Chapter 7: Plate Tectonicss3.amazonaws.com/zanran_storage/nlcs.k12.oh.us/ContentPages/... · Do this lab to learn about clues that can be used to reassemble a supercontinent. 1. Collect

180 CHAPTER 7 Plate Tectonics

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sections

1 Continental Drift

2 Seafloor SpreadingLab Seafloor Spreading Rates

3 Theory of Plate TectonicsLab Predicting TectonicActivity

Virtual Lab Where do mostearthquake epicenters andvolcanoes occur?

Will this continent split?Ol Doinyo Lengai is an active volcano in theEast African Rift Valley, a place where Earth’scrust is being pulled apart. If the pulling con-tinues over millions of years, Africa will sepa-rate into two landmasses. In this chapter,you’ll learn about rift valleys and other cluesthat the continents move over time.

Pretend you’re a journalist with anaudience that assumes the continents have never moved.Write about the kinds of evidence you’ll need to convincepeople otherwise.

Science Journal

Plate Tectonics

Bourseiller/Durieux/Photo Researchers

This CD-ROM is an editableMicrosoft® PowerPoint®

presentation that includes:• a pre-made presentation for

every chapter• interactive graphics• animations• audio clips• image bank• all new section and chapter

questions• Standardized Test Practice• transparencies• pre-lab questions for all labs• Foldables directions• links to blue.msscience.com

About the Photo

Pulling Apart Africa This is a photo of Ol DoinyoLengai, an active volcano in the East African RiftValley. This valley is a location where Earth’scrust is being pulled apart. Should this continueover millions of years, Africa will separate intotwo landmasses.

Energy The transfer of energy in Earth’s interiorsets up massive convection currents in the man-tle. These currents are thought to be the drivingforce that causes movement of Earth’s plates.

Science Journal The con-tinent shapes appear to match,plant and animal fossils fromSouth America and Africa arevery similar, and broad areas ofrock in Africa and SouthAmerica are the same type. Allof this suggests that these conti-nents were once joined.

Chapter Vocabularycontinental drift, p. 182Pangaea, p. 182seafloor spreading, p. 187plate tectonics, p. 190plate, p. 190lithosphere, p. 190asthenosphere, p. 190convection current, p. 195

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CHAPTER 7 Plate Tectonics 181

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Plate Tectonics Make the following Foldable to help identify what you already

know, what you want to know, and what you learned about plate tectonics.

Fold a vertical sheet of paper from side to side. Make thefront edge about1.25 cm shorter thanthe back edge.

Turn lengthwise and fold into thirds.

Unfold and cut only the layer along both folds to make three tabs.

Label each tab.

Identify Questions Before you read the chap-ter, write what you already know about platetectonics under the left tab of your Foldable, andwrite questions about what you’d like to knowunder the center tab. After you read the chapter,list what you learned under the right tab.

Know? Learned?Like to know?

STEP 4

STEP 3

STEP 2

STEP 1

Start-Up Activities

Preview this chapter’s contentand activities at blue.msscience.com

Reassemble an ImageCan you imagine a giant landmass that brokeinto many separate continents and Earth scientists working to reconstruct Earth’s past?Do this lab to learn about clues that can beused to reassemble a supercontinent.

1. Collect interesting photographs from anold magazine.

2. You and a partner each select one photo,but don’t show them to each other. Theneach of you cut your photos into pieces nosmaller than about 5 cm or 6 cm.

3. Trade your cut-up photo for your partner’s.

4. Observe the pieces, and reassemble thephotograph your partner has cut up.

5. Think Critically Write a paragraphdescribing the characteristics of the cut-up photograph that helped you putthe image back together. Think of otherexamples in which characteristics ofobjects are used to match them up with other objects.

Bourseiller/Durieux/Photo Researchers

Purpose Use the Launch Lab tointroduce students to the ideathat Earth’s continents may haveonce been connected.

LinguisticPreparation Ask students tobring in old magazines.Materials photos from discardedmagazines, scissorsTeaching Strategy Have studentscompare their results with thoseof other students. Discuss differ-ences and similarities.Safety Precautions Remind stu-dents to handle scissors with care.

Think CriticallyThe shape and content of the piecesenable students to see how they fittogether. Students may suggest jigsawpuzzles or broken artifacts.

AssessmentProcess Ask students to write intheir Science Journals the char-acteristics that make some pho-tographs easier to fit togetherthan others. Use PerformanceAssessment in the ScienceClassroom, p. 99.

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Dinah Zike Study Fold

Student preparation materials forthis Foldable are available in theChapter FAST FILE Resources.




! Describe the hypothesis of continental drift.

! Identify evidence supporting continental drift.

The hypothesis of continental driftled to plate tectonics—a theory thatexplains many processes in Earth.

Review Vocabularycontinent: one of the six or seven great divisions of land on theglobe

New Vocabulary• continental drift

• Pangaea

Continental Drift

Figure 1 This illustration represents how the continentsonce were joined to formPangaea. This fittingtogether of continentsaccording to shape isnot the only evidencesupporting the pastexistence of Pangaea.

PA

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NorthAmerica

South America Africa

Eurasia

India

AntarcticaAustralia

Evidence for Continental DriftIf you look at a map of Earth’s surface, you can see that

the edges of some continents look as though they could fittogether like a puzzle. Other people also have noticed this fact.For example, Dutch mapmaker Abraham Ortelius noted the fitbetween the coastlines of South America and Africa more than400 years ago.

Pangaea German meteorologist Alfred Wegener (VEG nur)thought that the fit of the continents wasn’t just a coincidence.He suggested that all the continents were joined together atsome time in the past. In a 1912 lecture, he proposed thehypothesis of continental drift. According to the hypothesis ofcontinental drift, continents have moved slowly to their currentlocations. Wegener suggested that all continents once were connected as one large landmass, shown in Figure 1, that brokeapart about 200 million years ago. He called this large landmassPangaea (pan JEE uh), which means “all land.”

Who proposed continental drift?

Tie to Prior KnowledgeConnecting Continents Have stu-dents recall the general shapes ofAfrica and South America. Ask ifthey can recall how the easterncoast of South America seems tofit into the western coast ofAfrica. Display a world map, ask-ing again if students can makethe connection.

BellringerSection Focus Transparenciesalso are available on the Interactive Chalkboard CD-ROM.L2

Plate Tectonics

If you were interested in the fossils of an animal that liked warmweather, would you think of digging in Antarctica? Archaeologistshave found many interesting fossils there, including parts of ahadrosaur, a dinosaur previously found only in the Americas.

A Cold DigSection FocusTransparency11

1. Antarctica has a very inhospitable climate. Why might fossils ofwarm-weather animals be found there?

2. What are some reasons that the climate of Antarctica mightchange in the future?

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Section 1 Resource Manager

Chapter FAST FILE ResourcesTransparency Activity, p. 44Directed Reading for Content Mastery,

pp. 19, 20Note-taking Worksheets, pp. 33–35

MiniLAB, p. 3Enrichment, p. 30Reinforcement, p. 27Lab Activity, pp. 9–11

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Answer Alfred Wegener

SECTION 1 Continental Drift 183

Use an AnalogyPuzzle Pieces Have students recallthat when putting together a jig-saw puzzle, they use the shapes ofthe puzzle pieces and the pictureon the box as clues. When think-ing about evidence of continentaldrift, the puzzle pieces are analo-gous to the shapes of the conti-nents, and the picture clues areanalogous to evidence found oneach continent.

