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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 Today
The 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.