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CH 9 Plate tectonics • 1915 – Alfred Wegener proposed his • • • • continental drift hypothesis 200 mya one large super-continent Pangaea (all lands) began to break up Panthalassa – all oceans Not widely accepted b/c he couldn’t explain why it occurred Dies in 1930 trying to figure it out Evidence • Jig-saw puzzle pieces fit • SA and Africa • Fossil evidence • Mesosaurus and Glossopteris • Climatic evidence • Glaciers and coal deposits • Rock evidence • Age, type, alignment of mts Plate Tectonics • Move slowly about 5 cm or 2.5” per year • Plate movement causes EQs, volcanoes, mts • Lithosphere – plates • outer and rigid • Crust and upper mantle • Moves over the asthenosphere • Asthenosphere • below and plastic like • Lower mantle Boundary Types 1. Transform • • • • Plates slide past one another Stress involved is shearing Forms Eqs Ex: San Andreas Fault 2. Divergent • • • • Plates move away from each other Stress involved is tension Forms mid ocean ridges and rift zones Ex: Mid-Atlantic Ridge and Iceland 3. Convergent • Plates move towards each other • Stress involved is compression • 3 different results! A) oceanic-continental crusts meet • Subduction (denser plate [oceanic] dives below) • Ocean trench forms offshore • Continental volcanic arcs • Ex: Cascades, Andes B) Oceanic-oceanic crusts meet • Subduction occurs and one plate dives down • Ocean trench forms • Volcanic island arcs • Ex: Japan, Aleutians C) Continental-continental crusts meet • • • • No subduction Collision boundary Mountains form Ex: Appalachians, Himalayas • Pie crust meets hand tossed) • Thinner and more sweet • Holds up better to washings • Able to leap tall buildings in a single bound. • Makes Jessica Alba look like a ho. Plate Tectonic Evidence 1. Paleomagnetism • Polar wandering & Polar reversals 2. Earthquake patterns • Earthquakes occur @ plate boundaries 3. Ocean Drilling • Glomar Challenger & Harry Hess • 180 million (oceanic) vs. 4 billion (continental) 4. Hot spots—example = Hawaii • Only one active volcano • Evidence of movement & direction • Islands further from hot spot are oldest Causes of Plate Motion • Scientists generally agree that convection in the mantle is the basic driving force for plate movement. • The unequal heating within the earth causes this. • Read about slab-pull & ridge-push. • Read about mantle convection.
