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?

Why was Wegener’shypothesis rejected?

Vocabulary◆ continental drift◆ Pangaea

More than 300 years ago, mapmakers produced world maps thataccurately showed the shapes of the continents. Looking at these maps,people noticed that some continents fit together like pieces of a jigsawpuzzle. Few people thought much about this observation until theearly twentieth century. Then, scientists began to look again at the fitof the continents and think about what it might mean.

The Continental PuzzleA German scientist, Alfred Wegener, also noticed thesimilarity between the coastlines on opposite sides ofthe South Atlantic Ocean. As you can see in Figure 1,the shapes of South America and Africa are an approx-imate fit. In 1915, Wegener proposed his radicalhypothesis of continental drift. According toWegener’s hypothesis of continental drift, the conti-nents had once been joined to form a singlesupercontinent. He called this supercontinentPangaea, meaning all land.

Wegener also hypothesized that about 200 millionyears ago Pangaea began breaking into smaller conti-nents. The continents then drifted slowly to theirpresent positions.

South America

Africa

Hypothesis Evidence

a. continental puzzle

b.

c.

d. ?

?

?

ContinentalDrift

Figure 1 A Curious Fit Thismap shows the best fit of SouthAmerica and Africa at a depthof about 900 meters. Areas ofoverlap appear in brown.

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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 Before Its 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

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Section 9.1

Origins of the Idea of Continental DriftScientists rarely come up with ideas entirely ontheir own. They always draw upon the works ofprevious scientists. Even Sir Isaac Newton, thefather of physics, said “If I have seen fartherthan others it is because I have stood on theshoulders of giants.” The hypothesis ofcontinental drift was no exception. Manypeople, including Francis Bacon and mapmakerAntonio Snider-Pellegrini, had commentedupon the fit of continents around the Atlantic

Ocean. Benjamin Franklin, in 1782, proposedsomething similar to continental drift: “Thecrust of the Earth must be a shell floating on afluid interior. . . . Thus the surface of the globewould be capable of being broken anddistorted by the violent movements of thefluids on which it rested.” Frank Taylorproposed the idea in 1908 that the continentshad moved, and he published it in 1910. AlfredWegener’s ideas combined and built upon theresearch of many other geoscientists.

Facts and Figures

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Evidence for Continental DriftWegener presented a variety of evidence to support the hypothesis ofcontinental drift. His evidence included similar fossils, types of rock,and traces of glaciation on widely separated landmasses.

Matching Fossils Fossil evidence for continental driftincludes several fossil organisms found on different landmasses.Wegener reasoned that these organisms could not have crossed the vastoceans presently separating the continents. An example is Mesosaurus,a reptile whose fossil remains are limited to eastern South America andsouthern Africa, as shown in Figure 2. Scientists think that Mesosauruslived in freshwater lakes and shallow bays. It could not have swumacross the vast, salty Atlantic Ocean. Therefore, Wegener argued, SouthAmerica and Africa must have been joined.

The distribution of other types of fossils also supported Wegener’shypothesis. For example, fossils of Glossopteris, a small plant, are foundtoday in South America, southern Africa, and India. Yet the character-istics of Glossopteris seeds make it very unlikely that the seeds couldhave blown or floated long distances across the oceans. Fossils of theland reptile Lystrosaurus show a similar pattern of distribution.

In Wegener’s time, the idea of land bridges was the accepted expla-nation for similar fossils being found on different landmasses.However, if land bridges did exist between South America and Africa,their remnants should still lie below sea level. But no signs of such landbridges have ever been found in the Atlantic Ocean.

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

Figure 2 Fossil Evidence Fossilsof Mesosaurus have been foundon both sides of the SouthAtlantic and nowhere else in theworld. Fossil remains of this andother organisms on the continentsof Africa and South Americaappear to link these landmasses atsome time 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 continents to 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 . . .

Mesosaurus occurs onlyin eastern South America

and southern Africa.

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Rock Types Anyone who has worked a jigsaw puzzle knows thatthe pieces must fit together to form a complete picture. The picture inthe continental drift puzzle is one of matching rock types and moun-tain belts. If the continents were once part of Pangaea, the rocks foundin a particular region on one continent should closely match in ageand type those in adjacent positions on the adjoining continent.

