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|part of speech:
||(used with a singular verb) a theory that scientists use to explain the movement of continents, the eruption of volcanoes, and other changes or events in the earth's geology. The theory of plate tectonics says that the earth's crust is made up of separate sections that are like very large plates. These sections move around constantly because they float on melted rock. The movement of these sections of the crust causes earthquakes.