The primary cause of the motion of tectonic plates is convection currents in the mantle below. Circulating currents in the mantle (along which hot material rises from the center to the surface, cools, and then sinks back down) cause the plates (which float on top of the mantle) to move around. As they do so, they either drift away from eachother, or collide with eachother. Where they drift away, material from the mantle seeps through, cools, and becomes part of the lithosphere (the plates). Where thy collide, the denser oceanic plate sinks below the other plate (either another oceanic plate or a continental plate), melts, and becomes part of the mantle. The gravitational pull on the subducted plate, caused by its high density relative to the mantle, also contributes to the motion of the plates as the subducted edge is pulled downward. Tidal forces, centrifugal force, and other forces related to the Earth’s rotation have also been suggested as possible causes of the motion of tectonic plates.
Acording to the theory of plate tectonics, the Earth’s crust is made up of a dozen plates upon which the continents and oceans rest. The surface beneath these plates–the mantel–is made up of a hot, soft lava like substance, which is continously moving, like a conveyor belt, driven by heat and other forces within the Earth’s core. Because of this constant movement, the plates are constantly shifting, at a rate of approximately 1cm (.5 in)-15cm (6in) a year, in different directions.
One theory is that as new material is formed at ocean ridges (magma breaks the surface at plate boundaries and cools) it pushes the plates apart. This is called ridge oush. As this new material cools, it becomes denser and settles, forming subduction zones as it moves down. The force created by this may also be responsible for plate movement and is called slab pull.
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