Island Arc

Relief map of Kurile Islands

A cursory examination of an atlas shows that many islands in the Pacific, especially those close to continental land masses, are arranged in arcs.  Examples include the Aleutians, the Kuriles, the Ryukyus, and most of the Netherlands East Indies.

According to modern tectonic theory, these island arcs form where oceanic crust is slowly being subducted into the mantle of the Earth.  Loose sediment scraped from the top of the crust piles up at the point of subduction (producing what geologists call a melange), and the water in the subducted rock lowers its melting point to form large volumes of magma.  Thus, an island arc has a region of deep water to one side (an oceanic trench) and a line of volcanoes running through the islands.  Deep earthquakes are common.

The subducting crust sometimes generates a plastic flow of mantle rock behind the island arc. This produces a small spreading center that pulls apart the crust behind the island arc, producing a back-arc basin.  For example, the Sea of Japan appears to be an inactive back-arc basin behind the Japan island arc. 

Island arcs may involve subduction of oceanic crust under another section of oceanic crust or a section of continental crust.  When oceanic crust is subducted under oceanic crust, as is the case with the Marianas island arc, the arc usually consists of small volcanic islands.  When oceanic crust is subducted under continental crust, the island arc often consists of large islands with a significant sedimentary basement complex, as with Japan or the Netherlands East Indies.


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