Earth Science Textbook Chapter 4 Plate Tectonics

, Chapter Tectonics Male mounts for Important ( mum ol Earth surface And or worm . As a bash lor content know ( of plate 15 derived from the fit of the the location of earthquakes , and es and me distribution oi fossils . rot . and zones is . Students know Earth is composed 01 several layers a cold , as hot , mantle and a dense . core . Students know the we of and move at rates of centimeter per year I moons to movements In the mantle . Students know events , such as earthquakes . eruptions and mountain , rerun from plate motions . Student ! know how to ma or features of ( in ( mountains . faults , a volcanoes in terms of plate tectonics . Marry phenomena on Earth fau are by the transfer ! energy through radiation Ind . As a basis for understanding this ( map . know heal lrom Earth reaches the surface through The Imp in the marks California San fault . Two pieces of crust Slide past eath other along liar hull .

I Ill Sh HUI Plate Tectonics , Fin on the . BIG What are plates , and how do their movements change our surface ?

Check What You Know Imagine knocking a hardboiled egg against a table so that the shell cracks in several places . Then suppose that you slice the egg in half to make a cross section of the shell , egg white , and yolk . Explain how your sliced egg with the cracked shell can serve as a model of Earth . 127

The images shown here represent some of the key terms in this chapter . You can use this vocabulary skill to help you understand the meaning al some key terms in this chapter . Use Greek Word Origins Many science words come to English from ancient Greek . in this chapter you will learn the word lithosphere . You probably know the meaning of sphere , a globe or ball . The Greek word litho means stone . Therefore . the lithosphere is the stony outer shell , or sphere , that forms Earth surface . sphere lithosphere stone globe the stony outer part of Earth Learn these Greek words to help you remember the key terms . Greek Root I Meaning I Examples am he i bum Apply It ! Review the Greek roots and meanings in the chart . Then predict the meaning of seismic waves . As you read , revise your definition as needed . Use these Greek words to help you figure out unfamiliar words in this chapter .

Chapter Vocabulary . waves . mantle asthenosphere outer ( ore inner core ' IV ?

current ?

continental drift Se , run 07137 spreading Hench subduction 55 . van ' theory plate ( tanks fan ?

spreading boundary rift valley boundary boundary Build . vy Use 0125 identify Supporting Evidence Science textbooks often describe the evidence that supports a theory or hypothesis . Remember that scientific evidence includes data and facts that have been confirmed by observation or experiments . You can use a graphic organizer like the one below to you understand how supporting evidence is related to a theory . Include a title I supporting evidence on the left the hypothesis or theory on the right This chapter discusses Wegener hypothesis of continental drift . Title Evidence Hypothesis Shape of continents All continents WEFE once joined together but have since drifted apart . Apply It ! Review the graphic organizer and answer the questions . What is a good title for the graphic organizer ?

What information should you include in the ovals on the left ?

As you read Section complete the graphic organizer on continental drift . In Section , create a graphic organizer to explain the theory of spreading .

Standards Investigation Make a Model of Earth In this chapter , you will learn how movements deep within Earth help to form mountains and other surface features . As you read this chapter , you will build a model that shows Earth interior . Your Goal To build a model that shows Earth surface features , as well as a cutaway view of Earth interior Your model must be built to scale to show the layers of Earth interior include at least three of the plates that form surface , as well as two landmasses or continents show how the plates push together , pull apart , or slide past each other and indicate their direction of movement a be built following the safety guidelines in Appendix A Plan It ! Think about the materials you could use to make a model . How will you show what happens beneath the crust ?

As you learn about floor spreading and plate tectonics , add the appropriate features to your model .

Section Earth Interior win Standards Fo ( ALI Students know Earth is composed of several layers . a cold , brittle lithosphere a hot . mantle and a dense , metallic core . How have geologists learned about Earth inner structure ?

What are the characteristics at Earth crust . mantle . and core ?

Key Terms I seismic waves I pressure I trust I mantle I lithosphere asthenosphere I outer ( ore inner core FIGURE Lava From inside Earth A lava flow like this one on Mount , Hawaii , transfers some of the heat oi Earth interior to the surface . 73 Standards How Do Scientists Find Out inside Earth ?

Your teacher will provide you with three closed film canisters . Each canister contains a different material Vour goal is to determine what is inside each though you ca directly observe what it contains . Tape a paper label on each canister . To gather evidence about what is in the canisters , you may tap , roll , shake , or weigh them . Record your observations . What differences do you notice between the canisters ?

Apart from their appearance on the outside . are the canisters similar in any way ?

How did you obtain this evidence ?

think It Over From your observations , what can you infer about the contents of the canisters ?

How is a canister like Earth ?

i eruptions like at Mount in Hawaii wonder . What ?

Yet this question is very difficult to answer . Much as geologists would like to . they not dig zi hole to the center of . The in , far tin surf . The deepest mine in the , a gold mine in South Africa , it of . But that only the surface . You would have to more than times that reach .

Exploring Inside Earth Geologists have used this main types of evidence to learn about Earth interior direct evidence from rock samples and indirect evidence from seismic waves . Geologists have used this evidence to build a picture of interior . Evidence From Rock Samples Rocks from inside Earth give geologists clues about Earth structure . Geologists have drilled holes as much as kilometers into Earth . The drills Mu up samples of rock . From these samples . geologists can , learn about conditions deep inside Earth . where these rocks , by , and formed . In addition . inside Earth blast rock studying how seismic waves travel to the surface from depths of more than kilometers . 53 These rocks provide clues about the interior . Path of seismic waves , Earthquake Evidence Front Seismic Waves ( can not look inside Earth . Instead , they must rely on indirect methods of observation . Have you ever hung a picture on at wall ?