DiscussionContinental Drift Consider whyWegener’s idea of continentaldrift was rejected? Scientists at thetime thought Wegener’s observationscould be explained by something otherthan continental drift. Also, Wegenerfailed to provide a reasonable or believ-able mechanism to explain how conti-nental drift might occur.


A Controversial Idea Wegener’s ideas about continentaldrift were controversial. It wasn’t until long after Wegener’sdeath in 1930 that his basic hypothesis was accepted. The evidence Wegener presented hadn’t been enough to convincemany people during his lifetime. He was unable to explainexactly how the continents drifted apart. He proposed that thecontinents plowed through the ocean floor, driven by the spin ofEarth. Physicists and geologists of the time strongly disagreedwith Wegener’s explanation. They pointed out that continentaldrift would not be necessary to explain many of Wegener’sobservations. Other important observations that came latereventually supported Wegener’s earlier evidence.

Fossil Clues Besides the puzzlelike fit of the continents,fossils provided support for continental drift. Fossils of the reptile Mesosaurus have been found in South America andAfrica, as shown in Figure 2. This swimming reptile lived infreshwater and on land. How could fossils of Mesosaurus befound on land areas separated by a large ocean of salt water? Itprobably couldn’t swim between the continents. Wegenerhypothesized that this reptile lived on both continents whenthey were joined.

How do Mesosaurus fossils support the past existence of Pangaea?

Figure 2 Fossil remains ofplants and animals that lived inPangaea have been found on morethan one continent. Evaluate How do the locations of Glossopteris, Mesosaurus,Kannemeyerid, Labyrinthodont,and other fossils support Wegener’shypothesis of continental drift?

Topic: Continental DriftVisit for Web links to information about thecontinental drift hypothesis.

Activity Research and write abrief report about the initial reac-tions, from the public and scientificcommunities, toward Wegener’scontinental drift hypothesis.

blue.msscience.com

KannemeyeridGlossopteris

MesosaurusLabyrinthodont

Answer Fossils of Mesosaurus, a fresh-water and land animal, have been foundin widespread areas separated byoceans of salt water, through whichthey could not swim.

Caption AnswerFigure 2 Matching fossils on widelyseparated continents provide evidencethat these land masses were once joined.

Quick DemoPaper “Plates”Materials paper plate, red marker,blue marker, green marker, pairof scissors, 13! " 9! baking pan,access to waterEstimated Time 10 minutesProcedure Draw a large red cir-cle on the paper plate. Draw ablue circle inside the red circle.Draw a green circle inside theblue circle. Using scissors cutthe paper plate in half. Fill thebaking pan halfway with water.Place the two plate semicirclesinto the pan to demonstrate ourfloating continents.

Visually Impaired In order to help visuallyimpaired students better understand how shapesthat fit together can be used to help reconstructPangaea, make clay models of the continentalmasses that formed the supercontinent. Makesure the edges of the continents clearly match.Have students work with a partner to reconstructthe clay “Pangaea.” L2



A Widespread Plant Another fossil that supports thehypothesis of continental drift is Glossopteris (glahs AHP tur us).Figure 3 shows this fossil plant, which has been found in Africa,Australia, India, South America, and Antarctica. The presence ofGlossopteris in so many areas also supported Wegener’s idea thatall of these regions once were connected and had similar climates.

Climate Clues Wegener used continental drift to explainevidence of changing climates. For example, fossils of warm-weather plants were found on the island of Spitsbergen in theArctic Ocean. To explain this, Wegener hypothesized thatSpitsbergen drifted from tropical regions to the arctic. Wegeneralso used continental drift to explain evidence of glaciers foundin temperate and tropical areas. Glacial deposits and rock sur-faces scoured and polished by glaciers are found in SouthAmerica, Africa, India, and Australia. This shows that parts ofthese continents were covered with glaciers in the past. Howcould you explain why glacial deposits are found in areas whereno glaciers exist today? Wegener thought that these continentswere connected and partly covered with ice near Earth’s southpole long ago.

Rock Clues If the continents were connected at one time, thenrocks that make up the continents should be the same in locationswhere they were joined. Similar rock structures are found on different continents. Parts of the Appalachian Mountains of theeastern United States are similar to those found in Greenland andwestern Europe. If you were to study rocks from eastern SouthAmerica and western Africa, you would find other rock structuresthat also are similar. Rock clues like these support the idea that thecontinents were connected in the past.

Figure 3 This fossil plant,Glossopteris, grew in a temperateclimate.

Interpreting Fossil DataProcedure1. Build a three-layer

landmass using clay ormodeling dough.

2. Mold the clay into mountain ranges.

3. Place similar “fossils” intothe clay at various loca-tions around the landmass.

4. Form five continents fromthe one landmass. Also,form two smaller land-masses out of differentclay with different mountain ranges and fossils.

5. Place the five continentsand two smaller land-masses around the room.

6. Have someone who didnot make or place thelandmasses make a modelthat shows how they oncewere positioned.

7. Return the clay to its container so it can be used again.

AnalysisWhat clues were useful inreconstructing the originallandmass?

184 CHAPTER 7 Plate TectonicsMartin Land/Science Source/Photo Researchers

Fossils of Lystrosaurus, a small rep-tile that lived about 200 millionyears ago, have been found inSouth Africa, Antarctica, and India.

Purpose Students reaffirm thatgeologic clues can be used toshow how continents that arenow separate were once joined.

KinestheticMaterials three colors of mod-eling clay or modeling dough;objects such as macaroni, smallbuttons, or peanuts to use asfossils; spatula for cutting land-masses apartTeaching Strategy Review theprocedure for making the land-masses with students in classbefore having them completethe activity at home.AnalysisPossible answer: I looked for clues inthe pattern of fossils and “mountainranges.”

AssessmentOral Have students describethe characteristics they used to reconstruct the originallandmass. Use PerformanceAssessment in the ScienceClassroom, p. 143.

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Ancient Antarctica In the 1960s, evidencewas found in Antarctica that indicated awarm climate existed there 200 millionyears ago. Glossopteris fossils and rocks con-taining coal were found only 200 milesfrom the south pole.

ActivityBoundary Discussion Organizestudents into four groups andassign each group one of the fol-lowing topics to master andpresent to the class: convergentboundaries, divergent bound-aries, transform boundaries, andthe driving mechanism of platetectonics. Interpersonal

LSCOOP LEARNL2

Challenge New evidence suggests that mountainsin southwest South America match up with theAppalachians. Have students research and write areport about how this came about. Possible answer:Before colliding with the African Plate, the North AmericanPlate may have been positioned west of South America.

PL3


Check for UnderstandingVisual-Spatial Make photocopiesof the globe from 250 millionyears ago, 135 million years ago,and present day. Using thesecopies, refresh the students onthe ideas of continental drift.

ReteachOutlines Have students outlinethe section, including all of theimportant points. Have pairsexchange outlines and then usethe outlines to quiz one another.

InterpersonalLSL1

Self Check1. Explain how Wegener used climate clues to support his

hypothesis of continental drift.

2. Describe how rock clues were used to support thehypothesis of continental drift.

3. Summarize the ways that fossils helped support thehypothesis of continental drift.

4. Think Critically Why would you expect to see similarrocks and rock structures on two landmasses that wereconnected at one time.

SummaryEvidence for Continental Drift

• Alfred Wegener proposed in his hypothesis of continental drift that all continents wereonce connected as one large landmass calledPangaea.

• Evidence of continental drift came from fossils, signs of climate change, and rockstructures from different continents.

How could continents drift?

• During his lifetime, Wegener was unable toexplain how, when, or why the continentsdrifted.

• After his death, advances in technology permitted new ideas to be developed to help explain his hypothesis.

