evidence for plate tectonics as it turns out, alfred was right
TRANSCRIPT
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Evidence for Plate Tectonics
As it turns out, Alfred was right.
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1. Alfred Wegener’s Evidence for Continental Drift
• The puzzle-like fit of some of the continents (such as those along the Atlantic Ocean)
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1. Alfred Wegener’s Evidence for Continental Drift
• The puzzle-like fit of some of the continents (such as those along the Atlantic Ocean)
• Mountain ranges that match up when continents are fit together
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1. Alfred Wegener’s Evidence for Continental Drift
• The puzzle-like fit of some of the continents (such as those along the Atlantic Ocean)
• Mountain ranges that match up when continents are fit together
• Evidence of glaciers in areas that can’t have glaciers today (glaciers in Australia?)
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More evidence for Continental Drift:
• Fossils of identical animals (ones that can’t swim or fly) in far away places
• Coal, deserts and salt deposits in places they couldn’t form today
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Problems with the idea?
• How do you get the solid rock of the continents to “drift” through the solid rock of the ocean floors?
• What mechanism can possibly move so much rock?
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N. America Europe
Ocean
2. Harry Hess and the Sea Floor
• Mid-Ocean Ridges—People expected the ocean floor to be shaped like a bowl.
Instead, Hess found tall mountain ranges in the middle of the ocean.
North America
Europe
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• Hess also noticed flat-topped mountains below the surface (“guyots”)
Hess reasoned that the flat-topped mountains could be explained if they had been flattened by wave erosion at the surface, then dropped below sea level at a later time.
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Topography of the Atlantic Ocean FloorSeamounts and Guyots
Mid-Atlantic Ridge
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Sea Floor Spreading• Hess speculated that the features seen on the
ocean floor could be explained if the sea floor was spreading away from mid-ocean ridges, with new ocean floor being made at the ridge.
Hot, upwelling mantle material raises the surface and fills in the gaps as spreading occurs
8:40-13:17
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3. Evidence from Paleomagnetism (“fossil magnetism”)
• Magnetic minerals pointing in weird directions in continental rocks—they should be magnetized to the north, but some pointed in other directions. “Polar Wandering”?
• Magnetism within rocks of different ages on the same continent were not consistent.
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The “Last Nail”: Mirror-image magnetic stripes along mid-ocean ridges!
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From Continent to Rift to Ocean: Animations 2.3, 2.5
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4. The oldest sea floor is next to the continents; the youngest is along the mid-ocean ridges.
The oldest sea floor rocks found are only about 200 MY old.
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But if new ocean floor is being made at the ridges, and the Earth isn’t getting any bigger, where is
the old ocean floor going?
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The Tonga Trench
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5. Benioff Zones: Hugo Benioff noticed that earthquake foci near ocean trenches plot along a descending line. Earthquakes can only happen in brittle rocks.
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• Mantle rocks at these depths are “plastic”, not brittle, so earthquakes cannot occur in them.
• The earthquakes are due to oceanic lithosphere diving down (subducting) at the trench, thus solving the ocean crust problem.
13:18-19:55
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6. Worldwide distribution of earthquake epicenters
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• Earthquake epicenters “frame” the plates. Earthquakes are much less common inside the plate boundaries.
19:56-22:23
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7. Hot Spots (Mantle Plumes)• Certain places on Earth
stood out as exceptions to the rule
• ex. Hawaii—constant volcanic and seismic activity, but not at a plate boundary
• But why? Instead of ruining the new theory, The answer provided great confirmation of plate movement
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• The Hawaiian hot spot records the movement of the Pacific plate as a linear series of islands and seamounts.
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Development of Island Chains above Mantle Plumes--Animation
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