![]() A mature island arc develops into a chain of relatively large islands (such as Japan or Indonesia) as more and more volcanic material is extruded and sedimentary rocks accumulate around the islands. The magma, which is lighter than the surrounding mantle material, rises through the mantle and the overlying oceanic crust to the ocean floor where it creates a chain of volcanic islands known as an island arc. It mixes with the overlying mantle, and the addition of water to the hot mantle lowers the crust’s melting point and leads to the formation of magma (flux melting). The significant volume of water within the subducting material is released as the subducting crust is heated. ![]() The subducted lithosphere descends into the hot mantle at a relatively shallow angle close to the subduction zone, but at steeper angles farther down (up to about 45°). There is commonly an ocean trench along the boundary as the crust bends downwards. Often it is the older and colder plate that is denser and subducts beneath the younger and warmer plate. The types are ocean-ocean, ocean-continent, and continent-continent.Īt an ocean-ocean convergent boundary, one of the plates (oceanic crust and lithospheric mantle) is pushed, or subducted, under the other (Figure 4.6.1). Modified from "Physical Geology" by Steven Earle*Ĭonvergent boundaries, where two plates are moving toward each other, are of three types, depending on the type of crust present on either side of the boundary - oceanic or continental.
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