An island arc is a chain of volcanic islands formed over a subduction zone where oceanic crust is subducting beneath oceanic crust.The volcanoes form an arc shaped line curving away from the trench. When cold dense oceanic crust converges with less dense oceanic crust it sinks back down into the mantle at a trench and is reincorporated into the mantle.
Mantle convection currents combine with slab pull to keep the plate moving downwards. A pattern of earthquakes with increasing depth follows the descending slab- Benioff zone. As the plate descends it takes sea sediments with it or scrapes them off as an accretionnary prism. The sediments and minerals in the slab release fluids which cause the mantle above to melt and magma to rise to the surface. Where it erupts it forms a volcano.
Fluids released from the slab enable the melting of the mantle wedge aboveto produce a calc-alkaline melt. Volcanoes tend to erupt explosively. Notable Island Arcs include The Antlles Arc in The Caribbean and The Aleutian Arc in Alaska.