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In geologic terms, a plate is a large, rigid slab of solid rock. The word tectonics comes from the Greek 'to build.' Putting these two words together, we get the term plate tectonics, which to how the Earth's surface is built of plates. The theory of plate tectonics that the Earth's outermost layer is into a dozen or more large and small plates that are moving to one another as they ride atop hotter, more mobile material.