The outer surface of the earth is made of 12 major plates and several smaller plates. These plates move towards each other (convergent boundaries), away from each other (spreading or divergent boundaries) or slide past each other (transform boundaries). Volcanoes are associated with convergent boundaries, where ocean plates are subducted under ocean or continental plates, with spreading boundaries, where new ocean plates are created, and with hot spots. A complete description of the Theory of Plate Tectonics is presented in Volcano Lessons.
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