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  • 2. Earth (planet)INTRODUCTIONEarth (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 swirlingwhite clouds floating above blue oceans. About 71 percent of Earth’s surface is covered bywater, which is essential to life. The rest is land, mostly in the form of continents that riseabove the oceans.Earth’s surface is surrounded by a layer of gases known as the atmosphere, which extendsupward from the surface, slowly thinning out into space. Below the surface is a hot interior ofrocky material and two core layers composed of the metals nickel and iron in solid and liquidform.Unlike the other planets, Earth has a unique set of characteristics ideally suited to supportinglife as we know it. It is neither too hot, like Mercury, the closest planet to the Sun, nor toocold, 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 trapheat 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’satmosphere distinguishes it from the planet Venus, which is otherwise much like Earth. Venusis about the same size and mass as Earth and is also neither too near nor too far from theSun. But because Venus has too much heat-trapping carbon dioxide in its atmosphere, itssurface 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 possibilitythat life may once have existed on other planets or their moons, or may exist today inprimitive form. Mars, for example, has many features that resemble river channels, indicatingthat liquid water once flowed on its surface. If so, life may also have evolved there, andevidence for it may one day be found in fossil form. Water still exists on Mars, but it is frozenin polar ice caps, in permafrost, and possibly in rocks below the surface.For thousands of years, human beings could only wonder about Earth and the otherobservable planets in the solar system. Many early ideas—for example, that the Earth was asphere and that it traveled around the Sun—were based on brilliant reasoning. However, itwas only with the development of the scientific method and scientific instruments, especially inthe 18th and 19th centuries, that humans began to gather data that could be used to verifytheories 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. Andwith the use of telescopes, new planets such as Uranus, Neptune, and Pluto were discovered.
  • 3. In the second half of the 20th century, more advances in the study of Earth and the solarsystem 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 withscientific instruments. Astronauts landed on the Moon and gathered ancient rocks thatrevealed much about the early solar system. During this remarkable advancement in humanhistory, 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 andmoons has provided new insights about Earth, just as the study of the Sun and other stars likeit 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 mostgeologically active of all the planets and moons in the solar system. Earth is constantlychanging. 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’satmosphere. For example, Earth at one time had the same amount of carbon dioxide in itsatmosphere as Venus now has, but early forms of life helped remove this carbon dioxide overmillions of years. These life forms also added oxygen to Earth’s atmosphere and made itpossible for animal life to evolve on land.A variety of scientific fields have broadened our knowledge about Earth, includingbiogeography, climatology, geology, geophysics, hydrology, meteorology, oceanography, andzoogeography. Collectively, these fields are known as Earth science. By studying Earth’satmosphere, its surface, and its interior and by studying the Sun and the rest of the solarsystem, 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 GALAXYEarth is the third planet from the Sun, after Mercury and Venus. The average distancebetween Earth and the Sun is 150 million km (93 million mi). Earth and all the other planets inthe solar system revolve, or orbit, around the Sun due to the force of gravitation. The Earthtravels at a velocity of about 107,000 km/h (about 67,000 mph) as it orbits the Sun. All butone of the planets orbit the Sun in the same plane—that is, if an imaginary line were extendedfrom the center of the Sun to the outer regions of the solar system, the orbital paths of theplanets 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) fromthe Sun; at minimum distance Earth is about 147 million km (about 91 million mi) from the
  • 4. Sun. If Earth orbited the Sun in a perfect circle, it would always be the same distance from theSun.The solar system, in turn, is part of the Milky Way Galaxy, a collection of billions of starsbound together by gravity. The Milky Way has armlike discs of stars that spiral out from itscenter. The solar system is located in one of these spiral arms, known as the Orion arm, whichis about two-thirds of the way from the center of the Galaxy. In most parts of the NorthernHemisphere, this disc of stars is visible on a summer night as a dense band of light known asthe Milky Way.Earth is the fifth largest planet in the solar system. Its diameter, measured around theequator, is 12,756 km (7,926 mi). Earth is not a perfect sphere but is slightly flattened at thepoles. Its polar diameter, measured from the North Pole to the South Pole, is somewhat lessthan the equatorial diameter because of this flattening. Although Earth is the largest of thefour planets—Mercury, Venus, Earth, and Mars—that make up the inner solar system (theplanets closest to the Sun), it is small compared with the giant planets of the outer solarsystem—Jupiter, Saturn, Uranus, and Neptune. For example, the largest planet, Jupiter, has adiameter at its equator of 143,000 km (89,000 mi), 11 times greater than that of Earth. Afamous atmospheric feature on Jupiter, the Great Red Spot, is so large that three Earthswould fit inside it.Earth has one natural satellite, the Moon. The Moon orbits the Earth, completing onerevolution in an elliptical path in 27 days 7 hr 43 min 11.5 sec. The Moon orbits the Earthbecause of the force of Earth’s gravity. However, the Moon also exerts a gravitational force onthe Earth. Evidence for the Moon’s gravitational influence can be seen in the ocean tides. Apopular theory suggests that the Moon split off from Earth more than 4 billion years ago whena 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 runsbetween the North and South poles. The period of one complete rotation is defined as a dayand takes 23 hr 56 min 4.1 sec. The period of one revolution around the Sun is defined as ayear, or 365.2422 solar days, or 365 days 5 hr 48 min 46 sec. Earth also moves along withthe Milky Way Galaxy as the Galaxy rotates and moves through space. It takes more than 200million years for the stars in the Milky Way to complete one revolution around the Galaxy’scenter.Earth’s axis of rotation is inclined (tilted) 23.5° relative to its plane of revolution around theSun. This inclination of the axis creates the seasons and causes the height of the Sun in thesky at noon to increase and decrease as the seasons change. The Northern Hemispherereceives the most energy from the Sun when it is tilted toward the Sun. This orientationcorresponds 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,
  • 5. corresponding to summer in the Southern Hemisphere and winter in the Northern Hemisphere.Fall and spring occur in between these orientations.EARTH’S ATMOSPHEREThe atmosphere is a layer of different gases that extends from Earth’s surface to theexosphere, 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) andoxygen (21 percent). The remaining 1 percent of atmospheric gases consists of argon (0.9percent); carbon dioxide (0.03 percent); varying amounts of water vapor; and trace amountsof hydrogen, nitrous oxide, ozone, methane, carbon monoxide, helium, neon, krypton, andxenon.Layers of the AtmosphereThe layers of the atmosphere are the troposphere, the stratosphere, the mesosphere, thethermosphere, and the exosphere. The troposphere is the layer in which weather occurs andextends from the surface to about 16 km (about 10 mi) above sea level at the equator. Abovethe troposphere is the stratosphere, which has an upper boundary of about 50 km (about 30mi) above sea level. The layer from 50 to 90 km (30 to 60 mi) is called the mesosphere. At analtitude of about 90 km, temperatures begin to rise. The layer that begins at this altitude iscalled 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 theexosphere. The thermosphere and the exosphere overlap with another region of theatmosphere known as the ionosphere, a layer or layers of ionized air extending from almost60 km (about 50 mi) above Earth’s surface to altitudes of 1,000 km (600 mi) and more.Earth’s atmosphere and the way it interacts with the oceans and radiation from the Sun areresponsible for the planet’s climate and weather. The atmosphere plays a key role insupporting life. Almost all life on Earth uses atmospheric oxygen for energy in a processknown as cellular respiration, which is essential to life. The atmosphere also helps moderateEarth’s climate by trapping radiation from the Sun that is reflected from Earth’s surface. Watervapor, carbon dioxide, methane, and nitrous oxide in the atmosphere act as “greenhousegases.” Like the glass in a greenhouse, they trap infrared, or heat, radiation from the Sun inthe 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 oflife.Other gases in the atmosphere are also essential to life. The trace amount of ozone found inEarth’s stratosphere blocks harmful ultraviolet radiation from the Sun. Without the ozonelayer, life as we know it could not survive on land. Earth’s atmosphere is also an importantpart of a phenomenon known as the water cycle or the hydrologic cycle. See also Atmosphere.
