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Most of these island arcs are formed as one oceanic tectonic plate subducts another one and, in most cases, produces magma at depths below the over-riding plate. However, this is only true for those island arcs that are part of the group of mountain belts which are called volcanic arcs, a term which is used when all the elements of the arc-shaped mountain belt are composed of volcanoes. For example, large parts of the Andes-Central American-Canadian mountain chain may be known as a volcanic arc, but they are not islands (being situated upon and along a continental area) and are thus not classified as an island arc. On the other hand, the Aegean or Hellenic arc in the Mediterranean area, composed of numerous islands such as Crete, is an island arc, but is not volcanic. Parallel to it is the South Aegean Volcanic Arc, which is the volcanic island arc of the same tectonic system.
There is some debate about the usefulness of the distinction between island arcs and volcanic arcs. The term "volcanic island arc" is merely a sub-classification of "island arc." Island arcs are tectonically created arc-shaped mountain belts that are partly below sea level. Essentially, they represent a specific geographic-topographic situation in which a mountain belt is partly submerged in ocean. Many of these are composed of volcanoes, and can thus be further classified as volcanic island arcs.
In the subduction zone, the dehydration of the subducting slab releases volatiles into the overriding mantle wedge. The addition of volatile components (e.g., water) induces partial melting by lowering the solidus of the wedge. This process, called flux melting, generates varying degrees of 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. One of the theories to explain the arc shape views this as a consequence of the geometry of the spherical plate crumpling along a line on a spherical surface, but only the more broadly shaped arcs can be explained in this way.
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.
Ocean 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 repeatedly in the geological history of the Earth.
|Island arc||Country||Trench||Basin or marginal sea||Overriding Plate||Subducting plate|
|Aleutian Islands||United States||Aleutian Trench||Bering Sea||North American Plate||Pacific Plate|
|Kuril Islands||Russia||Kuril-Kamchatka Trench||Sea of Okhotsk||North American Plate||Pacific Plate|
|Japanese Archipelago||Japan||Japan Trench?Nankai Trough||Sea of Japan||North American Plate, Eurasian Plate||Pacific Plate, Philippine Sea Plate|
|Ryukyu Islands||Japan||Ryukyu Trench||East China Sea (Okinawa Trough)||Eurasian Plate||Philippine Sea Plate|
|Philippine Islands||Philippines||Philippine Trench||South China Sea, Celebes Sea||Eurasian Plate||Philippine Sea Plate|
|Sunda Islands||Indonesia||Java Trench||Java Sea, Flores Sea||Eurasian Plate||Australian Plate|
|Andaman and Nicobar Islands||India||Northern Java Trench||Andaman Sea||Eurasian Plate||Indo-Australian Plate|
|Izu Islands and Bonin Islands (Ogasawara Islands)||Japan||Izu-Ogasawara Trench||Philippine Sea Plate||Pacific Plate|
|Mariana Islands||United States||Mariana Trench||Philippine Sea Plate||Pacific Plate|
|Bismarck Archipelago||Papua New Guinea||New Britain Trench||Pacific Plate||Australian Plate|
|Solomon Islands (archipelago)||Solomon Islands||San Cristobal Trench||Pacific Plate||Australian Plate|
|New Hebrides||Vanuatu||New Hebrides Trench||Pacific Plate||Australian Plate|
|Tonga islands||Tonga||Tonga Trench||Australian Plate||Pacific Plate|
|Antilles||Puerto Rico Trench||Caribbean Sea||Caribbean Plate||North American Plate, South American Plate|
|South Sandwich Islands||United Kingdom||South Sandwich Trench||Scotia Sea||Scotia Plate||South American Plate|
|Aegean or Hellenic arc||Greece||Eastern Mediterranean Trench||Aegean Sea||Aegean Sea Plate or Hellenic Plate||African Plate|
|South Aegean Volcanic Arc||Greece||Eastern Mediterranean Trench||Aegean Sea||Aegean Sea Plate or Hellenic Plate||African Plate|
Remains of former island arcs have been identified at some locations. The table below mention a selection of these.
|Chaitenia||Chile, Argentina||Accreted to Patagonia in the Devonian.|
|Insular Islands||Canada, United States||Accreted to North America in the Cretaceous.|
|Intermontane Islands||Canada, United States||Accreted to North America in the Jurassic.|