If you have , you know that you can knock on the wall to locate the wooden beam neath the plaster that will support the picture . When you knock on the wall . you listen carefully for a change in the sound . To study Earth interior , geologists also use an indirect method . But instead of knocking on walls , they use seismic waves . When earthquakes occur . they produce seismic waves ( mik ) Geologists record the seismic waves and study how they travel through Earth . Different types of seismic behave differ . The speed of the waves and the paths they take reveal the structure of the planet . You will learn about the different types seismic waves in Chapter . Using data from seismic waves . geologists have learned that Earth interior is made up of layers . Each layer surrounds the layers beneath it . much like the layers ofan onion . in Figure you can see how seismic waves travel through the layers that make up Earth . what causes waves ! 133

For Links on the at Earth Visit Web Flaunt Pressure and Depth The deeper this swimmer goes . the greater the pressure horn the surrounding water . Comparing and Contrasting How is the water in the swimming pool similar to Earth interior ?

How is it different ?

Depth 1340 A Journey to the Center of Earth The three main layers of Earth are the crust . the mantle . and the core . These layers vary greatly in sin . composition . temperature . and pressure . if you could travel through these layers to the center of Earth , what would your trip he like ?

To begin . you will a vehicle that can travel through solid rock . The vehicle will carry instruments to record changes in temperature and pressure as you descend . Temperature As you start to tunnel the . the surrounding rock is cool . Then at about 20 meters down . your instruments report that the rock is getting warmer . For ' 40 motors that you from that point , the rises I Celsius degree . Th is rapid rise in temperature for several tens of kilometers . After that . the increases more slowly . but steadily . The high temperatures inside Earth are the result of heat left over from the formation of the planet . In addition . inside Earth release further heats the interior . Pressure During your journey to the center of Earth . your instruments record an increase in pressure in the surrounding rock . Pressure results from a force pressing on an area Because of the weight of the rock above . inside Earth increases as you go deeper , The deeper you go . the greater the pressure . Pressure inside Earth increases much as it does in the ming pool in .

The Crust in the ( rust . is the layer of rock that farms . The crust is a layer of solid rock that includes both land and the ocean . the crust you and mountains . The crust includes the soil and water on Earth . is much thinner than the that lies beneath it . In fact , you can think at as being similar to the skin onion . This crust is under high and the occur ) In most places . the crust is between and ( it can be up to 70 kilometers thick beneath . The crust ocean is ( i ( crust . crust consists ' of rocks such as Recall that basalt ( huh is a dark rock with ii lint . crust that the . mainly of nicks such as . that granite it an igneous rock that usually is light color and has a course texture . landing What is the main type of rod in oceanic trust ?

Flaunt Earth crust Half Dome in Yosemite National Park forms part of Earth continental crust . Soil and plants ( over much of the crust . 135 The Mantle Your journey downward continues . About 40 kilometers the . you cross a boundary . Below the boundary is the solid material of the mantle . at layer of hot rock . Earth mantle is made up of rock that is very hot , but solid . of the have different characteristics . Overall , the mantle is nearly thick . The lithosphere The uppermost part of the mantle and the crust together form it rigid layer called the lithosphere ( uh steer ) in Greek , means As you can see in Figure the lithosphere averages about 100 kilometers thick . Crust State Solid re ur are to Competition Oxygen . silicon . aluminum . Earths Reno , iron , sodium , interior is layers the ( run , mantle . outer ( and inner core . Interpreting Which of Earth layers is the ?

Inner Core sun solid ! Temperature . Composition tron . nickel Mums State Solid Density yon ) on , can . sun liquid Density Vern , Composition Iran . 136 The Asthenosphere Below the cold , brittle lithosphere . your vehicle encounters material that is hotter and under increasing pressure . As a result . the part of the mantle just beneath the lithosphere is less rigid than the rock above . Like road tar heat of the sun . the somewhat can bend like plastic . This soft layer is called the ( as THEN uh . In Greek . asthma weak . Although the is softer than the rest of the mantle . it solid . If you kicked it . you would stub your 108 . The Lower Mantle Beneath the asthenosphere . the mantle is solid . This solid material extends all the way to Earth core . and The lithosphere . the crust . rests on the material at the asthenosphere . Oceanic crust . Activity Creating Data Tables Imagine that you are in a vehicle that is tunneling deep into Earth interior . You stop several times on your trip to data . Copy the data table . for each depth , identify the layer and what that layer is made of . Then complete the table . Data Table

answer the questions depth inside Earth change ?

mantle and the core ?

Interpreting Date How does Earth interior ?

um Data Temperature Inside Earth The graph shows how temperatures change between Earth surface and the bottom of the mantle . on this graph , the temperature at Earth surface is Study the graph ( and then . Reading Graphs As you move tram left to right on the . how does . Estimating What is the temperature at the boundary between the sphere and the ?

Estimating What is the temperature at the boundary between the lower temperature change with depth in MIX ) Temperature ( Depth ( The Core After traveling through the , you reach Earth dense . core . The core is made mostly of the metals iron end nickel . it consists of two liquid outer core and inner core . the inner and outer core are kilometers thick . Outer Core and inner Core The outer core is a layer of molten metal that surrounds the inner core . Despite enormous pressure , the outer core is liquid . The inner core is a dense ball of solid metal . In the inner core . pressure the atoms of iron and nickel so much that they can not spread out and become liquid . Most of the current evidence suggests that both parts of the corr are made of iron and nickel . But scientists have found data showing that the core also contains substances such as oxygen . silicon . Scientists must seek more data before they decide which of these other substances is most abundant . Ruin What is the main between the outer 45219 rare and the Inner tore ?

lar Magnet Magnetic Field The pattern of iron was made by sprinkling them on paper placed under a bar magnet . Core and Earth Magnetic Field Consider an nary bar magnet . if you place it on a piece of paper and iron on the paper , the iron line up with the bar magnetic . Earth also has a magnetic . If you could the entire planet with iron , they would a similar pattern . In fact , the planet acts like a giant bar net . When you use a compass , the compass aligns with the lines of force in magnetic . Magnetic pole south pole Earth ! Magnetic Field like a magnet , Earth has north and south poles . mun ! Earth Magnetic Field Just as a bar magnet is surrounded by its own magnetic . magnetic field surrounds the planet . Relating Cause and Effect if you shifted the magnet beneath the paper , what would happen to the iron ?

