In 1910, Alfred Wegenerdeveloped the hypothesisthat all the continents hadbeen joined together in a singleland mass and have drifted apart.

This single land mass was calledPangaea (latin for “all lands”)

The idea that the continents are slowlydrifting apart is known as continentaldrift.

He published his evidence in abook called Origin ofContinents and Oceans in 1915.

EvidenceFrom

Landforms• Wegener piecedtogether maps thatshowed mountainranges thatmatched up inSouth Africa andSouth America.

European coal mines matched upwith coal mines in North America.

Original Pangaean Coal Deposits in the "Gray Band"

•A fossil is a trace of an organismthat has been preserved in rock.

• Fossils for Mesosaurus andLystrosaurus were found in placesnow separated by oceans.

Evidence From Fossils

Neither reptile could have swum longdistances across salt water.

Evidence From Fossils• Another example wasGlossopteris which is a fernlikeplant that lived 250 millionyears ago. It was found in rocksin Africa, Australia, India and Antarctica.

• The seeds of this plant could not have beencarried by the wind and are too fragile tosurvive an ocean trip.

Evidence From Fossils

• Wegener concluded that fossils of thereptiles separated by an ocean that theycould not have swum and the widespreadoccurrence of the fern’s fossils supportedhis hypothesis of continental drift.

Evidence From Climate• Spitsbergen is an island in theArtic Ocean north of Norway.

• The island is ice-covered withharsh polar climates.

• Fossils of tropical plants werefound on this island. When theseplants lived 300 million yearsago, the island must have had awarm and mild climate.

•Wegener concluded that theisland must have been locatednear the equator.

Evidence From Climate• Geologists also found evidencethat at the same time it was warmerin Spitsbergen, it was much colderin South Africa.

• There is evidence that continentalglaciers once covered South Africa.

•Continental Glaciers are thicklayers of ice that coveredhundreds of thousands ofsquare kilometers.

•The climate of South Africa todayis too mild to have sustainedcontinental glaciers.

Explanations• Wegener also attempted to explain howthe drift took place and offer a newexplanation for how mountains form.

• He thought that when the driftingcontinents collide, their edges fold orcrumble.

•The folding continents slowly push uphuge chunks of rock to form mountains.

Scientists Reject Wegener’s Hypothesis

What forced or pulled the continentsapart?

Wegener thought there were centrifugaland tidal forces moving the continents, buthad no mechanism to prove it. Otherscientists calculated that these forces couldnever be strong enough to move landmasses.

Geologists would have to reject their own theories of howmountains form.

The fact that his work crossed disciplines exposed him to theterritoriality of those disciplines. The authorities in the various areasattacked him as an interloper that did not fully grasp their ownsubject. More importantly however, was that even the possibility ofContinental Drift was a huge threat to the established authorities ineach of the disciplines.

One of Alfred Wegener's critics, the geologist R. ThomasChamberlain, could not have summarized this threat any better :

"If we are to believe in Wegener's hypothesis we must forgeteverything which has been learned in the past 70 years and start allover again."

Scientists Reject Wegener’s Hypothesis

Geologists in the early 1900 thought that theEarth was cooling and shrinking.

Scientists Reject Wegener’s Hypothesis

It suggested that the Earth had been in a molten state, andfeatures such as mountains formed as it cooled andshrank. As the interior of the Earth cooled and shrank, therigid crust would have to shrink and crumple. Thecrumpling could produce features such as mountainranges.True geologists were unwilling to believe that the evidencealready collected about mountain formation could beexplained away by an unqualified scientist (Wegener was atrained astronomer and meteorologist, not geologist).

Scientists Reject Wegener’s Hypothesis• According to this idea, Wegener said thecrust would wrinkle all over the Earth.

• Wegener noted that mountains are formedin narrow bands along the edges of thecontinents.

• Wegener’s theory was rejected until the1950s when new evidence about the Earth’sstructure caused scientists to reconsider thehypothesis.