Volcanic Arc

A volcanic arc is a chain of volcanoes positioned in an arc shape as seen from above. Offshore volcanoes form islands, resulting in a volcanic island arc. Generally they 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.

These should not be confused with hotspot volcanic chains, where volcanoes 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) to the northwest are smaller and more lush than the recently-created (400,000 years ago) Hawaii island itself, which is more rocky. Volcanic arcs do not generally exhibit such a simple age pattern.

There are two types of volcanic arcs:

• oceanic arcs form when oceanic crust subducts beneath other oceanic crust on an adjacent plate, creating a volcanic island arc. (Not all island arcs are volcanic island arcs.)
• continental arcs form when oceanic crust subducts beneath continental crust on an adjacent plate, creating an arc-shaped mountain belt. (Not all mountain belts are formed this way.)

In some situations, a single subduction zone may show both aspects along its length, as part of a plate subducts beneath a continent and part beneath adjacent oceanic crust.

(The term "volcanic arc" is often confused with the term "island arc". The former is a series of volcanoes, but not necessarily offshore. The latter is a series of islands, not necessarily composed solely of volcanoes.)

Volcanoes are present in almost any mountain belt, but this does not make it a volcanic arc. Often there are isolated, but impressively huge volcanoes in a mountain belt. For instance, Vesuvius and the Etna volcanoes in Italy are part of separate but different kinds of mountainous volcanic ensembles.

The active front of a volcanic arc is the belt where volcanism develops at a given time. Active fronts may move over time (millions of years) changing their distance from the oceanic trench as well as their width.