the hypothetical supercontinent Pangaea. Other scientists, such as British geophysicist Harold Jeffreys, argued that continental drift was physically impossible. Paleontologists, such as American George Gaylord Simpson, said that the distribution of fossils could be explained by other means.

Theory of Seafloor Spreading

After World War II, geophysical evidence began to accumulate that confirmed the lateral motion of continents and indicated the young age of oceanic crust. This evidence led to the theories of seafloor spreading and plate tectonics in the 1960s. American marine geologists Robert S. Dietz and Harry H. Hess proposed the seafloor spreading hypothesis, the concept that the oceanic crust is created as the seafloor spreads apart along midocean ridges. American oceanographers Bruce C. Heezen, Marie Tharp, and others prepared detailed maps of the ocean floors and the mid-Atlantic ridge and rift system, a mountainous chain found throughout the ocean. These maps provided additional evidence that seemed to support the continental drift theory. Further evidence came from paleomagnetism, the record of the orientation of earth's magnetic field recorded in rocks. In the 1950s, British geophysicist S. Keith Runcorn determined that this evidence indicated that the continents had moved relative to the earth’s magnetic poles and to each other. British marine geophysicists Fred J. Vine and Drummond Matthews described the record of changes in the earth’s magnetic field when they discovered “magnetic stripes” formed at spreading centers of the mid-ocean ridges, leading to the Vine-Matthews hypothesis. Magnetic stripes were also independently described by Canadian geophysicist Lawrence Morley and confirmed by American marine geologist Walter Pitman and others. These stripes indicated reversals of the direction of the earth’s magnetic field recorded in rock as new ocean crust was created at mid-ocean ridges. Scientists used paleomagnetism and seafloor spreading to determine that the continents had moved relative to the magnetic poles and to each other.

Theory of Plate Tectonics

Canadian geophysicist J. Tuzo Wilson and American geophysicist Jason Morgan, among others, proposed the outline of the theory of plate tectonics in the 1960s. This theory stated that the earth’s lithosphere is made up of several rigid plates. These plates slide and move over a less-rigid layer called the asthenosphere. A plate may be composed entirely of oceanic crust, like the Pacific Plate, or of part ocean crust and part continental crust, like the North American Plate. New ocean crust is generated at ocean ridges (underwater mountain chains formed by the young ocean crust). Older ocean crust sinks down, or subducts, into the earth’s mantle at subduction zones, which are found at the deepest parts of the ocean, called trenches. As the plates move, they collide and form mountains. The plates recycle crust, generate volcanoes, and move past each other along faults. Using satellites, scientists can now measure movement of the continental plates in centimeters per year. Plate boundaries are the sites of most of the earth's earthquakes and the majority of earth's volcanoes. The continents are made of remelted sediments and partially melted oceanic crust, forming a lower density layer that has collected through time. The mechanism that drives the earth’s crustal plates is still not known, but geologists can use plate tectonics to explain most geologic activity. See also Earth.

Earth as a Planetary Body

The full recognition by scientists of earth as a planetary body, combining the fields of solar-system astronomy and geology, is perhaps the latest revolution in the earth sciences. Although scientists have recognized earth as a planet for centuries, space exploration that began in the 1960s created a new view of the earth. Photographs of earth taken from space had a profound effect on how people saw the earth. The exploration of neighboring moons and planets has led to a new understanding of the earth as an evolving planet.

Contributed By:
Joanne Bourgeois

Global Warming

GlobalGlobal Warming

INTRODUCTION

Global Warming, increase in the average temperature of the atmosphere, oceans, and landmasses of Earth. The planet has warmed (and cooled) many times during the 4.65 billion years of its history. At present Earth appears to be facing a rapid warming, which most scientists believe results, at least in part, from human activities. The chief cause of this warming is thought to be the burning of fossil fuels, such as coal, oil, and natural gas, which releases into the atmosphere carbon dioxide and other substances known as greenhouse gases. As the atmosphere becomes richer in these gases, it becomes a better insulator, retaining more of the heat provided to the planet by the Sun.

The average surface temperature of Earth is just below 15°C (59°F). Over the last century, this average has risen by about 0.6 Celsius degree (1 Fahrenheit degree). Scientists predict further warming of 1.4 to 5.8 Celsius degrees (2.5 to 10.4 Fahrenheit degrees) by the year 2100. This temperature rise is expected to melt polar ice caps and glaciers as well as warm the oceans, all of which will expand ocean volume and raise sea level by an estimated 9 to 100 cm (4 to 40 in), flooding some coastal regions and even entire islands. Some regions in warmer climates will receive more rainfall than before, but soils will dry out faster between storms. This soil desiccation may damage food crops, disrupting food supplies in some parts of the world. Plant and animal species will shift their ranges toward the poles or to higher elevations seeking cooler temperatures, and species that cannot do so may become extinct. The potential consequences of global warming are so great that many of the world's leading scientists have called for international cooperation and immediate action to counteract the problem.

THE GREENHOUSE EFFECT

Greenhouse Effect

The energy that lights and warms Earth comes from the Sun. Most of the energy that floods onto our planet is short-wave radiation, including visible light. When this energy strikes the surface of Earth, the energy changes from light to heat and warms Earth. Earth’s surface, in turn, releases some of this heat as long-wave infrared radiation.

Carbon Cycle

Photosynthesis plays a crucial role in the carbon cycle. Carbon continuously circulates in the earth’s ecosystem. In the atmosphere, it exists as colorless, odorless carbon dioxide gas, which is used by plants in the process of photosynthesis. Animals acquire the carbon stored in plant tissue when they eat and exhale carbon dioxide as a by-product of metabolism. Although some carbon is removed from circulation temporarily as coal, petroleum, fossil fuels, gas, and limestone deposits, cellular respiration and photosynthesis balance to keep the amount of atmospheric carbon relatively stable. Industrialization, however, has contributed additional carbon dioxide to the environment.

Much of this long-wave infrared radiation makes it all the way back out to space, but a portion remains trapped in Earth’s atmosphere. Certain gases in the atmosphere, including water vapor, carbon dioxide, and methane, provide the trap. Absorbing and reflecting infrared waves radiated by Earth, these gases conserve heat as the glass in a greenhouse does and are thus known as greenhouse gases. As the concentration of these greenhouse gases in the atmosphere increases, more heat energy remains trapped below. All life on Earth relies on this greenhouse effect—without it, the planet would be colder by about 33 Celsius degrees (59 Fahrenheit degrees), and ice would cover Earth from pole to pole. However, a growing excess of greenhouse gases in Earth’s atmosphere threatens to tip the balance in the other direction—toward continual warming.

III

TYPES OF GREENHOUSE GASES

Greenhouse gases occur naturally in the environment and also result from human activities. By far the most abundant greenhouse gas is water vapor, which reaches the atmosphere through evaporation from oceans, lakes, and rivers.

Carbon dioxide is the next most abundant greenhouse gas. It flows into the atmosphere from many natural processes, such as volcanic eruptions; the respiration of animals, which breathe in oxygen and exhale carbon dioxide; and the burning or decay of organic matter, such as plants. Carbon dioxide leaves the atmosphere when it is absorbed into ocean water and through the photosynthesis of plants, especially trees. Photosynthesis breaks up carbon dioxide, releasing oxygen into the atmosphere and incorporating the carbon into new plant tissue.

Industrial Smokestacks

Carbon dioxide, sulfur dioxide, and other types of contaminants pouring from industrial smokestacks contribute largely to the world’s atmospheric pollution. Carbon dioxide contributes significantly to global warming, while sulfur dioxide emissions are the principal cause of acid rain in the northeastern United States.

Kim Westerskov/Oxford Scientific Films

Humans escalate the amount of carbon dioxide released to the atmosphere when they burn fossil fuels, solid wastes, and wood and wood products to heat buildings, drive vehicles, and generate electricity. At the same time, the number of trees available to absorb carbon dioxide through photosynthesis has been greatly reduced by deforestation, the long-term destruction of forests by indiscriminate cutting of trees for lumber or to clear land for agricultural activities.

Ultimately, the oceans and other natural processes absorb excess carbon dioxide in the atmosphere. However, human activities have caused carbon dioxide to be released to the atmosphere at rates much faster than that at which Earth’s natural processes can cycle this gas. In 1750 there were about 281 molecules of carbon dioxide per million molecules of air (abbreviated as parts per million, or ppm). In 2006 two major scientific organizations—the World Meteorological Organization (WMO) and the United States National Oceanic and Atmospheric Administration (NOAA)—reported that levels of carbon dioxide in the atmosphere had hit a record high. Using different measurement techniques, the WMO said carbon dioxide levels had risen to 377 ppm, an annual increase of 1.8 ppm, and the NOAA reported a figure of 381 ppm for a yearly increase of 2.6 ppm. If current predictions prove accurate, by the year 2100 carbon dioxide will reach concentrations of more than 540 to 970 ppm. At the highest estimation, this concentration would be triple the levels prior to the Industrial Revolution, the widespread replacement of human labor by machines that began in Britain in the mid-18th century and soon spread to other parts of Europe and to the United States.

Photosynthesis

Methane is an even more effective insulator, trapping over 20 times more heat than does the same amount of carbon dioxide. Methane is emitted during the production and transport of coal, natural gas, and oil. Methane also comes from rotting organic waste in landfills, and it is released from certain animals, especially cows, as a byproduct of digestion. Since the beginning of the Industrial Revolution in the mid-1700s, the amount of methane in the atmosphere has more than doubled.

Nitrous oxide is a powerful insulating gas released primarily by burning fossil fuels and by plowing farm soils. Nitrous oxide traps about 300 times more heat than does the same amount of carbon dioxide. The concentration of nitrous oxide in the atmosphere has increased 17 percent over preindustrial levels.

In addition, greenhouse gases are produced in many manufacturing processes. Perfluorinated compounds result from the smelting of aluminum. Hydrofluorocarbons form during the manufacture of many products, including the foams used in insulation, furniture, and car seats. Refrigerators built in some developing nations still use chlorofluorocarbons as coolants. In addition to their ability to retain atmospheric heat, some of these synthetic chemicals also destroy Earth’s high-altitude ozone layer, the protective layer of gases that shields Earth from damaging ultraviolet radiation. For most of the 20th century these chemicals have been accumulating in the atmosphere at unprecedented rates. But since 1995, in response to regulations enforced by the Montréal Protocol on Substances that Deplete the Ozone Layer and its amendments, the atmospheric concentrations of many of these gases are either increasing more slowly or decreasing.

Scientists are growing concerned about other gases produced from manufacturing processes that pose an environmental risk. In 2000 scientists identified a substantial rise in atmospheric concentrations of a newly identified synthetic compound called trifluoromethyl sulfur pentafluoride. Atmospheric concentrations of this gas are rising quickly, and although it still is extremely rare in the atmosphere, scientists are concerned because the gas traps heat more effectively than all other known greenhouse gases. Perhaps more worrisome, scientists have been unable to confirm the industrial source of the gas.

MEASURING GLOBAL WARMING

GOES Weather Satellite

Broadcasters use data from meteorological satellites to predict weather and to broadcast storm warnings when necessary. Satellites such as the Geostationary Operational Environmental Satellite (GOES) collect meteorological and infrared information about the atmosphere and the ocean. A camera on the GOES is continuously pointed at Earth, broadcasting satellite images of cloud patterns both day and night. Here, the GOES-C satellite is being encapsulated inside its payload fairing aboard a Delta rocket.