5. Compare and contrast the locations of fossils of thetemperate plant Glossopteris, as shown in Figure 2, with the climate that exists at each location today.

How could continents drift?Although Wegener provided evidence to support his hypothe-

sis of continental drift, he couldn’t explain how, when, or whythese changes, shown in Figure 4, took place. The idea suggestedthat lower-density, continental material somehow had to plowthrough higher-density, ocean-floor material. The force behindthis plowing was thought to be the spin of Earth on its axis—anotion that was quickly rejected by physicists. Because other scien-tists could not provide explanations either, Wegener’s idea of con-tinental drift was initially rejected. The idea was so radicallydifferent at that time that most people closed their minds to it.

Rock, fossil, and climate clues were the main types of evi-dence for continental drift. After Wegener’s death, more clueswere found, largely because of advances in technology, and newideas that related to continental drift were developed. You’lllearn about a new idea, seafloor spreading, in the next section.

Figure 4 These computer models show the probable coursethe continents have taken. On thefar left is their position 250 millionyears ago. In the middle is theirposition 135 million years ago. At right is their current position.

SECTION 1 Continental Drift 185blue.msscience.com/self_check_quiz

1. Fossils of warm-weather plantsfound on islands in the Arctic Oceanand glacial features found in placessuch as Africa supported the ideathat continents drift.

2. Rock structures on different conti-nents are similar.

3. Fossils of the same terrestrial organ-ism were found on widely separatedcontinents.

4. The same rock structure they sharedwhen attached would appear onboth halves after the landmass splitapart.

5. Possible answer: compare–all loca-tions are on landmasses;contrast–some locations are in tem-perate climates, others are in arid,semiarid, or polar climates.

Content Have groups of stu-dents write and perform skits inwhich one student playsWegener introducing his theory,and others play scientistsdebunking it. Encourage stu-dents to use visual props. UsePerformance Assessment inthe Science Classroom, p. 147.

Figure 4 What is happening toeastern Africa at present? It issplitting apart.

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Mapping the Ocean FloorIf you were to lower a rope from a boat until it reached the

seafloor, you could record the depth of the ocean at that partic-ular point. In how many different locations would you have todo this to create an accurate map of the seafloor? This is exactlyhow it was done until World War I, when the use of soundwaves was introduced by German scientists to detect sub-marines. During the 1940s and 1950s, scientists began usingsound waves on moving ships to map large areas of the oceanfloor in detail. Sound waves echo off the ocean bottom—thelonger the sound waves take to return to the ship, the deeperthe water is.

Using sound waves, researchers discovered an underwatersystem of ridges, or mountains, and valleys like those found onthe continents. In some of these underwater ridges are ratherlong rift valleys where volcanic eruptions and earthquakesoccur from time to time. Some of these volcanoes actually arevisible above the ocean surface. In the Atlantic, the Pacific, andin other oceans around the world, a system of ridges, called themid-ocean ridges, is present. These underwater mountainranges, shown in Figure 5, stretch along the center of much ofEarth’s ocean floor. This discovery raised the curiosity of manyscientists. What formed these mid-ocean ridges?

How were mid-ocean ridges discovered?

! Explain seafloor spreading.! Recognize how age and mag-

netic clues support seafloorspreading.

Seafloor spreading helps explainhow continents moved apart.

Review Vocabularyseafloor: portion of Earth’s crustthat lies beneath ocean waters

New Vocabulary• seafloor spreading

Seafloor Spreading

Figure 5 As the seafloorspreads apart at a mid-oceanridge, new seafloor is created. Theolder seafloor moves away fromthe ridge in opposite directions.

Tie to Prior KnowledgeOcean Floor Ask students if theyever have walked into an ocean,lake, or river. Have themdescribe how the floor of thebody of water felt. Students maymention steep and gentle slopes, rocks,bars or ridges of sand, and depressions.Explain that the entire oceanfloor has such features, and ittook special tools to map it.


Plate Tectonics

Until recently, the bottom of the sea was impossible to see. Newtechnology has improved the view, and today we have a better idea ofwhat is going on there. This photo shows one feature of the oceanfloor—a deep-sea vent.

The Main EventSection FocusTransparency22

1. What is occurring in the photograph?

2. What features on land are similar to this deep-sea vent?

3. Judging from the photo, what do you think conditions aroundthis vent are like?

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Chapter FAST FILE ResourcesTransparency Activity, p. 45Directed Reading for Content Mastery, p. 20Enrichment, p. 31

Reinforcement, p. 28Lab Worksheets, pp. 5–6

Mathematics Skill Activities, p. 5Science Inquiry Labs, pp. 23–24

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Answer using sound waves

ActivityVolcano Map Show students amap with the location of volca-noes indicated. Have studentsuse it to determine subductionareas. Visual-SpatialLSL2

SECTION 2 Seafloor Spreading 187

The Seafloor Moves In the early 1960s,Princeton University scientist Harry Hess sug-gested an explanation. His now-famous theory isknown as seafloor spreading. Hess proposedthat hot, less dense material below Earth’s crustrises toward the surface at the mid-ocean ridges.Then, it flows sideways, carrying the seaflooraway from the ridge in both directions, as seen inFigure 5.

As the seafloor spreads apart, magma is forcedupward and flows from the cracks. It becomessolid as it cools and forms new seafloor. As newseafloor moves away from the mid-ocean ridge, itcools, contracts, and becomes denser. Thisdenser, colder seafloor sinks, helping to form theridge. The theory of seafloor spreading was latersupported by the following observations.

How does new seafloor form atmid-ocean ridges?

Evidence for SpreadingIn 1968, scientists aboard the research ship Glomar

Challenger began gathering information about the rocks on theseafloor. Glomar Challenger was equipped with a drilling rig thatallowed scientists to drill into the seafloor to obtain rock sam-ples. Scientists found that the youngest rocks are located at themid-ocean ridges. The ages of the rocks become increasinglyolder in samples obtained farther from the ridges, adding to theevidence for seafloor spreading.

Using submersibles along mid-ocean ridges, new seafloorfeatures and life-forms also were discovered there, as shown inFigure 6. As molten material is forced upward along the ridges,it brings heat and chemicals that support exotic life-forms indeep, ocean water. Among these are giant clams, mussels, andtube worms.

Magnetic Clues Earth’s magnetic fieldhas a north and a south pole. Magnetic

lines, or directions, of force leave Earth near the south pole andenter Earth near the north pole. During a magnetic reversal, thelines of magnetic force run the opposite way. Scientists havedetermined that Earth’s magnetic field has reversed itself manytimes in the past. These reversals occur over intervals of thou-sands or even millions of years. The reversals are recorded inrocks forming along mid-ocean ridges.

Figure 6 Many new discoverieshave been made on the seafloor.These giant tube worms inhabitareas near hot water vents alongmid-ocean ridges.

SECTION 2 Seafloor Spreading 187

Curie Point Find outwhat the Curie point isand describe in yourScience Journal whathappens to iron-bearingminerals when they areheated to the Curie point.Explain how this isimportant to studies ofseafloor spreading.

Ralph White/CORBIS

Answer Magma moves upward andout of cracks in the seafloor. As it solidi-fies on the surface, new seafloor forms.Older seafloor moves away from theridge.