  • 6. The Atmosphere and the Water CycleThe 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 andreused for billions of years. Very little water has been created or lost during this period oftime. 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 evaporateand enter the atmosphere as water vapor. Some of this water vapor falls as precipitationdirectly 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 onthe land. Due to the force of gravity, the water in the rivers eventually empties back into theoceans. Melted snow or rain also may enter the ground. Groundwater may be stored forhundreds or thousands of years, but it will eventually reach the surface as springs or smallpools known as seeps. Even snow that forms glacial ice or becomes part of the polar caps andis kept out of the cycle for thousands of years eventually melts or is warmed by the Sun andturned into water vapor, entering the atmosphere and falling again as precipitation. All waterthat falls on land eventually returns to the ocean, completing the water cycle.EARTH’S SURFACEEarth’s surface is the outermost layer of the planet. It includes the hydrosphere, the crust,and the biosphere.HydrosphereThe hydrosphere consists of the bodies of water that cover 71 percent of Earth’s surface. Thelargest of these are the oceans, which contain over 97 percent of all water on Earth. Glaciersand 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 36times more plentiful than water found in lakes, inland seas, rivers, and in the atmosphere aswater vapor. Only 0.017 percent of all the water on Earth is found in lakes and rivers. And amere 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 andagriculture. 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.CrustThe 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
  • 7. mi) thick. The oceanic crust is thinner than the continental crust. Its average thickness is 5 to10 km (3 to 6 mi). The crust has a definite boundary called the Mohorovi i discontinuity, orsimply the Moho. The boundary separates the crust from the underlying mantle, which is muchthicker and is part of Earth’s interior.Oceanic crust and continental crust differ in the type of rocks they contain. There are threemain types of rocks: igneous, sedimentary, and metamorphic. Igneous rocks form whenmolten rock, called magma, cools and solidifies. Sedimentary rocks are usually created by thebreakdown of igneous rocks. They tend to form in layers as small particles of other rocks or asthe mineralized remains of dead animals and plants that have fused together over time. Theremains of dead animals and plants occasionally become mineralized in sedimentary rock andare recognizable as fossils. Metamorphic rocks form when sedimentary or igneous rocks arealtered by heat and pressure deep underground.Oceanic crust consists of dark, dense igneous rocks, such as basalt and gabbro. Continentalcrust consists of lighter-colored, less dense igneous rocks, such as granite and diorite.Continental crust also includes metamorphic rocks and sedimentary rocks.BiosphereThe biosphere includes all the areas of Earth capable of supporting life. The biosphere rangesfrom about 10 km (about 6 mi) into the atmosphere to the deepest ocean floor. For a longtime, scientists believed that all life depended on energy from the Sun and consequently couldonly exist where sunlight penetrated. In the 1970s, however, scientists discovered variousforms of life around hydrothermal vents on the floor of the Pacific Ocean where no sunlightpenetrated. They learned that primitive bacteria formed the basis of this living community andthat the bacteria derived their energy from a process called chemosynthesis that did notdepend on sunlight. Some scientists believe that the biosphere may extend relatively deepinto Earth’s crust. They have recovered what they believe are primitive bacteria from deeplydrilled holes below the surface.Changes to Earth’s SurfaceEarth’s surface has been constantly changing ever since the planet formed. Most of thesechanges have been gradual, taking place over millions of years. Nevertheless, these gradualchanges have resulted in radical modifications, involving the formation, erosion, and re-formation of mountain ranges, the movement of continents, the creation of hugesupercontinents, and the breakup of supercontinents into smaller continents.The weathering and erosion that result from the water cycle are among the principal factorsresponsible for changes to Earth’s surface. Another principal factor is the movement of Earth’scontinents and seafloors and the buildup of mountain ranges due to a phenomenon known as
  • 8. plate tectonics. Heat is the basis for all of these changes. Heat in Earth’s interior is believed tobe responsible for continental movement, mountain building, and the creation of new seafloorin ocean basins. Heat from the Sun is responsible for the evaporation of ocean water and theresulting precipitation that causes weathering and erosion. In effect, heat in Earth’s interiorhelps build up Earth’s surface while heat from the Sun helps wear down the surface.WeatheringWeathering is the breakdown of rock at and near the surface of Earth. Most rocks originallyformed in a hot, high-pressure environment below the surface where there was little exposureto water. Once the rocks reached Earth’s surface, however, they were subjected totemperature changes and exposed to water. When rocks are subjected to these kinds ofsurface conditions, the minerals they contain tend to change. These changes constitute theprocess of weathering. There are two types of weathering: physical weathering and chemicalweathering.Physical weathering involves a decrease in the size of rock material. Freezing and thawing ofwater in rock cavities, for example, splits rock into small pieces because water expands whenit 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 clayminerals, resulting in a new substance with totally different properties than the parentfeldspar. Chemical weathering is of significance to humans because it creates the clayminerals that are important components of soil, the basis of agriculture. Chemical weatheringalso causes the release of dissolved forms of sodium, calcium, potassium, magnesium, andother chemical elements into surface water and groundwater. These elements are carried bysurface water and groundwater to the sea and are the sources of dissolved salts in the sea.ErosionErosion 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 alonger period of time than other forms of erosion. When water from rain or melted snowmoves downhill, it can carry loose rock or soil with it. Erosion by running water forms thefamiliar gullies and V-shaped valleys that cut into most landscapes. The force of the runningwater removes loose particles formed by weathering. In the process, gullies and valleys arelengthened, widened, and deepened. Often, water overflows the banks of the gullies or riverchannels, resulting in floods. Each new flood carries more material away to increase the size ofthe valley. Meanwhile, weathering loosens more and more material so the process continues.
  • 9. Erosion by glacial ice is less common, but it can cause the greatest landscape changes in theshortest amount of time. Glacial ice forms in a region where snow fails to melt in the springand summer and instead builds up as ice. For major glaciers to form, this lack of snowmelt hasto occur for a number of years in areas with high precipitation. As ice accumulates andthickens, it flows as a solid mass. As it flows, it has a tremendous capacity to erode soil andeven solid rock. Ice is a major factor in shaping some landscapes, especially mountainousregions. Glacial ice provides much of the spectacular scenery in these regions. Features suchas horns (sharp mountain peaks), arêtes (sharp ridges), glacially formed lakes, and U-shapedvalleys 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 ofan area determines the distribution, amount, and kind of precipitation that the area receivesand thus the type and rate of weathering. An area with an arid climate erodes differently thanan area with a humid climate. The elevation of an area also plays a role by determining thepotential energy of running water. The higher the elevation the more energetically water willflow 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 erosionhave affected the area—determines its overall appearance. Relatively young landscapes tendto be more rugged and angular in appearance. Older landscapes tend to have more roundedslopes and hills. The oldest landscapes tend to be low-lying with broad, open river valleys andlow, rounded hills. The overall effect of the wearing down of an area is to level the land; thetendency is toward the reduction of all land surfaces to sea level.Plate TectonicsOpposing this tendency toward leveling is a force responsible for raising mountains andplateaus and for creating new landmasses. These changes to Earth’s surface occur in theoutermost solid portion of Earth, known as the lithosphere. The lithosphere consists of thecrust and another region known as the upper mantle and is approximately 65 to 100 km (40to 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 thewhole apple.Scientists believe that the lithosphere is broken into a series of plates, or segments. Accordingto the theory of plate tectonics, these plates move around on Earth’s surface over long periodsof time. Tectonics comes from the Greek word, tektonikos, which means “builder.”
