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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.