248 Chapter 9

9.1 Continental Drift

Reading StrategySummarizing Copy the table. Fill it in asyou read to summarize the evidence ofcontinental drift.

Key ConceptsWhat is the hypothesis ofcontinental drift?

What evidence supportedcontinental drift?

Vocabulary◆ continental drift◆ Pangaea

Will California eventually slide into the ocean? Have continentsreally drifted apart over the centuries? Early in the twentieth century,most geologists thought that the positions of the ocean basins and con-tinents were fixed. During the last few decades, however, new data havedramatically changed our understanding of how Earth works.

An Idea Before Its TimeThe idea that continents fit together like pieces ofa jigsaw puzzle came about when better worldmaps became available. Figure 1 shows the twomost obvious pieces of this jigsaw puzzle. However,little significance was given this idea until 1915,when Alfred Wegener, a German scientist, pro-posed his radical hypothesis of continental drift.

Wegener’s continental drift hypothesis statedthat the continents had once been joined to forma single supercontinent. He called this supercon-tinent Pangaea, meaning all land.

Wegener also hypothesized that about 200 mil-lion years ago Pangaea began breaking into smallercontinents. These continents then drifted to theirpresent positions, as shown on page 250. Wegenerand others collected much evidence to supportthese claims. Let’s examine their evidence.

South America

Africa

Hypothesis Evidence

a. continental puzzle

b.

c.

d. ?

?

?

ContinentalDrift

Figure 1 A Curious Fit This map shows the best fit of SouthAmerica and Africa at a depth of about 900 meters. The areaswhere continents overlap appearin brown. Inferring Why are there areas ofoverlap?

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FOCUS

Section Objectives9.1 Describe the hypothesis of

continental drift.9.2 Evaluate the evidence in

support of continental drift.9.3 Identify the main objections

to Wegener’s hypothesis ofcontinental drift.

Build VocabularyWord Forms Before students read thissection, ask them to write a sentence ortwo describing the meaning of the worddrift. Then have them write a predictionfor what they think continental driftmeans. After students read the section,have them examine their predictions anddiscuss whether their predictions mustbe changed.

Reading Strategya. continental puzzleb. matching fossilsc. matching rocks and structuresd. ancient climates

INSTRUCT

An Idea BeforeIts TimeUse VisualsFigure 1 Point out the small areas ofbrown and light blue between Africaand South America. Ask: What couldcause the brown-shaded regions ofoverlap? (accumulation of sedimentsdeposited by rivers and stretching of theplates) What do you think the lightblue areas represent? (the continentalshelf)Visual

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Reading Focus

1

Section 9.1

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Evidence: The Continental Puzzle Wegener first thoughtthat the continents might have been joined when he noticed the sim-ilarity between the coastlines on opposite sides of the South AtlanticOcean. He used present-day shorelines to show how the continents fittogether. However, his opponents correctly argued that erosion con-tinually changes shorelines over time.

Evidence: Matching Fossils Fossil evidence for conti-nental drift includes several fossil organisms found on differentlandmasses. Wegener reasoned that these organisms could not havecrossed the vast oceans presently separating the continents. An exam-ple is Mesosaurus, an aquatic reptile whose fossil remains are limitedto eastern South America and southern Africa, as shown in Figure 2.If Mesosaurus had been able to swim well enough to cross the vastSouth Atlantic Ocean, its fossils should be more widely distributed.This is not the case. Therefore, Wegener argued, South America andAfrica must have been joined somehow.

The idea of land bridges was once the most widely accepted expla-nation for similar fossils being found on different landmasses. Mostscientists believed that during a recent glacial period, the lowering ofsea level allowed animals to cross the narrow Bering Strait betweenAsia and North America. However, if land bridges did exist betweenSouth America and Africa, their remnants should still lie below sealevel. But no signs of such land bridges have ever been found in theAtlantic Ocean.

How does the distribution of Mesosaurus fossilsprovide evidence for continental drift?

Figure 2 Location ofMesosaurus Fossils ofMesosaurus have been found onboth sides of the South Atlanticand nowhere else in the world.Fossil remains of this and otherorganisms on the continents ofAfrica and South America appearto link these landmasses at sometime in Earth’s history.

South America

Africa

Q If all the continents were oncejoined as Pangaea, what did therest of Earth look like?

A When all the continentswere together, there must alsohave been one huge ocean sur-rounding them. This ocean iscalled Panthalassa (pan = all,thalassa = sea). Today all thatremains of Panthalassa is thePacific Ocean, which has beendecreasing in size since thebreakup of Pangaea.

Evidence:Matching FossilsPurpose Students compare twogroups of fossils from two continentsto identify those fossils that are commonto both continents.

