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At Subduction Boundaries As you may recall from Chapter 8, subduction boundaries are places where lithospheric plates collide. The diagrams below show the two processes by which volcanoes form: first, when an oceanic plate is forced beneath a continental plate, and second, when one oceanic plate is forced beneath another oceanic plate. At a subduction boundary, volcanoes always form on the overriding plate. Where an oceanic plate collides with a continental plate, the volcanoes form on the overriding continental plate. The Cascade Range, which extends along the Pacific coast from northern California to British Columbia, Canada, is made up of volcanoes that have formed at an oceanic-continental subduction boundary. Where two oceanic plates collide, the volcanoes create a chain of volcanic islands, called a volcanic island arc, on the overriding plate. The Mariana Islands in the Pacific Ocean are an example of a volcanic island arc formed at such a subduction boundary. CLASSZONE.COM Observe an animation of volcanism at a subduction zone. Keycode: ES0902 Volcanic Activity at a Subduction Boundary BETWEEN AN OCEANIC PLATE AND A CONTINENTAL PLATE 4 Oceanic lithosphere 1 Continental lithosphere Some of the magma reaches Earth’s surface, and volcanoes form on the overriding continental plate. Asthenosphere Water in the subducted rock is released into the asthenosphere. 2 The water lowers melting temperatures 3 The magma is less dense of materials in the asthenosphere, leading to the formation of magma. than its surroundings, so it rises. BETWEEN OCEANIC PLATES The process by which magma forms at an oceanic-oceanic subduction boundary is similar to the process at an oceanic-continental boundary. Notice that the difference between the two processes occurs at step 4. 4 Oceanic lithosphere Oceanic lithosphere Magma that reaches Earth’s surface is underwater. Thus, an arc of volcanic islands forms on the overriding oceanic plate. Asthenosphere Chapter 9 Volcanoes 195