A volcanic arc (also known as a magmatic arc) is a belt of volcanoes formed above a subducting oceanic tectonic plate, with the belt arranged in an arc shape as seen from above. Volcanic arcs typically parallel an oceanic trench, with the arc located further from the subducting plate than the trench. The oceanic plate is saturated with water, mostly in the form of hydrous minerals such as micas, amphiboles, and serpentines. As the oceanic plate is subducted, it is subjected to increasing pressure and temperature with increasing depth. The heat and pressure break down the hydrous minerals in the plate, releasing water into the overlying mantle. Volatiles such as water drastically lower the melting point of the mantle, causing some of the mantle to melt and form magma at depth under the overriding plate. The magma ascends to form an arc of volcanoes parallel to the subduction zone. Volcanic arcs are distinct from volcanic chains formed over hotspots in the middle of a tectonic plate. Volcanoes often form one after another as the plate moves over the hotspot, and so the volcanoes progress in age from one end of the chain to the other. The Hawaiian Islands form a typical hotspot chain, with the older islands to the northwest and Hawaii Island itself, which is just 400,000 years old, at the southeast end of the chain over the hotspot. Volcanic arcs do not generally exhibit such a simple age-pattern. There are two types of volcanic arcs:
intraoceanic arcs (primitive arcs) form when oceanic crust subducts beneath other oceanic crust on an adjacent plate, creating a volcanic island arc. continental arcs form when oceanic crust subducts beneath continental crust on an adjacent plate, creating an arc-shaped mountain belt. In some situations, a single subduction zone may show both aspects along its length, as part of a plate subducts beneath a continent and part beneath adjacent oceanic crust. The Aleutian Islands and adjoining Alaskan Peninsula are an example of such a subduction zone. The active front of a volcanic arc is the belt where volcanism develops at a given time. Active fronts may move over time (millions of years), changing their distance from the oceanic trench as well as their width.
Tectonic setting
A volcanic arc is part of an arc-trench complex, which is the part of a subduction zone that is visible at the Earth's surface. A subduction zone is where a tectonic plate composed of relatively thin, dense oceanic lithosphere sinks into the Earth's mantle beneath a less dense overriding plate. The overriding plate may be either another oceanic plate or a continental plate. The subducting plate, or slab, sinks into the mantle at an angle, so that there is a wedge of mantle between the slab and the overriding plate. The boundary between the subducting plate and the overriding plate coincides with a deep and narrow oceanic trench. 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 within the subducting slab with the seismic hypocenters located at increasing depth under the island arc: these quakes define the Wadati–Benioff zones. The volcanic arc forms on the overriding plate over the point where the subducting plate reaches a depth of roughly 120 kilometres (75 mi) and is a zone of volcanic activity between 50 and 200 kilometers (31 and 124 mi) in width. The shape of a volcanic arc is typically convex towards the subducting plate. This is a consequence of the spherical geometry of the Earth. The subducting plate behaves like a flexible thin spherical shell, and such a shell be bent downwards by an angle of θ, without tearing or wrinkling, only on a circle whose radius is θ/2. This means that arcs where the subducting slab descends at a shallower angle will be more tightly curved. Prominent arcs whose slabs subduct at about 45 degrees, such as the Kuril Islands, the Aleutian Islands, and the Sunda Arc, have a radius of about 20 to 22 degrees. Volcanic arcs are divided into those in which the overriding plate is continental (Andean-type arcs) and those in which the overriding plate is oceanic (intraoceanic or primitive arcs). The crust beneath the arc is up to twice as thick as average continental or oceanic crust: The crust under Andean-type arcs is up to 80 kilometers (50 mi) thick, while the crust under intraoceanic arcs is 20 to 35 kilometers (12 to 22 mi) thick. Both shortening of the crust and magmatic underplating contribute to thickening of the crust. Volcanic arcs are characterized by explosive eruption of calc-alkaline magma, though young arcs sometimes erupt tholeiitic magma and a few arcs erupt alkaline magma. Calc-alkaline magma can be distinguished from tholeiitic magma, typical of mid-ocean ridges, by its higher aluminium and lower iron content and by its high content of large-ion lithophile elements, such as potassium, rubidium, caesium, strontium, or barium, relative to high-field-strength elements, such as zirconium, niobium, hafnium, rare-earth elements (REE), thorium, uranium, or tantalum. Andesite is particularly characteristic of volcanic arcs, though it sometimes also occurs in regions of crustal extension. In the rock record, volcanic arcs can be recognized from their thick sequences of volcaniclastic rock (formed by explosive volcanism) interbedded with greywackes and mudstones and by their calc-alkaline composition. In more ancient rocks that have experienced metamorphism and alteration of their composition (metasomatism), calc-alkaline rocks can be distinguished by their content of trace elements that are little affected by alteration, such as chromium or titanium, whose content is low in volcanic arc rocks. Because volcanic rock is easily weathered and eroded, older volcanic arcs are seen as plutonic rocks, the rocks that formed underneath the arc (e.g. the Sierra Nevada batholith), or in the sedimentary record as lithic sandstones. Paired metamorphic belts, in which a belt of high-temperature, low-pressure metamorphism is located parallel to a belt of low-temperature, high-pressure metamorphism, preserve an ancient arc-trench complex in which the high-temperature, low-pressure belt corresponds to the volcanic arc.
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