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Theory of Plate Tectonics Plate Tectonics • The theory of plate tectonics states that the Earth’s crust is divided into broken pieces called plates. • Earth’s surface has about a dozen rigid plates • Plates move slowly (a few centimeters per year) • These rigid pieces of the lithosphere drift around like “rafts” on the plastic-like asthenosphere (mantle). Plates consist of the crust plus the upper mantle • This layer is called the lithosphere • About 100 km (60 miles) thick • Composition is Fe silicates (basalt) except for… – Continents; mostly Al silicates (granite) – Continents “float” on the rest of the lithosphere Below the lithosphere is the asthenosphere • Same composition but is partly melted (plastic like) • Able to flow very slowly • Flow caused by convection currents – Heated rock expands and rises – Cool rock contracts and sinks – The plate above is carried along on the flow – Continents ride along on the plates Convection Currents • The force that drives the plate movement are convection currents.