Island arc
An island arc is a type of archipelago formed by plate tectonics as one oceanic crustal tectonic plate subducts under another and is melted into magma by the intense heat. Oceanic plates are composed of basalt, a lighter material than the material that composes the upper mantle. When melted this material becomes very buoyant and rises up to the surface as magma where it becomes lava and forms volcano chains in the shape of an arc. Weathering and erosion of these volcanic rocks produces the not uncommon black-green beaches composed of olivine sand eroded from the volcanic cones.
On the ocean side of the island arc is a deep trench where the subducted plate sinks below the subducting plate. This trench is created by the friction of the subducting plate pulling the leading edge of the overbearing plate downward. Great frictional forces heat the rock on both plates in this area.
Oceans that are being reduced by the subduction of plates are called 'remnant oceans' as they will slowly be shrunken out of existence and crushed in the orogenic collision. This process has happend over and over in the geologic history of the Earth.
The Japanese island chain is a good example of an island arc.