Plate Tectonics A Unifying Theory. Early Theory Contraction Theory – Young earth was made up of...
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Transcript of Plate Tectonics A Unifying Theory. Early Theory Contraction Theory – Young earth was made up of...
Plate Tectonics
A Unifying Theory
Early Theory• Contraction Theory–Young earth was made up of molten rock –Contracted (got smaller) as it cooled
forming mountain ranges.
Early Theory
• Realized that the earth was heated by the breakdown of radioactive elements– Earth might be getting
warmer not cooler and expanding
Alfred Wegener
• Wasn’t buying it!• In 1910, Started
lecturing and writing about his new hypothesis
The Continental Drift Hypothesis
• Continents were all one supercontinent called Pangaea.–Pangaea fragmented
and pieces drifted to the current location of the continents
Problems with Continental Drift• Scientist could not explain what caused
continents to move• Didn’t believe the evidence was conclusive
Evidence for Continental Drift
• Matching Coastlines– South America and
Africa• Matching Geology– Similar age of rocks
on the coast– Similar geologic
features
Evidence for Continental Drift• Fossil Evidence–Glossopteris fossils in South Africa, South
America, Antarctica and India
Evidence for Continental Drift• Glacial Deposits–Grooves from glacial sheet movement
match up
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)
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
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
How fast do the plates move?