This is an image the ocean floor of the Earth, showing island arcs being formed.
Click on image for full size
Image from: NOAA/NESDIS/National Geophysical Data Center, Boulder, CO
Island Arc Formation
There are two ways in which a group of islands can form.
1.) As a lithospheric slab is being subducted, the slab melts when the edges reach a depth which is sufficiently hot. Hot, remelted material from the subducting slab rises and leaks into the crust, forming a series of volcanoes. These volcanoes can make a chain of islands called an "island arc". Examples of island arcs are the Japanese islands, the Kuril Islands, and the Aleutian Islands of Alaska, shown here.
Island Arcs are formed on the opposing edge of a subducted slab. For 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.
2.) 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.
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