Scientists think that movements in the liquid outer core create Earth magnetic field . As you can see in Figure , the magnetic field affects the whole Earth . Vocabulary use Greet wont Origins Use what you know about the Greek words and to write the of . Reviewing Key Concepts El El El El Why is it difficult to determine Earth inner ?

inferring How are seismic waves used to provide evidence about Earth interior ?

a . Listing List Earth three main layers . Comparing and Contrasting How are the lithosphere and the similar ?

How are they different ?

909 Asses ment . leading am . writing an Classifying Classify each of the following layers as liquid , rigid . or solid but able to slowly . lithosphere . lower mantle , miter core , inner core . Writing in Write a of to the center of term . Your layers of lAll you travel surface . 13 !

Section Convection in the Mantle . Students know heat from How . ca ! Hea cause Earth Inl ! reaches the surface Motion In a i ' Carefully pour some hot water . to a small shallow pan . Fill a How is heat transferred ?

plastic cup about half full cold water . Place the cup in at causes currents pan , in Earth mantle ?

Allow the water to stand for two minutes until all motion ' Fill a plastic dropper with some ' food coloring . Then , holding ' the dropper under the I surface and slightly away from the edge of the cup . gently ' squeeze a small droplet of the food coloring into the water . Observe the water for one minute . Add another droplet at the water surface in the middle of the cup and observe again . Think It Over Inferring How do you explain what happened to the droplets of food coloring ?

Why do you think the second droplet moved in a way that was different from the way the first droplet moved ?

Earth molten outer core is nearly as hot as the surface of the sun . What makes an object become hotter ?

Whether the object is Earth core or it cooking pot . the cause is the an object is heated . the particles that make up the object move faster . The particles have more energy . Have you ever touched a hot pot accidentally ?

so . you have learned for yourself ( in a painful way ) that heat moves . In this case . it moved from the hot pot to your hand . The movement of energy from El warmer object to a cooler object is called heat transfer . To explain how heat moves from Earth core through the mantle . you need to know how heat is transferred .

Types of Heat Transfer Heat always moves from a warmer substance to cooler stance . For , holding an ice tube will make your hand begin to feel cold in a few seconds . But is the coldness in the ice cube moving to your hand ?

No ! Cold is the absence of heat . It the heat in your hand that moves to the ice cube . This is one of the ways that heat is transferred There three types of heat transfer radiation . conduction . and convection . Radiation The transfer of energy through space is called radiation . Heat transfer by radiation takes place with no direct Contact between a heat source and an obiect . Sunlight is tion that warms Earth surface . Other familiar forms of tion include the heat you feel around a flame or open lire . Conduction Heat transfer within a material or between that are touching is called conduction . For example . a spoon in a pot of soup heats up by conduction . as shown in Figure . Heat moves from the hot soup and the pot to the that make up the spoon . The particles near the bottom of the spoon vibrate faster as they are heated , so they bump into other particles and heat them , too . Gradually the spoon heats up . When your hand touches the spoon , conduction transfers heat from the spoon directly to your skin . Then you feel the heat . Conduction also causes an ice cube in your hand to melt . Conduction is for some of the heat fer inside . my what is conduction ?

Haunt conduction . the heated particles of a substance transfer heat through with other particles in the substance . Conduction heats the spoon and the pot itself . That why you need a min to protect your hand from the hot handle .

( Go ' um For More on currents In the mantle ( Web Code Haunt I ( Differences in temperature and density cause convection currents . In the pot , currents arise because the soup close to the heat is hotter and less dense than the soup near the surface . Convection can also by within and gates . Convection is heat transfer by the ull currents within at . During , of to . This heat front one part of the to another . transfer by is by and within at . Density is a ilk uru of how much mans their is in a volume substance . For ' rock is more than ' at given oi rock has more mass than the same volume of water . When at liquid or gas is heated , the faster and spread . As at . particles of the heated occupy more space . The density decreases . But when cools . its particles slowly and settle together more . As ( its increases . Convection Currents you soup on it slow , in density lion in the soup . as shown in Figure . As the soup at the bottom of the pot ( hot . it expands and . Thu warm , less dense soup ( upward and over the . soup . At the ' warm cools . ing denser . Then gravity pulls this cooler . denser soup back down the bottom tit put . where it is again . A ' as soup to the of the pot and the warmer . less dense soup rises . A convection current is the that transfers heat within . Heating and cooling of the . changes in the the force of gravity combine to set convection currents in motion . as long ax hunt is added . Without heal , eventually stop . What is the role of gravity In creating convection currents ?

Reviewing Key Concepts i vi our ) Convection Currents in Earth in mantle , large amounts of heat are by convection currents , as shown in . Heat from the core and the mantle itself causes convection currents in the mantle . How can mantle ?

Recall that ' as you go deeper in the mantle . Rock in the lower mantle is hotter and therefore less dense than the rock above it . As a result . solid rock rises slowly from the bottom of the mantle toward the top . The hot rock eventually cools and sinks back through the mantle . of rising and sinking take place nver millions of years . tion like these have been moving inside for most of Earth history . Convection currents noun Mantle Convection Convection currents rise and sink through the mantle and the liquid outer core Applying concepts What part of Earth Interior is like the soup In the pot ?

What part is like the burner an the stove ?

Assessment Section Vocabulary Use Greek word origins Use what you about the Greek words and to write the of Tracing Meat Flow Convection currents may keep the air inside your home at a comfortable temperature . Air is made up of gases , so it is a fluid . Regardless oi the type of home heating system . heated air circulates through a room by convection . You may have tried to adjust the flow of air in a stuffy room by opening a window . when you did so , you were use of convection currents . With an adult family member , study how your home is heated . Look tor evidence of convection currents a . Listing What are lhe three of heat ?

Explaining How is heat through space ?

a . Defining What current ?

Relating Cause and Effect In general . what happens to the density of as it becomes hotter ?