Curie Point The Curie point is the tem-perature above which iron-bearing min-erals lose their magnetism. As lava coolsinto rock on the ocean floor, the rockacquires a magnetic field like that ofEarth’s. If these rocks are reheatedbeyond the Curie point, the magneticsignature they acquired at the time theyformed is lost.Research Have students researchand write about the scientist forwhom the Curie point is named.French physicist Pierre Curie P

Mapping the Seafloor Harry Hess(1906–1969) was a geologistwho collected important dataabout the seafloor during WorldWar II, when he was captain of anavy transport vessel. His shipwas fitted with a new devicecalled a fathometer, an echosounder that showed ocean-floor depth under the ship. Thedata was meant to allow troopships to get close to shore, butHess used it to map the floor ofthe Pacific Ocean.

Certain species of green turtles swim from SouthAmerica to Ascension Island in the South Atlantic tolay their eggs. The turtles may have started this tripwhen these land masses were much closer. As theseafloor spread, the instinctive trip became longer.

Figure 6 What allows the exoticlife shown to exist? Heat frommolten material forced upward alongrift valley.

Challenge Have students research the scientificand technological advances that led to the theoryof seafloor spreading. Have students display theirdiscoveries on a time line. Logical-Mathematical

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1. Magnetic reversals recorded in iron-bearing minerals show that newrock was being formed at the ridgesover time.

2. The rocks get older as you move far-ther from the mid-ocean ridge.

3. Hot, dense material is forcedupward at mid-ocean ridges. It thenmoves sideways, carrying the

seafloor away from the ridge inboth directions.

4. The molten material is less densethan the surrounding rock. So it isforced upward toward the surface.

5. Continental drift hypothesis pro-vided no mechanism for movement;seafloor spreading explained howcontinents have separated over

time as ocean basins enlarged.6. If North America continues to move

away from the Mid-Atlantic Ridgeat 1.25 cm/y, these two featureswill separate by an additional 250million centimeters or 2,500 kilo-meters in 200 million years.

Caption AnswerFigure 7 It shows the rock continuallyformed and moved away from the ridgeover time.

Self Check1. Summarize What properties of iron-bearing

minerals on the seafloor support the theory of seafloorspreading?

2. Explain how the ages of the rocks on the ocean floorsupport the theory of seafloor spreading.

3. Summarize How did Harry Hess’s hypothesis explainseafloor movement?

4. Explain why some partly molten material rises towardEarth’s surface.

5. Think Critically The ideas of Hess, Wegener, and oth-ers emphasize that Earth is a dynamic planet. How isseafloor spreading different from continental drift?

SummaryMapping the Ocean Floor

• Mid-ocean ridges, along the center of theocean floor, have been found by using sound waves, the same method once used to detect submarines during World War I.

• Harry Hess suggested, in his seafloor spreading hypothesis, that the seafloormoves.

Evidence for Spreading

• Scientists aboard Glomar Challenger providedevidence of spreading by discovering that the youngest rocks are located at ridges andbecome increasingly older farther from theridges.

• Magnetic alignment of rocks, in alternatingstrips that run parallel to ridges, indicatesreversals in Earth’s magnetic field and provides further evidence of seafloor spreading.

6. Solve One-Step Equations North America is movingabout 1.25 cm per year away from a ridge in the middle of the Atlantic Ocean. Using this rate, howmuch farther apart will North America and the ridge be in 200 million years?

Magnetic Time Scale Iron-bearing minerals,such as magnetite, that are found in the rocks of theseafloor can record Earth’s magnetic field directionwhen they form. Whenever Earth’s magnetic fieldreverses, newly forming iron minerals will recordthe magnetic reversal.

Using a sensing device called a magnetometer(mag nuh TAH muh tur) to detect magnetic fields,scientists found that rocks on the ocean floor showmany periods of magnetic reversal. The magneticalignment in the rocks reverses back and forth overtime in strips parallel to the mid-ocean ridges, asshown in Figure 7. A strong magnetic reading isrecorded when the polarity of a rock is the same asthe polarity of Earth’s magnetic field today. Because

of this, normal polarities in rocks show up as large peaks. Thisdiscovery provided strong support that seafloor spreading wasindeed occurring. The magnetic reversals showed that new rockwas being formed at the mid-ocean ridges. This helped explainhow the crust could move—something that the continental drifthypothesis could not do.

Figure 7 Changes in Earth’smagnetic field are preserved in rockthat forms on both sides of mid-ocean ridges. Explain why this is considered tobe evidence of seafloor spreading.

Normal magnetic polarityReverse magnetic polarity

blue.msscience.com/self_check_quiz188 CHAPTER 7 Plate Tectonics

Performance Assess students’understanding by challengingthem to draw concept maps that contain the main ideas inthis section. Use PerformanceAssessment in the ScienceClassroom, p. 161.

Check for UnderstandingKinesthetic Have students makea concept map that includes evi-dence for seafloor spreadingusing the following phrases: agesincrease away from ridge, pat-tern of magnetic field reversals,mid-ocean ridge, pattern of ages,and reverses back and forth.

ReteachSeafloor Model Have pairs of stu-dents construct three-dimen-sional models of the seafloor at amid-ocean ridge.

KinestheticLSCOOP LEARNL1

DiscussionPlate Boundaries and EarthquakesWhy are earthquakes “associ-ated” with plate boundaries?Plates do not move smoothly. Instead,they stick and catch on each other. Whenthe plates are “stuck,” strain, or potentialenergy, builds up in the rocks. When theplates move again, this energy isreleased as an earthquake.


Seaflccr Spreading RatesHow did scientists use their know-ledge of seafloor spreading and mag-netic field reversals to reconstruct Pangaea? Try this lab to see how you can determine where a continent mayhave been located in the past.

Real-WorldQuestion

Can you use clues, such as magnetic field reversals on Earth, to help recon-struct Pangaea?

Goals! Interpret data about magnetic field

reversals. Use these magnetic clues to reconstruct Pangaea.

Materialsmetric rulerpencil

Procedure1. Study the magnetic field graph above. You

will be working only with normal polarityreadings, which are the peaks above thebaseline in the top half of the graph.

2. Place the long edge of a ruler vertically onthe graph. Slide the ruler so that it lines upwith the center of peak 1 west of the Mid-Atlantic Ridge.

3. Determine and record the distance andage that line up with the center of peak1 west. Repeat this process for peak 1 eastof the ridge.

4. Calculate the average distance and age forthis pair of peaks.

5. Repeat steps 2 through 4 for the remainingpairs of normal-polarity peaks.

6. Calculate the rate of movement in cm peryear for the six pairs of peaks. Use the for-mula rate ! distance/time. Convert kilome-ters to centimeters. For example, tocalculate a rate using normal-polarity peak5, west of the ridge:

rate !"10 m

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Conclude and Apply1. Compare the age of igneous rock found near

the mid-ocean ridge with that of igneous rockfound farther away from the ridge.

2. If the distance from a point on the coast ofAfrica to the Mid-Atlantic Ridge isapproximately 2,400 km, calculate how longago that point in Africa was at or near theMid-Atlantic Ridge.

3. How could you use this method toreconstruct Pangaea?

6

150 100 50 0 50 100125 75 25 25 75 125 150

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11

2

3 45

6

12 8 4 0

Age (millions of years)

Distance (km)

Normalpolarity

Reversepolarity

West East

Mid

-Atl

anti

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dge

210 6 2 1 3

45

67

89

1011

12

LAB 189

Real-World Question

Purpose Students can interpretmagnetic field reversals in rockto determine the rate of seafloorspreading.

Logical-Mathematical

Process Skills make tables, usegraphs, predict, observe, infer,use numbers, interpret data

Time Required 50 to 60 minutes

Teaching Strategy Note thatthe rate of movement (half the spreading rate) is about 1.25 cm/yr.

