Avolcanic arc (also known as amagmatic arc[1]: 6.2 ) is a belt ofvolcanoes formed above asubducting oceanictectonic plate,[2] with the belt arranged in an arc shape as seen from above. Volcanic arcs typically parallel anoceanic 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 asmicas,amphiboles, andserpentines. 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 themantle, causing some of the mantle to melt and formmagma 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 overhotspots 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. TheHawaiian 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:
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. TheAleutian Islands and adjoiningAlaskan Peninsula are an example of such a subduction zone.
The active front of a volcanic arc is the belt wherevolcanism develops at a given time. Active fronts may move over time (millions of years), changing their distance from theoceanic trench as well as their width.
A volcanic arc is part of anarc-trench complex, which is the part of asubduction zone that is visible at the Earth's surface. A subduction zone is where atectonic plate composed of relatively thin, denseoceanic lithosphere sinks into theEarth's mantle beneath a less dense overriding plate. The overriding plate may be either another oceanic plate or a continental plate. The subducting plate, orslab, sinks into the mantle at an angle, so that there is awedge of mantle between the slab and the overriding plate.[1]: 5
The boundary between the subducting plate and the overriding plate coincides with a deep and narrowoceanic trench. This trench is created by the gravitational pull of the relatively dense subducting plate pulling the leading edge of the plate downward.[3]: 44–45 Multipleearthquakes occur within the subducting slab with theseismichypocenters located at increasing depth under the island arc: these quakes define theWadati–Benioff zones.[3]: 33 The volcanic arc forms on the overriding plate over the point where the subducting plate reaches a depth of roughly 120 kilometres (75 mi)[4] and is a zone of volcanic activity between 50 and 200 kilometers (31 and 124 mi) in width.[5]
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 theKuril Islands, theAleutian Islands, and theSunda Arc, have a radius of about 20 to 22 degrees.[6]
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. Bothshortening of the crust andmagmatic underplating contribute to thickening of the crust.[1]: 6
Volcanic arcs are characterized by explosive eruption ofcalc-alkaline magma, though young arcs sometimes erupttholeiitic magma[7] and a few arcs eruptalkaline magma.[8] Calc-alkaline magma can be distinguished from tholeiitic magma, typical ofmid-ocean ridges, by its higheraluminium and loweriron content[9]: 143–146 and by its high content oflarge-ion lithophile elements, such aspotassium,rubidium,caesium,strontium, orbarium, relative to high-field-strength elements, such aszirconium,niobium,hafnium,rare-earth elements (REE),thorium,uranium, ortantalum.[10]Andesite is particularly characteristic of volcanic arcs, though it sometimes also occurs in regions of crustal extension.[11]
In the rock record, volcanic arcs can be recognized from their thick sequences ofvolcaniclastic rock (formed by explosive volcanism) interbedded withgreywackes andmudstones and by their calc-alkaline composition. In more ancient rocks that have experiencedmetamorphism 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 aschromium ortitanium, whose content is low in volcanic arc rocks.[7] Because volcanic rock is easilyweathered anderoded, older volcanic arcs are seen asplutonic rocks, the rocks that formed underneath the arc (e.g. theSierra Nevadabatholith),[12] or in the sedimentary record aslithic sandstones.[13]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.[7]
In asubduction zone, loss of water from the subducted slab inducespartial melting of the overriding mantle and generates low-density,calc-alkaline magma that buoyantly rises to intrude and be extruded through thelithosphere of the overriding plate. Most of the water carried downwards by the slab is contained in hydrous (water-bearing) minerals, such asmica,amphibole, orserpentinite minerals. Water is lost from the subducted plate when the temperature and pressure become sufficient to break down these minerals and release their water content. The water rises into the wedge of mantle overlying the slab and lowers the melting point of mantle rock to the point where magma is generated.[1]: 5.3
While there is wide agreement on the general mechanism, research continues on the explanation for focused volcanism along a narrow arc some distance from the trench.[1]: 4.2 [14] The distance from the trench to the volcanic arc is greater for slabs subducting at a shallower angle, and this suggests that magma generation takes place when the slab reached a critical depth for the breakdown of an abundant hydrous mineral. This would produce an ascending "hydrous curtain" that accounts for focused volcanism along the volcanic arc. However, some models suggest that water is continuously released from the slab from shallow depths down to 70 to 300 kilometers (43 to 186 mi), and much of the water released at shallow depths producesserpentinization of the overlying mantle wedge.[1]: 4.2.42 According to one model, only about 18 to 37 percent of the water content is released at sufficient depth to produce arc magmatism. The volcanic arc is then interpreted as the depth at which the degree of melting becomes great enough to allow the magma to separate from its source rock.[5]
It is now known that the subducting slab may be located anywhere from 60 to 173 kilometers (37 to 107 mi) below the volcanic arc, rather than a single characteristic depth of around 120 kilometers (75 mi), which requires more elaborate models of arc magmatism. For example, water released from the slab at moderate depths might react with amphibole minerals in the lower part of the mantle wedge to produce water-richchlorite. This chlorite-rich mantle rock is then dragged downwards by the subducting slab, and eventually breaks down to become the source of arc magmatism.[4] The location of the arc depends on the angle and rate of subduction, which determine where hydrous minerals break down and where the released water lowers the melting point of the overlying mantle wedge enough for melting.[15]
The location of the volcanic arc may be determined by the presence of a cool shallow corner at the tip of the mantle wedge, where the mantle rock is cooled by both the overlying plate and the slab. Not only does the cool shallow corner suppress melting, but its high stiffness hinders the ascent of any magma that is formed. Arc volcanism takes place where the slab descends out from under the cool shallow corner, allowing magma to be generated and rise through warmer, less stiff mantle rock.[14]
Magma may be generated over a broad area but become focused into a narrow volcanic arc by a permeability barrier at the base of the overriding plate. Numerical simulations suggest that crystallization of rising magma creates this barrier, causing the remaining magma to pool in a narrow band at the apex of the barrier. This narrow band corresponds to the overlying volcanic arc.[16]
Two classic examples of oceanic island arcs are theMariana Islands in the western Pacific Ocean and theLesser Antilles in the western Atlantic Ocean. TheCascade Volcanic Arc in western North America and theAndes along the western edge of South America are examples of continental volcanic arcs. The best examples of volcanic arcs with both sets of characteristics are in the North Pacific, with the Aleutian Arc consisting of theAleutian Islands and their extension theAleutian Range on theAlaska Peninsula, and the Kuril–Kamchatka Arc comprising theKuril Islands and southernKamchatka Peninsula.
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| Basalts by tectonic setting | |
| Basalts by form and flow | |
| Basalts by chemistry | |
| Important minerals | |