Describe form . Identifying Name two of Earth in which convection currents take place . Relating Cause and Effect What causes convection currents in the mantle ?

Predicting What will happen to the currents in the mantle if interior eventually cools clown ?

143 Section Drifting Continents ( i ( Standards Focus Students know evidence of plate tectonics is derived train the of the continents the location of earthquakes , volcanoes . and ocean ridges and the distribution 09 fossils , rock types . and ancient climatic zones . what was Alfred hypothesis about the continents ?

what evidence supported hypothesis ?

Why was hypothesis rejected by most scientists of his day ?

Key Terms continental I limit How Are Earth Continents Linked Together ?

Find the oceans and the seven continent on a globe showing Earth physical features . How much of the globe is occupied by the Pacific Ocean ?

Does most of Earth dry land lie in the Northern or Hemisphere ?

Find the points or areas where most ol the continents are connected . Find the points at several of the continents almost touch . but are not connected . Examine the globe more closely . Find the great belt of mountains running from north to south along the western side of North and South America . can you find another great belt of mountains on the globe ?

It Over Posing Questions What questions can you pose about how oceans . continents . and mountains are distributed on Earth surface ?

Five hundred years ago . the we voyages of Columbus and other explorers changed the map of the world . The continents of Europe . Asia . and Africa were already known to Soon were also showing the outlines of the of North and South America . Looking at these world maps . many people wondered why the coasts of several matched so neatly . For example . the coasts of Africa and South America look as if they could together like pieces . In the 17005 , geologists thought that conti had always remained in the same place . But early in the . one scientist became curious about the matching shapes of the continents

Mule 10 Continental Today continent provide clues about Earth history . Observing Which coastline of continents seem to match up like pu ! pieces ?

Hint Refer to the map in figure . Vi Continental Drift In 1910 , a young German scientist named Alfred guh nur ) attempted to solve the jigsaw puzzle of the continents . He that Earth continents had moved ! hypothesis was that all the continents were once joined together in a single and have since drifted apart . Wegener idea that the continents slowly moved over surface became known as continental drift . According to , the continents drifted together to form the ( pan Hill uh ) means aU lands . According to , about 300 million years ago . This was the time when reptiles and winged insects first appeared . Tropical forests . which later formed coal deposits . covered large parts of Earth surface . Over tens of millions of years , began to break apart . The pieces of slowly moved toward their . These pieces became the continents as they are today . How did attempt to prove his idea ?

Wegener evidence from different fields to support his ideas about continental drift . He studied land features , fossils . and evidence of climate change . In . published his evidence for continental drift in a book called The Origin of Continents and Oceans . I For links on drift Visit Web ( ode 145

Evidence to Continental Drift KI ! Ocean Folded tau can Fossils and rocks found on different continents provide evidence that Earth landmasses once were joined together in the . Inferring What do the mountain ranges in Africa and south America show , according to hypothesis ?

Evidence From Land Features As shown in Figure . mountains and other features on the continents provided for continental drift . For example , when Wegener pieced together maps of Africa and South America . he noticed that mountain ranges on both continents line up . He noticed that European coal match up with coal in North America . Evidence From Fossils Wegener also used fossils to suppon his argument for continental drift . A fund ! is any trace of an ancient organism that has been preserved in rock . For example . tuh ris ) was a plant that lived 250 million years ago . fossils have been found in rocks in Africa , South America . Australia , India , and The occurrence of on these widely separated masses convinced that had existed . Other examples include fossils of the water reptiles and fossils have also been found in now separated by . Neither reptile could have swum distances across salt water . Wegener inferred that these reptiles lived on a single landmass that has since split apart .

Evidence From Ancient climatic zones used evidence of past climates to support his hypothesis . Wegener found this evidence by studying and rocks . Evidence front fossils showed that , an island in the Arctic Ocean , once had a warm climate . Today , much of is covered with ice . But scientists found fossils of plants there . When these plants lived . about 300 million years ago . the island must have been closer to the equator . where it is warm . Wegener concluded that must have readied its location in the Arctic through continental drift . Evidence from rocks showed that South Africa . which has a warm climate today . was once very cold . Geologists found deep scratches in rocks and sediment left behind by glaciers . This showed that continental glaciers once covered South Africa . Continental glaciers an thick layers of ice that cover all or part of a continent . But the climate of South Africa is too mild today for continental glaciers to form . that continental drift had moved South Africa away from the South Pole . Indian How would continental drift meet I continent ' climate ! 147

Wegener Hypothesis Rejected Wegener attempted to explain how continental drift took place . He suggested that the continents plowed across the . Unfortunately . could not provide a satisfactory explanation for the force pushes or pulls the continents . could not identify the cause of continental drift . most geologists rejected his idea . Geologists also idea because it contradicted their ideas about how form . More than 30 years would pass before geologists accepted new evidence that supported ideas . In the early ( many geologists thought that formed Earth was slowly cooling and shrinking . According to this . mountains formed when Lhe crust wrinkled like the skin of at apple . Wegener said that if these geologists were correct . then should be found all over Earth surface . But Al usually occur in narrow bands along the edges of ?

i arm . proposed that when collide , and to his book um his their edges crumple and fold . The folding Continents push up death in 1930 , huge mountains . section Assessment Target Reading Skill Identity Supporting a . Explaining Why did most scientists reject Evident What supporting evidence did you ?

include in graphic fur the . Making Judgments Do you think the ' hypothesis of continental drift ?

scientists time should have Key concepts accepted his ?

Why or why not ?

Who proposed the concept of continental drill ?

Summarizing According to the hypothesis of rig I continental drift . how would a world map have changed over the last 250 million years ?

i Him I a . Reviewing What evidence supported the ' dim ?

and tracing paper , trace the outlines of the . it How did fossil ma , bad he A a Oman . Label the continents . Then use scissors to ?

arming or coal haw carefully cut your map along the edges of the found the ice of ( coal only in warm . Use ' i I ) to explain how coal could be found so near to the South Pole . continents . Throw away the Atlantic Ocean . Place the two remaining pieces on a dark and ask family members to try to fit the two halves together . Explain to them about continental drift and .

