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Plate Tectonics and Continental Drift Plate Tectonics • The surface of the Earth is composed of about a dozen major rigid, moving crustal plates and several smaller plates •Plates a. Slabs of Earth’s lithosphere (crust and upper mantle) b. Average thickness is 100 km (62 miles) c. Most plates support both continent and ocean Continental Drift states that the continents have drifted and still are drifting apart Why do the plates move? 1. Due to tremendous heat, rock in the asthenosphere is like hot taffy 2. This allows plates to ride on top of hot, flowing rock. 3. Plates move because heat is being released from deep inside the earth. 4. Convection currents causes hot material to rise and expand (plates diverge) and cooler material to sink and contract (plates converge). Continental Drift _________ Wegener proposed the theory that the crustal plates are moving over the mantle. This was supported by fossil and rock type evidence; also matching of coastline shapes. Convection Currents The force responsible for plate movement is __________. Plate Boundaries The following two slides show maps of the current major plate boundaries and their relative motions. You should be able to describe various landforms created by different plate movements.