5 - 2 Motion of the plate

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The surface of the solid earth is covered by more than 10 plates which are about 100km thick. The crust is, so to speak, put on these plates. A plate is newly produced by the mantle rising at an oceanic ridge, and it sinks down and disappears into the mantle at a trench. (See Figure below.)
An oceanic ridge is, so to say, a huge crack in the crust, which forms a very long chain of mountains, for example, under the Pacific Ocean, the Atlantic Ocean and the Indian Ocean. At these oceanic ridges, the plate is divided right and left, and accordingly hot mantle comes up from the depths.
A trench is a very, very deep ditch under the sea water. The plate sinks down at this trench into the mantle because of its own weight.
The motion of a plate is very slow; i.e., several cm/year or 10cm/year at most. Plate tectonics is a theory (or logic) to explain the seismic and volcanic activities and the formation of large chains of mountains on the earth.

Motion of the plate

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