  • 10. According to the theory, the lithosphere is divided into large and small plates. The largestplates include the Pacific plate, the North American plate, the Eurasian plate, the Antarcticplate, 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 moveaway from each other, they collide or move against each other, or they slide past each otheras 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 ofthe oceans and continents.When Plates Pull ApartWhen the plates pull apart, two types of phenomena occur depending on whether themovement takes place in the oceans or on land. When plates pull apart on land, deep valleysknown as rift valleys form. An example of a rift valley is the Great Rift Valley that extendsfrom 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 iscreated 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 tomove apart and to create new seafloor. According to this explanation, molten rock known asmagma rises from Earth’s interior to form hot spots beneath the ocean floor. As two oceanicplates pull apart from each other in the middle of the oceans, a crack, or rupture, appears andforms the mid-ocean ridges. These ridges exist in all the world’s ocean basins and resemblethe seams of a baseball. The molten rock rises through these cracks and creates new seafloor.When Plates CollideWhen 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, ittriggers the melting of the surrounding lithosphere and a region just below it known as theasthenosphere. These pockets of molten crust rise behind the margin through the overlyingplate, 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 denseroceanic crust will dive below the continental crust. If both plates are oceanic crust, then eithermay be subducted. If both are continental crust, subduction can continue for a while but will
  • 11. eventually end because continental crust is not dense enough to be forced very far into theupper mantle.The results of this subduction process are readily visible on a map showing that 80 percent ofthe world’s volcanoes rim the Pacific Ocean where plates are colliding against each other. Thesubduction zone created by the collision of two oceanic plates—the Pacific plate and thePhilippine plate—can also create a trench. Such a trench resulted in the formation of thedeepest 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 collisionof 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 (29,035ft) above sea level.When Plates Slide Past Each OtherFinally, 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 becauseneither 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 inCalifornia 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) peryear. But over millions of years this gradual movement results in radical changes. Currentplate movement is making the Pacific Ocean and Mediterranean Sea smaller, the AtlanticOcean larger, and the Himalayan Mountains higher.EARTH’S INTERIORThe interior of Earth plays an important role in plate tectonics. Scientists believe it is alsoresponsible for Earth’s magnetic field. This field is vital to life because it shields the planet’ssurface from harmful cosmic rays and from a steady stream of energetic particles from theSun known as the solar wind.Composition of the InteriorEarth’s interior consists of the mantle and the core. The mantle and core make up by far thelargest part of Earth’s mass. The distance from the base of the crust to the center of the coreis about 6,400 km (about 4,000 mi).
  • 12. Scientists have learned about Earth’s interior by studying rocks that formed in the interior androse to the surface. The study of meteorites, which are believed to be made of the samematerial 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 aboutthe composition of the interior. The sudden movement of rocks during an earthquake causesvibrations that transmit energy through the Earth in the form of waves. The way these wavestravel 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 itknown 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 (about1,800 mi). Scientists believe the asthenosphere is made up of mushy plastic-like rock withpockets of molten rock. The term asthenosphere is derived from Greek and means “weaklayer.” The asthenosphere’s soft, plastic quality allows plates in the lithosphere above it toshift and slide on top of the asthenosphere. This shifting of the lithosphere’s plates is thesource of most tectonic activity. The asthenosphere is also the source of the basaltic magmathat 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 minorcomponents 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 about2,260 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 solidform. The inner core and the outer core also contain a small percentage of radioactivematerial. The existence of radioactive material is one of the sources of heat in Earth’s interiorbecause as radioactive material decays, it gives off heat. Temperatures in the inner core maybe as high as 6650°C (12,000°F).The Core and Earth’s MagnetismScientists believe that Earth’s liquid iron core is instrumental in creating a magnetic field thatsurrounds 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 andnatural philosopher William Gilbert. Gilbert proposed the idea to explain why the magnetizedneedle in a compass points north. According to Gilbert, Earth’s magnetic field creates amagnetic north pole and a magnetic south pole. The magnetic poles do not correspond to thegeographic North and South poles, however. Moreover, the magnetic poles wander and arenot always in the same place. The north magnetic pole is currently close to Ellef RingnesIsland in the Queen Elizabeth Islands near the boundary of Canada’s Northwest Territorieswith Nunavut. The south magnetic pole lies just off the coast of Wilkes Land, Antarctica.
  • 13. Not only do the magnetic poles wander, but they also reverse their polarity—that is, the northmagnetic pole becomes the south magnetic pole and vice versa. Magnetic reversals haveoccurred at least 170 times over the past 100 million years. The reversals occur on averageabout 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 berelated to Earth’s rotation and changes in the flow of liquid iron in the outer core.Some scientists theorize that the flow of liquid iron in the outer core sets up electrical currentsthat produce Earth’s magnetic field. Known as the dynamo theory, this theory appears to bethe best explanation yet for the origin of the magnetic field. Earth’s magnetic field operates ina region above Earth’s surface known as the magnetosphere. The magnetosphere is shapedsomewhat like a teardrop with a long tail that trails away from the Earth due to the force ofthe solar wind.Inside the magnetosphere are the Van Allen radiation belts, named for the American physicistJames A. Van Allen who discovered them in 1958. The Van Allen belts are regions wherecharged particles from the Sun and from cosmic rays are trapped and sent into spiral pathsalong the lines of Earth’s magnetic field. The radiation belts thereby shield Earth’s surfacefrom these highly energetic particles. Occasionally, however, due to extremely strongmagnetic fields on the Sun’s surface, which are visible as sunspots, a brief burst of highlyenergetic particles streams along with the solar wind. Because Earth’s magnetic field linesconverge and are closest to the surface at the poles, some of these energetic particles sneakthrough and interact with Earth’s atmosphere, creating the phenomenon known as an aurora.EARTH’S PASTOrigin of EarthMost scientists believe that the Earth, Sun, and all of the other planets and moons in the solarsystem formed about 4.6 billion years ago from a giant cloud of gas and dust known as thesolar nebula. The gas and dust in this solar nebula originated in a star that ended its life in aviolent 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 heavierelements, such as carbon and oxygen. All of the chemical elements we know were originallymade in the star that became a supernova. Our bodies are made of these same chemicalelements. Therefore, all of the elements in our solar system, including all of the elements inour 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 solarnebula. As these clumps came together and grew larger, they caused the solar nebula tocontract in on itself. The contraction caused the cloud of gas and dust to flatten in the shapeof a disc. As the clumps continued to contract, they became very dense and hot. Eventuallythe atoms of hydrogen became so dense that they began to fuse in the innermost part of the
  • 14. cloud, and these nuclear reactions gave birth to the Sun. The fusion of hydrogen atoms in theSun is the source of its energy.Many scientists favor the planetesimal theory for how the Earth and other planets formed outof this solar nebula. This theory helps explain why the inner planets became rocky while theouter planets, except for Pluto, are made up mostly of gases. The theory also explains why allof the planets orbit the Sun in the same plane.According to this theory, temperatures decreased with increasing distance from the center ofthe 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 otherheavy compounds that make up rock, could condense out—that is, could change from a gas toa solid or liquid. Due to the force of gravity, small clumps of this rocky material eventuallycame 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 theybecame the four inner rocky planets. The inner region, however, was still too hot for otherlight elements, such as hydrogen and helium, to be retained. These elements could only existin the outermost part of the disc, where temperatures were lower. As a result two of the outerplanets—Jupiter and Saturn—are mostly made of hydrogen and helium, which are also thedominant elements in the atmospheres of Uranus and Neptune.The Early EarthWithin the planetesimal Earth, heavier matter sank to the center and lighter matter rosetoward the surface. Most scientists believe that Earth was never truly molten and that thistransfer of matter took place in the solid state. Much of the matter that went toward thecenter contained radioactive material, an important source of Earth’s internal heat. As heaviermaterial moved inward, lighter material moved outward, the planet became layered, and thelayers 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 underwenta period when they were bombarded by meteorites, the rocky debris left over from theformation of the solar system. The impact craters created during this period of heavybombardment 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 theMoon 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 Earththat melted the outer part of the planet and created the crust. By 4 billion years ago, both theoceanic and continental crust had formed, and the oldest rocks were created. These rocks areknown as the Acasta Gneiss and are found in the Canadian territory of Nunavut. Due to the
  • 15. meteorite bombardment, the early Earth was too hot for liquid water to exist and so it wasimpossible for life to exist.Geologic TimeGeologists divide the history of the Earth into three eons: the Archean Eon, which lasted fromaround 4 billion to 2.5 billion years ago; the Proterozoic Eon, which lasted from 2.5 billion to543 million years ago; and the Phanerozoic Eon, which lasted from 543 million years ago tothe present. Each eon is subdivided into different eras. For example, the Phanerozoic Eonincludes the Paleozoic Era, the Mesozoic Era, and the Cenozoic Era. In turn, eras are furtherdivided into periods. For example, the Paleozoic Era includes the Cambrian, Ordovician,Silurian, Devonian, Carboniferous, and Permian Periods.The Archean Eon is subdivided into four eras, the Eoarchean, the Paleoarchean, theMesoarchean, and the Neoarchean. The beginning of the Archean is generally dated as the ageof the oldest terrestrial rocks, which are about 4 billion years old. The Archean Eon ended 2.5billion years ago when the Proterozoic Eon began. The Proterozoic Eon is subdivided into threeeras: the Paleoproterozoic Era, the Mesoproterozoic Era, and the Neoproterozoic Era. TheProterozoic Eon lasted from 2.5 billion years ago to 543 million years ago when thePhanerozoic Eon began. The Phanerozoic Eon is subdivided into three eras: the Paleozoic Erafrom 543 million to 248 million years ago, the Mesozoic Era from 248 million to 65 millionyears ago, and the Cenozoic Era from 65 million years ago to the present.Geologists base these divisions on the study and dating of rock layers or strata, including thefossilized remains of plants and animals found in those layers. Until the late 1800s scientistscould only determine the relative ages of rock strata. They knew that in general the top layersof 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 rocklayers. The fossil record helped scientists determine how organisms evolved or when theybecame extinct. By studying rock layers around the world, geologists and paleontologists sawthat the remains of certain animal and plant species occurred in the same layers, but wereabsent or altered in other layers. They soon developed a fossil index that also helpeddetermine the relative ages of rock layers.Beginning in the 1890s, scientists learned that radioactive elements in rock decay at a knownrate. 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 determinedthrough stratigraphy and the fossil index and assigned absolute ages to the various strata. Asa result scientists were able to assemble Earth’s geologic time scale from the Archean Eon tothe present.
