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Chapter 7: Changing Continents and Climates
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19th Century Geology
• Law of Superposition – 19th century geologists assumed that strata lower in the stratigraphic exposure were older than the overlying strata
• Law of Uniformitarianism – assumes that processes that weathered, eroded, and deposited rock in the past were the same as those operating today
• These laws used to develop geologic time scale
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Alfred Wegener
• 1912 Theory of Horizontal Displacement of the Continents – similarities between Atlantic coastlines
• Theory of Continental Drift (1912-1929)– Crust of continents composed of relatively
light rocks. Ocean floors largely composed of dense basalts. Lighter continental rocks could float on viscous mantle of Earth and override oceanic crust
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– Permian glacial deposits in Brazil, South Africa, India, and Australia – indicates that tropical areas once glaciated, so they must have been close to the poles at one time
– Similarities in the shapes of continents, their geology, and fossils suggest closer proximity in the past
– Geodetic measurements of latitude and longitude of Greenland indicated westward movement of the landmass
– Continental Drift led to Plate Tectonics
Alfred Wegener
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• Pangaea
• Gondwanaland – Africa, South America, Australia, Antarctica, and India
• Laurasia – North America, Europe, and Asia
• Pangaea formed 195 mya – began to break up 180 mya with formation of Atlantic Midoceanic Ridge
Alfred Wegener
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Figure 7.3
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Vindication – Alfred Wegener
• Lystrosaurus – Triassic reptile –fossils found in Antarctica, Africa, and India – Antarctica must have been warmer in the past
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• Galaxioidea – freshwater fish family – found in southern South America, South Africa, Australia, and New Zealand – must have been connected in the past
• WWII – sea floor mapping by submarines
Vindication – Alfred Wegener
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• Paleomagnetism – ancient igneous rocks are
little compasses • iron or titanium point to their magnetic north• compare this to current magnetic north to
determine where continents used to lie
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Figure 7.6
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Glaciation• Radiocarbon dating – based on measuring
the amount of the radioactive isotope carbon 14 found in plant and animal remains
• Greenland ice cores extend 100,000 years• Antarctic ice cores extend 400,000 years• Ice cores provide record of atmospheric
chemistry• 18O isotopes deposited during warmer
times – 16O isotopes deposited during cooler times
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Figure 7.7
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Figure 7.9
Relationship between atmospheric temperature and 18O isotopes