Materials 2 groups of photographs orsamples of fossils, including at least onetype of fossil found in both groups

Procedure Have students examine thetwo groups of fossils. Tell them that thetwo groups were found on differentcontinents. Ask them to identify anyfossils that were found on bothcontinents. Have students infer theimplications of this observation.

Expected Outcome Students shouldinfer that the two continents had to beconnected at some point in the pastwhen the organism in the fossil lived.Visual, Logical

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Customize for Inclusion Students

Visually Impaired Puzzle pieces of continentscan be made out of sandpaper by gluing amap onto the back of a piece of sandpaperand cutting out the continents. This learningtool can be used by both visually impaired

students and students who learn tactilely.Remind students who use these pieces that thepiece must be held with the rough side downfor correct geographical orientation of thecontinent.

Answer to . . .

Figure 1 Areas where there are riversor streams have deposited largeamounts of sediments.

Mesosaurus occurs onlyin eastern South America

and southern Africa.

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NorthAmerica

AfricaSouthAmerica

AntarcticaAustralia

India

Europe

250 Million Years AgoPangaea consisted of all themajor continents.

200 Million Years Ago Therifting that eventually resulted inthe Atlantic Ocean occurred overan extended period of time. Thefirst rift developed betweenNorth America and Africa.

100 Million Years AgoContinued rifting of the southernlandmasses sent India on anorthward journey.

Asia

TethysSea

PA

NG

AE A

G ON

DW

A

NA L A N D

L A U R A S I A

NorthAmerican

plate

Africanplate

SouthAmerican

plate

Antarctic plate

Pacificplate

Eurasianplate50 Million Years Ago

Australia began to separatefrom Antarctica.

Present A modern map showsthat India has collided withAsia, creating the Himalayas.

Australian-Indianplate

NorthAmerica

Africa

SouthAmerica

Antarctica

Australia

India

Europe Asia

250 Chapter 9

Figure 3 Pangaea broke up gradually over aperiod of 200 million years.

A

B

C

D

E

Breakup of Pangaea

250 Chapter 9

Use VisualsFigure 3 Have students study the mapsshowing the breakup of Pangaea. Ask:In the breakup of Pangaea, whatcontinents appear to have separatedfirst? (North America and Africa) Whatocean began to form when NorthAmerica and Africa separated? (AtlanticOcean) How was India formed? (Indiabroke away from Gondwanaland. It movednorth and eventually collided with Asia.)Visual, Logical

Some students may think that thecontinents have remained in approx-imately the same positions since thebreakup of Pangaea. Make transparenciesof the five parts of Figure 3. Superimposethe transparencies two at a time to showstudents the changes. Ask students tocome up to the projection to point outchanges in the location of continentsfrom one transparency to another.Ask which continent has moved thefarthest. (Asia)Visual, Logical

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Section 9.1 (continued)

Recently, a unique species of purple frogthat lives underground was discovered insouthwestern India. DNA analysis showed thatthe frog was related to a group of frogs thatlive only in the Seychelles Islands off the

eastern coast of Africa and almost 3000 kmacross the Indian Ocean from India. Biologiststhink that the two frog populations areadditional evidence for continental drift.

Facts and Figures

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Evidence: Rock Types and Structures Anyone who hasworked a jigsaw puzzle knows that the pieces must fit together to forma clear picture. The clear picture in the continental drift puzzle is oneof matching rock types and mountain belts. If the continents existedas Pangaea, the rocks found in a particular region on one continentshould closely match in age and type those in adjacent positions onthe adjoining continent.

Rock evidence for continental drift exists in the form of sev-eral mountain belts that end at one coastline, only to reappear on alandmass across the ocean. For example, the Appalachian mountainbelt runs northeastward through the eastern United States, ending offthe coast of Newfoundland, as shown in Figure 4A. Mountains of thesame age with similar rocks and structures are found in the BritishIsles and Scandinavia. When these landmasses are fit together as inFigure 4B, the mountain chains form a nearly continuous belt.

Evidence: Ancient Climates Wegener was a meteorologist,so he was interested in obtaining data about ancient climates to sup-port continental drift. And he did find evidence for dramatic globalclimate changes. Wegener found glacial deposits showing thatbetween 220 million and 300 million years ago, ice sheets coveredlarge areas of the Southern Hemisphere. Layers of glacial till werefound in southern Africa and South America, as well as in India andAustralia. Below these beds of glacial debris lay scratched and groovedbedrock carved by the ice. In some locations, the scratches andgrooves showed that the ice had moved from what is now the sea ontoland. It is unusual for large continental glaciers to move from the sea

How does the location of mountain chains provideevidence of continental drift?

