Island arc

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This article is about the geographic feature. For the scientific journal, see Island Arc (journal).
The Ryukyu Islands form an island arc.

An island arc is a type of archipelago, often composed of a chain of volcanoes, with arc-shaped alignment, situated parallel and close to a boundary between two converging tectonic plates.

Most of these island arcs are formed as one oceanic tectonic plate subducts another one and, in most cases, produces magma at depth 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.[citation needed]

Classification[edit]

There is some debate about the usefulness of the distinction between island arcs and volcanic arcs.[citation needed] 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.

Tectonic formation[edit]

Following is a brief description of a subduction zone and its relation with volcanism, one of the more common causes of (volcanic) island arcs (for more information, see the page covering subduction):

In the subduction zone, loss of volatiles from the subducted slab induces partial melting of the overriding mantle. This process, called flux melting, 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. 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.[citation needed]

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.[citation needed]

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.[citation needed]

Examples[edit]

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 TrenchNankai 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
Egean, 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

Ancient Island arcs[edit]

See also[edit]