Volcanic arc

The oceanic plate is saturated with water, mostly in the form of hydrous minerals such as micas, amphiboles, and serpentines.

The heat and pressure break down the hydrous minerals in the plate, releasing water into the overlying mantle.

Active fronts may move over time (millions of years), changing their distance from the oceanic trench as well as their width.

[3]: 44–45  Multiple earthquakes occur within the subducting slab with the seismic hypocenters located at increasing depth under the island arc: these quakes define the Wadati–Benioff zones.

[1]: 5.3 While there is wide agreement on the general mechanism, research continues on the explanation for focused volcanism along a narrow arc some distance from the trench.

[1]: 4.2.42  According to one model, only about 18 to 37 percent of the water content is released at sufficient depth to produce arc magmatism.

The volcanic arc is then interpreted as the depth at which the degree of melting becomes great enough to allow the magma to separate from its source rock.

For example, water released from the slab at moderate depths might react with amphibole minerals in the lower part of the mantle wedge to produce water-rich chlorite.

This chlorite-rich mantle rock is then dragged downwards by the subducting slab, and eventually breaks down to become the source of arc magmatism.

Not only does the cool shallow corner suppress melting, but its high stiffness hinders the ascent of any magma that is formed.

Arc volcanism takes place where the slab descends out from under the cool shallow corner, allowing magma to be generated and rise through warmer, less stiff mantle rock.

[14] Magma may be generated over a broad area but become focused into a narrow volcanic arc by a permeability barrier at the base of the overriding plate.