Plate Tectonics

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.