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Plate Tectonics

Oceanic-oceanic Plates

Recall that plates in continental areas are thicker and less dense than in oceanic areas. When two oceanic plates converge (an oceanic-oceanic convergent margin) one of the plates subducts into a trench. The subducted plate sinks downward into the mantle where it begins to melt. Molten rock from the melting plate rises toward the surface and forms a chain of volcanic islands, or a volcanic island arc, behind the ocean trench. Subduction of the Pacific plate below the North American plate along the coast of Alaska formed the Aleutian Trench and the Aleutian Islands, a volcanic island arc. At oceanic-oceanic divergent margins, sea floor spreading occurs and the ocean slowly grows wider. Today, Europe and North America move about 3 in (7.6 cm) farther apart every year as the Atlantic Ocean grows wider.

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Science EncyclopediaScience & Philosophy: Planck mass to PositPlate Tectonics - Continental Drift Versus Plate Tectonics, An Overview Of Tectonic Theory, Proofs Of Tectonic Theory, Rates Of Plate Movement