NASA

As early as 1896 scientists suggested that burning fossil fuels might change the composition of the atmosphere and that an increase in global average temperature might result. The first part of this hypothesis was confirmed in 1957, when researchers working in the global research program called the International Geophysical Year sampled the atmosphere from the top of the Hawaiian volcano Mauna Loa. Their instruments indicated that carbon dioxide concentration was indeed rising. Since then, the composition of the atmosphere has been carefully tracked. The data collected show undeniably that the concentrations of greenhouse gases in the atmosphere are increasing.

Scientists have long suspected that the global climate, the long-term average pattern of temperature, was also growing warmer, but they were unable to provide conclusive proof. Temperatures vary widely all the time and from place to place. It takes many years of climate observations to establish a trend. Records going back to the late 1800s did seem to show a warming trend, but these statistics were spotty and untrustworthy. Early weather stations often were located near cities, where temperature measurements were affected by the heat emitted from buildings and vehicles and stored by building materials and pavements. Since 1957, however, data have been gathered from more reliable weather stations, located far away from cities, and from satellites. These data have provided new, more accurate measurements, especially for the 70 percent of the planetary surface that is ocean water (see Satellite, Artificial). These more accurate records indicate that a surface warming trend exists and that, moreover, it has become more pronounced. Looking back from the end of the 20th century, records show that the ten warmest years of the century all occurred after 1980, and the three hottest years occurred after 1990, with 2005 being the warmest year of all.

Greenhouse gas concentrations are increasing. Temperatures are rising. But does the gas increase necessarily cause the warming, and will these two phenomena continue to occur together? In 1988 the United Nations Environment Program and the World Meteorological Organization established a panel of 200 leading scientists to consider the evidence. In its Third Assessment Report, released in 2001, this Intergovernmental Panel on Climate Change (IPCC) concluded that global air temperature had increased 0.6 Celsius degree (1 Fahrenheit degree) since 1861. The panel agreed that the warming was caused primarily by human activities that add greenhouse gases to the atmosphere. The IPCC predicted in 2001 that the average global temperature would rise by another 1.4 to 5.8 Celsius degrees (2.5 to 10.4 Fahrenheit degrees) by the year 2100.

The IPCC panel cautioned that even if greenhouse gas concentrations in the atmosphere ceased growing by the year 2100, the climate would continue to warm for a period after that as a result of past emissions. Carbon dioxide remains in the atmosphere for a century or more before nature can dispose of it. If greenhouse gas emissions continue to increase, experts predict that carbon dioxide concentrations in the atmosphere could rise to more than three times preindustrial levels early in the 22nd century, resulting in dramatic climate changes. Large climate changes of the type predicted are not unprecedented; indeed, they have occurred many times in the history of Earth. However, human beings would face this latest climate swing with a huge population at risk.

EFFECTS OF GLOBAL WARMING

Shrinking Greenland Ice Sheet

The Greenland ice sheet underwent extensive surface melting from 1992 to 2002, according to the Arctic Climate Impact Assessment report released in 2004. Tinted areas in these satellite images show the extent of surface melting in 1992 and in 2002. The report warned that the melting of Greenland’s ice sheet and the melting of glaciers in Alaska and Canada are increasingly contributing to a rise in the world’s sea level.

Clifford Grabhorn/Courtesy of ACIA 2004

Scientists use elaborate computer models of temperature, precipitation patterns, and atmosphere circulation to study global warming. Based on these models, scientists have made several predictions about how global warming will affect weather, sea levels, coastlines, agriculture, wildlife, and human health.

Weathe

Storm Surge Barrier, The Netherlands

Some experts predict that an increase in global warming will result in unpredictable weather patterns, including storm surges in which wind piles up water in low-lying areas. The curved arms of the New Waterway Storm Surge Barrier in The Netherlands protect Rotterdam and other inland cities from flooding during large storms on the North Sea. Normally, the large, curved arms are retracted to allow ships from the North Sea to travel to ports along the New Waterway. When a dangerous storm is anticipated, the arms are swung out to block off the waterway and prevent large waves from pushing floodwaters inland.

ANP-Foto

Scientists predict that during global warming, the northern regions of the Northern Hemisphere will heat up more than other areas of the planet, northern and mountain glaciers will shrink, and less ice will float on northern oceans. Regions that now experience light winter snows may receive no snow at all. In temperate mountains, snowlines will be higher and snowpacks will melt earlier. Growing seasons will be longer in some areas. Winter and nighttime temperatures will tend to rise more than summer and daytime ones.

The warmed world will be generally more humid as a result of more water evaporating from the oceans. Scientists are not sure whether a more humid atmosphere will encourage or discourage further warming. On the one hand, water vapor is a greenhouse gas, and its increased presence should add to the insulating effect. On the other hand, more vapor in the atmosphere will produce more clouds, which reflect sunlight back into space, which should slow the warming process (see Water Cycle).

Greater humidity will increase rainfall, on average, about 1 percent for each Fahrenheit degree of warming. (Rainfall over the continents has already increased by about 1 percent in the last 100 years.) Storms are expected to be more frequent and more intense. However, water will also evaporate more rapidly from soil, causing it to dry out faster between rains. Some regions might actually become drier than before. Winds will blow harder and perhaps in different patterns. Hurricanes, which gain their force from the evaporation of water, are likely to be more severe. Against the background of warming, some very cold periods will still occur. Weather patterns are expected to be less predictable and more extreme.

Sea Levels

Flooding in Bangladesh

An increase in global warming will likely result in a rise in sea levels that could threaten many coastal areas around the world. Experts predict that parts of Bangladesh may become completely submerged if sea levels rise.

Express Newspapers/Getty Images

As the atmosphere warms, the surface layer of the ocean warms as well, expanding in volume and thus raising sea level. Warming will also melt much glacier ice, especially around Greenland, further swelling the sea. Sea levels worldwide rose 10 to 25 cm (4 to 10 in) during the 20th century, and IPCC scientists predict a further rise of 9 to 88 cm (4 to 35 in) in the 21st century.

Sea-level changes will complicate life in many coastal regions. A 100-cm (40-in) rise could submerge 6 percent of The Netherlands, 17.5 percent of Bangladesh, and most or all of many islands. Erosion of cliffs, beaches, and dunes will increase. Storm surges, in which winds locally pile up water and raise the sea, will become more frequent and damaging. As the sea invades the mouths of rivers, flooding from runoff will also increase upstream. Wealthier countries will spend huge amounts of money to protect their shorelines, while poor countries may simply evacuate low-lying coastal regions.

Even a modest rise in sea level will greatly change coastal ecosystems. A 50-cm (20-in) rise will submerge about half of the present coastal wetlands of the United States. New marshes will form in many places, but not where urban areas and developed landscapes block the way. This sea-level rise will cover much of the Florida Everglades.

Agriculture

A warmed globe will probably produce as much food as before, but not necessarily in the same places. Southern Canada, for example, may benefit from more rainfall and a longer growing season. At the same time, the semiarid tropical farmlands in some parts of Africa may become further impoverished. Desert farm regions that bring in irrigation water from distant mountains may suffer if the winter snowpack, which functions as a natural reservoir, melts before the peak growing months. Crops and woodlands may also be afflicted by more insects and plant diseases.

Animals and Plants

Animals and plants will find it difficult to escape from or adjust to the effects of warming because humans occupy so much land. Under global warming, animals will tend to migrate toward the poles and up mountainsides toward higher elevations, and plants will shift their ranges, seeking new areas as old habitats grow too warm. In many places, however, human development will prevent this shift. Species that find cities or farmlands blocking their way north or south may die out. Some types of forests, unable to propagate toward the poles fast enough, may disappear.

Human Health

In a warmer world, scientists predict that more people will get sick or die from heat stress, due less to hotter days than to warmer nights (giving the sufferers less relief). Diseases now found in the tropics, transmitted by mosquitoes and other animal hosts, will widen their range as these animal hosts move into regions formerly too cold for them. Today 45 percent of the world’s people live where they might get bitten by a mosquito carrying the parasite that causes malaria; that percentage may increase to 60 percent if temperatures rise. Other tropical diseases may spread similarly, including dengue fever, yellow fever, and encephalitis. Scientists also predict rising incidence of allergies and respiratory diseases as warmer air grows more charged with pollutants, mold spores, and pollens.

DEBATES OVER GLOBAL WARMING

Scientists do not all agree about the nature and impact of global warming. A few observers still question whether temperatures have actually been rising at all. Others acknowledge past change but argue that it is much too early to be making predictions for the future. Such critics may also deny that the evidence for the human contribution to warming is conclusive, arguing that a purely natural cycle may be driving temperatures upward. The same dissenters tend to emphasize the fact that continued warming could have benefits in some regions.

Scientists who question the global warming trend point to three puzzling differences between the predictions of the global warming models and the actual behavior of the climate. First, the warming trend stopped for three decades in the middle of the 20th century; there was even some cooling before the climb resumed in the 1970s. Second, the total amount of warming during the 20th century was only about half what computer models predicted. Third, the troposphere, the lower region of the atmosphere, did not warm as fast as the models forecast. However, global warming proponents believe that two of the three discrepancies have now been explained.

The lack of warming at midcentury is now attributed largely to air pollution that spews particulate matter, especially sulfates, into the upper atmosphere. These particulates, also known as aerosols, reflect some incoming sunlight out into space. Continued warming has now overcome this effect, in part because pollution control efforts have made the air cleaner.

The unexpectedly small amount of total warming since 1900 is now attributed to the oceans absorbing vast amounts of the extra heat. Scientists long suspected that this was happening but lacked the data to prove it. In 2000 the U.S. National Oceanic and Atmospheric Administration (NOAA) offered a new analysis of water temperature readings made by observers around the world over 50 years. Records showed a distinct warming trend: World ocean temperatures in 1998 were higher than the 50-year average by 0.2 Celsius degree (0.3 Fahrenheit degree), a small but very significant amount.

The third discrepancy is the most puzzling. Satellites detect less warming in the troposphere than the computer models of global climate predict. According to some critics, the atmospheric readings are right, and the higher temperatures recorded at Earth’s surface are not to be trusted. In January 2000 a panel appointed by the National Academy of Sciences to weigh this argument reaffirmed that surface warming could not be doubted. However, the lower-than-predicted troposphere measurements have not been entirely explained.

VII

EFFORTS TO CONTROL GLOBAL WARMING

The total consumption of fossil fuels is increasing by about 1 percent per year. No steps currently being taken or under serious discussion will likely prevent global warming in the near future. The challenge today is managing the probable effects while taking steps to prevent detrimental climate changes in the future.

Damage can be curbed locally in various ways. Coastlines can be armored with dikes and barriers to block encroachments of the sea. Alternatively, governments can assist coastal populations in moving to higher ground. Some countries, such as the United States, still have the chance to help plant and animal species survive by preserving habitat corridors, strips of relatively undeveloped land running north and south. Species can gradually shift their ranges along these corridors, moving toward cooler habitats.

There are two major approaches to slowing the buildup of greenhouse gases. The first is to keep carbon dioxide out of the atmosphere by storing the gas or its carbon component somewhere else, a strategy called carbon sequestration. The second major approach is to reduce the production of greenhouse gases.

Carbon Sequestration

The simplest way to sequester carbon is to preserve trees and to plant more. Trees, especially young and fast-growing ones, soak up a great deal of carbon dioxide, break it down in photosynthesis, and store the carbon in new wood. Worldwide, forests are being cut down at an alarming rate, particularly in the tropics. In many areas, there is little regrowth as land loses fertility or is changed to other uses, such as farming or building housing developments. Reforestation could offset these losses and counter part of the greenhouse buildup.

Many companies and governments in the United States, Norway, Brazil, Malaysia, Russia, and Australia have initiated reforestation projects. In Guatemala, the AES Corporation, a U.S.-based electrical company, has joined forces with the World Resources Institute and the relief agency CARE to create community woodlots and to teach local residents about tree-farming practices. The trees planted are expected to absorb up to 58 million tons of carbon dioxide over 40 years.

