Island arcs form where two plates oceanic plates are being dragged or pushed together. In some cases, one of the plates may include continental crust at some distance from the plate margin and this make the plate slightly more buoyant, whereas the entirely oceanic plate is denser and will be subducted. This situation can be seen around the northern and western margin of the Pacific plate where it is being subducted beneath the North American, Eurasian and Indo-Australian plates.
In some situations, both plates can include continental material such as where the Indo-Australia plate is subducting beneath the Eurasian plate.
Volcanic island arcs are created by volcanic activity associated with the subduction of the oceanic plate. Water released by hydrous minerals within the subducting plate triggers melting of the subducted slab and surrounding mantle, and diapirs of melt rise towards the surface. This melt is extruded onto the ocean floor and builds up volcanoes on the edge of the overriding plate.