Volcanic Arc

A volcanic arc is a curved belt of volcanoes that forms above a subducting oceanic tectonic plate, typically running parallel to, and further from the plate boundary than, the associated oceanic trench. The descending oceanic plate carries water locked in hydrous minerals such as micas, amphiboles, and serpentine. As the plate sinks and heats up, these minerals break down and release that water into the overlying mantle, and the added water lowers the mantle melting point enough to generate magma. That magma rises through the overriding plate, and where enough of it reaches the surface it builds the chain of volcanoes that defines the arc.