Carbon dioxide gas can also be sequestered directly. Carbon dioxide has traditionally been injected into oil wells to force more petroleum out of the ground or seafloor. Now it is being injected simply to isolate it underground in oil fields, coal beds, or aquifers. At one natural gas drilling platform off the coast of Norway, carbon dioxide brought to the surface with the natural gas is captured and reinjected into an aquifer from which it cannot escape. The same process can be used to store carbon dioxide released by a power plant, factory, or any large stationary source. Deep ocean waters could also absorb a great deal of carbon dioxide. The feasibility and environmental effects of both these options are now under study by international teams.

In an encouraging trend, energy use around the world has slowly shifted away from fuels that release a great deal of carbon dioxide toward fuels that release somewhat less of this heat-trapping gas. Wood was the first major source of energy used by humans. With the dawn of the Industrial Revolution in the 18th century, coal became the dominant energy source. By the mid-19th century oil had replaced coal in dominance, fueling the internal combustion engines that were eventually used in automobiles. By the 20th century, natural gas began to be used worldwide for heating and lighting. In this progression, combustion of natural gas releases less carbon dioxide than oil, which in turn releases less of the gas than do either coal or wood.

Nuclear energy, though controversial for reasons of safety and the high costs of nuclear waste disposal, releases no carbon dioxide at all. Solar power, wind power, and hydrogen fuel cells also emit no greenhouse gases. Someday these alternative energy sources may prove to be practical, low-pollution energy sources, although progress today is slow. See also Energy Supply, World; Wind Energy.

National and Local Programs

The developed countries are all working to reduce greenhouse emissions. Several European countries impose heavy taxes on energy usage, designed partly to curb such emissions. Norway taxes industries according to the amount of carbon dioxide they emit. In The Netherlands, government and industry have negotiated agreements aimed at increasing energy efficiency, promoting alternative energy sources, and cutting down greenhouse gas output.

In the United States, the Department of Energy, the Environmental Protection Agency, product manufacturers, local utilities, and retailers have collaborated to implement the Energy Star program. This voluntary program rates appliances for energy use and gives some money back to consumers who buy efficient machines. The Canadian government has established the FleetWise program to cut carbon dioxide emissions from federal vehicles by reducing the number of vehicles it owns and by training drivers to use them more efficiently.

Many local governments are also working against greenhouse emissions by conserving energy in buildings, modernizing their vehicles, and advising the public. Individuals, too, can take steps. The same choices that reduce other kinds of pollution work against global warming. Every time a consumer buys an energy-efficient appliance; adds insulation to a house; recycles paper, metal, and glass; chooses to live near work; or commutes by public transportation, he or she is fighting global warming.

International Agreements

International cooperation is required for the successful reduction of greenhouse gases. In 1992 at the Earth Summit in Rio de Janeiro, Brazil, 150 countries pledged to confront the problem of greenhouse gases and agreed to meet again to translate these good intentions into a binding treaty.

In 1997 in Japan, 160 nations drafted a much stronger agreement known as the Kyōto Protocol. This treaty, which went into force in February 2005, calls for the 38 industrialized countries that now release the most greenhouse gases to cut their emissions to levels 5 percent below those of 1990. This reduction is to be achieved no later than 2012. Initially, the United States voluntarily accepted a more ambitious target, promising to reduce emissions to 7 percent below 1990 levels; the European Union, which had wanted a much tougher treaty, committed to 8 percent; and Japan, to 6 percent. The remaining 122 nations, mostly developing nations, were not asked to commit to a reduction in gas emissions.

But in 2001 newly elected U.S. president George W. Bush renounced the treaty, saying that such carbon dioxide reductions in the United States would be too costly. He also objected that developing nations would not be bound by similar carbon dioxide reducing obligations. The Kyōto Protocol could not go into effect unless industrial nations accounting for 55 percent of 1990 greenhouse gas emissions ratified it. That requirement was met in 2004 when the cabinet of Russian president Vladimir Putin approved the treaty, paving the way for it to go into effect in 2005.

Some critics find the Kyōto Protocol too weak. Even if it were enforced immediately, it would only slightly slow the buildup of greenhouse gases in the atmosphere. Much stronger action would be required later, particularly because the developing nations exempted from the Kyōto rules are expected to produce half the world’s greenhouse gases by 2035. The most influential opponents of the protocol, however, find it too strong. Opposition to the treaty in the United States is spurred by the oil industry, the coal industry, and other enterprises that manufacture or depend on fossil fuels. These opponents claim that the economic costs to carry out the Kyōto Protocol could be as much as $300 billion, due mainly to higher energy prices. Proponents of the Kyōto sanctions believe the costs will prove more modest—$88 billion or less—much of which will be recovered as Americans save money after switching to more efficient appliances, vehicles, and industrial processes.

Behind the issue of cost lies a larger question: Can an economy grow without increasing its greenhouse gas emissions at the same time? In the past, prosperity and pollution have tended to go together. Can they now be separated, or decoupled, as economists say? In nations with strong environmental policies, economies have continued to grow even as many types of pollution have been reduced. However, limiting the emission of carbon dioxide has proved especially difficult. For example, The Netherlands, a heavily industrialized country that is also an environmental leader, has done very well against most kinds of pollution but has failed to meet its goal of reducing carbon dioxide output.

After 1997 representatives to the Kyōto Protocol met regularly to negotiate a consensus about certain unresolved issues, such as the rules, methods, and penalties that should be enforced in each country to slow greenhouse emissions. The negotiators designed a system in which nations with successful cleanup programs could profit by selling unused pollution rights to other nations. For example, nations that find further improvement difficult, such as The Netherlands, could buy pollution credits on the market, or perhaps earn them by helping reduce greenhouse gas emissions in less developed countries, where more can be achieved at less expense. Russia, in particular, stood to benefit from this system. In 1990 the Russian economy was in a shambles, and its greenhouse gas emissions were huge. Since then Russia has already cut its emissions by more than 5 percent below 1990 levels and is in a position to sell emission credits to other industrialized countries, particularly those in the European Union (EU).

Contributed By:
John Hart

Geomorphology

INTRODUCTION

Slope Development

Landforms and landscapes change over time as a result of various dynamic factors. These factors include tectonic movement, weather, erosion, and gravity. At any given moment, a landscape may include one or more of the features shown.

Geomorphology, scientific study of landforms and landscapes. The term usually applies to the origins and dynamic morphology (changing structure and form) of the earth's land surfaces, but it can also include the morphology of the seafloor and the analysis of extraterrestrial terrains. Sometimes included in the field of physical geography, geomorphology is really the geological aspect of the visible landscape. The science has developed in two distinctive ways that must be integrated in order for the whole picture of landscapes to emerge.

HISTORICAL GEOMORPHOLOGY

One approach to the science of landforms is by means of historical, cyclic geomorphology. The concepts involved were worked out at the turn of the 20th century by the American geologist William Morris Davis, who stated that every landform could be analyzed in terms of structure, process, and stage. The first two are also treated by process geomorphology, discussed below; but the third, by introducing the element of time, is subject to a far greater degree of interpretation. Davis argued that every landform underwent development through a predictable, cyclic sequence: youth, maturity, and old age.

Historical geomorphology relies on various chronological analyses, notably those provided by stratigraphic studies of the last 2 million years, known as the Quaternary period. The relative chronology usually may be worked out by observation of stratigraphic relationships, and the time intervals involved may then be established more precisely by dating methods such as historical records, radiocarbon analysis, tree-ring counting (dendrochronology), and paleomagnetic studies. By applying such methods to stratigraphic data, a quantitative chronology of events is constructed that furnishes a basis for calculating long-term rates of change.

III

PROCESS GEOMORPHOLOGY

This second branch of geomorphology analyzes contemporary dynamic processes at work in landscapes. The mechanisms involved—weathering and erosion—combine processes that are in some respects destructive and in others constructive. The bedrock and soil provide the passive material, whereas the climatic regime and crustal dynamics together provide the principal active variables.

UNDERLYING DYNAMICS

Folded Rocks

Geomorphologists study the shape of the earth’s surface and the various processes that change the landscape. For example when large pieces of the earth’s crust move laterally, they create huge compressional forces that can bend or even break rocks. Here, the sedimentary rock layers show an anticlinal fold, in which the layers bend downward from the crest.

V. Englebert/Photo Researchers, Inc.

In geomorphological processes, gravity is an all-pervading, essentially invariable energy factor; a second variable, energy flow is provided by solar radiation. The latter is expressed either as a direct thermal variable or, indirectly, through the hydrologic cycle, which involves evaporation of water from the ocean, atmospheric transport of water, precipitation as rain or snow, and a return to the ocean by various processes. A third energy factor is heat flow from the earth's interior. Although of a magnitude considerably less than solar energy, this heat flow ultimately is responsible for creating major geological structures such as faults, but rates of change tend to be quite low (usually less than 1 mm per year). Nonetheless, in particular zones—for example, along crustal-plate boundaries (see Plate Tectonics) such as the San Andreas fault—stress may build up until released catastrophically in violent displacements of up to 12 m (40 ft). Locally, heat flow from the interior is concentrated in eruptions of magma (molten rock), which produce a variety of volcanic landforms.

WEATHERING AND EROSION

Glacial Erosion

Glaciers, such as this icy formation in Switzerland, erode the land over which they move. Glacial erosion is caused by distinct processes, such as abrasion, crushing, and fracturing of the glacier. Climate changes affect the size of glaciers, and glaciers move by growing or shrinking depending on the climate. Moving glaciers erode and transport large quantities of rocks, sand, and other particles along their path. Glacial erosion processes form features such as hanging valleys, moraines, cirques, horns, and scoured rock faces.

Paolo Koch/Photo Researchers, Inc.

Weathering is often a combination of three processes: the mechanical process, as in the growth of ice or salt crystals or in thermal heating and cooling; the chemical process, as in acid-water solutions that tend to dissolve minerals such as calcite and feldspar; and the biological process, as in the effect of plant roots, which generate both mechanical and chemical energy. Erosion is the dislodging, removal, and transport of material, either in solution or in particle form. The energy to accomplish this may be provided in the form of raindrops, running water, wind, waves, or simply gravity (as in a landslide).

An eroding landmass tends to rise to compensate for the removal of the load, but it eventually stabilizes as land relief decreases and stream gradients decline. The resulting surface, almost flat, is called a peneplain. It may be interrupted, here and there, by isolated hills called monadnocks consisting of rocks especially resistant to erosion. The theoretical base level of such a surface—the ultimate grade of streams—is mean sea level. For a peneplain to form and not be destroyed by renewed erosion, sea level must remain stable for millions of years. However, since the end of the Quaternary Ice Age, 10,000 years ago, sea level has risen hundreds of feet.

Human-induced soil erosion is a feature of the present day and of the last few millennia, because clearing land of native vegetation or excessive grazing by domesticated animals exposes the soil to massive erosion. In this way some 3 billion metric tons of particulate material are washed from the surface of the U.S. alone each year. In undisturbed natural settings, on the other hand—notably in low-relief continental interiors—erosion rates are very slow (except in semiarid areas where thunderstorms produce flash floods). In structurally active belts such as in youthful mountains, which as a rule coincide with plate boundaries that recently collided or rifted, erosion rates may be enormous.

Of all the different processes acting on the earth's surface, rain and rivers are the most vigorous erosive agents. By contrast, although wave action on a rocky coast is often impressive, the rate of retreat of the shoreline is generally very slow. Sand dunes in the Sahara are also impressive, but the sand is only a relatively thin veneer; and the moraines left by giant continental glaciers are likewise only superficial scrapings of ancient soils. In general, without human interference, the landscape is stable.

