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continental drift the theory that the earth's continents shift their positions because of currents in the molten magma of the earth's mantle.
plate tectonics (used with a sing. verb) the geologic theory that the earth's crust consists of several independent plates floating on semiliquid magma, whose constant motion is the cause of continental drift, volcanic eruption, and the like.
seafloor spreading the creation of new oceanic crust formed by an upward flow and outward spread of magma along mid-ocean ridges.