Island arc

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Schematic section through the lithosphere in the area of ​​a subduction zone. The islands are formed by subduction volcanism (here not on oceanic, but on thinned continental crust ).

An island arch is a geotectonic structure of the upper crust of the earth , which is geographically expressed in the form of an arched island chain. As a result of plate tectonics , it occurs where an oceanic plate dips below another lithospheric plate at a certain angle ( subduction ), always on the non-submerged plate (upper plate). The arch-like formation of the structure is a consequence of the spherical shape of the earth. Island arcs are typically regions of high seismic and igneous activity.


The shape of the island arcs is in the geometry of spheres pieces on a spherical surface -founded. If you make a small incision in a table tennis ball and then press the flat piece in, a curved kink is created, the curvature of which points away from the incision.

Ruptures and crevices occur in the earth's crust along such kinks . Often the uppermost sediment residues are sheared off from the submerged plate and accumulate at the edge of the plate on top ( accretion wedge ). In addition, the friction between the two plates creates a high temperature. The sediments on the plate and their altered rocks contain a lot of water. It is released under the action of heat and pressure and moves upwards. There it leads to a lowering of the melting point in the jacket material of the top plate. This partially melts it into magma . Since the magma is hot, it has a lower density than the surrounding rock. This is comparable to air in a hot air balloon. It rises and eventually reaches the surface where volcanoes or plutons are formed. Island arcs are therefore of volcanic origin. In addition to active volcanoes, their greenish-black beaches are typical of island arches. They consist of the eroded basalt material of the volcanic cone.

On the oceanic side of the arch of the island, where the subducted plate dips under the plate on top, there is a deep sea channel . The part between the deep sea channel and the arch of the island is called the fore-arc , the part on the other side of the arch of the island is called the back-arc .

The shear stress can also simultaneously cause a parallel lateral displacement of the plate edges, whereby an expansion of the island arch takes place.

If an oceanic plate dips under a continental plate , there is usually no island arc, but a volcanic belt in the mountain range unfolded by the collision, as is the case e.g. B. in the Andes the case (see also active continental margin ) .

The arc-shaped chain of the Leeward Islands of the Lesser Antilles (satellite image) is a geographical expression of the subduction zone on the eastern edge of the Caribbean Plate . At the bottom right of the picture, Barbados can be seen as part of the non-magmatic Fore-Arc that protrudes above the sea surface.

Selection of island arches