Editing Subduction (section)

===Arc magmatism===
{{Main|Volcanic arc}}
Two kinds of arcs are generally observed on Earth: [[island arcs]] that form on the oceanic lithosphere (for example, the [[Mariana Islands|Mariana]] and the [[Tonga]] island arcs), and [[continental arc]]s such as the [[Cascade Volcanic Arc]], that form along the coast of continents. Island arcs (intraoceanic or primitive arcs) are produced by the subduction of oceanic lithosphere beneath another oceanic lithosphere (ocean-ocean subduction) while continental arcs (Andean arcs) form during the subduction of oceanic lithosphere beneath a continental lithosphere (ocean-continent subduction).{{sfn|Stern|2002|pp=24-25}} An example of a volcanic arc having both island and continental arc sections is found behind the [[Aleutian Trench]] subduction zone in Alaska.

[[Volcano]]es that occur above subduction zones, such as [[Mount St. Helens]], [[Mount Etna]], and [[Mount Fuji]], lie approximately one hundred kilometers from the trench in arcuate chains called [[volcanic arc]]s. Plutons, like Half Dome in Yosemite National Park, generally form 10–50&nbsp;km{{sfn|Stern|2002|pp=1–38}} below the volcanoes within the volcanic arcs and are only visible on the surface once the volcanoes have weathered away. The volcanism and plutonism occur as a consequence of the subducting oceanic slab dehydrating as it reaches higher pressures and temperatures. Once the oceanic slab reaches about 100&nbsp;km in depth,{{sfn|Stern|2002|pp=1–38}} hydrous minerals become unstable and release fluids into the asthenosphere. The fluids act as a flux for the rock within the asthenosphere and cause it to partially melt. The partially melted material is more buoyant and as a result will rise into the lithosphere, where it forms large magma chambers called diapirs. Some of the magma will make it to the surface of the crust where it will form volcanoes and, if eruptive on earth's surface, will produce andesitic lava. Magma that remains in the lithosphere long enough will cool and form plutonic rocks such as diorite, granodiorite, and sometimes granite.

The arc magmatism occurs one hundred to two hundred kilometers from the trench and approximately one hundred kilometers above the subducting slab.<ref>{{Cite news|url=https://www.sciencedaily.com/releases/2017/04/170407143316.htm|title=Volcanic arcs form by deep melting of rock mixtures: Study changes our understanding of processes inside subduction zones|work=ScienceDaily|access-date=2017-06-21|language=en}}</ref> Arcs produce about 10% of the total volume of magma produced each year on Earth (approximately 0.75 cubic kilometers),<!-- Could use more up-to-date estimates, but only with a reliable source citation.--> much less than the volume produced at mid-ocean ridges,<ref>{{cite book |last1=Fisher |first1=Richard V. |last2=Schmincke |first2=H.-U. |title=Pyroclastic rocks |date=1984 |publisher=Springer-Verlag |location=Berlin |isbn=3540127569 |page=5}}</ref> but they have formed most [[continental crust]].{{sfn|Stern|2002}} Arc volcanism has the greatest impact on humans because many arc volcanoes lie above sea level and erupt violently. [[Particulate|Aerosols]] injected into the stratosphere during violent eruptions can cause rapid cooling of Earth's [[climate]] and affect air travel.{{sfn|Stern|2002|pp=27-31}}

Arc-magmatism plays a role in Earth's [[Carbon cycle]] by releasing subducted carbon through volcanic processes. Older theory states that the carbon from the subducting plate is made available in overlying magmatic systems via decarbonation, where CO{{Subscript|2}} is released through silicate-carbonate metamorphism.<ref name="Frezzotti-2011"/> However, evidence from thermodynamic modeling has shown that the pressures and temperatures necessary for this type of metamorphism are much higher than what is observed in most subduction zones.<ref name="Frezzotti-2011">{{Cite journal |last1=Frezzotti |first1=M. L. |last2=Selverstone |first2=J. |last3=Sharp |first3=Z. D. |last4=Compagnoni |first4=R. |date=2011 |title=Carbonate dissolution during subduction revealed by diamond-bearing rocks from the Alps |url=http://dx.doi.org/10.1038/ngeo1246 |journal=Nature Geoscience |volume=4 |issue=10 |pages=703–706 |doi=10.1038/ngeo1246 |bibcode=2011NatGe...4..703F |issn=1752-0894|url-access=subscription }}</ref> Frezzoti et al. (2011) propose a different mechanism for carbon transport into the overriding plate via dissolution (release of carbon from carbon-bearing minerals into an aqueous solution) instead of decarbonation. Their evidence comes from the close examination of mineral and fluid inclusions in low-temperature (<600&nbsp;°C) diamonds and garnets found in an eclogite facies in the Alps. The chemistry of the inclusions supports the existence of a carbon-rich fluid in that environment, and additional chemical measurements of lower pressure and temperature facies in the same tectonic complex support a model for carbon dissolution (rather than decarbonation) as a means of carbon transport.<ref name="Frezzotti-2011" />