  • 16. PrecambrianThe Precambrian is a time span that includes the Archean and Proterozoic eons and beganabout 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 thatcan still be studied. Very little remains of Earth from the period of 4.6 billion to about 4 billionyears 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 rocklayers have been dated to 4.2 billion years.The Precambrian is also the longest chapter in Earth’s history, spanning a period of about 3.5billion years. During this timeframe, the atmosphere and the oceans formed from gases thatescaped from the hot interior of the planet as a result of widespread volcanic eruptions. Theearly atmosphere consisted primarily of nitrogen, carbon dioxide, and water vapor. As Earthcontinued 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 cometscontaining 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’ssurface as early as the middle Precambrian. About a billion years ago, the Earth’s plates werecentered around the South Pole and formed a supercontinent called Rodinia. Slowly, pieces ofthis supercontinent broke away from the central continent and traveled north, forming smallercontinents.Life originated during the Precambrian. The earliest fossil evidence of life consists ofprokaryotes, one-celled organisms that lacked a nucleus and reproduced by dividing, aprocess known as asexual reproduction. Asexual division meant that a prokaryote’s hereditarymaterial 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, butcertainly by about 3.5 billion years ago, and were anaerobic—that is, they did not requireoxygen 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 prokaryoteknown as cyanobacteria or blue-green algae. These cyanobacteria gradually introducedoxygen in the atmosphere as a result of photosynthesis. In shallow tropical waters,cyanobacteria formed mats that grew into humps called stromatolites. Fossilized stromatoliteshave been found in rocks in the Pilbara region of western Australia that are more than 3.4billion years old and in rocks of the Gunflint Chert region of northwest Lake Superior that areabout 2.1 billion years old.
  • 17. For billions of years, life existed only in the simple form of prokaryotes. Prokaryotes werefollowed by the relatively more advanced eukaryotes, organisms that have a nucleus in theircells and that reproduce by combining or sharing their heredity makeup rather than by simplydividing. Sexual reproduction marked a milestone in life on Earth because it created thepossibility of hereditary variation and enabled organisms to adapt more easily to a changingenvironment. The very latest part of Precambrian time some 560 million to 545 million yearsago saw the appearance of an intriguing group of fossil organisms known as the Ediacaranfauna. First discovered in the northern Flinders Range region of Australia in the mid-1940s andsubsequently found in many locations throughout the world, these strange fossils appear to bethe precursors of many of the fossil groups that were to explode in Earths oceans in thePaleozoic Era.Paleozoic EraAt the start of the Paleozoic Era about 543 million years ago, an enormous expansion in thediversity and complexity of life occurred. This event took place in the Cambrian Period and iscalled the Cambrian explosion. Nothing like it has happened since. Almost all of the majorgroups 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’sbody is designed, with heads, legs, rear ends, claws, tentacles, or antennae—also originatedduring this period.Fishes first appeared during the Paleozoic Era, and multicellular plants began growing on theland. Other land animals, such as scorpions, insects, and amphibians, also originated duringthis time. Just as new forms of life were being created, however, other forms of life weregoing out of existence. Natural selection meant that some species were able to flourish, whileothers 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 ofwarm, shallow seas, along with the buildup of oxygen in the atmosphere, may have aided thisexplosion of life forms. The shallow seas were created by the breakup of the supercontinentRodinia. During the Ordovician, Silurian, and Devonian periods, which followed the CambrianPeriod and lasted from 490 million to 354 million years ago, some of the continental piecesthat had broken off Rodinia collided. These collisions resulted in larger continental masses inequatorial regions and in the Northern Hemisphere. The collisions built a number of mountainranges, including parts of the Appalachian Mountains in North America and the CaledonianMountains of northern Europe.Toward the close of the Paleozoic Era, two large continental masses, Gondwanaland to thesouth and Laurasia to the north, faced each other across the equator. Their slow but eventfulcollision during the Permian Period of the Paleozoic Era, which lasted from 290 million to 248million years ago, assembled the supercontinent Pangaea and resulted in some of thegrandest mountains in the history of Earth. These mountains included other parts of the
  • 18. Appalachians and the Ural Mountains of Asia. At the close of the Paleozoic Era, Pangaearepresented over 90 percent of all the continental landmasses. Pangaea straddled the equatorwith a huge mouthlike opening that faced east. This opening was the Tethys Ocean, whichclosed as India moved northward creating the Himalayas. The last remnants of the TethysOcean can be seen in today’s Mediterranean Sea.The Paleozoic came to an end with a major extinction event, when perhaps as many as 90percent of all plant and animal species died out. The reason is not known for sure, but manyscientists believe that huge volcanic outpourings of lavas in central Siberia, coupled with anasteroid impact, were joint contributing factors.Mesozoic EraThe Mesozoic Era, beginning 248 million years ago, is often characterized as the Age ofReptiles because reptiles were the dominant life forms during this era. Reptiles dominated notonly on land, as dinosaurs, but also in the sea, in the form of the plesiosaurs andichthyosaurs, and in the air, as pterosaurs, which were flying reptiles.The Mesozoic Era is divided into three geological periods: the Triassic, which lasted from 248million to 206 million years ago; the Jurassic, from 206 million to 144 million years ago; andthe Cretaceous, from 144 million to 65 million years ago. The dinosaurs emerged during theTriassic Period and were one of the most successful animals in Earth’s history, lasting forabout 180 million years before going extinct at the end of the Cretaceous Period. The firstbirds and mammals and the first flowering plants also appeared during the Mesozoic Era.Before flowering plants emerged, plants with seed-bearing cones known as conifers were thedominant form of plants. Flowering plants soon replaced conifers as the dominant form ofvegetation during the Mesozoic Era.The Mesozoic was an eventful era geologically with many changes to Earth’s surface. Pangaeacontinued to exist for another 50 million years during the early Mesozoic Era. By the earlyJurassic Period, Pangaea began to break up. What is now South America began splitting fromwhat is now Africa, and in the process the South Atlantic Ocean formed. As the landmass thatbecame North America drifted away from Pangaea and moved westward, a long subductionzone extended along North America’s western margin. This subduction zone and theaccompanying arc of volcanoes extended from what is now Alaska to the southern tip of SouthAmerica. Much of this feature, called the American Cordillera, exists today as the easternmargin of the Pacific Ring of Fire. During the Cretaceous Period, heat continued to be releasedfrom the margins of the drifting continents, and as they slowly sank, vast inland seas formedin much of the continental interiors. The fossilized remains of fishes and marine molluskscalled ammonites can be found today in the middle of the North American continent becausethese areas were once underwater. Large continental masses broke off the northern part ofsouthern Gondwanaland during this period and began to narrow the Tethys Ocean. The largestof these continental masses, present-day India, moved northward toward its collision with