Section Spreading ' Students know evidence ol plate tectonics is derived from the lit ( the location ol earthquakes . volcanoes , and mid ocean ridges . and the distribution at , rock types , and ancient climatic zones . What IS the process at sea spreading ?

What Is the evidence for sea spreading ?

What happens at trenches Key Terms ridge ' sonar spreading trench subduction What Is the Effect of a Change in Density ?

Partially fill a sink or dishpan with water . Open up a dry washcloth in your hand . Does the washcloth feel light or heavy ?

Moisten one edge at the washcloth in the water . Then gently place the washcloth so that it floats on the water surface . Observe the washcloth carefully ( especially at its edges ) as it starts to sink . Remove the washcloth from the water and open it up in your hand . Is the mass of the washcloth the same as , less than , or greater than when it was dry ?

Think it Over observing How did the density change ?

What effect did this change in density have on the washcloth ?

in the . this is . is no light living things are generally . Ye ! areas of the arc teeming with life . One of these ' is an Rise . forms part ofthe ( coasts of and South America . ocean sinks through cracks . or vents . in the crust . The ( is heated ' with hot from the mantle . The hot water then spurts hack the ocean . Around ( of the creatures . Giant , may in the water . Nearby sit giant clams across . scuttle by . Surprisingly . the geological ( some of the . for Wi drift . FIGURE 13 The Floor Shrimp . crabs , and other organisms cluster near hot water vents in the ocean floor . 149

14 The is more then kilometers long . What Is unusual . Ocean Floor aim mean Ridges The East Rise is just one of many ridges A rig is undersea mountain chain that is part of at long system of mountains that winds beneath Earth oceans . In the . scientists mapped the ridges using sonar . Son is a device that bounces sound waves underwater objects and then records the echoes of these sound waves . The time it takes for the echo to arrive indicates the to the object . ridges curve like the seam of a baseball along the son . They extend into all of Earth . Figure shows the location of these . Most of the mountains in the ridge system lie hidden under hundreds of meters of water . But in a few places the ridge above the surface . For example , the island of Iceland is a part of the ocean ridge . Iceland rises above the in the North tic Ocean . A valley splits the top of some ridges .

What Is Spreading ?

The mapping of ridges made some scientists ous to know more about them . What are the ridges ?

How do they form ?

These scientists carefully examined maps of the ridge system . A few scientists began to think about the ocean floor in relation to the problem of continental drift . They began to think that maybe was right ! Perhaps the continents do move . In . one geologist proposed a radical idea . He suggested that a process he called spreading continually adds new material to the ocean . In spreading , the sea spreads apart along both sides of ridge as new crust is added . As a result . the ocean more like conveyor belts . carrying the continents along with them . Look at Figure to see the of spreading . spreading begins at a ridge , which forms along a crack in the oceanic . Along the ridge . molten material that forms several kilometers beneath the surface rises and erupts . At the same time , older rock moves outward on both sides of the ridge . As the molten material cools . it forms a strip of solid rock in the center of the ridge . When more molten material into the crack . it forms a new strip of rock ridge ( Go For More on spreading Web ( ode 15 Molten material erupts through the center of a ridge . This material harden to form the melt at the ocean floor , Applying to the rock along the ridge when new molten material ?

151 Rock formed when Earth magnetic field was normal Rock formed when Earth magnetic field was reversed 16 Magnetic stripes Magnetic stripes In the rock of the ocean show the direction of Earth magnetic field at the time the rock hardened . Interpreting Diagrams How are more matching stripes evidence of spreading ?

ridge Evidence for Spreading Several types of evidence supported the theory of eruptions of molten material . magnetic stripes in the rock of the ocean the ages of the rocks themselves . In the . this evidence led scientists to look again at hypothesis of continental drift . Evidence From Molten Material The scientists ered evidence that new material is indeed erupting along ocean ridges . The scientists to the ocean floor in Alvin . a small submarine . Alvin was built to withstand the crushing pressures four kilometers down in the ocean . in a valley . Alvin now found strange rocks shaped like pillows . Other rocks looked like toothpaste squeezed from a tube . Such rocks form only when molten material hardens quickly under water . These rocks showed that molten material has erupted again and again along the mid ridges . Evidence From Magnetic Stripes Patterns in the rocks of the ocean floor provided more support for ing . You read earlier that Earth behaves like a giant magnet . with a north pole and a south pole . Surprisingly . Earths have reversed themselves many times during history . The last happened years ago . What if the magnetic poles suddenly themselves today ?

You i would that your compass needle pointed south .

( the that up the iii lies in ul . in 16 . hold ( oi in held . The of the ocean This molten ( and ( ruck . lined up in iif ' pole . thi iron bit in place . ing ' ti ' then used instruments to retard the ' on both sides ( if i . of rack ) These ite with of th it when the magnetic field pointed . Look at 16 . Notice ' the ( sides . Evidence From Drilling Samples The limit oi ' ruck by drilling into the ocean . The ( drilling ship built in . gathered samples . The ( drilling sit ( In in the occult This teat has to digging hole intu the top oi the Empire State Building . from the sea up the pipes . Then the the age oi the in the . found that the ridge the were taken . the rocks were . The rocks were always in iii Ihr ridges . that has place . Radius Why does the rod of the . have a pattern of magnetic stripes ! Reversing Poles . Cut six short pieces , each about ( I long , from a length of audiotape . Tape one end of eath piece oi audiotape to a surface . The pieces should be spaced apart and lined up lengthwise in a single row . Touch a bar north pole to the lirst piece of audiotape . Then reverse the magnet and touch its south pole to the next piece . Repeat Step until you have applied the magnet to each piece oi audiotape . Sweep one end of the magnet about above the line of audiotape . Observe what happens . Making Models What characteristic of the ocean floor did you observe as you swept the magnet along the line of audiotape pieces ?