Contributed By:
Rhodes W. Fairbridge

All about islam

Islam

INTRODUCTION

The Qur’an

The inscription on buildings of verses from the Qur’an symbolizes the living presence of the holy book in Islamic society. This tower with decorative Qur’anic inscriptions is in Delhi, India.

Gillian Darley/Edifice/Corbis

Islam, one of the three major world religions, along with Judaism and Christianity, that profess monotheism, or the belief in a single God.

In the Arabic language, the word Islam means “surrender” or “submission”—submission to the will of God. A follower of Islam is called a Muslim, which in Arabic means “one who surrenders to God.” The Arabic name for God, Allah, refers to the God worshiped by Jews and Christians. Islam’s central teaching is that there is only one all-powerful, all-knowing God, and this God created the universe. This rigorous monotheism, as well as the Islamic teaching that all Muslims are equal before God, provides the basis for a collective sense of loyalty to God that transcends class, race, nationality, and even differences in religious practice. Thus, all Muslims belong to one community, the umma, irrespective of their ethnic or national background.

Within two centuries after its rise in the 7th century, Islam spread from its original home in Arabia into Syria, Egypt, North Africa, and Spain to the west, and into Persia, India, and, by the end of the 10th century, beyond to the east. In the following centuries, Islam also spread into Anatolia and the Balkans to the north, and sub-Saharan Africa to the south. The Muslim community comprises about 1 billion followers on all five continents, and Islam is the fastest-growing religion in the world. The most populous Muslim country is Indonesia, followed by Pakistan and Bangladesh. Beyond the Middle East, large numbers of Muslims live in India, Nigeria, the former republics of the Union of Soviet Socialist Republics (USSR), and China.

One of the reasons for the growth of the Muslim community has been its openness to new members. Children born to Muslim parents are automatically considered Muslim. At any time, a non-Muslim can convert to Islam by declaring himself or herself to be a Muslim. A person’s declaration of faith is sufficient evidence of conversion to Islam and need not be confirmed by others or by religious authorities.

THE TEACHINGS OF MUHAMMAD

Mecca, Saudi Arabia

The al-Haram Mosque in Mecca, Saudi Arabia, holds the holiest shrine of Islam, the Kaaba. As the birthplace of Islam’s founder, the Prophet Muhammad, Mecca is considered a holy city. It is a pilgrimage point for Muslims worldwide, who are expected to visit the city at least once if they are able to do so.

Mehmet Biber/Photo Researchers, Inc.

Around the year ad 570 Muhammad, the founding prophet of Islam, was born in Mecca, at the time the central city of the Arabian Peninsula. Some 40 years later Muhammad started preaching a new religion, Islam, which constituted a marked break from existing moral and social codes in Arabia. The new religion of Islam taught that there was one God, and that Muhammad was the last in a series of prophets and messengers. Through his messengers God had sent various codes, or systems of laws for living, culminating in the Qur’an (Koran), the holy book of Islam. These messengers were mortal men, and they included among many others Moses, the Hebrew prophet and lawgiver, and Jesus, whom Christians believe to be the son of God rather than a prophet.

Islam also taught that the Christian Bible (which includes the Hebrew Bible as the Old Testament and an additional 27 books referred to as the New Testament), and the Qur'an were all holy books. According to the Qur’an, the two earlier Scriptures had been altered over time from their original forms given by God, while the Qur'an would remain perfect, preserved by God from such distortion. In addition to distinguishing itself from the Hebrew and Christian traditions, the new religion taught that the God of Islam had provided humanity with the means to know good from evil, through the prophets and the Qur’an. Therefore, on the Day of Judgment people will be held accountable for their actions.

Muhammad’s teachings met with severe and hostile opposition, and in the year 622 he left Mecca and sought refuge in the city of Yathrib, as a number of his followers had already done. Upon Muhammad's arrival, the name Yathrib was changed to Medina (meaning “the city”). The date of Muhammad's immigration was later set as the beginning of the 12-month lunar Islamic calendar.

III

THE FIVE PILLARS

During the ten years between his arrival in Medina and his death in ad 632, Muhammad laid the foundation for the ideal Islamic state. A core of committed Muslims was established, and a community life was ordered according to the requirements of the new religion. In addition to general moral injunctions, the requirements of the religion came to include a number of institutions that continue to characterize Islamic religious practice today. Foremost among these were the five pillars of Islam, the essential religious duties required of every adult Muslim who is mentally able. The five pillars are each described in some part of the Qur’an and were already practiced during Muhammad's lifetime. They are the profession of faith (shahada), prayer (salat), almsgiving (zakat), fasting (sawm), and pilgrimage (hajj). Although some of these practices had precedents in Jewish, Christian, and other Middle Eastern religious traditions, taken together they distinguish Islamic religious practices from those of other religions. The five pillars are thus the most central rituals of Islam and constitute the core practices of the Islamic faith.

Many polemical descriptions of Islam have focused critically on the Islamic concept of jihad. Jihad, considered the sixth pillar of Islam by some Muslims, has been understood to mean holy war in these descriptions. However, the word in Arabic means 'to struggle' or 'to exhaust one's effort,' in order to please God. Within the faith of Islam, this effort can be individual or collective, and it can apply to leading a virtuous life; helping other Muslims through charity, education, or other means; preaching Islam; and fighting to defend Muslims. Western media of the 20th century continue to focus on the militant interpretations of the concept of jihad, whereas most Muslims do not.

The Profession of Faith

The absolute focus of Islamic piety is Allah, the supreme, all knowing, all-powerful, and above all, all-merciful God. The Arabic word Allah means “the God,” and this God is understood to be the God who brought the world into being and sustains it to its end. By obeying God's commands, human beings express their recognition of and gratitude for the wisdom of creation, and live in harmony with the universe.

The profession of faith, or witness to faith (shahada), is therefore the prerequisite for membership in the Muslim community. On several occasions during a typical day, and in the saying of daily prayers, a Muslim repeats the profession, 'I bear witness that there is no god but Allah and that Muhammad is his messenger.' There are no formal restrictions on the times and places these words can be repeated. To become a member of the Muslim community, a person has to profess and act upon this belief in the oneness of God and the prophethood of Muhammad. To be a true profession of faith that represents a relationship between the speaker and God, the verbal utterance must express genuine knowledge of its meaning as well as sincere belief. A person’s deeds can be subjected to scrutiny by other Muslims, but a person’s utterance of the profession of faith is sufficient evidence of membership in the Muslim community and cannot be challenged by other members of this community.

The Five Daily Prayers

Minaret of the Great Mosque at Sāmarrā’

This spiral minaret, where the muezzin once called the faithful to prayer, is the only surviving feature of the Great Mosque at Sāmarrā’, Iraq. At the time of its construction (848-852), the Great Mosque at Sāmarrā’ was the largest Islamic mosque in the world.

SEF/Art Resource, NY

The second pillar of Islam is the religious duty to perform five prescribed daily prayers or salat. All adult Muslims are supposed to perform five prayers, preceded by ritual cleansing or purification of the body at different intervals of the day. The Qur’anic references also mention the acts of standing, bowing, and prostrating during prayers and facing a set direction, known as qibla. The Muslims were first required to face Jerusalem during prayer, but already during Muhammad's lifetime they were commanded to face the Kaaba, an ancient shrine in the city of Mecca. The Qur’an also refers to the recitation of parts of the Qur’an as a form of prayer. However, even with its numerous references, the Qur’an alone does not give exact instructions for this central ritual of prayer.

The most detailed descriptions of the rituals for prayer derive from the example set by the prophet Muhammad and are preserved in later Islamic traditions. Some details of these rituals vary, however all Muslims agree that there are five required daily prayers to be performed at certain times of day: dawn (fajr or subh), noon (zuhr), midafternoon (asr), sunset (maghrib), and evening (isha). The dawn, noon, and sunset prayers do not start exactly at dawn, noon, and sunset; instead, they begin just after, to distinguish the Islamic ritual from earlier practices of worshiping the sun when it rises or sets.

Layout of a Mosque

Mosques are laid out in accordance with Muslim prayer. They are generally organized around a courtyard, a reminder of the courtyard of Muhammad’s house, which served as the first mosque. Muslims pray facing the holy city of Mecca, a direction known as the qibla. A mihrab, or prayer niche, indicates the qibla, and the main prayer hall stands on the qibla side. Worshipers, called to prayer by a crier from the minaret, may hear a sermon delivered from the mimbar near the mihrab. A mosque that has a vaulted hall, or eyvan on each side of its courtyard, as this mosque does, is known as a four-eyvan mosque.

A prayer is made up of a sequence of units called bowings (rak’as). During each of these units, the worshiper stands, bows, kneels, and prostrates while reciting verses from the Qur’an as well as other prayer formulas. With some variations among different Muslim sects, at noon, afternoon, and evening prayers, these units are repeated four times, while during the sunset prayer they are repeated three times, and at dawn only twice. The opening chapter of the Qur’an, al-Fatiha, is repeated in each unit in a prayer sequence. Each prayer concludes with the recitation of the profession of faith followed by the greeting 'may the peace, mercy, and blessings of God be upon you.'

Wherever Muslims live in substantial numbers throughout the world, the call to prayer, or adhan, is repeated five times a day by a muezzin (crier) from a mosque, the Muslim place of worship. Muslims are encouraged to pray together in mosques, but group prayer is only a religious obligation for the noon prayer on Friday. Women, travelers, sick Muslims, and those attending to the sick are granted license not to attend the Friday congregational prayer, although they may attend if they wish.

The Friday noon prayer is led by an imam, who is simply a prayer leader; this prayer differs from the usual noon prayers of the other days of the week. As a required part of the ritual at this congregational meeting, two sermons precede the prayer. On other days, Muslims can pray anywhere they wish, either individually or in groups. They must observe the rituals of praying at certain times of day, facing in the direction of Mecca, observing the proper order of prayers, and preparing through symbolic purification. Depending on the situation, this last ritual of ablution requires either total washing of the body or a less elaborate ritual washing of the hands, mouth, face, and feet.

In addition to the five required daily prayers, Muslims can perform non-obligatory prayers, some of which have fixed ritual formats and are performed before or after each of the five daily prayers. Others are performed at night, either individually or with other Muslims. These additional formal and informal prayers give expression to the primary function of prayer in Islam, which is personal communication with God for the purpose of maintaining the abiding presence of the divine in the personal lives of Muslims. The more formal aspects of prayer also serve to provide a disciplined rhythm that structures the day and fosters a sense of community and shared identity among Muslims.

Almsgiving

The third pillar of Islam is zakat, or almsgiving. A religious obligation, zakat is considered an expression of devotion to God. It represents the attempt to provide for the poorer sectors of society, and it offers a means for a Muslim to purify his or her wealth and attain salvation. The Qur’an, together with other Islamic traditions, strongly encourages charity and constantly reminds Muslims of their moral obligation to the poor, orphans, and widows; however, it distinguishes between general, voluntary charity (sadaqa) and zakat, the latter being an obligatory charge on the money or produce of Muslims. While the meaning of terms has been open to different interpretations, the Qur’an regularly refers to zakat, identifying specific ways in which this tax can be spent. These specific uses include spending zakat on the poor and the needy, on those who collect and distribute zakat, on those whom Muslims hope to win over and convert to Islam, on travelers, on the ransom of captives, to relieve those who are burdened with debts, and on the cause of God.

