Volcanic arc
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A volcanic arc is a chain of volcanic islands or mountains located near the edge of continents that are formed as the result of tectonic plate subduction. Subduction causes these arcs by creating magma as one tectonic plate sinks beneath another into the asthenosphere. The low-density magma is forced towards the surface through the overlying crust, and some erupts onto the surface to form volcanoes.
The magma is generated within the deep mantle wedge of the overriding plate. A subducting plate carries with it a lot of sediment and altered basalt and hydrated peridotite, all of which are rich in water. As the plate gets deeper and deeper, the water is released and migrates upwards. The water causes the melting point of the upper mantle rock (that is wedged between the two plates) to drop and magma is formed. It is this magma derived from the asthenosphere and enriched in water and other constituents from the subducting plate (and also probably contaminated by melted material from the overlying crust) that erupts in an arc volcano.
The type of igneous rock most characteristic of volcanic arcs is andesite, but a range of rock types from basalt to rhyolite is commonly present in the volcanoes. These rock types make up the calc-alkaline series. The presence of the calc-alkaline series is used as a tool to identify parts of the continental crust formed in volcanic arcs, long after the arcs themselves have vanished.
There are two types of volcanic arcs: oceanic (island) and continental. The Cascade Mountain Range in western North America is an example of a continental volcanic arc. An example of an island arc is the Aleutian Islands, a chain of islands that stretches nearly 2,500 km from the Alaskan Peninsula to the Near Islands. The arc contains approximately 80 major volcanic centers, nearly half of which have been active in historic times.