17 Drilling The Challenger was the first research ship designed to drill , samples from the oceanic crust farther formed oceanic crust Iles dosed to the ridge . tram the ridge . subduction ocean floor created along the ridge is destroyed at a . In the process of subduction . ocean floor sinks down beneath the trench into the mantle . Drawing Where would the densest ocean floor he found ?

Tri School HON subduction at Trenches How can the ocean keep getting wider and wider ?

The answer is that the ocean generally does not just keep spreading . Instead , the ocean sinks beneath deep water canyons called trenches . At a trench , the ocean bends downward . What occurs at ?

In taking tens of millions of of the sinks hack into the It trenches . The Process of subduction The process by which the ocean floor sinks beneath a trench and back into the mantle is called subduction ( sub DUK shun ) During duction . ocean closer to a ridge moves away from the ridge and toward a trench . spreading and subduction work together . They move the ocean as if it were on a giant conveyor belt . New ocean is hot . But as it moves away from the ocean ridge . it cools and becomes more dense . Eventually . as shown in Figure . gravity pulls this older . denser ocean down beneath the trench . The sinking ocean floor is like the washcloth in the Standards at the beginning of this section . As the dry washcloth floating on the water gets wet . its density increases and it begins to sink .

subduction and Earth oceans Together . tion and spreading change the size and shape of the oceans . These processes renew the ocean floor about every 200 million years . That is the time it takes for new rock to form at a mid ocean ridge , move across the ocean , and sink into trench . The vast Ocean covers almost one third of the planet . And yet it is shrinking . How can that be ?

The Pacific Ocean has many trenches around its . Subduction occurs through these trenches faster than new crust can be added . The result is that the width of the will gradually shrink over millions of years . As you can see in Figure I ) the Atlantic is expanding , The main reason for this is that the Atlantic Ocean has only a few short trenches . As it result . most of the spreading ocean has nowhere to go . in most places . the oceanic crust of the Atlantic Ocean is attached to the continental crust of the continents around the ocean . So as the Atlantic spreads . the ' spreading the continents along its edges also move . Over time . the dim , gump , and whole ocean gets wider . America is increasing by a few centimeters per year , FISH ! 19 grow Why is the Ocean shrinking ?

Section Assessment Target Reading Skill Identify Supporting Predicting Where would you expect to lind Evidence Create a graphic organizer for the the oldest rock on the ocean ?

theory of spreading . What evidence What is a trench ?

did you include ?

Relating Cause and Elise ! What happens to Reviewing Key come , oceanic crust at a trench ! III ?

Identifying Along what feature of ocean floor does spreading begin ?

In ' nce . Describing How is new rock added to the dex , ocean ?

Sequencing What are the steps in the if . I process of spreading ?

I . Reviewing What three of evidence be to dude um . I . antes noun along and near provided support for the sea the die spreading ?

Applying Concepts How do rocks along the central valley of the ridge provide evidence spreading ?

Directed inquiry Modeling Spreading colored markers sheets of unlined paper metric ruler Problem How does sea spreading add material to the ocean floor ?

Skills Focus making models Procedure a I . Draw stripes across one sheet of paper , parallel to the short sides of the paper . The stripes should vary in spacing and thickness . Fold the paper in half lengthwise and write the word Start at the top of both halves of the paper . Using the scissors , carefully an the paper in half along the fold line to two strips . Lightly fold the second sheet of paper into eighths . Then unfold it . leaving creases in the paper . Fold this sheet in half lengthwise . Starting at the fold , draw lines long on the middle crease and the two creases closest to the ends of the paper . Now carefully cut along the lines you drew . Unfold the paper . There should be three slits in the center of the paper .

. Put the two striped strips of paper together so their Stan labels touch one another . Insert the Start ends of the strips up through the center slit and then pull them toward the side slits . Insert the ends of the Strips into the side slits . Pull the ends of the strips and watch what happens at the center slit . I . Practice pulling the strips until you can make the two strips come up through the center and go down through the sides at the same time , Analyze and conclude . Making Models What feature of the ocean floor does the center slit stand for ?

What prominent feature of the ocean floor is missing from the model along the center slit ?

Making Models What do the side slits stand for ?

What does the space under the paper stand for ?

Comparing and Contrasting As shown by your model , how does the ocean floor close to the center slit differ from the ocean floor near a side slit ?

How does this difference affect the depth of the ocean ?

Melting Models What do the stripes on the strips stand for ?

Why is it important that your model have an identical pattern of stripes on both sides of the center slit ?

Applying Concepts Explain how differences in density and temperature provide some of the force needed to cause seafloor spreading and subduction . Communicating Use your own words to describe the process of spreading . What parts of the process were not shown by your model ?

More to Explore How could you modify your model to show an island that formed where a large amount of molten rock erupted from the ridge ?

How could you show what would happen to the island over a long period ol time ?

157 Section The Theory of Plate Tectonics Students . plates the size at continents and How Well Do the Continents Fit Together ?

of . I . Using a world map in an atlas . trace the menu in , shapes ofthe continents North America , South America , Africa . and Europe . what isthe theory or plate . Carefully cut apart the tectonics ?

what are the three types of plate . Piece together these as they may have looked belore split apart . creating the Atlantic Ocean . A . Attach your partial reconstruction of ' Dial ! to a piece of ' Obtain a map that shows the continental shell . The ' continental shelf is the apron of continental crust that extends ' under water around the edges of the continents . Trace around ' the shelves of the same continents used in Step . Repeat steps through to compare the lit of the continents with and without their continental shelves . Think it over Drawing Conclusions Do your observations support the idea that the continents were once joined together ?

when did they fit together better when you cut them out along their coastlines or along their continental shelves ?

Explain . Have you ever dropped a egg ?

If so , the eggshell probably cracked in many places . Earth , its solid outer shell . is not one unbroken layer . It is more like that cracked eggshell . it broken into jagged pieces . Them are cracks in the similar to those on the ocean . In the . scientists proposed that these cracks break the lithosphere into separate called plates . The along cracks in the lithosphere . As shown in Figure 20 , the plates carry the continents or parts of the ocean , or both . scientist combined what geologists knew about . plates . and drift into a single theory . Recall that a theory is a concept that explains a wide range of observations . A cracked eggshell !