Figure 4 A The AppalachianMountains run along the easternside of North America anddisappear off the coast ofNewfoundland. Mountains thatare similar in age and structureare found in the British Isles andScandinavia. B When theselandmasses are united as Pangaea,these ancient mountain chainsform a nearly continuous belt.

EuropeGreenlandNorth

America

SouthAmerica

AppalachianMountains

Africa

Scandinavia

CaledonianMountains

BritishIsles

NorthAmerica Africa

SouthAmerica

A B

Matching Mountain Ranges

For: Links on continental drift

Visit: www.SciLinks.org

Web Code: cjn-3091

Build Science SkillsUsing Models Havestudents use a child’sjigsaw puzzle withseveral large pieces todemonstrate matching rock types andmountain belts as follows. Studentsshould put the puzzle together on apiece of cardboard. After putting ittogether, the puzzle should be coveredwith another piece of cardboard andflipped over. On the back of the puzzle,students should draw lines representinga mountain belt that extends acrossseveral puzzle pieces. Students willunderstand when the puzzle is puttogether how mountain chains formcontinuous belts across land masses.Kinesthetic, Visual

Build Reading LiteracyRefer to p. 246D which provides theguidelines for relating cause and effect.

Relate Cause and Effect Have stu-dents read the section on pp. 251–252about ancient climates as evidencefor continental drift. Ask: Why didWegener believe that the existenceof glaciers in tropical regions of theSouthern Hemisphere was evidence ofcontinental drift rather than climaticchange? (The Northern Hemispherewas once tropical, as evidenced by coaldeposits that were formed from tropicalplants. If the Northern Hemisphere hadonce been closer to the equator, theSouthern Hemisphere probably had alsobeen further south, closer to the SouthPole. It was not likely that such a largechange in climate could have taken placewithout continental drift.)Logical

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Scientists think that 200 million years ago,what is now Pennsylvania was located farthersouth, near the equator. Fossils from coal fieldsin Pennsylvania show that the plants fromwhich the coal formed had large leaf-likestructures that are typical of tropical plants.The trunks of the plants had no growth rings,

also typical of tropical plants because thereis little seasonal temperature fluctuation toproduce the rings. Scientists believe that thesefossils are evidence that Pennsylvania once hada tropical climate and was located closer to theequator.

Facts and Figures

Answer to . . .

If mountain chains canbe continued across

present-day oceans, they provideevidence that the areas were onceconnected.

Download a worksheet oncontinental drift for students tocomplete, and find additionalteacher support from NSTA SciLinks.

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onto land. It is also interesting that much of the land area that showsevidence of this glaciation now lies near the equator in a subtropicalor tropical climate.

Could Earth have been cold enough to allow the formation of con-tinental glaciers in what is now a tropical region? Wegener rejected thisidea because, during this same time period, large tropical swampsexisted in the Northern Hemisphere. The lush vegetation of theseswamps eventually became the major coal fields of the eastern UnitedStates, Europe, and Siberia.

Wegener thought there was a better explanation for the ancient cli-mate evidence he observed. Thinking of the landmasses as asupercontinent, with South Africa centered over the South Pole, wouldcreate the conditions necessary to form large areas of glacial ice overmuch of the Southern Hemisphere. The supercontinent idea wouldalso place the northern landmasses nearer the tropics and account fortheir vast coal deposits, as shown in Figure 5.

Summarize the climate evidence for continentaldrift.

Figure 5 A The area of Pangaeacovered by glacial ice 300 millionyears ago. B The continents asthey are today. The white areasindicate where evidence of theold ice sheets exists. Interpreting Diagrams Wherewere the continents locatedwhen the glaciers formed?

Equator

NorthAmerica

Eurasia

AfricaSouthAmerica

India

Ice mass

Antarctica

Tethys Sea

Equator

Australia

Charting theAge of theAtlantic Ocean

Procedure1. The distance between

two locations acrossthe Atlantic Ocean,one in South Americaand one in Africa, isapproximately 4300 km.

2. Assume that these twolocations were oncejoined as part ofPangaea.

Analyze andConclude1. Calculating If the two

landmasses movedaway from each otherat a rate of 3.3 cm/y,how long did it takethese two locations tomove to their currentpositions?

2. Inferring Do youthink the Atlantic Oceanwould have formed at aconstant rate or wouldthat rate have variedover time? Why?

A

B

Glacier Evidence

Charting the Ageof the Atlantic Ocean

ObjectiveAfter completing this activity, studentswill be able to calculate the length oftime it takes two land masses toseparate, given the rate of spreading.

Skills Focus Calculating, Inferring

Prep Time none

Class Time 10 minutes

Teaching Tips You might want toreview conversion factors with students.

