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Origins of a Theory
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Observations
Hypothesis - a tentative idea or model, made to test its validity
Theory - a scientifically valid and TESTED principle
Law - a principle whose truth can be demonstrated mathematically
The “Jigsaw”
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Alfred Wegener
1915: The Origin of Continents and Oceans
Proposed his hypothesis of continental drift
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Continental drift hypothesis
Supercontinent Pangaea began breaking apart about 200 million years ago
Continents "drifted" to present positions
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Evidence used in support of continental drift hypothesis:
¸Fit of the continents
¸Paleoclimate
¸Rock type and
structural similarities
¸Fossil similarities
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Pangaea, Laurasia and Gondwana 200 million years ago
Paleoclimate evidence:
the “Gondwana sequence”
Paleoclimate evidence:
the “Gondwana sequence”
Paleoclimate evidence: continental glaciers
Tillite
Glacial striations
South Africa was centered over South Pole
Northern landmasses were closer to the equator – large coal deposits
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Paleoclimate evidence:
the “Gondwana sequence”
Fossil evidence - Mesosaurus
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More fossil evidence
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Glossopteris
Cynognathus
Lystrosaurus
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evidence from present-day organisms
marsupials
Kangaroos in Australia
Opossums in South America
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Matching Mountain Ranges in Laurasia
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The great debate
Objections to the continental drift hypothesis
Inability to provide a mechanism – tidal forces?
Wegener suggested that continents plowed through the
ocean crust, much like ice breakers cut through ice
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