SES4U Note: Plate Tectonics Tectonics: The study of the origin and arrangement of Earth’s broad structural features (e.g. mountains, folds, faults, continent shapes, earthquakes, etc.) Plate Tectonics Alfred Wegener, 1915: Proposed the idea of continental drift –200 myrs ago, 1.Continental Jigsaw puzzle: –Wegener noticed the similarity between coastlines on opposite sides of South Atlantic
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SES4U
Note: Plate Tectonics
Tectonics: The study of the origin and arrangement of Earth’s broad structural features (e.g. mountains, folds, faults, continent shapes, earthquakes, etc.)
Plate Tectonics
Alfred Wegener, 1915: Proposed the idea of continental drift
–200 myrs ago,
1.Continental Jigsaw puzzle:–Wegener noticed the similarity between coastlines on opposite sides of South Atlantic
2.Fossils:–Identical fossils found in Africa and S. America
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•Identical species lived in currently distant areas
3.Present Day Organisms:
–Ex/ kangaroos (Australia) & marsupial opossums (N. America)
4.Rock Types:–Identical rock types and rock ages …
5.Ancient Climates:–Evidence that massive ice sheets covered much of Southern Hemisphere and India during paleozoic era (300 myrs ago)
Until Wegener’s death in 1930, his publications received only criticism & skepticism.Why the skeptics?
–Wegener: moon’s gravity and the rotation of the Earth provide the force
–Wegener: Continents plow through ocean floor
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Seafloor Spreading•Harry Hess, 1962: Proposed that the ocean floor might be moving too (contrary to Wegener)–Ocean floor moves away from the mid-atlantic ridge like a conveyer belt
1.Ocean floor topography:–Via sonar technology, features in ocean floor were discovered:
2.Seafloor sediments:–More sediment exists further from oceanic ridge
3.Age of Seafloor:–Older rocks are far from ridge, younger rocks close–Oldest oceanic rocks are much younger than the continents --> where did the ancient ocean floor go?•It must have subducted under continents–See fig 17-7, p. 450!!!
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4.Paleomagnetism - the clincher–The study of ancient magnetic fields in the Earth.
Earth’s magnetic field:–Affects compass needles and magnetic metals (e.g. iron)–Protects earth from solar wind.–Ferromagnesiam minerals can act like compass needles
Magnetic Reversals:–Earth’s magnetic field periodically reverses its polarity! (north pole becomes south & vise versa)
–A rock solidifying during reversed polarity will be magnetized with reversed polarity.
–Magnetometer: device used to detect magnetic patterns in rocks on seafloor•Diagram of magnetic reversal pattern on seafloor rocks
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Putting 2 & 2 together•Seafloor spreading provided the answer to Wegener’s how question:–Continents are not plowing through the oceanic crust –
Plate Tectonics: The Theory TodayThe Earth’s crust is split into a dozen major plates + several smaller ones that move in different directions and at different rates.
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Lithosphere: Athenosphere:
Causes of Plate Motion (?)•Driving force: –Warm, buoyant rock rises and cooler, dense rock sinks
How does mantle convection drive plate motion?Three theories:
1. Slab Pull: as cold lithosphere subducts, its leading edge sinks and pulls the entire plate towards subduction zone.
Cold lithosphere sinks and pulls
2. Ridge Push: The new lithosphere at a ridge slides down the slope of the ridge - this pushes the rest of the plate.
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