Oceanic vs. Oceanic Boundary
When two plates converge one is usually subducted under the other and in the process a deep oceanic trench is formed. The Marianas Trench, for example, is a deep trench created as a result of the Philippine Plate subducting under the Pacific Plate. This type of convergence also results in the formation of undersea volcanoes. Over millions of years, however, the erupted lava and volcanic debris piles up on the ocean floor until a submarine volcano rises above sea level to form an island volcano.
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