The theory of plate tectonics states that the Earth's crust is rigid and is divided into about 12 main slabs or plates that float on an underlying plastic zone. Deformation occurs along the edges of the plates as they move and grind together. The San Andreas fault forms the boundary between the North American and Pacific plates, along which the two plates slide horizontally past each other (a transform plate boundary). New crust is created along spreading centers, another type of plate boundary where subsea mountain chains that rise above the ocean floor in many of world's oceans. Here new crust is created as the two plates move "spread" away from each other. Iceland is one place where a spreading center is above sea level. Crust is consumed in subduction zones, where the leading edge of one plate dives under another plate (e.g., the Andes on the west coast of South America). Earthquakes, volcanoes, and mountain belts are generally located near the edges of the plates. The direction and rate of movement (kinematics) of each plate is unique and fairly well known today, but the driving force or forces (dynamics) of the plates are not yet well understood. Plate tectonics implies that the Earth's crust is constantly moving and changing through time.
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