  • 19. southern Asia. As both the North Atlantic Ocean and South Atlantic Ocean continued to open,North and South America became isolated continents for the first time in 450 million years.Their westward journey resulted in mountains along their western margins, including theAndes of South America.Cenozoic EraThe Cenozoic Era, beginning about 65 million years ago, is the period when mammals becamethe dominant form of life on land. Human beings first appeared in the later stages of theCenozoic Era. In short, the modern world as we know it, with its characteristic geographicalfeatures and its animals and plants, came into being. All of the continents that we know todaytook shape during this era.A single catastrophic event may have been responsible for this relatively abrupt change fromthe Age of Reptiles to the Age of Mammals. Most scientists now believe that a huge asteroid orcomet 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 collisioncame with the discovery of a large impact crater off the coast of Mexico’s Yucatán Peninsulaand the worldwide finding of iridium, a metallic element rare on Earth but abundant inmeteorites, in rock layers dated from the end of the Cretaceous Period. The extinction of thedinosaurs 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 Periodlasted from about 65 million to about 1.8 million years ago. The Quaternary Period beganabout 1.8 million years ago and continues to the present day. These periods are furthersubdivided into epochs, such as the Pleistocene, from 1.8 million to 10,000 years ago, and theHolocene, from 10,000 years ago to the present.Early in the Tertiary Period, Pangaea was completely disassembled, and the modern continentswere all clearly outlined. India and other continental masses began colliding with southernAsia to form the Himalayas. Africa and a series of smaller microcontinents began colliding withsouthern Europe to form the Alps. The Tethys Ocean was nearly closed and began to resembletoday’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 itsactive 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. Theseearly humans began to migrate to other parts of the world between 2 million and 1.7 millionyears ago.The Quaternary Period marks the onset of the great ice ages. Many times, perhapsat least once every 100,000 years on average, vast glaciers 3 km (2 mi) thick invaded muchof North America, Europe, and parts of Asia. The glaciers eroded considerable amounts ofmaterial that stood in their paths, gouging out U-shaped valleys. Anatomically modern humanbeings, known as Homo sapiens sapiens, became the dominant form of life in the Quaternary
  • 20. Period. Most anthropologists (scientists who study human life and culture) believe thatanatomically modern humans originated only recently in Earth’s 4.6-billion-year history, about130,000 years ago.EARTH’S FUTUREWith the rise of human civilization about 8,000 years ago and especially since the IndustrialRevolution in the mid-1700s, human beings began to alter the surface, water, and atmosphereof Earth. In doing so, they have become active geological agents, not unlike other forces ofchange that influence the planet. As a result, Earth’s immediate future depends to a greatextent on the behavior of humans. For example, the widespread use of fossil fuels is releasingcarbon dioxide and other greenhouse gases into the atmosphere and threatens to warm theplanet’s surface. This global warming could melt glaciers and the polar ice caps, which couldflood coastlines around the world and many island nations. In effect, the carbon dioxide thatwas removed from Earth’s early atmosphere by the oceans and by primitive plant and animallife, and subsequently buried as fossilized remains in sedimentary rock, is being released backinto the atmosphere and is threatening the existence of living things.Even without human intervention, Earth will continue to change because it is geologicallyactive. 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 geologistspredict that in 200 million years the Red Sea will be the same size as the Atlantic Ocean istoday. Other scientists predict that the continent of Asia will break apart millions of years fromnow, and as it does, Lake Baikal in Siberia will become a vast ocean, separating twolandmasses that once made up the Asian continent.In the far, far distant future, however, scientists believe that Earth will become anuninhabitable planet, scorched by the Sun. Knowing the rate at which nuclear fusion occurs inthe Sun and knowing the Sun’s mass, astrophysicists (scientists who study stars) havecalculated that the Sun will become brighter and hotter about 3 billion years from now, whenit will be hot enough to boil Earth’s oceans away. Based on studies of how other Sun-like starshave 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 atmospherewill expand until it engulfs the planet Mercury. The Sun will then be 2,000 times brighter thanit is now and so hot it will melt Earth’s rocks. Earth will end its existence as a burnt cinder.Three billion years is the life span of millions of human generations, however. Perhaps bythen, humans will have learned how to journey beyond the solar system to colonize otherplanets in the Milky Way Galaxy and find another place to call “home.”
  • 21. EarthquakeINTRODUCTIONEarthquake, shaking of the Earth’s surface caused by rapid movement of the Earth’s rockyouter layer. Earthquakes occur when energy stored within the Earth, usually in the form ofstrain in rocks, suddenly releases. This energy is transmitted to the surface of the Earth byearthquake waves. The study of earthquakes and the waves they create is called seismology(from the Greek seismos, “to shake”). Scientists who study earthquakes are calledseismologists.The destruction an earthquake causes depends on its magnitude and duration, or the amountof shaking that occurs. A structure’s design and the materials used in its construction alsoaffect the amount of damage the structure incurs. Earthquakes vary from small, imperceptibleshaking to large shocks felt over thousands of kilometers. Earthquakes can deform theground, make buildings and other structures collapse, and create tsunamis (large sea waves).Lives may be lost in the resulting destruction.Earthquakes, or seismic tremors, occur at a rate of several hundred per day around the world.A worldwide network of seismographs (machines that record movements of the Earth) detectsabout 1 million small earthquakes per year. Very large earthquakes, such as the 1964 Alaskanearthquake, which caused millions of dollars in damage, occur worldwide once every fewyears. Moderate earthquakes, such as the 1989 tremor in Loma Prieta, California, and the1995 tremor in K be, Japan, occur about 20 times a year. Moderate earthquakes also causemillions of dollars in damage and can harm many people.In the last 500 years, several million people have been killed by earthquakes around theworld, including over 240,000 in the 1976 T’ang-Shan, China, earthquake. Worldwide,earthquakes have also caused severe property and structural damage. Adequate precautions,such as education, emergency planning, and constructing stronger, more flexible, safelydesigned structures, can limit the loss of life and decrease the damage caused byearthquakes.ANATOMY OF AN EARTHQUAKESeismologists examine the parts of an earthquake, such as what happens to the Earth’ssurface during an earthquake, how the energy of an earthquake moves from inside the Earthto the surface, how this energy causes damage, and the slip of the fault that causes theearthquake. Faults are cracks in Earth’s crust where rocks on either side of the crack havemoved. By studying the different parts and actions of earthquakes, seismologists learn moreabout their effects and how to predict and prepare for their ground shaking in order to reducedamage.
  • 22. Focus and EpicenterThe point within the Earth along the rupturing geological fault where an earthquake originatesis called the focus, or hypocenter. The point on the Earth’s surface directly above the focus iscalled the epicenter. Earthquake waves begin to radiate out from the focus and subsequentlyform along the fault rupture. If the focus is near the surface—between 0 and 70 km (0 and 40mi) deep—shallow-focus earthquakes are produced. If it is intermediate or deep below thecrust—between 70 and 700 km (40 and 400 mi) deep—a deep-focus earthquake will beproduced. Shallow-focus earthquakes tend to be larger, and therefore more damaging,earthquakes. This is because they are closer to the surface where the rocks are stronger andbuild up more strain.Seismologists know from observations that most earthquakes originate as shallow-focusearthquakes and most of them occur near plate boundaries—areas where the Earth’s crustalplates move against each other (see Plate Tectonics). Other earthquakes, including deep-focusearthquakes, can originate in subduction zones, where one tectonic plate subducts, or movesunder another plate.FaultsStress in the Earth’s crust creates faults, resulting in earthquakes. The properties of anearthquake depend strongly on the type of fault slip, or movement along the fault, that causesthe earthquake. Geologists categorize faults according to the direction of the fault slip. Thesurface between the two sides of a fault lies in a plane, and the direction of the plane isusually not vertical; rather it dips at an angle into the Earth. When the rock hanging over thedipping fault plane slips downward into the ground, the fault is called a normal fault. When thehanging wall slips upward in relation to the footwall, the fault is called a reverse fault. Bothnormal and reverse faults produce vertical displacements, or the upward movement of oneside of the fault above the other side, that appear at the surface as fault scarps. Strike-slipfaults are another type of fault that produce horizontal displacements, or the side by sidesliding movement of the fault, such as seen along the San Andreas fault in California. Strike-slip faults are usually found along boundaries between two plates that are sliding past eachother.WavesThe sudden movement of rocks along a fault causes vibrations that transmit energy throughthe Earth in the form of waves. Waves that travel in the rocks below the surface of the Earthare called body waves, and there are two types of body waves: primary, or P, waves, andsecondary, or S, waves. The S waves, also known as shearing waves, move the ground backand forth.