Matching types of rock in several mountain belts that todayare separated by oceans provide evidence for continental drift. Forexample, the Appalachian mountain belt in eastern North Americaends off the coast of Newfoundland, as shown in Figure 3A. Mountainsof the same age with similar rocks and structures are found in theBritish Isles and Scandinavia. When these landmasses are fitted togetheras in Figure 3B, the mountain chains form a nearly continuous belt.

Ancient Climates Wegener found evidence for dramatic globalclimate changes that supported his hypothesis. Wegener foundglacial deposits showing that between 220 million and 300 millionyears ago, ice sheets covered large areas of the SouthernHemisphere. Deposits of glacial till occurred at latitudes that todayhave temperate or even tropical climates: southern Africa, SouthAmerica, India, and Australia. Below these beds of glacial debris layscratched and grooved bedrock carved by the ice. In some locations,the scratches and grooves showed that the ice had moved from whatis now the sea onto land. It is unusual for large continental glaciers tomove from the sea onto land. It is also interesting that much of theland area that shows evidence of this glaciation now lies near theequator in a subtropical or tropical climate.

EuropeGreenlandNorth

America

SouthAmerica

AppalachianMountains

Africa

Scandinavia

CaledonianMountains

BritishIsles

NorthAmerica Africa

SouthAmerica

A B

For: Links on continental drift

Visit: www.SciLinks.org

Web Code: cjn-3091

Figure 3 A The AppalachianMountains run along the easternside of North America and end offthe coast of Newfoundland.Mountains that are similar in ageand structure are found in theBritish Isles and Scandinavia. B When these landmasses wereunited as Pangaea, these ancientmountain chains formed a nearlycontinuous belt.

Matching Mountain Ranges

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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. 250–251about ancient climates as evidence for continental drift. Ask: Why didWegener believe that the existence of glaciers in tropical regions of theSouthern Hemisphere was evidence ofcontinental drift rather than climaticchange? (The Northern Hemisphere was 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 there is 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 FiguresDownload a worksheet on continental drift for students tocomplete, and find additionalteacher support from NSTA SciLinks.

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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 4.

Summarize the climate evidence for continentaldrift.

Figure 4 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 across the 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 peryear, how long did ittake these two locationsto move to their currentpositions?

A

B

Glacier Evidence

Charting the Age of 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 have varied 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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Answer to . . .

Figure 4 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.

Section 9.1 Assessment

Rejection of Wegener’s HypothesisWegener’s hypothesis faced a great deal of criticism from other scien-tists. The main objection to Wegener’s hypothesis was that hecould not describe a mechanism capable of moving the continents.Wegener proposed that the tidal influence of the Moon was strongenough to give the continents a westward motion. However, physicistsquickly responded that tidal friction great enough to move the conti-nents 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, there was no evidence 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 evidence of continental drift.

During the years that followed Wegener’s hypothesis, major stridesin technology enabled scientists to map the ocean floor. Extensive dataon earthquake activity and Earth’s magnetic field also became avail-able. By 1967, these findings led to a new theory, known as platetectonics. The theory of plate tectonics proved that Wegener was cor-rect—the continents move. The theory also provided the frameworkfor understanding many other geologic processes, such as the forma-tion of the mountains shown in Figure 5.

How did Wegener try to explain continental drift?

Figure 5 Today, scientists knowthat plate movements helped topush up mountain ranges such asthe Canadian Rockies in BanffNational Park, Alberta, Canada.

Reviewing Concepts1. What is the hypothesis of continental

drift?

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

3. Why did scientists reject Wegener’scontinental drift hypothesis?

4. What was Pangaea?

Critical Thinking5. Applying Concepts How does the

occurrence of the same plant fossils in SouthAmerica and Africa 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.

252 Chapter 9

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Rejecting a HypothesisBuild 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

ASSESSEvaluate UnderstandingTo 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 evidence for continental drift.

Pangaea was a supercontinent made up 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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3

L2

Section 9.1 Assessment 5. 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 tropical latitudes, 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.

1. a hypothesis that proposes that the continents where once joined to form one 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

Answer to . . .

Wegener proposed thattidal forces due to the

moon might give the continents awestward motion and that thecontinents somehow pushed their wayacross the ocean floor.

Plate Tectonics 253

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

A

B

C

D

E

Breakup of Pangaea

Figure 6 Pangaea broke up gradually over aperiod of 200 million years.

For: Continental Drift activityVisit: PHSchool.comWeb Code: czp-3091

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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For: Continental Drift activityVisit: PHSchool.comWeb Code: czp-3091Students interact with mapsshowing continental drift online.