Conclude andApply

1. The nearer rock is to the ridge, theyounger it is.

2. about 192 million years ago, assum-ing a relatively constant rate ofspreading

3. Students could determine whenpoints on both coasts were at theridge. This would mark when at leastpart of Pangaea was intact.

LS

L2

LAB 189

Sample Data Table:

Performance Have your stu-dents measure the distancebetween a point on the eastcoast of the United States andthe Mid-Atlantic Ridge. Havethem determine when thatpoint was near the mid-oceanridge. Use PerformanceAssessment in the ScienceClassroom, p. 99.

Students can use a table such as this to record data.

Peak 1 2 3 4 5 6Distance west normal polarity 40 60 75 100 125 140

Distance east normal polarity 36 60 80 100 118 145

Average distance 38 60 78 100 122 142Age from scale (millions of years) 3.5 4.5 6.7 8.0 9.0 10.5

Rate of movement (cm/yr) 1.1 1.2 1.2 1.3 1.3 1.4


Plate TectonicsThe idea of seafloor spreading showed that more than just

continents were moving, as Wegener had thought. It was nowclear to scientists that sections of the seafloor and continentsmove in relation to one another.

Plate Movements In the 1960s, scientists developed a newtheory that combined continental drift and seafloor spreading.According to the theory of plate tectonics, Earth’s crust and partof the upper mantle are broken into sections. These sections,called plates, move on a plasticlike layer of the mantle. Theplates can be thought of as rafts that float and move on thislayer.

Composition of Earth’s Plates Plates are made of thecrust and a part of the upper mantle, as shown in Figure 8.These two parts combined are the lithosphere (LIH thuh sfihr).This rigid layer is about 100 km thick and generally is less densethan material underneath. The plasticlike layer below the litho-sphere is called the asthenosphere (as THE nuh sfihr). The rigidplates of the lithosphere float and move around on theasthenosphere.

Theory of Plate Tectonics

Continentalcrust

LithosphereLithosphere

AsthenosphereAsthenosphere Upper mantleUpper mantle

Lithosphere

Asthenosphere

Oceaniccrust

Mantle

LithosphereLithosphereLithosphere

Upper mantle

Figure 8 Plates of the lith-osphere are composed ofoceanic crust, continentalcrust, and rigid upper mantle.

! Compare and contrast differenttypes of plate boundaries.

! Explain how heat inside Earthcauses plate tectonics.

! Recognize features caused byplate tectonics.

Plate tectonics explains how manyof Earth’s features form.

Review Vocabularyconverge: to come togetherdiverge: to move aparttransform: to convert or change

New Vocabulary• plate tectonics

• plate

• lithosphere

• asthenosphere

• convection current


Tie to Prior KnowledgeEarthquakes Ask if anyone hasever experienced an earthquake.If so, have these students explainwhat happened. If no one has,explain that earthquakes causethe ground to shake, often caus-ing great damage. Tell studentsthat earthquakes often happenbecause of the movement ofplates.


Plate Tectonics

One of the most massive volcanic eruptions ever investigatedoccurred in a valley in southern Alaska in 1912. The eruption coveredover forty square miles with ash as deep as 210 meters and left thou-sands of vents (called fumaroles) in the valley spewing steam and gas.

Valley of Ten ThousandSmokes

Section FocusTransparency33

1. How did this valley get its name, the Valley of Ten ThousandSmokes?

2. Why don’t you see any smoke in the photograph?

3. Name some other places where there are volcanoes.

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Chapter FAST FILE ResourcesTransparency Activity, pp. 46, 48–49Directed Reading for Content Mastery,

pp. 21, 22Lab Activity, pp. 13–15MiniLAB, p. 4Enrichment, p. 32Lab Worksheets, pp. 7–8Reinforcement, p. 29

Cultural Diversity, p. 47Home and Community Involvement, p. 31Earth Science Critical Thinking/Problem Solving,

p. 8Reading and Writing Skill Activities, p. 27Performance Assessment in the Science Classroom,

p. 42Lab Management and Safety, p. 65

L2

SECTION 3 Theory of Plate Tectonics 191

Quick DemoContinental MovementMaterials globe or mapEstimated Time 10 minutesProcedure Obtain a globe ormake a map on which you canmove continent pieces from achild’s puzzle map. Use theglobe or map to demonstratecontinental movement.

PACIFICPLATE

NORTH AMERICANPLATE

SOUTHAMERICAN

PLATE

AFRICAN PLATE

EURASIAN PLATE

PACIFICPLATE

INDO-AUSTRALIANPLATE

ANTARCTICPLATE

ANTARCTICPLATE

PHILIPPINEPLATE

NORTHAMERICAN

PLATE

COCOS PLATE

SCOTIA PLATE

ARABIANPLATE

NAZCAPLATE

CARIBBEANPLATE

JUAN DE FUCAPLATE

Convergent boundaryDivergent boundaryTransform boundary

Plate BoundariesWhen plates move, they can interact in several ways. They

can move toward each other and converge, or collide. They alsocan pull apart or slide alongside one another. When the platesinteract, the result of their movement is seen at the plate bound-aries, as in Figure 9.

What are the general ways that plates interact?

Movement along any plate boundary means that changesmust happen at other boundaries. What is happening to theAtlantic Ocean floor between the North American and AfricanPlates? Compare this with what is happening along the westernmargin of South America.

Plates Moving Apart The boundary between two plates thatare moving apart is called a divergent boundary. You learnedabout divergent boundaries when you read about seafloorspreading. In the Atlantic Ocean, the North American Plate ismoving away from the Eurasian and the African Plates, as shownin Figure 9. That divergent boundary is called the Mid-AtlanticRidge. The Great Rift Valley in eastern Africa might become adivergent plate boundary. There, a valley has formed where acontinental plate is being pulled apart. Figure 10 shows a sideview of what a rift valley might look like and illustrates how thehot material rises up where plates separate.

Figure 9 This diagram shows themajor plates of the lithosphere, theirdirection of movement, and thetype of boundary between them.Analyze and Conclude Based onwhat is shown in this figure, what ishappening where the Nazca Platemeets the Pacific Plate?


Answer Plates can collide, pull apart,or move past one another.

Caption AnswerFigure 9 These plates are moving awayfrom each other.

The Indian Plate, which collidedwith Asia to form the Himalaya,continues to move at a rate ofalmost 5 cm per year. This massiveplate is moving twice as fast as yourfingernails grow!

Theory Development Many scientists contributedideas that led to plate tectonics theory. Have stu-dents select one from A.L. Du Toit, S.K.Runcorn, Bruce Heezen, Arthur Holmes, J. TuzoWilson, Jack Oliver, Lynn R. Sykes, Fred Vine,D.H. Matthews, and L.W. Morley and write aone-page report in their Science Journals abouthis contributions. PL2

English-Language Learners Pair English-languagelearning students with other students who cantake them through the steps of this lab and visu-ally assist them. After repeating steps 2 through 4of this lab a few times, encourage English-language-learning students to try some graphing.


Plates Moving Together If new crust is being added at onelocation, why doesn’t Earth’s surface keep expanding? As newcrust is added in one place, it disappears below the surface atanother. The disappearance of crust can occur when seafloorcools, becomes denser, and sinks. This occurs where two platesmove together at a convergent boundary.

When an oceanic plate converges with a less dense continen-tal plate, the denser oceanic plate sinks under the continentalplate. The area where an oceanic plate subducts, or goes down,into the mantle is called a subduction zone. Some volcanoesform above subduction zones. Figure 10 shows how this type ofconvergent boundary creates a deep-sea trench where one platebends and sinks beneath the other. High temperatures causerock to melt around the subducting slab as it goes under theother plate. The newly formed magma is forced upward alongthese plate boundaries, forming volcanoes. The Andes moun-tain range of South America contains many volcanoes. Theywere formed at the convergent boundary of the Nazca and theSouth American Plates.