The Qur’an provides less-detailed information about the kinds of things that are subject to the zakat tax or the precise share of income or property that should be paid as zakat. These determinations are provided in the traditions of the prophet Muhammad and have been the subject of elaborate discussions among Muslim legal experts, or jurists. For example, one-fortieth (2.5 percent) of the assets accumulated during the year (including gold, silver, and money) is payable at the end of the year, while one-tenth of the harvest of the land or date trees is payable at harvest time. Cattle, camels, and other domestic animals are subject to a more complex taxation system that depends on the animals in question, their age, the numbers involved, and whether they are freely grazing. Traditional zakat laws do not cover trade, but commercial taxes have been imposed by various Muslim governments throughout history.

Fasting

Last Day of Ramadan

Muslims pray in the upper gallery of a main mosque in the old walled city of Delhi, India. They are celebrating the end of Ramadan, the holy month of fasting as ordained by the Qur’an, the sacred scriptures of Islam.

REUTERS/CORBIS-BETTMANN

The fourth pillar of Islam is sawm, or fasting. Clear Qur’anic references to fasting account for the early introduction of this ritual practice. The Qur’an prescribes fasting during the month of Ramadan, the 9th month of the 12-month Islamic lunar year (see Calendar). The month of Ramadan is sacred because the first revelation of the Qur’an is said to have occurred during this month. By tradition the month starts with the sighting of the new moon by at least two Muslims. For the entire month, Muslims must fast from daybreak to sunset by refraining from eating, drinking, and sexual intercourse. Menstruating women, travelers, and sick people are exempted from fasting but have to make up the days they miss at a later date.

Festivities Ending Ramadan

Members of the Tarabin Bedouin tribe in Egypt prepare food for a three-day festival that marks the end of the Islamic holy month, Ramadan. During Ramadan adult Muslims fast from sunrise to sunset.

Ruth Fremson/AP/Wide World Photos

According to various traditional interpretations, the fast introduces physical and spiritual discipline, serves to remind the rich of the misfortunes of the poor, and fosters, through this rigorous act of worship, a sense of solidarity and mutual care among Muslims of all social backgrounds. Thus Muslims usually engage in further acts of worship beyond the ordinary during Ramadan, such as voluntary night prayer, reading sections from the Qur’an, and paying voluntary charity to the poor. Muslims may even choose to wake before daybreak to eat a meal that will sustain them until sunset. After the fasting ends, the holiday of breaking the fast, ‘id al-fitr, begins, lasting for three days.

At any time of year fasting is also required as a compensation for various offenses and violations of the law. Many Muslims also perform voluntary fasts at various times of the year as acts of devotion and spiritual discipline. However, such additional fasting is not required by Islamic law.

Pilgrimage to Mecca

Pilgrimage to the Kaaba

Muslims consider the Kaaba—a small sanctuary near the center of the Great Mosque in Mecca— to be the most sacred spot on earth. Muslim legend teaches that the ancient religious patriarchs Abraham and Ishmael built the shrine using foundations first laid by Adam. Muslims all over the world orient themselves toward the Kaaba while praying, and every able Muslim is expected to make a pilgrimage to the Kaaba at least once in his or her lifetime. This picture shows pilgrimage ceremonies, which consist of several days of rituals and festivals during the Islamic month of pilgrimages, Dhu al-Hijja.

Mehmet Biber/Photo Researchers, Inc.

The fifth pillar requires that Muslims who have the physical and financial ability should perform the pilgrimage, or hajj, to Mecca at least once in a lifetime. The ritual of pilgrimage was practiced by Arabs before the rise of Islam and continues from the early days of Islam. The hajj is distinct from other pilgrimages. It must take place during the 12th lunar month of the year, known as Dhu al-Hijja, and it involves a set and detailed sequence of rituals that are practiced over the span of several days. All of the pilgrimage rituals take place in the city of Mecca and its surroundings, and the primary focus of these rituals is a cubical structure called the Kaaba. According to Islamic tradition, the Kaaba, also referred to as the House of God, was built at God's command by the prophet Ibrahim (Abraham of the Hebrew and Christian Bibles) and his son Ismail (see Ishmael).

The Qur’an provides detailed descriptions of various parts of the ritual, and it portrays many of these rituals as reenactments of the activities undertaken by Ibrahim and Ismail in the course of building the Kaaba. Set into one corner of the Kaaba is the sacred Black Stone, which according to one Islamic tradition was given to Ibrahim by the angel Gabriel. According to another Islamic tradition this stone was first set in place by Adam.

Once pilgrims arrive in Mecca, ritual purification is performed. Many men shave their heads, and most men and women put on seamless white sheets. This simple and common dress symbolizes the equality of all Muslims before God, a status further reinforced by the prohibition of jewelry, perfumes, sexual intercourse, and hunting. After this ritual purification, Muslims circle the Kaaba seven times, run between al-Safa and al-Marwa, two hills overlooking the Kaaba, seven times, and perform several prayers and invocations. This ritual is a reenactment of the search by Hagar for water to give her son Ismail.

After these opening rituals, the hajj proper commences on the seventh day and continues for the next three days. Again, it starts with the performance of ritual purification followed by a prayer at the Kaaba mosque. The pilgrims then assemble at Mina, a hill outside Mecca, where they spend the night. The next morning they go to the nearby plain of Arafat, where they stand from noon to sunset and perform a series of prayers and rituals. The pilgrims then head to Muzdalifa, a location halfway between Arafat and Mina, to spend the night. The next morning, the pilgrims head back to Mina, on the way stopping at stone pillars symbolizing Satan, at which they throw seven pebbles.

Dome of the Rock

The oldest extant Islamic structure, the Dome of the Rock stands on the sacred rock in Jerusalem where the Prophet Muhammad is believed to have ascended to heaven. Caliph Abd al-Malik built the mosque during the late 7th century. The mosque’s basic octagonal design encloses a central space topped by a dome. A rich mosaic decoration covers the outer walls.

Israel Ministry of Tourism

The final ritual is the slaughter of an animal (sheep, goat, cow, or camel). This is a symbolic reenactment of God's command to Ibrahim to sacrifice his son Ismail, which Ibrahim and Ismail duly accepted and were about to execute when God allowed Ibrahim to slaughter a ram in place of his son. Most of the meat of the slaughtered animals is to be distributed to poor Muslims. The ritual sacrifice ends the hajj and starts the festival of the sacrifice, ‘id al-adha. The festivals of breaking fast (‘id al-fitr) at the end of Ramadan and ‘id al-adha are the two major Islamic festivals celebrated by Muslims all over the world.

During the pilgrimage most Muslims visit Medina, where the tomb of the Prophet is located, before returning to their homes. If the pilgrimage rituals are performed at any time of the year other than the designated time for hajj, the ritual is called umra. Although umra is considered a virtuous act, it does not absolve the person from the obligation of hajj. Most pilgrims perform one or more umras before or after the hajj proper.

Many Muslims pilgrims also travel to Jerusalem, which is the third sacred city for Islam. Muslims believe Muhammad was carried to Jerusalem in a vision. The Dome of the Rock houses the stone from which Muhammad is believed to have ascended to heaven and Allah in a night journey. Some Muslims perform pilgrimages to the Dome of the Rock and to other shrines where revered religious figures are buried. Some of these shrines are important primarily to the local populations, whereas others draw Muslims from distant regions. There are no standard prescribed rituals for these pilgrimages nor are they treated as obligatory acts of worship.

THE MOSQUE

Mosque of Córdoba in Spain

This mosque in Córdoba, Spain, was begun in ad 786, while the city was the capital of Moorish Spain. Although the mosque became a Christian cathedral after the Roman Catholics of Spain captured Córdoba in 1236, the building retains its Islamic heritage. The mosque features columns that support horseshoe-shaped arches decorated with stripes of alternating colors. Layered in two tiers, these distinctly Moorish arches convey a light and airy feeling to the interior of the building.

Adam Lubroth/Art Resource, NY

Of all Muslim institutions, the mosque is the most important place for the public expression of Islamic religiosity and communal identity. A mosque is a physical manifestation of the public presence of Muslims and serves as a point of convergence for Islamic social and intellectual activity. The Arabic word for mosque is masjid, which means a 'place of prostration' before God. Mosques are mentioned in the Qur’an, and the earliest model for a mosque was the residence that the prophet Muhammad built when he moved to Medina. This first mosque was an enclosure marked as a special place of worship. A small part of the mosque was sectioned off to house the Prophet and his family, and the remaining space was left open as a place for Muslims to pray.

Although later mosques developed into complex architectural structures built in diverse styles, the one requirement of all mosques continues to be based on the earliest model: a designation of space for the purpose of prayer. The early mosque served an equally important function that thousands of mosques continue to serve today: The mosque is a place where Muslims foster a collective identity through prayer and attend to their common concerns. A Muslim city typically has numerous mosques but only a few congregational or Friday mosques where the obligatory Friday noon prayers are performed.

Mosque in Burkina Faso

The Grand Mosque of Bobo-Dioulasso in Burkina Faso is built of mud brick, the local building material. About half the people in Burkina Faso are Muslims.

J. Hartley/Panos Pictures

As Islam spread outside Arabia, Islamic architecture was influenced by the various architectural styles of the conquered lands, and both simple and monumental mosques of striking beauty were built in cities of the Islamic world. Despite the borrowings from diverse civilizations, certain common features became characteristic of most mosques and thus serve to distinguish them from the sacred spaces of other religions and cultures.

Mosque in Nouakchott

A mosque in Nouakchott, the capital of Mauritania. Islam is the state religion of Mauritania and is professed by nearly all Mauritanians.

Christian Sappa/RAPHO

The most important characteristic of a mosque is that it should be oriented toward Mecca. One or more niches (mihrab) on one of the walls of the mosque often serve as indicators of this direction, called qibla. When the imam leads the prayers he usually faces one of these niches. Next to the mihrab, a pulpit (minbar) is often provided for the delivery of sermons (khutba). Many mosques also have separate areas for performing ritual ablution, and separate sections for women. In many mosques, several rows of columns are used to mark the way for worshipers to line up behind the imam during prayer.

Mosques usually have one or more minarets, or towers, from which the muezzin calls Muslims to prayer five times a day. In addition to their functional use, these minarets have become distinguishing elements of mosque architecture. In large mosques in particular, minarets have the effect of tempering the enormity and magnificence of the domed structure by conveying to the viewer the elevation of divinity above the pretensions of human grandeur.

Most mosques also have a dome, and the line connecting the center of the dome to the niche is supposed to point toward Mecca. Throughout the world there are many mosques that are not actually directed toward Mecca, but such misalignment is due to inaccurate methods for determining the direction of Mecca and does not imply a disregard for this requirement. The mosque is not a self-contained unit, nor is it a symbolic microcosm of the universe, as are some places of worship in other religions. Rather, the mosque is always built as a connection with Mecca, the ultimate home of Muslim worship that metaphorically forms the center of all mosques. See Islamic Art and Architecture.

THE GOD OF ISLAM

Islamic doctrine emphasizes the oneness, uniqueness, transcendence, and utter otherness of God. As such, God is different from anything that the human senses can perceive or that the human mind can imagine. The God of Islam encompasses all creation, but no mind can fully encompass or grasp him. God, however, is manifest through his creation, and through reflection humankind can easily discern the wisdom and power behind the creation of the world. Because of God’s oneness and his transcendence of human experience and knowledge, Islamic law forbids representations of God, the prophets, and among some Muslims, human beings in general. As a result of this belief, Islamic art came to excel in a variety of decorative patterns including leaf shapes later stylized as arabesques, and Arabic script. In modern times the restrictions on creating images of people have been considerably relaxed, but any attitude of worship toward images and icons is strictly forbidden in Islam.