Expected Outcome The two continentstook more than 130 million years toseparate.

Analyze and Conclude1. 130.3 million years2. The rate would probably havevaried over time because the drivingmechanism was most likely not uniform.Few Earth processes are uniform overtime.Logical

For Enrichment

Have students research the followingquestion: Pangaea began to break upand South America and Africa began toseparate 200 million years ago. Whattypes of living organisms were found onEarth when the two continents reachedtheir current positions?

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Section 9.1 (continued)

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Section 9.1 Assessment

Rejecting a HypothesisWegener’s drift hypothesis faced a great deal of criticism from otherscientists. One objection was that Wegener could not describe a mech-anism that was capable of moving the continents across the globe.Wegener proposed that the tidal influence of the Moon was strongenough to give the continents a westward motion. However, physicistsquickly responded that tidal friction of the size needed to move thecontinents would stop Earth’s rotation.

Wegener also proposed that the larger and sturdier continentsbroke through the oceanic crust, much like ice breakers cut throughice. However, no evidence existed to suggest that the ocean floor wasweak enough to permit passage of the continents without the oceanfloors being broken and deformed in the process.

Most scientists in Wegener’s day rejected his hypothesis. However,a few geologists continued to search for additional evidence of conti-nents in motion.

A New Theory Emerges During the years that followedWegener’s hypothesis, major strides in technology enabled scientiststo map the ocean floor. Extensive data on earthquake activity andEarth’s magnetic field also became available. By 1968, these findingsled to a new theory, known as plate tectonics. This theory provides theframework for understanding most geologic processes, such as the for-mation of the mountains shown in Figure 6.

Why was Wegener’s hypothesis rejected?

Q Some day will the continentscome back together and form asingle landmass?

A Yes, but not anytime soon.Based on current plate motions,it appears that the continentsmay meet up again in thePacific Ocean—in about300 million years.

Figure 6 Mountain ranges arecommonly formed at plateboundaries. This photographshows part of the CanadianRockies in Banff National Park,Alberta, Canada.

Reviewing Concepts1. What is the hypothesis of continental

drift?

2. List the evidence that supported thehypothesis of continental drift.

3. What was one of the main objections toWegener’s continental drift hypothesis?

4. What is Pangaea?

Critical Thinking5. Applying Concepts Would the occurrence

of the same plant fossils in South America andAfrica support continental drift? Explain.

6. Drawing Conclusions How did Wegenerexplain the existence of glaciers in the southernlandmasses, and the lush tropical swamps inNorth America, Europe, and Siberia?

Descriptive Paragraph Write a para-graph describing Pangaea. Include thelocation and climate of Pangaea. Use theequator as your reference for position.

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5. Yes, a land plant most likely could not travelacross a large ocean such as the Atlantic. Ifthe plant is found in both Africa and SouthAmerica, those areas had to have been joinedwhen the plant was growing.6. It is difficult to imagine that Earth hadcooled enough to form glaciers in tropicallatitudes, so in order to explain the glaciers,those areas had to have been closer to thepoles than in the present day. Also, the glacialgrooves indicate the ice was coming from anarea that at present is ocean. Large continentalglaciers form only on land, so that area hadto be land.

Section 9.1 Assessment

1. a hypothesis that proposes that thecontinents where once joined to formone supercontinent2. matching continental outlines, matchingfossils, matching rocks and structures, ancientclimates3. He could not provide a mechanism toexplain the movement of the continents.4. the supercontinent proposed by Wegener’shypothesis of continental drift

Rejecting aHypothesisBuild Science SkillsUsing Tables and Graphs Havestudents make a table listing the reasonswhy Wegener’s hypothesis was criticizedby some people and accepted by others.Intrapersonal, Verbal

ASSESSEvaluateUnderstandingTo assess students’ knowledge of sectioncontent, have them write two or threesentences describing each of the fourlines of evidence for Wegener’scontinental drift hypothesis.

ReteachHave students explain in their ownwords why Figure 2 shows evidencefor continental drift.

Pangaea was a supercontinent madeup of all the major continents joinedtogether. It began breaking into smallercontinents about 200 million years ago.Pangaea was located near the SouthPole. The southern part of Pangaea,made up of South America, Africa, India,Australia, and Antarctica, had a coldclimate with large continental glaciers.

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L2

3

L2

Answer to . . .

Figure 5 The continents were nearthe South Pole when the glaciersformed.

Glaciers in southernSouth America, southern

Africa, India, and Australia are found inareas that now have tropical climates.There is also evidence for tropicalclimates and coal swamps in areas thatare now at higher latitudes, such asnorthern Europe and the northeasternUnited States.

He could not provide amechanism for the

movement of the continents.

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