Solving the Problem???

Recall the Launch Lab you performedat the beginning of this chapter.

While you were trying to fit pieces of acut-up photograph together, what cluesdid you use?

Identifying the Problem Take a copy of a map of the world and

cut out each continent. Lay them on atabletop and try to fitthem together, usingtechniques you used in the Launch Lab.You will find that the pieces of your Earth puzzle—thecontinents—do not fit together well. Yet,several of the areas on some continents fittogether extremely well.

Take out anotherworld map—onethat shows the continental shelvesas well as the conti-nents. Copy it andcut out the continents,this time including thecontinental shelves.

Solving the Problem1. Does including the

continental shelves solve the problem of fitting the con-tinents together?

2. Why should con-tinental shelvesbe included withmaps of the continents?

How well do the continents fit together?


Topic: Earthquakes andVolcanoesVisit for Weblinks to recent news or magazinearticles about earthquakes andvolcanic activity related to platetectonics.

Activity Prepare a groupdemonstration about recent volcanic and earthquake events.Divide tasks among group mem-bers. Find and copy maps, dia-grams, photographs, and charts to highlight your presentation.Emphasize the locations of events and the relationship toplate tectonics.

blue.msscience.com

Use Science WordsWord Use Have students look upthe words diverge and convergeand use each word in a sentence.Then discuss how these mean-ings relate to plate boundaries.Possible answers: Two paths diverge at afork in the road; traffic will converge inthe center of the intersection. Plates con-verge, or come together, at some bound-aries and diverge, or move apart, atothers.

Answers1. Yes, most fit together when conti-

nental shelves are included.2. The continental shelves are the edges

of continents. Present-day coastlinesresult from sea-level changes.

Purpose to demonstrate compressionforces that can form folded mountainsMaterials two slabs of clay (5 cm thick andabout 30 cm long), wax paperPreparation Place the clay slabs on waxpaper to make them easier to slide.

Procedure Lay the two clay pieces flat on atable. Have students predict what will hap-pen when they are forced together. Pushthe two pieces together.Expected Outcome Students will see foldsand breaks form as the pieces of clay arepushed together.

AssessmentWhat landforms are the folds in the clayanalogous to on Earth’s surface? foldedmountains



NGS TITLEVISUALIZING PLATE BOUNDARIES

Figure 10

A RIFT VALLEY When continental plates pull apart,they can form rift valleys. The African continent isseparating now along the East African Rift Valley.

B y diverging at some boundaries and convergingat others, Earth’s plates are continually—butgradually—reshaping the landscape around

you. The Mid-Atlantic Ridge, for example, was formedwhen the North and South American Plates pulled apart from the Eurasian and African Plates (see globe).Some features that occur along plate boundaries—rift valleys, volcanoes, and mountain ranges—areshown on the right and below.

SEAFLOOR SPREADING A mid-ocean ridge, likethe Mid-Atlantic Ridge, forms where oceanicplates continue to separate. As rising magma(yellow) cools, it forms new oceanic crust.

Continental plate

Oceanicplate

EurasianPlate

AfricanPlate

NorthAmericanPlate

South AmericanPlate

Mid

-Atla

nticRidge

NorthAmericanPlate

EurasianPlate

AfricanPlate

South AmericanPlate

Mid

-Atla

nticRidge

SUBDUCTION Where oceanic and continentalplates collide, the oceanic plate plunges beneaththe less dense continental plate. As the platedescends, molten rock (yellow) forms and risestoward the surface, creating volcanoes.

CONTINENTAL COLLISION Where two continentalplates collide, they push up the crust to formmountain ranges such as the Himalaya.

SECTION 3 Theory of Plate Tectonics 193Davis Meltzer

Visualizing PlateBoundaries

Have students examine thepictures and read the captions.Then ask the following ques-tions.

How would you predict thesize of the Atlantic Ocean willchange over the next 100 millionyears? Why? The Atlantic Ocean willbecome larger because sea-floor spread-ing is occurring along the Mid-AtlanticRidge.

The Andes mountains arefound along the west coast ofSouth America. How did thismountain chain form? The plateboundary along the west coast of SouthAmerica is a convergent boundary, whichresults in the formation of mountainsand volcanoes.

ActivitySurtsey Have small groupsresearch the history of Surtsey, asmall island in the NorthAtlantic Ocean. Ask them todraw a map of the island’s loca-tion and write a summary of howthe island formed, describing thetype of plate boundary and thevolcanic activity involved.

InterpersonalLSCOOP LEARN

L2



Where Plates Collide A subduction zone also can formwhere two oceanic plates converge. In this case, the colder, older,denser oceanic plate bends and sinks down into the mantle. TheMariana Islands in the western Pacific are a chain of volcanicislands formed where two oceanic plates collide.

Usually, no subduction occurs when two continental platescollide, as shown in Figure 10. Because both of these plates areless dense than the material in the asthenosphere, the two platescollide and crumple up, forming mountain ranges. Earthquakesare common at these convergent boundaries. However, volca-noes do not form because there is no, or little, subduction. TheHimalaya in Asia are forming where the Indo-Australian Platecollides with the Eurasian Plate.

Where Plates Slide Past Each Other The third type ofplate boundary is called a transform boundary. Transformboundaries occur where two plates slide past one another. Theymove in opposite directions or in the same direction at differentrates. When one plate slips past another suddenly, earthquakesoccur. The Pacific Plate is sliding past the North American Plate,forming the famous San Andreas Fault in California, as seen inFigure 11. The San Andreas Fault is part of a transform plateboundary. It has been the site of many earthquakes.

Figure 11 The San AndreasFault in California occurs along thetransform plate boundary wherethe Pacific Plate is sliding past theNorth American Plate.

This photograph shows an aerial view of the San Andreas Fault.

San Andreas Fault

SanFrancisco

North American plate

Pacific plate

San Francisco Bay

Overall, the two plates are movingin roughly the same direction. Explain Why, then, do the redarrows show movement in oppositedirections?

Craig Aurness/CORBIS

Caption AnswerFigure 11 The western side (PacificPlate) of the fault is moving faster thanthe eastern side (North American Plate).

Rotation of Earth Because stu-dents feel as though they arestanding perfectly still onEarth’s surface, they often for-get that they are moving inseveral ways at the same time.Remind students that, as theysit in class, Earth is rotating at464 m/s, speeding around thesun at 29.8 km/s, and the tec-tonic plate on which they sit issliding around on Earth’s plastic-like mantle.

Figure 11 Have students studythe photograph of the SanAndreas Fault and then describeevidence that shows the plateson either side of the fault aremoving. Students should see thatstreams and other features that cross thefault are offset because of movement.

Mathematics The deepest point on Earth’s sur-face is the bottom of the Mariana Trench, 11.2km below sea level. Have students find Earth’shighest point. Mt. Everest is 8.8 km above sea level. Afterstudents determine which is bigger, have themdraw a scale diagram showing Mt. Everest in thetrench. Their drawings should show how manykilometers Mt. Everest’s top would be below sealevel. 2.4 km



Causes of Plate TectonicsMany new discoveries have been made about Earth’s crust

since Wegener’s day, but one question still remains. What causesthe plates to move? Scientists now think they have a good idea.They think that plates move by the same basic process thatoccurs when you heat soup.