  • 23. Earthquakes also contain surface waves that travel out from the epicenter along the surface ofthe Earth. Two types of these surface waves occur: Rayleigh waves, named after Britishphysicist Lord Rayleigh, and Love waves, named after British geophysicist A. E. H. Love.Surface waves also cause damage to structures, as they shake the ground underneath thefoundations of buildings and other structures.Body waves, or P and S waves, radiate out from the rupturing fault starting at the focus of theearthquake. P waves are compression waves because the rocky material in their path movesback and forth in the same direction as the wave travels alternately compressing andexpanding the rock. P waves are the fastest seismic waves; they travel in strong rock at about6 to 7 km (about 4 mi) per second. P waves are followed by S waves, which shear, or twist,rather than compress the rock they travel through. S waves travel at about 3.5 km (about 2mi) per second. S waves cause rocky material to move either side to side or up and downperpendicular to the direction the waves are traveling, thus shearing the rocks. Both P and Swaves help seismologists to locate the focus and epicenter of an earthquake. As P and Swaves move through the interior of the Earth, they are reflected and refracted, or bent, just aslight waves are reflected and bent by glass. Seismologists examine this bending to determinewhere the earthquake originated.On the surface of the Earth, Rayleigh waves cause rock particles to move forward, up,backward, and down in a path that contains the direction of the wave travel. This circularmovement is somewhat like a piece of seaweed caught in an ocean wave, rolling in a circularpath onto a beach. The second type of surface wave, the Love wave, causes rock to movehorizontally, or side to side at right angles to the direction of the traveling wave, with novertical displacements. Rayleigh and Love waves always travel slower than P waves andusually travel slower than S waves.CAUSESMost earthquakes are caused by the sudden slip along geologic faults. The faults slip becauseof movement of the Earth’s tectonic plates. This concept is called the elastic rebound theory.The rocky tectonic plates move very slowly, floating on top of a weaker rocky layer. As theplates collide with each other or slide past each other, pressure builds up within the rockycrust. Earthquakes occur when pressure within the crust increases slowly over hundreds ofyears and finally exceeds the strength of the rocks. Earthquakes also occur when humanactivities, such as the filling of reservoirs, increase stress in the Earth’s crust.Elastic Rebound TheoryIn 1911 American seismologist Harry Fielding Reid studied the effects of the April 1906California earthquake. He proposed the elastic rebound theory to explain the generation ofcertain earthquakes that scientists now know occur in tectonic areas, usually near plate
  • 24. boundaries. This theory states that during an earthquake, the rocks under strain suddenlybreak, creating a fracture along a fault. When a fault slips, movement in the crustal rockcauses vibrations. The slip changes the local strain out into the surrounding rock. The changein strain leads to aftershocks (smaller earthquakes that occur after the initial earthquake),which are produced by further slips of the main fault or adjacent faults in the strained region.The slip begins at the focus and travels along the plane of the fault, radiating waves out alongthe rupture surface. On each side of the fault, the rock shifts in opposite directions. The faultrupture travels in irregular steps along the fault; these sudden stops and starts of the movingrupture give rise to the vibrations that propagate as seismic waves. After the earthquake,strain begins to build again until it is greater than the forces holding the rocks together, thenthe fault snaps again and causes another earthquake.Predicting EarthquakesSeismologists try to predict how likely it is that an earthquake will occur, with a specified time,place, and size. Earthquake prediction also includes calculating how a strong ground motionwill affect a certain area if an earthquake does occur. Scientists can use the growing catalogueof recorded earthquakes to estimate when and where strong seismic motions may occur. Theymap past earthquakes to help determine expected rates of repetition. Seismologists can alsomeasure movement along major faults using global positioning satellites (GPS) to track therelative movement of the rocky crust of a few centimeters each year along faults. Thisinformation may help predict earthquakes. Even with precise instrumental measurement ofpast earthquakes, however, conclusions about future tremors always involve uncertainty. Thismeans that any useful earthquake prediction must estimate the likelihood of the earthquakeoccurring in a particular area in a specific time interval compared with its occurrence as achance event.The elastic rebound theory gives a generalized way of predicting earthquakes because it statesthat a large earthquake cannot occur until the strain along a fault exceeds the strengthholding the rock masses together. Seismologists can calculate an estimated time when thestrain along the fault would be great enough to cause an earthquake. As an example, after the1906 San Francisco earthquake, the measurements showed that in the 50 years prior to 1906,the San Andreas fault accumulated about 3.2 meters (10 feet) of displacement, or movement,at points across the fault. The maximum 1906 fault slip was 6.5 meters (21 feet), so it wassuggested that 50 years x 6.5 meters/3.2 meters (21 feet/10 feet), about 100 years, wouldelapse before sufficient energy would again accumulate to produce a comparable earthquake.Scientists have measured other changes along active faults to try and predict future activity.These measurements have included changes in the ability of rocks to conduct electricity,changes in ground water levels, and changes in variations in the speed at which seismic wavespass through the region of interest. None of these methods, however, has been successful inpredicting earthquakes to date.
  • 25. Seismologists have also developed field methods to date the years in which past earthquakesoccurred. In addition to information from recorded earthquakes, scientists look into geologichistory for information about earthquakes that occurred before people had instruments tomeasure them. This research field is called paleoseismology (paleo is Greek for “ancient”).Seismologists can determine when ancient earthquakes occurred.The Earth’s InteriorSeismologists also study earthquakes to learn more about the structure of the Earth’s interior.Earthquakes provide a rare opportunity for scientists to observe how the Earth’s interiorresponds when an earthquake wave passes through it. Measuring depths and geologicstructures within the Earth using earthquake waves is more difficult for scientists than ismeasuring distances on the Earth’s surface. However, seismologists have used earthquakewaves to determine that there are four main regions that make up the interior of the Earth:the crust, the mantle, and the inner and outer core.The intense study of earthquake waves began during the last decades of the 19th century,when people began placing seismographs at observatories around the world. By 1897scientists had gathered enough seismograms from distant earthquakes to identify that P and Swaves had traveled through the deep Earth. Seismologists studying these seismograms laterin the late 19th and early 20th centuries discovered P wave and S wave shadow zones—areason the opposite side of the Earth from the earthquake focus that P waves and S waves do notreach. These shadow zones showed that the waves were bouncing off some large geologicinterior structures of the planet.EXTRATERRESTRIAL QUAKESSeismic events similar to earthquakes also occur on other planets and on their satellites.Scientific missions to Earth’s moon and to Mars have provided some information related toextraterrestrial quakes. The current Galileo mission to Jupiter’s moons may provide evidenceof quakes on the moons of Jupiter.Between 1969 and 1977, scientists conducted the Passive Seismic Experiment as part of theUnited States Apollo Program. Astronauts set up seismograph stations at five lunar sites. Eachlunar seismograph detected between 600 and 3,000 moonquakes every year, a surprisingresult because the Moon has no tectonic plates, active volcanoes, or ocean trench systems.Most moonquakes had magnitudes less than about 2.0 on the Richter scale. Scientists usedthis information to determine the interior structure of the Moon and to examine the frequencyof moonquakes.Besides the Moon and the Earth, Mars is the only other planetary body on which seismographshave been placed. The Viking 1 and 2 spacecraft carried two seismographs to Mars in 1976.
