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They are long, curved chains of oceanic islands associated with intense volcanic and seismic activity and orogenic (mountain-building) processes.
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An island arc typically has a land mass or a partially enclosed, unusually shallow sea on its concave side.
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Along the convex side there almost invariably exists a long, narrow deep-sea trench.
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The greatest ocean depths are found in these depressions of the seafloor, as in the case of the Mariana (deepest trench in the world) and Tonga trenches.
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Prime examples of this form of geologic feature include the Aleutian-Alaska Arc and the Kuril-Kamchatka Arc.