Convection Inside Earth Soup that is cooking in a pan on thestove contains currents caused by an unequal distribution of heatin the pan. Hot, less dense soup is forced upward by the surround-ing, cooler, denser soup. As the hot soup reaches the surface, itcools and sinks back down into the pan. This entire cycle of heat-ing, rising, cooling, and sinking is called a convection current. Aversion of this same process, occurring in the mantle, is thought tobe the force behind plate tectonics. Scientists suggest thatdifferences in density cause hot, plasticlike rock to be forcedupward toward the surface.

Moving Mantle Material Wegener wasn’t able to come upwith an explanation for why plates move. Today, researcherswho study the movement of heat in Earth’s interior have pro-posed several possible explanations. All of the hypotheses useconvection in one way or another. It is, therefore, the transfer ofheat inside Earth that provides the energy to move plates andcauses many of Earth’s surface features. One hypothesis isshown in Figure 12. It relates plate motion directly to the move-ment of convection currents. According to this hypothesis, con-vection currents cause the movements of plates.

Figure 12 In one hypothesis,convection currents occur through-out the mantle. Such convectioncurrents (see arrows) are the driv-ing force of plate tectonics.

Core

Ridge

Trench

Trench

Mantle

Modeling ConvectionCurrentsProcedure

1. Pour water into a clear,colorless casserole dishuntil it is 5 cm from the top.

2. Center the dish on a hotplate and heat it. WARN-ING: Wear thermal mittsto protect your hands.

3. Add a few drops of foodcoloring to the waterabove the center of the hot plate.

4. Looking from the side ofthe dish, observe whathappens in the water.

5. Illustrate your observationsin your Science Journal.

Analysis1. Determine whether any

currents form in the water.2. Infer what causes the cur-

rents to form.

Purpose Students model andobserve currents. Visual-SpatialMaterials clear glass casseroledish, water, hot plate, food col-oring, thermal mittsTeaching Strategy Have studentsnote any movement in thewater.Safety Precautions Students mustwear thermal mitts. Be sure thedish is stove-top safe.Analysis1. Some students will observe cur-

rents; others won’t.2. The transfer of thermal energy from

the burner to the dish warms thewater near the bottom of the dish.The cooler, denser water at the topof the dish sinks, displacing thewarmer, less dense water, whichthen moves toward the top of thedish. As the warmer water cools, itbecomes denser and sinks to startthe cycle again.

AssessmentProcess Direct students to addinformative labels to the draw-ings they made of their observa-tions. The labels should benumbered and in sequence,explaining the steps in the for-mation and movement of con-vection currents. Use Perfor-mance Assessment in theScience Classroom, p. 127.

LSL2

Write-Draw-Discuss This strategy encouragesstudents to actively participate in readingand lectures, assimilating content creatively.Have students write about an idea, clarify it,then make an illustration or drawing. Askstudents to share responses with the classand display several examples. Have studentsWrite-Draw-Discuss about the causes ofplate tectonics.

Hawaiian Terms The Hawaiian Islands are volca-noes that formed as a result of magma risingthrough a “hot spot” in the middle of a plate.Some volcanic rocks have names that were madecommon in Hawaii. Pahoehoe (pa-hoe-ee-hoe-ee),from the Hawaiian word meaning “rope,” formsin linear ridges. Aa (ah-ah) forms with sharp,jagged surfaces.


Features Caused by Plate TectonicsEarth is a dynamic planet with a hot interior. This heat leads

to convection, which powers the movement of plates. As theplates move, they interact. The interaction of plates producesforces that build mountains, create ocean basins, and cause vol-canoes. When rocks in Earth’s crust break and move, energy isreleased in the form of seismic waves. Humans feel this releaseas earthquakes. You can see some of the effects of plate tectonicsin mountainous regions, where volcanoes erupt, or where land-scapes have changed from past earthquake or volcanic activity.

What happens when seismic energy is releasedas rocks in Earth’s crust break and move?

Normal Faults and Rift Valleys Tension forces, which areforces that pull apart, can stretch Earth’s crust. This causes largeblocks of crust to break and tilt or slide down the broken sur-faces of crust. When rocks break and move along surfaces, a faultforms. Faults interrupt rock layers by moving them out of place.Entire mountain ranges can form in the process, called fault-block mountains, as shown in Figure 13. Generally, the faultsthat form from pull-apart forces are normal faults—faults inwhich the rock layers above the fault move down when com-pared with rock layers below the fault.

Rift valleys and mid-ocean ridges can form where Earth’scrust separates. Examples of rift valleys are the Great Rift Valleyin Africa, and the valleys that occur in the middle of mid-oceanridges. Examples of mid-ocean ridges include the Mid-AtlanticRidge and the East Pacific Rise.

Figure 13 Fault-block moun-tains can form when Earth’s crustis stretched by tectonic forces. Thearrows indicate the directions ofmoving blocks. Name the type of force that occurswhen Earth’s crust is pulled in oppo-site directions.

Craig Brown/Index Stock

Caption AnswerFigure 13 tension

Make a ModelSeafloor Spreading Have studentsuse paper to make a model of adivergent boundary withseafloor spreading. The modelshould be dynamic and showhow spreading occurs. Studentscan draw parallel ridges on a long pieceof paper and construct a mechanism forthe paper to be drawn upward from bothsides through a slot (the plate boundary),revealing “new” parallel ridges as itemerges. Accept any workable design.

Visual-SpatialLSL2

Challenge Have students place a piece of long,plastic tubing into the neck of a strong balloonand secure the tube so no air can escape from theballoon. Have them place the balloon at the bot-tom of an aquarium tank, with the tubing extend-ing over the top of the tank. Then have studentsadd alternating 2-mm layers of moist sand anddry sand to the bottom of the tank, covering the

balloon. If they wish, students can build a smallcity on the top sand layer. Then have one studentslowly blow up the balloon until the sand beginsto crack to simulate an earthquake. Another student can videotape the “earthquake,” and stu-dents can prepare a documentary showing howthe quake affected the “city.” Kinesthetic

LSCOOP LEARNL3

Answer earthquakes

Make a ModelStrike-Slip Faults Provide claywith which students can makemodels to show the movementof plates at a strike-slip fault.Have them include surface fea-tures offset by the movement.

KinestheticLSL1

Mountains and Volcanoes Compression forces squeezeobjects together. Where plates come together, compression forcesproduce several effects. As continental plates collide, the forces thatare generated cause massive folding and faulting of rock layers intomountain ranges such as the Himalaya, shown in Figure 14, or theAppalachian Mountains. The type of faulting produced is gener-ally reverse faulting. Along a reverse fault, the rock layers above thefault surface move up relative to the rock layers below the fault.

What features occur where plates converge?

As you learned earlier, when two oceanic plates converge,the denser plate is forced beneath the other plate. Curved chains of volcanic islands called island arcs form above the sink-ing plate. If an oceanic plate converges with a continental plate,the denser oceanic plate slides under the continental plate.Folding and faulting at the continental plate margin can thickenthe continental crust to produce mountain ranges. Volcanoesalso typically are formed at this type of convergent boundary.

Asthenosphere

Lithosphere

Continental crust

Oceanic crust

Mou

ntai

n ra

nge

High

pla

teau


Volcanologist This per-son’s job is to study volca-noes in order to predicteruptions. Early warning ofvolcanic eruptions givesnearby residents time toevacuate. Volcanologistsalso educate the publicabout the hazards of vol-canic eruptions and tellpeople who live near volca-noes what they can do tobe safe in the event of aneruption. Volcanologiststravel all over the world to study new volcanic sites.

Figure 14 The Himalaya still areforming today as the Indo-AustralianPlate collides with the Eurasian Plate.