  • 26. Unfortunately, the instrument on Viking 1 failed to return signals to Earth. The instrument onViking 2 operated, but in one year, only one wave motion was detected. Scientists wereunable to determine the interior structure of Mars with only this single event.FaultFault (geology), crack in the crust of the earth along which there has been movement of therocks on either side of the crack. A crack without movement is called a joint. Faults occur on awide scale, ranging in length from millimeters to thousands of kilometers. Large-scale faultsresult from the movement of tectonic plates, continent-sized slabs of the crust that move ascoherent pieces.HOW FAULTS ARE CREATEDFaults are created by stress in the earth’s crust. Stress is a force, such as squeezing orstretching, that changes the shape of an object. When a material is stressed, the material mayrespond in three different ways. It can deform (stretch or compress) elastically, which meansthat when the stress is removed, the material goes back to its original shape. An elasticdeformation is, therefore, reversible. Alternatively, a stressed material can deforminelastically, which means that when the stress is removed, the material stays in its new,deformed shape. An inelastic deformation is irreversible. Lastly, a stressed material canfracture, or break into pieces.Most solid materials, including rocks, deform elastically under small stress. Ductile materialsare materials that will deform inelastically under moderate stress and will fracture underhigher stress. Brittle materials fracture with little or no inelastic deformation. Rocks tend to bebrittle when they are cold and become more ductile when they are hot. Rocks also tend tobecome more ductile when they are under pressure. For these reasons, most rocks are brittlenear the surface of the earth where there is less heat and pressure, and they become moreductile with depth. Most faults occur in the top 10 km (6 mi) of the crust. Below this depth,most rocks bend and fold in response to stress.Rocks can experience three different kinds of stress: tension, compression, and shearing.Tension pulls rocks apart, pushing opposite sides away from each other. Compressionsqueezes rocks, pushing opposite sides toward each other. Shearing pushes opposite sidespast each other in opposite but parallel directions.TsunamiTsunami, Japanese word, meaning “harbor wave” and used as the scientific term for seismicsea wave generated by an undersea earthquake or possibly an undersea landslide or volcaniceruption. When the ocean floor is tilted or offset during an earthquake, a set of waves iscreated similar to the concentric waves generated by an object dropped into the water. Mosttsunamis originate along the Ring of Fire, a zone of volcanoes and seismic activity, 32,500 km
  • 27. (24,000 mi) long, that encircles the Pacific Ocean. Since 1819, about 40 tsunamis have struckthe Hawaiian Islands.A tsunami can have wavelengths, or widths, of 100 to 200 km (60 to 120 mi), and may travelhundreds of kilometers across the deep ocean, reaching speeds of about 725 to 800 km/h(about 450 to 500 mph). Upon entering shallow coastal waters, the wave, which may havebeen only about half a meter (a foot or two) high out at sea, suddenly grows rapidly. Whenthe wave reaches the shore, it may be 15 m (50 ft) high or more. Tsunamis have tremendousenergy because of the great volume of water affected. They are capable of obliterating coastalsettlements.Tsunamis should not be confused with storm surges, which are domes of water that riseunderneath hurricanes or cyclones and cause extensive coastal flooding when the stormsreach land. Storm surges are particularly devastating if they occur at high tide.Richter ScaleRichter Scale, method of ranking the strength or size of an earthquake. The Richter scale, alsoknown as the local magnitude scale, was devised in 1935 by the American seismologistCharles F. Richter to rank earthquakes occurring in California. Richter and his associates latermodified it to apply to earthquakes anywhere in the world.The Richter scale ranks earthquakes based on how much the ground shakes 100 km (60 mi)from the earthquake’s epicenter, the site on the earth’s surface directly above theearthquake’s origin. The amount of ground movement is measured by an instrument called aseismograph. Seismographs can detect movements as small as about 0.00001 mm (about0.000004 in) to movements as large as about 1 m (about 40 in). In order to deal withnumbers in such a broad range, the Richter scale is a logarithmic scale—each increase of 1 onthe Richter scale represents a tenfold increase in movement. Thus, an earthquake registering7 on the scale is 10 times as strong as an earthquake registering 6, and the earth moves 10times as far.Earthquakes of magnitude 5 are considered moderate, while quakes of magnitude 6 areconsidered large, quakes of magnitude 7 are considered major, and quakes of magnitude 8 orlarger are considered great. For example, the Los Angeles earthquake of 1994 was amagnitude 6.7 earthquake and the San Francisco earthquake of 1906 was a magnitude 7.9earthquake. Although there is no upper limit to the Richter scale, earthquakes of magnitude 8or greater are rare. Worldwide, they occur only about once a year. Scientists believe that thecrust cannot store enough energy to release a magnitude 10 earthquake. There is also nolower limit on the Richter scale. An earthquake one-tenth the size of a magnitude 1earthquake would be a magnitude 0 earthquake, and an earthquake one tenth that size would
  • 28. be a magnitude -1 earthquake. Earthquakes with negative Richter scale magnitudes occurevery day, but are so small that they are difficult to detect.The amount of energy released by an earthquake is related to how much the earth moves.The energy released by an earthquake increases 32 fold for each increase of 1 on the Richterscale. Thus, an earthquake registering 7 on the Richter scale releases 32 times as muchenergy as an earthquake registering 6, even while the earth moves only 10 times as far. Theamount of energy released by a magnitude 4.3 earthquake is equivalent to the energyreleased by the atomic bomb that destroyed Hiroshima, Japan, which is equivalent to about 20kilotons of TNT. The largest earthquakes recorded to date measured about 9.5 and released asmuch energy as 66,000,000 Hiroshima-sized atomic bombs. It is estimated that a magnitude12 earthquake would release enough energy to split the earth in half.Human ActivitiesFault rupture is not the only cause of earthquakes; human activities can also be the direct orindirect cause of significant earthquakes. Injecting fluid into deep wells for waste disposal,filling reservoirs with water, and firing underground nuclear test blasts can, in limitedcircumstances, lead to earthquakes. These activities increase the strain within the rock nearthe location of the activity so that rock slips and slides along pre-existing faults more easily.While earthquakes caused by human activities may be harmful, they can also provide usefulinformation. Prior to the Nuclear Test Ban treaty, scientists were able to analyze the travel andarrival times of P waves from known earthquakes caused by underground nuclear test blasts.Scientists used this information to study earthquake waves and determine the interiorstructure of the Earth.Scientists have determined that as water level in a reservoir increases, water pressure inpores inside the rocks along local faults also increases. The increased pressure may cause therocks to slip, generating earthquakes. Beginning in 1935, the first detailed evidence ofreservoir-induced earthquakes came from the filling of Lake Mead behind Hoover Dam on theNevada-Arizona state border. Earthquakes were rare in the area prior to construction of thedam, but seismographs registered at least 600 shallow-focus earthquakes between 1936 and1946. Most reservoirs, however, do not cause earthquakes.DISTRIBUTIONSeismologists have been monitoring the frequency and locations of earthquakes for most ofthe 20th century. Seismologists generally classify naturally occurring earthquakes into one oftwo categories: interplate and intraplate. Interplate earthquakes are the most common; theyoccur primarily along plate boundaries. Intraplate earthquakes occur where the crust is
  • 29. fracturing within a plate. Both interplate and intraplate earthquakes may be caused by tectonicor volcanic forces.Tectonic EarthquakesTectonic earthquakes are caused by the sudden release of energy stored within the rocksalong a fault. The released energy is produced by the strain on the rocks due to movementwithin the Earth, called tectonic deformation. The effect is like the sudden breaking andsnapping back of a stretched elastic band.Volcanic EarthquakesVolcanic earthquakes occur near active volcanoes but have the same fault slip mechanism astectonic earthquakes. Volcanic earthquakes are caused by the upward movement of magmaunder the volcano, which strains the rock locally and leads to an earthquake. As the fluidmagma rises to the surface of the volcano, it moves and fractures rock masses and causescontinuous tremors that can last up to several hours or days. Volcanic earthquakes occur inareas that are associated with volcanic eruptions, such as in the Cascade Mountain Range ofthe Pacific Northwest, Japan, Iceland, and at isolated hot spots such as Hawaii.LOCATIONSSeismologists use global networks of seismographic stations to accurately map the focuses ofearthquakes around the world. After studying the worldwide distribution of earthquakes, thepattern of earthquake types, and the movement of the Earth’s rocky crust, scientists proposedthat plate tectonics, or the shifting of the plates as they move over another weaker rockylayer, was the main underlying cause of earthquakes. The theory of plate tectonics arose fromseveral previous geologic theories and discoveries. Scientists now use the plate tectonicstheory to describe the movement of the Earth’s plates and how this movement causesearthquakes. They also use the knowledge of plate tectonics to explain the locations ofearthquakes, mountain formation, and deep ocean trenches, and to predict which areas will bedamaged the most by earthquakes. It is clear that major earthquakes occur most frequently inareas with features that are found at plate boundaries: high mountain ranges and deep oceantrenches. Earthquakes within plates, or intraplate tremors, are rare compared with thethousands of earthquakes that occur at plate boundaries each year, but they can be very largeand damaging.Earthquakes that occur in the area surrounding the Pacific Ocean, at the edges of the Pacificplate, are responsible for an average of 80 percent of the energy released in earthquakesworldwide. Japan is shaken by more than 1,000 tremors greater than 3.5 in magnitude eachyear. The western coasts of North and South America are very also active earthquake zones,with several thousand small to moderate earthquakes each year.