Islamic Monotheism

Ardabīl Carpet

This carpet was woven for the tomb-mosque of Shah Tahmasp at Ardabīl, Iran, in 1539 and 1540. Carpets like this often had more than 100 knots per sq cm (250 per sq in), so a team of weavers was needed to finish a carpet in a reasonable amount of time. The central design is a medallion shape, a traditional motif for mosque carpets.

Bridgeman Art Library, London/New York

Before Islam, many Arabs believed in a supreme, all-powerful God responsible for creation; however, they also believed in lesser gods. With the coming of Islam, the Arab concept of God was purged of elements of polytheism and turned into a qualitatively different concept of uncompromising belief in one God, or monotheism. The status of the Arabs before Islam is considered to be one of ignorance of God, or jahiliyya, and Islamic sources insist that Islam brought about a complete break from Arab concepts of God and a radical transformation in Arab belief about God.

Islamic doctrine maintains that Islam’s monotheism continues that of Judaism and Christianity. However, the Qur’an and Islamic traditions stress the distinctions between Islam and later forms of the two other monotheistic religions. According to Islamic belief, both Moses and Jesus, like others before them, were prophets commissioned by God to preach the essential and eternal message of Islam. The legal codes introduced by these two prophets, the Ten Commandments and the Christian Gospels, took different forms than the Qur’an, but according to Islamic understanding, at the level of doctrine they are the same teaching. The recipients of scriptures are called the people of the book or the 'scriptured' people. Like the Jews and the Christians before them, the Muslims became scriptured when God revealed his word to them through a prophet: God revealed the Qur’an to the prophet Muhammad, commanding him to preach it to his people and later to all humanity.

Although Muslims believe that the original messages of Judaism and Christianity were given by God, they also believe that Jews and Christians eventually distorted them. The self-perceived mission of Islam, therefore, has been to restore what Muslims believe is the original monotheistic teaching and to supplant the older legal codes of the Hebrew and Christian traditions with a newer Islamic code of law that corresponds to the evolving conditions of human societies. Thus, for example, Islamic traditions maintain that Jesus was a prophet whose revealed book was the Christian New Testament, and that later Christians distorted the original scripture and inserted into it the claim that Jesus was the son of God. Or to take another example, Muslims maintain that the strict laws communicated by Moses in the Hebrew Bible were appropriate for their time. Later, however, Jesus introduced a code of behavior that stressed spirituality rather than ritual and law.

Mosque in Bosnia and Herzegovina

The Ottomans conquered most of Bosnia in 1463, and by 1483 controlled most of Herzegovina as well. The two territories, then separated, remained provinces of the Ottoman Empire for the next 400 years. Here, a mosque built by the Ottomans stands near Mostar. Mostar was severely damaged as a result of the civil war that followed Bosnia and Herzegovina’s declaration of independence from Yugoslavia in 1992.

THE BETTMANN ARCHIVE

According to Muslim belief, God sent Muhammad with the last and perfect legal code that balances the spiritual teachings with the law, and thus supplants the Jewish and Christian codes. According to the teachings of Islam, the Islamic code, called Sharia, is the final code, one that will continue to address the needs of humanity in its most developed stages, for all time. The Qur’an mentions 28 pre-Islamic prophets and messengers, and Islamic traditions maintain that God has sent tens of thousands of prophets to various peoples since the beginning of creation. Some of the Qur’anic prophets are familiar from the Hebrew Bible, but others are not mentioned in the Bible and seem to be prophetic figures from pre-Islamic Arabia.

For the Muslim then, Islamic history unfolds a divine scheme from the beginning of creation to the end of time. Creation itself is the realization of God's will in history. Humans are created to worship God, and human history is punctuated with prophets who guarantee that the world is never devoid of knowledge and proper worship of God. The sending of prophets is itself understood within Islam as an act of mercy. God, the creator and sustainer, never abandons his creations, always providing human beings with the guidance they need for their salvation in this world and a world to come after this one. God is just, and his justice requires informing people, through prophets, of how to act and what to believe before he holds them accountable for their actions and beliefs. However, once people receive the teachings of prophets and messengers, God's justice also means that he will punish those who do wrong or do not believe and will reward those who do right and do believe. Despite the primacy of justice as an essential attribute of God, Muslims believe that God’s most fundamental attribute is mercy.

Humanity’s Relationship to God

Mosque in Tajikistan

A majority of Tajikistan’s inhabitants are ethnic Tajiks, who are predominantly Muslim. Under Soviet rule (1921-1991), religion was severely restricted; mosques were closed and religious practice was prohibited outside of state-sanctioned places of worship. Restrictions were eased somewhat in the mid-1980s, but it was not until Tajikistan became independent in 1991 that Muslims were again able to freely practice their religion.

V. Khristoforov/TASS/SOVFOTO-EASTFOTO

According to Islamic belief, in addition to sending prophets, God manifests his mercy in the dedication of all creation to the service of humankind. Islamic traditions maintain that God brought the world into being for the benefit of his creatures. His mercy toward humanity is further manifested in the privileged status God gave to humans. According to the Qur’an and later traditions, God appointed humankind as his vice regents (caliphs) on earth, thus entrusting them with the grave responsibility of fulfilling his scheme for creation.

The Islamic concept of a privileged position for humanity departs from the early Jewish and Christian interpretations of the fall from Paradise that underlie the Christian doctrine of original sin. In the biblical account, Adam and Eve fall from Paradise as a result of disobeying God’s prohibition, and all of humanity is cast out of Paradise as punishment. Christian theologians developed the doctrine that humankind is born with this sin of their first parents still on their souls, based upon this reading of the story. Christians believe that Jesus Christ came to redeem humans from this original sin so that humankind can return to God at the end of time. In contrast, the Qur’an maintains that after their initial disobedience, Adam and Eve repented and were forgiven by God. Consequently Muslims believe that the descent by Adam and Eve to earth from Paradise was not a fall, but an honor bestowed on them by God. Adam and his progeny were appointed as God's messengers and vice regents, and were entrusted by God with the guardianship of the earth.

Angels

The nature of humankind’s relationship to God can also be seen clearly by comparing it with that of angels. According to Islamic tradition, angels were created from light. An angel is an immortal being that commits no sins and serves as a guardian, a recorder of deeds, and a link between God and humanity. The angel Gabriel, for example, communicated God's message to the prophet Muhammad. In contrast to humans, angels are incapable of unbelief and always obey God. Some followers of Islam view Satan as an angel who was unusual in his ability to defy God, while others view him as a jinn, or spirit created by God from smokeless fire, who roamed among the angels.

Despite these traits, Islamic doctrine holds that humans are superior to angels. According to Islamic traditions, God entrusted humans and not angels with the guardianship of the earth and commanded the angels to prostrate themselves to Adam. Satan, together with the other angels, questioned God's appointment of fallible humans to the honorable position of viceregency. Being an ardent monotheist, Satan disobeyed God and refused to prostrate himself before anyone but God. For this sin, Satan was doomed to lead human beings astray until the end of the world. According to the Qur’an, God informed the angels that he had endowed humans with a knowledge angels could not acquire.

Islamic Theology

Kazimayn Mosque

The gold-domed Kazimayn Mosque, pictured at night, is near Baghdād in Iraq. This famous building, begun in the 11th century and completed in the 19th century, contains the tombs of revered Shia Muslim leaders.

Mehmet Biber/Photo Researchers, Inc.

For centuries Muslim theologians have debated the subjects of justice and mercy as well as God’s other attributes. Initially, Islamic theology developed in the context of controversial debates with Christians and Jews. As their articulations of the basic doctrines of Islam became more complex, Muslim theologians soon turned to debating different interpretations of the Qur’an among themselves, developing the foundations of Islamic theology.

Mosque in Vladikavkaz

This mosque serves members of the Muslim minority in Vladikavkaz, the capital of the Russian republic of Alania (North Ossetia).

SOVFOTO-EASTFOTO

Recurring debates among Islamic scholars over the nature of God have continued to refine the Islamic concepts of God’s otherness and Islamic monotheism. For example, some theologians interpreted Qur’anic attributions of traits such as hearing and seeing to God metaphorically to avoid comparing God to created beings. Another controversial theological debate focused on the question of free will and predestination. One group of Muslim theologians maintained that because God is just, he creates only good, and therefore only humans can create evil. Otherwise, this group argued, God’s punishment of humans would be unjust because he himself created their evil deeds. This particular view was rejected by other Muslim theologians on the grounds that it limits the scope of God's creation, when the Qur’an clearly states that God is the sole creator of everything that exists in the world.

Mosque of Muhammad Ali, Cairo

The Mosque of Muhammad Ali stands within the walls of the Citadel in Cairo. Built between 1830 and 1857, it is the largest and grandest of the four mosques contained in the Citadel. Cairo has been an Islamic cultural center for more than 1000 years.

Richard Evans

Another controversial issue was the question of whether the Qur’an was eternal or created in time. Theologians who were devoted to the concept of God's oneness maintained that the Qur’an must have been created in time, or else there would be something as eternal as God. This view was rejected by others because the Qur’an, the ultimate authority in Islam, states in many places and in unambiguous terms that it is the eternal word of God.

Many other theological controversies occupied Muslim thinkers for the first few centuries of Islam, but by the 10th century the views of Islamic theologian al-Ashari and his followers, known as Asharites, prevailed and were adopted by most Muslims. The way this school resolved the question of free will was to argue that no human act could occur if God does not will it, and that God's knowledge encompasses all that was, is, or will be. This view also maintains that it is God's will to create the power in humans to make free choices. God is therefore just to hold humans accountable for their actions. The views of al-Ashari and his school gradually became dominant in Sunni, or orthodox, Islam, and they still prevail among most Muslims. The tendency of the Sunnis, however, has been to tolerate and accommodate minor differences of opinion and to emphasize the consensus of the community in matters of doctrine.

As is the case with any religious group, ordinary Muslims have not always been concerned with detailed theological controversies. For ordinary Muslims the central belief of Islam is in the oneness of God and in his prophets and messengers, culminating in Muhammad. Thus Muslims believe in the scriptures that God sent through these messengers, particularly the truth and content of the Qur’an. Whatever their specific religious practices, most Muslims believe in angels, the Day of Judgment, heaven, paradise, and hell.

THE PROPHET MUHAMMAD

Belief in the message of Muhammad comes second only to belief in the one God. Muhammad was born around the year 570 and was orphaned at an early age. He was eventually raised by his uncle, who had religious prominence within the main Quraysh tribe of Mecca but was of modest financial means. At age 25, Muhammad married Khadija, a well-to-do, 40-year-old woman. At age 40, during a retreat in the hills outside Mecca, Muhammad had his first experience of Islam. The angel Gabriel appeared to a fearful Muhammad and informed him that he was God's chosen messenger. Gabriel also communicated to Muhammad the first revelation from God. Terrified and shaken, Muhammad went to his home. His wife became the first person to accept his message and convert to Islam. After receiving a series of additional revelations, Muhammad started preaching the new religion, initially to a small circle of relatives and friends, and then to the general public.

The Meccans first ignored Muhammad, then ridiculed him. As more people accepted Muhammad's call, the Meccans became more aggressive. After failing to sway Muhammad away from the new religion they started to persecute his less prominent followers. When this approach did not work, the opposing Meccans decided to persecute Muhammad himself. By this time, two main tribes from the city of Yathrib, about 300 km (200 mi) north of Mecca, had invited Muhammad to live there. The clan leaders invited Muhammad to Yathrib as an impartial religious authority to arbitrate disputes. In return, the leaders pledged to accept Muhammad as a prophet and thus support the new religion of Islam.