Ric Ergenbright/CORBIS

Answer continental–continental: highmountain ranges; oceanic–oceanic:island arcs; oceanic–continental: moun-tains and volcanoes

Moving Mountains As the Indo-Australian Plate continues topush into and under theEurasian Plate, the EurasianPlate is thrust up over it. Indiamoves into Asia at a rate of 3.7to 5.4 cm/yr, thrusting theHimalaya about 1 cm highereach year. But because forces oferosion wear down the moun-tains by about the same amountannually, they remain about thesame height.

Geography Have students search the Internet,newspapers, and magazines for stories aboutearthquakes and volcanic eruptions that haveoccurred as a result of plate movements alongfaults. Have students summarize their findings inwritten paragraphs to share in class. Then havethem make a bulletin board display that includesa map showing the location of these tectonicevents. Visual-SpatialLSL2


Volcanologist Ask students toresearch to find out the typeand level of degree required tobecome a volcanologist. Thenhave them research the courserequirements and duration ofsuch a degree at a university oftheir choice.



Direction of Forces Inwhich directions do forcesact at convergent, diver-gent, and transform bound-aries? Demonstrate theseforces using wooden blocksor your hands.

Figure 15 Most of the move-ment along a strike-slip fault isparallel to Earth’s surface. Whenmovement occurs, human-builtstructures along a strike-slip fault are offset, as shown here in this road.

Strike-Slip Faults At transform boundaries, two platesslide past one another without converging or diverging. Theplates stick and then slide, mostly in a horizontal direction,along large strike-slip faults. In a strike-slip fault, rocks onopposite sides of the fault move in opposite directions, or inthe same direction at different rates. This type of fault move-ment is shown in Figure 15. One such example is the San Andreas Fault. When plates move suddenly, vibrations aregenerated inside Earth that are felt as an earthquake.

Earthquakes, volcanoes, and mountain ranges are evidenceof plate motion. Plate tectonics explains how activity insideEarth can affect Earth’s crust differently in different locations.You’ve seen how plates have moved since Pangaea separated.Is it possible to measure how far plates move each year?

Testing for Plate TectonicsUntil recently, the only tests scientists could use to check for

plate movement were indirect. They could study the magneticcharacteristics of rocks on the seafloor. They could study volca-noes and earthquakes. These methods supported the theory thatthe plates have moved and still are moving. However, they didnot provide proof—only support—of the idea.

New methods had to be discovered to be able to measurethe small amounts of movement of Earth’s plates. Onemethod, shown in Figure 16, uses lasers and a satellite. Now,scientists can measure exact movements of Earth’s plates of aslittle as 1 cm per year.

Roger Ressmeyer/CORBIS

DiscussionLiving on a Fault Would you liveor construct a building along astrike-slip fault? Explain. Mostwould not because of the danger of earth-quakes and the likelihood of destruction.Why might so many people livealong the San Andreas Fault?Possible answers: Some people werethere before the danger was known; somethink the danger is minimal, some mightbe unable to relocate; some may thinkthat other factors (climate, economicadvantages) outweigh the negatives ofearthquake danger.

Direction of Forces Convergent:toward each other; divergent: awayfrom each other; transform: sliding pasteach other.

Moving Plates Creepmeters, lasers,and satellites are used to measureplate movements. They haveshown that the Pacific Plate hasbeen sliding past the NorthAmerican Plate along the SanAndreas Fault at a rate of 1.2 to3.3 cm/yr. If that continues, thePacific Plate will continue tomove north relative to the NorthAmerican Plate, bringing LosAngeles up next to San Franciscoin about 27 million years.

Challenge Challenge students to research the1992 Landers, California, earthquake. In theirScience Journals, have them write a summaryexplaining why this quake is important to thestudy of plate tectonics. It may indicate that a new faultis forming and that the Pacific–North American Plate bound-ary may be shifting eastward. Logical-Mathematical

LSL3


Check for UnderstandingInterpersonal Students can takethe answers they came up with inthe section review and pair upwith other students to discusstheir answers.

ReteachConveyor Belt The movement ofitems on a supermarket con-veyor belt is analogous to themovement of Earth’s plates atopconvection currents in the man-tle. The belt is like the currents,and the moving groceries arelike the plates moving on top ofthe currents. Have students drawdiagrams showing these paral-lels. Visual-SpatialLSL1

Pulse laser

Outboundlaser light

Outboundlaser light

Returnlaser lightReturn

laser light

LAGEOS Satellite

Orbit

Telescope Telescope

Detector

Self Check1. Describe what occurs at plate boundaries that are

associated with seafloor spreading.

2. Describe three types of plate boundaries where vol-canic eruptions can occur.

3. Explain how convection currents are related to platetectonics.

4. Think Critically Using Figure 9 and a world map,determine what natural disasters might occur inIceland. Also determine what disasters might occur inTibet. Explain why some Icelandic disasters are notexpected to occur in Tibet.

SummaryPlate Tectonics

• The theory of plate tectonics states that sections of the seafloor and continents move as plates on a plasticlike layer of the mantle.

Plate Boundaries

• The boundary between two plates movingapart is called a divergent boundary.

• Plates move together at a convergent boundary.

• Transform boundaries occur where two platesslide past one another.

Causes of Plate Tectonics

• Convection currents are thought to cause themovement of Earth’s plates.

Features Caused by Plate Tectonics

• Tension forces cause normal faults, rift valleys,and mid-ocean ridges at divergent boundaries.

• At convergent boundaries, compression forcescause folding, reverse faults, and mountains.

• At transform boundaries, two plates slide pastone another along strike-slip faults.

Figure 16 When using the Satellite Laser Ranging System, scientistson the ground aim laser pulses at a satellite. The pulses reflect off thesatellite and are used to determine a precise location on the ground.


5. Predict Plate tectonic activity causes many eventsthat can be dangerous to humans. One of theseevents is a seismic sea wave, or tsunami. Learn howscientists predict the arrival time of a tsunami in acoastal area.

6. Use a Word Processor Write three separate descrip-tions of the three basic types of plate boundaries—divergent boundaries, convergent boundaries, andtransform boundaries. Then draw a sketch of an example of each boundary next to your description.

Current Data Satellite LaserRanging System data show thatHawaii is moving toward Japanat a rate of about 8.3 cm peryear. Maryland is moving awayfrom England at a rate of 1.7 cmper year. Using such methods,scientists have observed thatthe plates move at rates rangingfrom about 1 cm to 12 cm peryear.

blue.msscience.com/self_check_quiz

1. Plates move apart; rift-type volcaniceruptions.

2. Divergent; ocean–continent conver-gent; ocean–ocean convergent

3. Their movement in the mantlecauses the movement of platesabove them.

4. Iceland: volcanoes; Tibet: earth-quakes; volcanoes are unlikely inTibet because two continentalplates converge there, with little orno subduction.

5. Answers should include the ideathat scientists monitor underseaearthquakes, then estimate the

potential arrival times of tsunamisin coastal areas.

6. Descriptions and sketches shouldmatch divergent boundaries, con-vergent boundaries, and transformboundaries as shown in Figure 10and Figure 11.

Oral Copy portions of Figure 10showing different plates and theboundaries that they form. Havestudents identify the boundariesand the typical landforms formedby each type of interaction. UsePerformance Assessment inthe Science Classroom p. 89.


Documents

Chapter 7: Plate Tectonicss3.amazonaws.com/zanran_storage/nlcs.k12.oh.us/ContentPages/... · Do this lab to learn about clues that can be used to reassemble a supercontinent. 1. Collect