<P> A volcanic arc is a chain of volcanoes formed above a subducting plate, positioned in an arc shape as seen from above . Offshore volcanoes form islands, resulting in a volcanic island arc . Generally, volcanic arcs result from the subduction of an oceanic tectonic plate under another tectonic plate, and often parallel an oceanic trench . The oceanic plate is saturated with water, and volatiles such as water drastically lower the melting point of the mantle . As the oceanic plate is subducted, it is subjected to greater and greater pressures with increasing depth . This pressure squeezes water out of the plate and introduces it to the mantle . Here the mantle melts and forms magma at depth under the overriding plate . The magma ascends to form an arc of volcanoes parallel to the subduction zone . </P> <P> These should not be confused with hotspot volcanic chains, where volcanoes often form one after another in the middle of a tectonic plate, as the plate moves over the hotspot, and so the volcanoes progress in age from one end of the chain to the other . The Hawaiian Islands form a typical hotspot chain; the older islands (tens of millions of years old) to the northwest are smaller and more lush than the recently created (400,000 years ago) Hawaii island itself, which is more rocky . Hotspot volcanoes are also known as "intra-plate" volcanoes, and the islands they create are known as Volcanic Ocean Islands . Volcanic arcs do not generally exhibit such a simple age - pattern . </P>

What part does water play in volcanism at arcs
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