This is an image the ocean floor of the Earth, showing trenches associated with island arcs.
Click on image for full size
Image from: NOAA/NESDIS/National Geophysical Data Center, Boulder, CO
Island Arc Formation
One of the processes of island formation involves lithospheric subduction. As a lithospheric slab is being subducted, a trench is formed at the interface with the adjoining slab. The slab undergoes melting when the edges reach a depth of the interior which is sufficiently hot. Hot, remelted material from the subducting slab rises and leaks into the crust, forming a series of volcanoes on the other side of the trench, each associated with the subducting slab of lithosphere.
Island Arcs are formed on the opposing edge of a subducted slab by a series of volcanos which pour out enough material to form substantial amounts of land. Examples of island arcs are the Japanese islands, the Kuril Islands, and the Aleutian Islands of Alaska, shown here. In each case, there is an associated subducting slab and a trench. The trenches for these island arcs can barely be made out in this map.
The second way in which islands are formed is via plumes or hot spots in the lithosphere. The Hawaiian Islands are an example of this type of island formation. In this case, there is no associated subducting slab nor trench.
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