An island arc
is a geographic
feature formed by plate tectonics
as one oceanic
tectonic plate subducts (subduction
) 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 boyant and rises up to the surface where it becomes lava
and forms volcanos in the shape of an arc. Due to the reactions of the metamorphic and igneous rocks, it is not uncommon to find black-green beaches composed of olivine eroded from 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 frictinal 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. This process has happend over and over in the geologic history of the Earth.
Japan is a good example of an island arc.