How Plates Move The theory of plate tectonic ( tek ) states that pieces of Earth lithosphere are in slow . constant motion . driven by convection currents in the mantle . OThe theory of plate tectonics explains the . movement , and tion of Earth plates . What forte is great enough to move Earth plates ?

Geologists think that movement of convection currents in the mantle is the major cause of plate motion . Mantle motion is transferred to the lithosphere at its boundary with the sphere . The plates are carried along in mud the same way that ice floats on water . During subduction . gravity pulls one edge of a plate down into the mantle . The rest of the plate also moves . This slow movement is similar to what pens in a pot of soup when gravity causes the cooler . denser soup near the surface to sink . As the plates more . they change Earth . These changes include earthquakes . volcanoes . mountain ranges , and trenches . Future 10 Plate boundaries divide the lithosphere Into large plates Interpreting Map which plates Include both and I ocean floor ?

150 Plate Boundaries The edges of Earth plates meet at plate boundaries . Plate boundaries extend deep into the lithosphere . Faults form along these boundaries . Faults are breaks in Earth crust where rocks have slipped past each other . There are three of plate boundaries spreading boundaries . colliding boundaries . and sliding boundaries . A different type of plate movement occurs along each type of boundary . Scientists have used instruments on satellites to measure plate motion . The plates move about I to 24 centimeters per year in response to movements in the mantle . The North ican and Eurasian plates are moving apart at a rate of centimeters per year . That about as fast as your grow . But these plates have been moving for millions of years . Boundaries The place where two plates move apart is called a spreading Must spreading aries occur along the ridges where ing occurs . Spreading boundaries are also called divergent Paint It boundaries . Plate Tectonics Spreading boundaries also occur on land When a Plate movements have built many ing boundary develops on land . two of Earth plates slide ' 07 apart . A deep valley called a forms along the surfaces and ocean . For . Rift Valley in East Africa happen , continue , marks at deep crack in the African continent . pull apart ?

When two plates ol oceanic crust collide . one pint occurs as oceanic plat is beneath the other . move apart along the ridge . Trench ridge Colliding Boundaries The place where two plates come together , or collide . is called a colliding boundary . Another term for colliding boundary is convergent boundary . When two plates collide . the density of the plates determines which one comes out on top . There are three types of collision between . In the first type of collision , two plates made up of oceanic crust meet at a trench . The plate that is more dense sinks under the other plate . The reason for this is that oceanic crust becomes cooler and denser during spreading . In the second type of collision . on oceanic plate collides with a continental plate . Subduction occurs as the denser oceanic plate sinks beneath the less dense continental plate . In the third type of collision . two plates carrying continental crust collide . but subduction does not take place . Neither piece of crust is dense enough to sink very far into the mantle . Instead . the collision squeezes the crust into mighty mountain ranges . A sliding boundary is a place where two plates slip past each other . moving in opposite directions . Along sliding boundaries , crust is neither created nor destroyed . Sliding boundaries can also be called boundaries . hill i What features town when two plans come together ?

Spending Boundary A rift valley forms when two pieces of continental crust pull apart . Boundary Two plates slide past each other . Calculating a Rate To calculate the rate of plate motion , divide the distance the plate moves bythe time it takes to move that distance . For example , a plate takes million years to move 156 . Calculate its rate of motion . 56 years Practice Problem The Pacific plate is sliding past the North American plate . it has taken 10 million years for the plate to . What is the Pacific plate rate of motion ?

an per year laundry Mu ( plates collide . forming I mountain range . rust Plate Motions Over Time The movement of plates has greatly changed Earth surface . Geologists have evidence that . before existed , other formed and split apart over billions of years . formed when Earth drifted together about 260 million years ago . Then . about 225 million years ago , began to break apart . 315 Figure 22 shows how major landmasses have moved since the breakup of . 22 Continental Drift Geologists have used computer modeling to trace the movements of the continents since the breakup of . Posing Questions What questions would you need to order to predict where the continents will be in so million years ?

Go . ye 135 million van Ago Fur Drift activity Visit Web Code section Assessment Vocabulary Skill Use Word Origins This chapter is about the theory of plate tectonics . Use what you know about the Greek word origin of tectonics to explain the meaning of Pia i . Calculating a Rate There are two Reviewing Key Concepts Islands on opposite sides of a a . Defining What in plates ?

A Summarizing What is the theory of plate tectonics ?

Relating Cause and Him What do scientists think causes the movement of Earth plates ?

What are the three types of plate boundaries ?

1153 Ocean . During the last million years , the distance between the islands has increased by 200 kilometers . Calculate the rate at which the two plates are moving apart It . Describing Describe the type of movement that occurs at each type of plate boundary . Predicting is likely to occur at a plat boundary where oceanic crust collides with continental crust ?

Skills Lab Directed Inquiry Modeling Mantle Convection Currents Materials . large bottle or container with tap water small glass jar red food coloring aluminum foil or plastic wrap small pieces of paper or several paper hole punches Problem How might convection in Earth mantle affect tectonic plates ?

Skills FOCUS models . observing Procedure . Fill the large jar about half full with cold tap water . Partly fill the small jar with hot tap water and stir in drops of food coloring . add enough hot water to fill the jar to the brim . Cover the top of the jar with aluminum foil or plastic wrap and secure with a band . Carefully lower the jar into the bottle of tap water . Place the pieces of paper on the surface of the water . Without disturbing the water , use the tip of the pencil to two small holes about in diameter in the material covering the jar . Predict what will happen to the colored water and to the pieces of paper on the surface . Observe the contents of the jar , as well as the paper pieces on the surface of the water . Analyze and conclude . Observing Describe what happened to the colored water and to the pieces of paper after the holes were punched in the material covering the jar . Drawing Conclusions How did your prediction compare with what actually happened to the colored water and pieces of paper ?

