Plate Tectonics A Unifying Theory. Early Theory Contraction Theory – Young earth was made up of...
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![Page 1: Plate Tectonics A Unifying Theory. Early Theory Contraction Theory – Young earth was made up of molten rock – Contracted (got smaller) as it cooled forming.](https://reader036.fdocuments.us/reader036/viewer/2022062423/5697c0261a28abf838cd59fd/html5/thumbnails/1.jpg)
Plate Tectonics
A Unifying Theory
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Early Theory• Contraction Theory–Young earth was made up of molten rock –Contracted (got smaller) as it cooled
forming mountain ranges.
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Early Theory
• Realized that the earth was heated by the breakdown of radioactive elements– Earth might be getting
warmer not cooler and expanding
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Alfred Wegener
• Wasn’t buying it!• In 1910, Started
lecturing and writing about his new hypothesis
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The Continental Drift Hypothesis
• Continents were all one supercontinent called Pangaea.–Pangaea fragmented
and pieces drifted to the current location of the continents
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Problems with Continental Drift• Scientist could not explain what caused
continents to move• Didn’t believe the evidence was conclusive
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Evidence for Continental Drift
• Matching Coastlines– South America and
Africa• Matching Geology– Similar age of rocks
on the coast– Similar geologic
features
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Evidence for Continental Drift• Fossil Evidence–Glossopteris fossils in South Africa, South
America, Antarctica and India
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Evidence for Continental Drift• Glacial Deposits–Grooves from glacial sheet movement
match up
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Evidence for Continental Drift• Paleomagnetism- The
magnetic poles of the earth have changed over time– Igneous rocks magnetize
based the location of the poles while they are cooling. (mineral grains will point to magnetic north)
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The Missing Piece• Paleomagnetism in
the sea floor– Sea floor was made
up of bands of igneous basalt with alternating polarities
– Bands were symmetric across the mid-Atlantic ridge
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Sea Floor Spreading• Rocks on either side of
the ridge were moving away from each other
• New molten rock was filling the gap as the floor spread.
• Older rocks are farther from the ridge and new rocks are closer
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How fast do the plates move?