Hegira

In the year 622, Muhammad immigrated to Yathrib, and the name of the city was changed to Medina, meaning city of the Prophet. This date was designated by later Muslims as the beginning of the Muslim calendar, year one of hegira (Arabic hijra, “immigration”). Only two years after Muhammad's arrival in Medina, the core community of Muslims started to expand. At Medina, in addition to preaching the religious and moral message of Islam, Muhammad organized an Islamic society and served as head of state, diplomat, military leader, and chief legislator for the growing Muslim community. Hostilities soon broke out between the Muslims in Medina and the powerful Meccans. In 630, after a series of military confrontations and diplomatic maneuvers, the Muslims in Medina extended their authority over Mecca, the most important city of Arabia at the time. Before Muhammad died in 632, the whole Arabian Peninsula was united for the first time in its history, under the banner of Islam.

Muhammad’s Humanity

Early accounts of Muhammad contain some stories that describe supernatural events such as his night journey from Mecca to Jerusalem and his subsequent ascent to heaven on the back of a supernatural winged horse. Despite such stories, the primary focus of the biographies, as well as Islamic doctrine in general, is on the humanity of Muhammad.

Like all prophets before him, Muhammad was a mortal man, commissioned by God to deliver a message to his people and to humanity. Like other prophets, Muhammad was distinguished from ordinary people by certain powers and faculties. For example, Muslims believe that the distinction of being sinless was granted to Muhammad by God to support his career as a prophet. Thus Muhammad is portrayed in the Qur’an as a person who makes mistakes but who does not sin against God. However, God corrected Muhammad’s mistakes or errors in judgment, so that his life serves as an example for future Muslims to follow. This emphasis on Muhammad's humanity serves as a reminder that other humans can reasonably aspire to lead a good life as he did.

VII

THE QUR’AN

Muslim Boys Studying the Qur’an

The Qur’an is at the center of Muslim life. Muslims recite verses from it in their daily prayers and at important public and private events. Many Muslims also memorize this holy scripture so that they can keep it in their hearts. Before touching the holy book, Muslims follow rituals for purification, including washing and preparing the mind, body, and spirit. Care must be taken that the Qur’an does not come into contact with any unclean substance, and it is never to be laid upon the ground.

Piers Benatar/Panos Pictures

As with other prophets and messengers, God supported Muhammad by allowing him to work miracles and thus prove that he was a genuine prophet. The singular miracle of Muhammad and the ultimate proof of the truthfulness of Islam is the Qur’an. In accordance with the words of the scripture itself, Muslims believe that the Qur’an is the timeless word of God, “the like of which no human can produce.” This trait of the scripture, called inimitability (i'jaz), is based on belief in the divine authorship of the Qur’an. Unlike earlier religions, the miracle of Islam is a literary miracle, and Muhammad's other supernatural acts are subordinate to it.

This belief in the unique nature of the Qur’an has led Muslims to devote great intellectual energies to the study of its contents and form. In addition to interpreting the scripture and deriving doctrines and laws from it, many disciplines within Qur’anic studies seek to understand its linguistic and literary qualities as an expression of its divine origins.

The Format of the Holy Book

Illustrated Text of the Qur’an

This beautifully decorated page comes from a Qur’an of the late 8th century or early 9th century. Muslims believe that the Qur’an is an infallible transcription of God’s message to Muhammad. As the messenger of God and seal of the prophets, Muhammad was charged with the responsibility of relaying this message to all believers. Divided into 114 suras, or chapters, the Qur’an is meant to be recited or chanted as part of Islamic worship.

Bojan Brecelj/Corbis

The Qur’an is made up of 114 chapters, called suras, which appear, from the second chapter onward, roughly in order of length, beginning with the longest and ending with the shortest chapters. The first chapter, al-Fatiha (“the Opening”), is a short chapter that is recited during each of the five daily prayers and in many other ritual prayers. All but one chapter begin with the formula 'in the name of God, the Merciful Lord of Mercy' (bism Allah al-Rahman al-Rahim). Each chapter is divided into verses called ayat (singular aya, meaning “sign” or “proof”). With few exceptions the verses are randomly organized without a coherent narrative thread.

A typical chapter of the Qur’an may address any combination of the following themes: God and creation, prophets and messengers from Adam to Jesus, Muhammad as a preacher and as a ruler, Islam as a faith and as a code of life, disbelief, human responsibility and judgment, and society and law. Later Muslim scholars have argued that the text’s timelessness and universality explain the lack of narrative coherence and the randomness of the topics. In other words, the multiple meanings of the Qur’an transcend linear narrative as they transcend any particular historical moment.

The Qur’an and the Bible

Islam recognizes the divine origins of the earlier Hebrew and Christian Scriptures and represents itself as both a restoration and a continuation of their traditions. Because of this, the Qur’an draws on biblical stories and repeats many biblical themes. In particular, the stories of several biblical prophets appear in the Qur’an, some in a condensed form; other stories, such as those of Abraham, Moses, and Jesus, are given in elaborate detail and even with subtle revisions of the biblical accounts.

One of the important differences between the Qur’anic and biblical stories of Abraham's sacrifice of his son, for example, is that the Qur’an suggests this son is Ishmael, from whom Arabs are descended, and not Isaac, from whom the tribes of Israel are descended. A more substantial difference relates to the Islamic story of Jesus, who according to the Qur’an is a mortal, human prophet. The Islamic faith categorically rejects the idea that God was ever born, as opposed to Christian belief that Jesus was born the son of God. Islam also rejects the idea that God shared his divinity with any other being.

Another important idea elaborated in the Qur’an and later Islamic doctrine, in conscious distinction from the biblical accounts, is that although prophets are capable of human errors, God protects them from committing sins and also protects them from excruciating suffering or humiliating experiences. God would not abandon his prophets in times of distress. Therefore, the Qur’an maintains that God interfered to save Jesus from torture and death by lifting him to heaven and replacing him on the cross with someone who looked like him.

The Preservation of the Qur’an

From its inception during the lifetime of Muhammad, Islamic doctrine gave priority to the preservation of the scripture. As a result, one of the earliest expressions of religiosity focused on studying, reciting, and writing down the scripture. When Muhammad died, the preservation of the scripture was also a conscious concern among his companions and successors. Early historical sources refer to immediate efforts undertaken by successors of Muhammad to collect the chapters of the Qur’an, which were written down by his various companions.

Within about two decades after the death of the Prophet, various existing copies of parts of the Qur’an were collected and collated by a committee of close companions of Muhammad who were known for their knowledge of the Qur’an. This committee was commissioned by the third successor of Muhammad, Uthman ibn Affan, and the committee’s systematic effort is the basis of the codified official text currently used by Muslims. The thematic randomness of the verses and chapters of the Qur’an in its current format clearly illustrates that the early companions who produced this official version of the Qur’an were primarily concerned with establishing the text and made no attempt to edit its contents in order to produce a coherent narrative. Because of this, scholars agree that the Uthmanic text genuinely reflects, both in its content and form, the message that Muhammad preached.

Interpretations of the Qur’an

Despite the consensus among Muslims on the authenticity of the current format of the Qur’an, they agree that many words in the Qur’an can be interpreted in equally valid ways. The Arabic language, like other Semitic languages, has consonants and vowels, and the meanings of words are derived from both. For several centuries, the written texts of the Qur’an showed only the consonants, without indicating the vowel marks. As a result, there are different ways in which many words can be vocalized, with different meanings; this allows for various legitimate interpretations of the Qur’an.

One of the disciplines for the study of the Qur’an is exclusively dedicated to the study and documentation of acceptable and unacceptable variant readings. According to Muslim scholars, there are some 40 possible readings of the Qur’an, of which 7 to 14 are legitimate. The legitimacy of different possible interpretations of the scripture is supported by a statement in the Qur’an that describes verses as either unambiguously clear, or as ambiguous because they carry a meaning known only to God. Therefore, with the exception of a small number of unquestionably clear injunctions, the meaning of the Qur’anic verses is not always final.

The Qur’an is the primary source of authority, law and theology, and identity in Islam. However, in many cases it is either completely silent on important Islamic beliefs and practices or it gives only general guidelines without elaboration. This is true of some of the most basic religious obligations such as prayer, which the Qur’an prescribes without details. Details elaborating on the teachings and laws of the Qur’an are derived from the sunna, the example set by Muhammad’s life, and in particular from hadith, the body of sayings and practices attributed to him.

VIII

HADITH

Shrine of Abbas

The shrine of Abbas in Karbalā, Iraq, is a pilgrimage site for Shia Muslims. Abbas was the son of Ali, the son-in-law of the prophet Muhammad. Abbas was martyred and buried in Karbalā. His tomb is in a domed chamber that shimmers with silver mirrors.

Eddie Adams/Leo de Wys, Inc.

As the second source of authority in Islam, hadith complements the Qur’an and provides the most extensive source for Islamic law. The ultimate understanding of the Qur’an depends upon the context of Muhammad’s life and the ways in which he demonstrated and applied its message. There is evidence that Muhammad's sayings and practices were invoked by his companions to answer questions about Islam. Unlike the Qur’an, however, in the early periods hadith was circulated orally, and no attempts were made to establish or codify it into law until the beginnings of the second century of Islam.

Due to the late beginnings of the efforts to collect and compile reports about Muhammad's traditions, Muslim scholars recognize that the authenticity of these reports cannot be taken for granted. Many spurious reports were often deliberately put into circulation to support claims of various political and sectarian groups. Other additions resulted from the natural tendency to confuse common practices that predated Islam with new Islamic laws and norms. The fading of memory, the dispersion of the companions of the prophet over vast territories, and the passing away of the last of these companions also contributed to the problem of authenticating Muhammad’s traditions.

Preaching in the Mosque

Book illustration was an essential Islamic art, which flourished from the 7th to the 18th century. This manuscript page shows Abu Zayd preaching in the mosque of Samarqand. Islamic art focused on book arts rather than easel painting because it was believed that art should serve a function, that of education.

Bridgeman/Art Resource, NY

To establish the authority of hadith on firmer ground, Muslim scholars developed several disciplines dedicated to examining and verifying the relative authenticity of various reports attributed to the Prophet. The contents of sayings, as well as the reliability of those who transmitted them, were carefully scrutinized, and the hadiths were classified into groups granted varying degrees of authenticity, ranging from the sound and reliable to the fabricated and rejected. This systematic effort culminated in the 9th century, some 250 years after the death of Muhammad, in the compilation of several collections of sound (sahih) hadith. Of six such highly reliable compilations, two in particular are considered by Muslims to be the most important sources of Islamic authority after the Qur’an. These are Sahih Muslim and Sahih Bukhari (the sound books of Muslim and Bukhari).

Jumeirah Mosque

The Jumeirah Mosque is located in the city of Dubai, in the United Arab Emirates. Dubai is the chief port and commercial center of the Emirates.

SIME/Schmid Reinhard/4Corners Images

Historically, the compilation of hadith went hand in hand with the elaboration of Islamic law and the parallel development of Islamic legal theory. Initially, neither the law nor its procedures were systematically elaborated, although there can be little doubt that both the Qur’an and hadith were regularly invoked and used to derive laws that governed the lives of Muslims. By the beginning of the 9th century, the use of these two sources was systematized and a complex legal theory was introduced. In its developed form, this theory maintains that there are four sources from which Islamic law is derived. These are, in order of priority, the Qur’an, the hadith, the consensus of the community (ijma), and legal analogy (qiyas). Functional only when there is no explicit ruling in the Qur’an or hadith, consensus confers legitimacy retrospectively on historical practices of the Muslim community. In legal analogy, the causes for existing Islamic rulings are applied by analogy to similar cases for which there are no explicit statements in either the Qur’an or hadith. Using these methods, a vast and diverse body of Islamic law was laid out covering various aspects of personal and public life.

