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Continental drift


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Continental drift

  1. 1. • In 1910, Alfred Wegenerdeveloped the hypothesisthat all the continents hadbeen joined together in a singleland mass and have drifted apart.
  2. 2. • This single land mass was calledPangaea (latin for “all lands”)
  3. 3. • The idea that the continents areslowly drifting apart is known ascontinental drift.•He published his evidence in a bookcalled Origin of Continents andOceans in 1915.
  4. 4. Evidence From Landforms• Wegener pieced together maps thatshowed mountain ranges thatmatched up in South Africa andSouth America.•European coal mines matched upwith coal mines in North America.
  5. 5. Evidence From Fossils• A fossil is a trace of an organism that hasbeen preserved in rock.• Fossils for Mesosaurusand Lystrosaurus werefound in places now separated by oceans. • Neither reptile could have swum long distances across salt water.
  6. 6. 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 been carried by the wind and are too fragile to survive an ocean trip.
  7. 7. 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.
  8. 8. Evidence From Climate• Spitsbergen is an island in the Artic Ocean north ofNorway.• The island is ice-covered with harsh polarclimates.• Fossils of tropical plants were found on this island.When these plants lived 300 million years ago, theisland must have had a warm and mild climate.•Wegener concluded that the island must have beenlocated near the equator.
  9. 9. Evidence From Climate• Geologists also found evidence that at the sametime it was warmer in Spitsbergen, it was muchcolder in South Africa.• There is evidence that continental glaciers oncecovered South Africa. •Continental Glaciers are thick layers of ice that covered hundreds of thousands of square kilometers.•The climate of South Africa today is too mild tohave sustained continental glaciers.
  10. 10. 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.
  11. 11. Scientists Reject Wegener’s Hypothesis • What forced or pulled the continents apart? • Geologists would have to reject their own theories of how mountains form. • Geologists in the early 1900 thought that the Earth was cooling and shrinking.
  12. 12. Scientists Reject Wegener’s Hypothesis• According to this idea, the crust wouldwrinkle all over the Earth.• Mountains are formed in narrow bandsalong the edges of the continents.• Wegener’s theory was rejected until inthe 1950’s when new evidence about theEarth’s structure caused scientists toreconsider the hypothesis.