An
island arc is a type of
archipelago formed as one oceanic
tectonic plate subducts under another and produces
magma at depth below the over-riding plate. Island arcs that develop along the edges of a continent (for example, large parts of the Andes/ Central American/ Canadian mountain chain) may be known as a
volcanic arc, though most people find the distinction of little benefit.
In the subduction zone, loss of volatiles from the subducted slab induces partial melting of the overriding mantle and generates low-density,
calc-alkaline magma that buoyantly rises to intrude and be extruded through the lithosphere of the overriding plate. The resulting
volcano chain has the shape of an arc parallel to the convergent plate boundary and convex toward the subducting plate. This is a consequence of the geometry of the spherical plate crumpling along a line on a spherical surface.
On the subducting side of the island arc is a deep and narrow
oceanic trench, which is the trace at the Earth’s surface of the boundary between the downgoing and overriding plates. This trench is created by the gravitational pull of the relatively dense subducting plate pulling the leading edge of the plate downward. Multiple
earthquakes occur along this subduction boundary with the
seismic hypocenters located at increasing depth under the island arc: these quakes define the
Wadati-Benioff zones.
Oceans basins that are being reduced by subduction are called 'remnant oceans' as they will slowly be shrunken out of existence and crushed in the subsequent
orogenic collision. This process has happened over and over in the geologic history of the Earth.
Examples
Ancient Island arcs