Inferring What type of heat transfer took place in the bottle ! Describe how the transfer occurred . Making Models Which part of your model represents a tectonic plate ?

which part represents Earth mantle ?

Communicating How well do you think this lab modeled the movement of Earth plates ?

What similarities exist between this model and actual plate movement ?

What factors you able to model in this lab ?

Design an Experiment Repeat this activity . but develop a plan to measure the temperature of the water inside the large jar . Is there a difference in temperature between the water surface and the water near the top of the small jar ?

Do you observe any change in the convection currents as the water temperature changes ?

your teachers approval , carry out your plan . 0163 Chapter I Earth Interior Key Concepts I ( med two main types of In Faith interior from rock samples and The layers ' mantle , and the i in size , temperature . and . The crust is a layer of ruck includes both dry land and . is hot . but solid . Scientists divide the mantle ) The con of thi iron and liquid outer core and ii solid inner . Key Terms . wave ! I I ( tut inner core Convect on and the Mantle ( Key Concepts I Il . hI ) and convection . cooling , in ( and to set ( currents in motion . the and i ) in . Key Terms I ' density iii current Dri Continents Key Concepts that all the had once been joined together in a single drifted apart . 1640 Study Guide Earth plates are large til the lithosphere that move slowly , producing faults . mountain ranges . Volcanoes . and , evident to suppon his ideas about drift . He land I . fossil ) and evidence of . could not provide for tom that pushes ur pulls the . Key Terms drift , Spreading Key Concepts In ) the sea Hour spreads along hull ) as new is add As , halts . the with them . I types iii eruptions oi . the of the floor . and ages ' a taking tens of millions of yearn part sinks hack into the mantle at . Key terms ) sonar deep ocean trench Key Th of plate the formation , and of Earth . Thou at three kind tic . and sliding . A type of plate urs along . Key Terms plate tectonics I rift valley colliding

Review and Assessment ) Target Reading Skill Supporting Evidence ( the graphic ' on the theory of plate . Evidence . um For . om Wei ( ode The Theory of Tectonics Theory Plates are in slow . constant motion . moving away from each other , toward each other . and sliding out each other . Reviewing Key Terms Choose the letter of the best answer . HINT I . Thu upper mantle is a . Si . mist . nu ! The of heart by I . i of particles ni is . I . ocean place at a lower mantle . ridges , valleys . HINT I . The that powers , crash into each Al A A ) i ' a colliding boundary . the between the mantle and the . sliding ) Complete the following sentences so that your answers clearly explain the key terms . of he main mu up continental is . Heat is in the by convection currents . are . the lAIn ( ii HINT . which was . crux ! return to the mantle at ocean trenches . which an . A rill ' spreading boundary , whichis ' in Science Prediction Now that you have learned about the theory of plate tectonics . write a paragraph predicting what the shape and positions of Earth continents will be 50 million years in the future . VIII BIZ Id Plate Tectonics 155

Review and Assessment Checking Concepts 11 . kinds of indirect evidence do geologists use In study the structure of Earth ?

11 . I low do temperature and pressure change as you go deeper into Forth ?

13 . happens in Earth interior to produce Earths magnetic field ?

Describe the layer where the is produced . 14 . Why are there convection currents in the mantle ?

15 . How do magnetic on the ?

Why are these stripes ?

16 . What type of feature will form where two continental plates collide ?

Thinking Critically 17 . Comparing and Contrasting How are and continental crust alike ?

How do they differ ! sequencing Place the in correct order ! they . lIl at Earth surface and move toward the center inner core , mantle . lithosphere . 19 . Predicting In the below . a plate of oceanic crust is a plate of continental crust . will happen ?

Why ?

Relating Cause and What do many geologists think is the driving of plate tectonics ?

Explain . 21 . Making Judgments lu plate tectonics as a . What is theory ?

How is plain tectonic at ' drift considered a theory ?

Hint Refer to the Skills I more on theories . 166 Mat 22 . a Rate it takes , lU ( years tor plate to move About . Calculate the rate of plate motion . Applying Skills Use the map to answer Questions . that new plate is rhyming in the menu . The part plate Australia is the part of the plate . Maps In what direction is the part ofthe plane moving ?

in what is the part carrying lndia moving ?

24 , Predicting As India and move in different . what type of plate boundary will form between them ?

25 . Inferring failures could occur where the northern part or the is colliding with the plate ?

Investigation ' Performance Assessment your model lo the class . Point the types of plate boundaries your model . i . the plate and landforms that result in these areas .

Choose the letter that best answers the question or completes the statement . An island on the plate moves a distance of 550 in 50 years . What is the plate rate of speed ! A 44 per year I I per year per year I I per year . The subduction of oceanic crust beneath continental crust occurs A continental ( is denser than oceanic crust . I oceanic crust is denser than continental crust . the heavy mountains on crust push the oceanic crust down . I ) oceanic crust is warmer than continental . Which statement hest explains how convection currents can occur in Earth mantle ?

A The mantle is made up of hot liquid . The mantle is made of hot but solid rock that can slowly . The density of ( at temperature increases in the mantle . rotation causes mantle convection . Which following is evidence for spreading ?

A matching patterns ol magnetic stripes in the ocean floor volcanic eruptions along ridges older rock found farther from ridges all ofthe above . Wegener thought the continents moved because fossils of the same organisms are found on widely separated continents . ner use of fossil evidence is an example of at ) A prediction . I observation . inference . controlled experiment . Standards Practice at in Earth outer core hat is transferred mainly A seismic waves . I radiation . conduction . convection currents . Which of the following best describes the process in the diagram below ?

A plates a boundary . I Colliding plates form volcanoes . Spreading plate form a ridge . Spreading plates form a rift valley . I . Today . the Mediterranean Sea lies between Europe and Africa . But the African plate is moving toward the Eurasian plate at it me ofa few centimeters per year . Predict how this area will change in 100 million years . In your answer . first explain how the Mediterranean Sea will change . Then explain what will happen on land . Chapman