In addition to the laws pertaining to the five pillars, Islamic law covers areas such as dietary laws, purity laws, marriage and inheritance laws, commercial transaction laws, laws pertaining to relationships with non-Muslims, and criminal law. Jews and Christians living under Muslim rule are subject to the public laws of Islam, but they have traditionally been permitted to run their internal affairs on the basis of their own religious laws.

THE SPREAD OF ISLAM

Spread of Islam

In the 7th and 8th centuries the religion of Islam spread through conversion and military conquest throughout the Middle East and North Africa. By 733, just 100 years after the death of Muhammad, the founder of Islam, an ordered Islamic state stretched from India in the east to Spain in the west.

Since its inception Islam has been perceived by Muslims to be a universal code. During Muhammad's lifetime, two attempts were made to expand northward into the Byzantine domain and its capital in Constantinople, and within ten years after Muhammad’s death, Muslims had defeated the Sassanids of Persia and the Byzantines, and had conquered most of Persia, Iraq, Syria, and Egypt. The conquests continued, and the Sassanian Empire was soon after destroyed and the influence of Byzantium was largely diminished (see Byzantine Empire). For the next several centuries intellectuals and cultural figures flourished in the vast, multinational Islamic world, and Islam became the most influential civilization in the world.

The Rightly Guided Caliphs

Mosque at Mazār-e Sharīf

Muslims from all over the country make pilgrimages to the 15th-century mosque at Mazār-e Sharīf in northern Afghanistan. The religious significance of the site derives from the belief that the tomb of Ali, fourth caliph of Islam and son-in-law of Muhammad, the founder of Islam, lies within the mosque. More than 99 percent of the population of Afghanistan practices Islam.

George Hunter/ALLSTOCK, INC.

The first four successors of Muhammad, known as rightly guided caliphs, ruled for some 30 years (see Caliphate). Their rule, together with that of Muhammad, is considered by most Muslims to constitute the ideal Islamic age. The second caliph, Umar, ruled from ad 634 to 644; he is credited with being the first caliph to found new Islamic cities, Al Başra (ad 635) and Kūfah (ad 638). The administration of the eastern and western Islamic provinces was coordinated from these two sites. After the third caliph, Uthman, was murdered by a group of Muslim mutineers, the fourth caliph, Ali, succeeded to power and moved his capital to Kūfah in Iraq. From this capital he fought the different opposition factions. Among the leaders of these factions, Mu’awiyah, governor of the rich province of Syria and a relative of Uthman, outlasted Ali. After Ali’s death in 661, Mu’awiyah founded the Umayyad dynasty, which ruled a united Islamic empire for almost a century. Under the Umayyads the Islamic capital was shifted to Damascus. See Spread of Islam.

Shia Islam

The followers of Ali were known as the Shia (partisans) of Ali. Although they began as a political group, the Shia, or Shia Muslims, became a sect with specific theological and doctrinal positions. A key event in the history of the Shia and for all Muslims was the tragic death at Karbala of Husayn, the son of Ali, and Muhammad's daughter Fatima. Husayn had refused to recognize the legitimacy of the rule of the Umayyad Yazid, the son of Mu’awiyah, and was on his way to rally support for his cause in Kūfah. His plans were exposed before he arrived at Kūfah, however, and a large Umayyad army met him and 70 members of his family at the outskirts of the city. The Umayyads offered Husayn the choice between a humiliating submission to their rule or a battle and definite death. Husayn chose to fight, and he and all the members of his family with him were massacred. The incident was of little significance from a military point of view, but it was a defining moment in the history of Shia Islam. Although not all Muslims are Shia Muslims, all Muslims view Husayn as a martyr for living up to his principles even to death.

The Twelver Shia, or Ithna-‘Ashariyya, is the largest of the Shia Muslim sects. They believe that legitimate Islamic leadership is vested in a line of descent starting with Muhammad's cousin and son-in-law, Ali, through Ali's two sons, Hasan and Husayn, and then through Husayn's descendants. These were the first 12 imams, or leaders of the Shia Muslim community. The Shia Muslims believe that Muhammad designated all 12 successors by name and that they inherited a special knowledge of the true meaning of the scripture that was passed from father to son, beginning with the Prophet himself. This family, along with its loyal followers and representatives, has political authority over the Shia Muslims.

Sunni Islam

Sunni Islam was defined during the early Abbasid period (beginning in ad 750), and it included the followers of four legal schools (the Malikis, Hanafis, Shafi’is, and Hanbalis). In contrast to the Shias, the Sunnis believed that leadership was in the hands of the Muslim community at large. The consensus of historical communities, not the decisions of political authorities, led to the establishment of the four legal schools. In theory a Muslim could choose whichever school of Islamic thought he or she wished to follow and could change this choice at will. The respect and popularity that the religious scholars enjoyed made them the effective brokers of social power and pitched them against the political authorities.

After the first four caliphs, the religious and political authorities in Islam were never again united under one institution. Their usual coexistence was underscored by a mutual recognition of their separate spheres of influence and their respective duties and responsibilities. Often, however, the two powers collided, and invariably any social opposition to the elite political order had religious undertones.

Sufism

Whirling Dervishes

In the Middle Ages the great Sufi orders, which had several million adherents, were established; about 100 orders still exist, many of them in Turkey and Iran. One of the most influential founders of orders was the Persian poet Jalal al-Din Muhammad Rumi, who, in addition to composing poetry and other works, instituted devotional dances, particularly those of the whirling dervishes.

BBC Worldwide Americas, Inc.

An ascetic tradition called Sufism, which emphasized personal piety and mysticism and contributed to Islamic cultural diversity, further enriched the Muslim heritage. In contrast to the legal-minded approach to Islam, Sufis emphasized spirituality as a way of knowing God. During the 9th century Sufism developed into a mystical doctrine, with direct communion or even ecstatic union with God as its ideal. One of the vehicles for this experience is the ecstatic dance of the Sufi whirling dervishes. Eventually Sufism later developed into a complex popular movement and was institutionalized in the form of collective, hierarchical Sufi orders.

Rumi

Sufism, or Islamic mysticism, influenced the lyrical poetry of 13th-century Persian writer Jalal al-Din Rumi. He explored spiritual concepts such as the meaning of life and the ultimate need for the human soul to unite with God. Rumi often used the second person “you” in his poetry, but he frequently disguised the identity of the “you.” For example, the subject of Rumi’s poem 1245 (recited by an actor) may be a human lover, God, another part of himself, or a combination of all three.

The Sufi emphasis on intuitive knowledge and the love of God increased the appeal of Islam to the masses and largely made possible its extension beyond the Middle East into Africa and East Asia. Sufi brotherhoods multiplied rapidly from the Atlantic coast to Indonesia; some spanned the entire Islamic world, others were regional or local. The tremendous success of these fraternities was due primarily to the abilities and humanitarianism of their founders and leaders, who not only ministered to the spiritual needs of their followers but also helped the poor of all faiths and frequently served as intermediaries between the people and the government.

The Abbasid Dynasty

Córdoba, Spain

The Moorish history of the city of Córdoba in Spain dates from the 8th century, when the city became a Muslim caliphate. The Moorish influence can still be seen in much of the architecture, including the city’s famous cathedral, originally an impressive mosque.

Poseidon Pictures London

Islamic culture started to evolve under the Umayyads, but it grew to maturity in the first century of the Abbasid dynasty. The Abbasids came to power in ad 750 when armies originating from Khorāsān, in eastern Iran, finally defeated the Umayyad armies. The Islamic capital shifted to Iraq under the Abbasids. After trying several other cities, the Abbasid rulers chose a site on the Tigris River on which the City of Peace, Baghdād, was built in 762. Baghdād remained the political and cultural capital of the Islamic world from that time until the Mongol invasion in 1258, and for a good part of this time it was the center of one of the great flowerings of human knowledge. The Abbasids were Arabs descended from the Prophet's uncle, but the movement they led involved Arabs and non-Arabs, including many Persians, who had converted to Islam and who demanded the equality to which they were entitled in Islam.

Courtyard, Madrasa, Eşfahān

A madrasa is a place for learning and prayer. This view into the courtyard of the Madrasa Chahār Bāgh in Eşfahān, Iran, shows the domed mosque, central pool, and rooms around the courtyard for study and accommodation. The madrasa was built from 1706 to 1714.

Art Resource, NY

The Abbasids distributed power more evenly among the different ethnicities and regions than the Umayyads had, and they demonstrated the universal inclusiveness of Islamic civilization. They achieved this by incorporating the fruits of other civilizations into Islamic political and intellectual culture and by marking these external influences with a distinctly Islamic imprint.

Baghdād, Iraq

Baghdād became the capital city of the newly created kingdom of Iraq in 1921, but the city’s history dates back many centuries more. Built in ad 762 on a fertile plain next to the Tigris River in central Iraq, Baghdād is the country's largest city and its center of transportation and manufacturing. In 1991, however, heavy bombing during the Persian Gulf War destroyed much of the city's industry and transportation network.

Barry Iverson/Woodfin Camp and Associates, Inc.

As time passed, the central control of the Abbasids was reduced and independent local leaders and groups took over in the remote provinces. Eventually the rival Shia Fatimid caliphate was established in Egypt, and the Baghdād caliphate came under the control of expanding provincial dynasties. The office of the caliph was nonetheless maintained as a symbol of the unity of Islam, and several later Abbasid caliphs tried to revive the power of the office.

Persian Manuscript

During the rule of the Abbasid caliphs, from 750 to 1258, Islamic culture flourished. This 13th-century Persian manuscript was created during this period. Islamic art used input from neighboring cultures including the Persians in the development of a cohesive Islamic style of art.

Bibliotheque Nationale, Paris/Laurie Platt Winfrey, Inc./Woodfin Camp and Associates, Inc.

In 1258, however, a grandson of Mongol ruler Genghis Khan named Hulagu, encouraged by the kings of Europe, led his armies across the Zagros Mountains of Iran and destroyed Baghdād. According to some estimates, about 1 million Muslims were murdered in this massacre. In 1259 and 1260 Hulagu's forces marched into Syria, but they were finally defeated by the Mamluks of Egypt, who had taken over the Nile Valley. For the next two centuries, centers of Islamic power shifted to Egypt and Syria and to a number of local dynasties. Iraq became an impoverished, depopulated province where the people took up a transitory nomadic lifestyle. Iraq did not finally experience a major cultural and political revival until the 20th century.

THE PRESENCE OF ISLAM IN THE 20TH CENTURY

Many of the accepted Islamic religious and cultural traditions were established between the 7th and 10th centuries, during the classical period of Islamic history. However, Islamic culture continued to develop as Islam spread into new regions and mixed with diverse cultures. The 19th-century occupation of most Muslim lands by European colonial powers was a main turning point in Muslim history. The traditional Islamic systems of governance, social organization, and education were undermined by the colonial regimes. Nation-states with independent governments divided the Muslim community along new ethnic and political lines.

Today about 1 billion Muslims are spread over 40 predominantly Muslim countries and 5 continents, and their numbers are growing at a rate unmatched by that of any other religion in the world. Despite the political and ethnic diversity of Muslim countries, a core set of beliefs continues to provide the basis for a shared identity and affinity among Muslims. Yet the radically different political, economic, and cultural conditions under which contemporary Muslims live make it difficult to identify what constitutes standard Islamic practice in the modern world. Many contemporary Muslims draw on the historical legacy of Islam as they confront the challenges of modern life. Islam is a significant, growing, and dynamic presence in the world. Its modern expressions are as diverse as the world in which Muslims live.

Contributed By:
Ahmad S. Dallal

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