  • 30. Intraplate earthquakes are less frequent than plate boundary earthquakes, but they are stillcaused by the internal fracturing of rock masses. The New Madrid, Missouri, earthquakes of1811 and 1812 were extreme examples of intraplate seismic events. Scientists estimate thatthe three main earthquakes of this series were about magnitude 8.0 and that there were atleast 1,500 aftershocks.EFFECTSGround shaking leads to landslides and other soil movement. These are the main damage-causing events that occur during an earthquake. Primary effects that can accompany anearthquake include property damage, loss of lives, fire, and tsunami waves. Secondaryeffects, such as economic loss, disease, and lack of food and clean water, also occur after alarge earthquake.Ground Shaking and LandslidesEarthquake waves make the ground move, shaking buildings and causing poorly designed orweak structures to partially or totally collapse. The ground shaking weakens soils andfoundation materials under structures and causes dramatic changes in fine-grained soils.During an earthquake, water-saturated sandy soil becomes like liquid mud, an effect calledliquefaction. Liquefaction causes damage as the foundation soil beneath structures andbuildings weakens. Shaking may also dislodge large earth and rock masses, producingdangerous landslides, mudslides, and rock avalanches that may lead to loss of lives or furtherproperty damage.FireAnother post-earthquake threat is fire, such as the fires that happened in San Francisco afterthe 1906 earthquake and after the devastating 1923 Tokyo earthquake. In the 1923earthquake, about 130,000 lives were lost in Tokyo, Yokohama, and other cities, many infirestorms fanned by high winds. The amount of damage caused by post-earthquake firedepends on the types of building materials used, whether water lines are intact, and whethernatural gas mains have been broken. Ruptured gas mains may lead to numerous fires, and firefighting cannot be effective if the water mains are not intact to transport water to the fires.Fires can be significantly reduced with pre-earthquake planning, fire-resistant buildingmaterials, enforced fire codes, and public fire drills.Tsunami Waves and FloodingAlong the coasts, sea waves called tsunamis that accompany some large earthquakes centeredunder the ocean can cause more death and damage than ground shaking. Tsunamis areusually made up of several oceanic waves that travel out from the slipped fault and arrive one
  • 31. after the other on shore. They can strike without warning, often in places very distant fromthe epicenter of the earthquake. Tsunami waves are sometimes inaccurately referred to astidal waves, but tidal forces do not cause them. Rather, tsunamis occur when a major faultunder the ocean floor suddenly slips. The displaced rock pushes water above it like a giantpaddle, producing powerful water waves at the ocean surface. The ocean waves spread outfrom the vicinity of the earthquake source and move across the ocean until they reach thecoastline, where their height increases as they reach the continental shelf, the part of theEarth’s crust that slopes, or rises, from the ocean floor up to the land. Tsunamis wash ashorewith often disastrous effects such as severe flooding, loss of lives due to drowning, anddamage to property.Earthquakes can also cause water in lakes and reservoirs to oscillate, or slosh back and forth.The water oscillations are called seiches (pronounced saysh). Seiches can cause retainingwalls and dams to collapse and lead to flooding and damage downstream.DiseaseCatastrophic earthquakes can create a risk of widespread disease outbreaks, especially inunderdeveloped countries. Damage to water supply lines, sewage lines, and hospital facilitiesas well as lack of housing may lead to conditions that contribute to the spread of contagiousdiseases, such as influenza (the flu) and other viral infections. In some instances, lack of foodsupplies, clean water, and heating can create serious health problems as well.REDUCING DAMAGEEarthquakes cannot be prevented, but the damage they cause can be greatly reduced withcommunication strategies, proper structural design, emergency preparedness planning,education, and safer building standards. In response to the tragic loss of life and great cost ofrebuilding after past earthquakes, many countries have established earthquake safety andregulatory agencies. These agencies require codes for engineers to use in order to regulatedevelopment and construction. Buildings built according to these codes survive earthquakesbetter and ensure that earthquake risk is reduced.Engineers minimize earthquake damage to buildings by using flexible, reinforced materialsthat can withstand shaking in buildings. Since the 1960s, scientists and engineers have greatlyimproved earthquake-resistant designs for buildings that are compatible with modernarchitecture and building materials. They use computer models to predict the response of thebuilding to ground shaking patterns and compare these patterns to actual seismic events, suchas in the 1994 Northridge, California, earthquake and the 1995 K be, Japan, earthquake. Theyalso analyze computer models of the motions of buildings in the most hazardous earthquakezones to predict possible damage and to suggest what reinforcement is needed.
  • 32. Structural DesignGeologists and engineers use risk assessment maps, such as geologic hazard and seismichazard zoning maps, to understand where faults are located and how to build near themsafely. Engineers use geologic hazard maps to predict the average ground motions in aparticular area and apply these predicted motions during engineering design phases of majorconstruction projects. Engineers also use risk assessment maps to avoid building on majorfaults or to make sure that proper earthquake bracing is added to buildings constructed inzones that are prone to strong tremors. They can also use risk assessment maps to aid in theretrofit, or reinforcement, of older structures.In urban areas of the world, the seismic risk is greater in nonreinforced buildings made ofbrick, stone, or concrete blocks because they cannot resist the horizontal forces produced bylarge seismic waves. Fortunately, single-family timber-frame homes built under modernconstruction codes resist strong earthquake shaking very well. Such houses have laterallybraced frames bolted to their foundations to prevent separation. Although they may suffersome damage, they are unlikely to collapse because the strength of the strongly jointedtimber-frame can easily support the light loads of the roof and the upper stories even in theevent of strong vertical and horizontal ground motions.Emergency Preparedness PlansEarthquake education and preparedness plans can help significantly reduce death and injurycaused by earthquakes. People can take several preventative measures within their homesand at the office to reduce risk. Supports and bracing for shelves reduce the likelihood ofitems falling and potentially causing harm. Maintaining an earthquake survival kit in the homeand at the office is also an important part of being prepared.In the home, earthquake preparedness includes maintaining an earthquake kit and makingsure that the house is structurally stable. The local chapter of the American Red Cross is agood source of information for how to assemble an earthquake kit. During an earthquake,people indoors should protect themselves from falling objects and flying glass by taking refugeunder a heavy table. After an earthquake, people should move outside of buildings, assemblein open spaces, and prepare themselves for aftershocks. They should also listen foremergency bulletins on the radio, stay out of severely damaged buildings, and avoid coastalareas in the event of a tsunami.In many countries, government emergency agencies havedeveloped extensive earthquake response plans. In some earthquake hazardous regions, suchas California, Japan, and Mexico City, modern strong motion seismographs in urban areas arenow linked to a central office. Within a few minutes of an earthquake, the magnitude can bedetermined, the epicenter mapped, and intensity of shaking information can be distributed viaradio to aid in response efforts.