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Section 15-3 The Theory of Plate Tectonics Plate Tectonics Plate tectonics is the theory that explains how large pieces of the Earth's outermost layer, called tectonic plates, move and change shape. Tectonic Plate Boundaries A boundary is a place where tectonic plates touch. 3 types of boundaries: convergent, divergent, & transform The type of boundary depends on how the tectonic plates move relative to one another. Tectonic plates can collide, separate, or slide past each other. Convergent Boundary When two tectonic plates collide, the boundary between them is a convergent boundary. There are three types of convergent boundaries: continental-continental, continental-oceanic, and oceanicoceanic. Continental-Continental Collisions When two tectonic plates with continental crust collide, they buckle and thicken, pushing the continental crust upward. Continental-Oceanic Collisions When a plate with oceanic crust collides with a plate with continental crust, the denser oceanic crust sinks into the asthenosphere. This convergent boundary has a special name: the subduction zone. Oceanic-Oceanic Collisions When two tectonic plates with oceanic lithosphere collide, one of the plates with oceanic lithosphere is subducted, or sinks, under the other plate. Divergent Boundary When two tectonic plates separate, the boundary between them is called a divergent boundary. New sea floor forms at divergent boundaries. The mid-ocean ridges are the most common type of divergent boundary. Transform Boundary When two tectonic plates slide past each other horizontally, the boundary between them is called a transform boundary. The San Andreas Fault is a good example of a transform boundary. This fault marks the place where the Pacific and North American plates are sliding past each other. Possible Causes of Tectonic Plate Motion Ridge push, convection, and slab pull are 3 possible driving forces of plate tectonics. Because of ridge push, the oceanic lithosphere slides downhill under the force of gravity. Convection causes the oceanic lithosphere to move sideways and away from the midocean ridge. Possible Causes of Tectonic Plate Motion, cont. Because oceanic lithosphere is denser than the asthenosphere, the edge of the tectonic plate that contains oceanic lithosphere sinks and pulls the rest of the tectonic plate with it in a process called slab pull. Tracking Tectonic Plate Motion Tectonic movements are usually measured in centimeters per year. The plates move 3-5 cm per year. Scientists use a network of satellites called GPS to measure the rate of tectonic plate movement.