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Continental drift

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A hypothesis that originally proposed that the continents had once been joined to form a single supercontinent; The supercontinent broke into pieces, which drifted into their present-day positions.

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Pangaea

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The proposed supercontinent that 200 million years ago began to break apart and form the present landmasses.

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Sonar

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An electronic depth-sounding mechanism. Sonar is an acronym for sound navigation and ranging. Sonar calculates ocean depth by recording the time it takes for an energy pulse to reach the ocean floor and return.

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Deep-ocean trench

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A surface feature in the seafloor produced by the descending plate during subduction.

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Mid-ocean ridge

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A continuous elevated zone on the floor of all the major ocean basins and varying in width from 1000-4000 kilometers. The rifts at the crests of ridges represent divergent plate boundaries.

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Rift valley

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Deep faulted structure found along the axes of divergent plate boundaries. Rift valleys can develop on the seafloor or on land.

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Sea-floor spreading

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The process by which plate tectonics produces new oceanic lithosphere at the ocean ridges.

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Subduction

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The process by which oceanic crust sinks beneath a trench and back into the mantle at a colliding plate boundary.

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Paleomagnetism

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The study of changes in Earth’s magnetic field, as shown by patterns of magnetism in rocks that have formed over time.

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Plate

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One of numerous rigid sections of the lithosphere that moves as a unit over material of the asthenosphere.