Editing Subduction (section)

==Characteristics and effects==

===Metamorphism===
{{main|Subduction zone metamorphism}}
Subduction zones host a unique variety of rock types created by the high-pressure, low-temperature conditions a subducting slab encounters during its descent.<ref>Zheng, Y.-F., Chen, Y.-X., 2016. Continental versus oceanic subduction zones. National Science Review 3, 495-519.</ref> The metamorphic conditions the slab passes through in this process create and destroy water bearing (hydrous) mineral phases, releasing water into the mantle. This water lowers the melting point of mantle rock, initiating melting.<ref name=sdsu>{{cite web|title=How Volcanoes work – Subduction Zone Volcanism|url=http://www.geology.sdsu.edu/how_volcanoes_work/subducvolc_page.html|publisher=San Diego State University Department of Geological Science|access-date=2021-04-11|archive-date=2018-12-29|archive-url=https://web.archive.org/web/20181229025459/http://www.geology.sdsu.edu/how_volcanoes_work/subducvolc_page.html|url-status=dead}}</ref> Understanding the timing and conditions in which these dehydration reactions occur is key to interpreting mantle melting, volcanic arc magmatism, and the formation of continental crust.<ref name=Mibe>{{cite journal|last1=Mibe|first1=Kenji|title= Slab melting versus slab dehydration in subduction zones|doi=10.1073/pnas.1010968108|journal=Proceedings of the National Academy of Sciences|date=2011|volume=108|issue=20|pages=8177–8182|display-authors=etal|pmid=21536910|pmc=3100975|doi-access=free}}</ref>

A [[metamorphic facies]] is characterized by a stable mineral assemblage specific to a pressure-temperature range and specific starting material. Subduction zone [[metamorphism]] is characterized by a low temperature, [[Ultra-high-pressure metamorphism|high-ultrahigh pressure metamorphic]] path through the [[zeolite]], prehnite-pumpellyite, [[blueschist]], and [[eclogite]] facies stability zones of subducted oceanic crust.<ref name="Zheng, Y 2017">Zheng, Y.-F., Chen, R.-X., 2017. Regional metamorphism at extreme conditions: Implications for orogeny at convergent plate margins. Journal of Asian Earth Sciences 145, 46–73.</ref> [[Zeolite]] and prehnite-pumpellyite facies assemblages may or may not be present, thus the onset of metamorphism may only be marked by blueschist facies conditions.<ref name="Winter 541">{{cite book|last1=Winter|first1=John D.|title=Principles of Igneous and Metamorphic Petrology|publisher=Prentice Hall|isbn=978-0-321-59257-6|pages=541–548|year=2010}}</ref> Subducting slabs are composed of basaltic crust topped with [[pelagic sediments]];<ref>{{cite book|last1=Reynolds|first1=Stephen|title=Exploring Geology|publisher=McGraw-Hill|isbn=978-0073524122|pages=124|date=2012-01-09}}</ref> however, the pelagic sediments may be accreted onto the forearc-hanging wall and not subducted.<ref>{{cite journal|last1=Bebout|first1=Grey E.|title=Metamorphic Chemical Geodynamics of Subduction|journal=Earth and Planetary Science Letters|date=May 31, 2007|volume=260|issue=3–4 |pages=375|doi=10.1016/j.epsl.2007.05.050|bibcode=2007E&PSL.260..373B}}</ref> Most metamorphic phase transitions that occur within the subducting slab are prompted by the dehydration of hydrous mineral phases. The breakdown of hydrous mineral phases typically occurs at depths greater than 10&nbsp;km.<ref name="Peacock 12-15">{{cite book |last1=Peacock|first1=Simon M.|chapter=Thermal Structure and Metamorphic Evolution of Subducting Slabs |pages=12–15 |editor-last1=Eiler |editor-first1=John |title=Inside the subduction factory |date=1 January 2004 |series=Geophysical Monograph Series |volume=138 |publisher=American Geophysical Union |isbn=9781118668573}}</ref> Each of these metamorphic facies is marked by the presence of a specific stable mineral assemblage, recording the metamorphic conditions undergone but the subducting slab. Transitions between facies cause hydrous minerals to dehydrate at certain pressure-temperature conditions and can therefore be tracked to melting events in the mantle beneath a volcanic arc.

===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" />

===Earthquakes and tsunamis===
[[File:Global subducted slabs USGS.png|thumb|Global map of subduction zones, with subducted slabs contoured by depth|350 px]]
{{Main|Megathrust earthquake}}
Elastic strain caused by plate convergence in subduction zones produces at least three types of earthquakes. These are deep earthquakes, megathrust earthquakes, and outer rise earthquakes. Deep earthquakes happen within the crust, megathrust earthquakes on the subduction interface near the trench, and outer rise earthquakes on the subducting lower plate as it bends near the trench.

Anomalously deep events are a characteristic of subduction zones, which produce the deepest quakes on the planet. Earthquakes are generally restricted to the shallow, brittle parts of the crust, generally at depths of less than twenty kilometers. However, in subduction zones quakes occur at depths as great as {{Convert|700|km|mi|abbr=on}}. These quakes define inclined zones of seismicity known as [[Wadati–Benioff zone]]s which trace the descending slab.{{sfn|Stern|2002|pp=17-18}}

Nine of the ten largest earthquakes of the last 100 years were subduction zone megathrust earthquakes. These included the [[1960 Valdivia earthquake|1960 Great Chilean earthquake]] which at M 9.5 was the largest earthquake ever recorded, the [[2004 Indian Ocean earthquake and tsunami]], and the [[2011 Tōhoku earthquake and tsunami]]. The subduction of cold oceanic lithosphere into the mantle depresses the local [[geothermal gradient]] and causes a larger portion of Earth's crust to deform in a more brittle fashion than it would in a normal geothermal gradient setting. Because earthquakes can occur only when a rock is deforming in a brittle fashion, subduction zones can cause large earthquakes. If such a quake causes rapid deformation of the sea floor, there is potential for [[tsunami]]s. The largest tsunami ever recorded happened due to a [[2004 Indian Ocean earthquake and tsunami|mega-thrust earthquake on December 26, 2004]]. The earthquake was caused by subduction of the Indo-Australian plate under the Eurasian plate, but the tsunami spread over most of the planet and devastated the areas around the Indian Ocean. Small tremors which cause small, nondamaging tsunamis, also occur frequently.{{sfn|Stern|2002|pp=17-18}}

A study published in 2016 suggested a new parameter to determine a subduction zone's ability to generate mega-earthquakes.<ref>{{Cite journal|last1=Bletery|first1=Quentin|last2=Thomas|first2=Amanda M.|last3=Rempel|first3=Alan W.|last4=Karlstrom|first4=Leif|last5=Sladen|first5=Anthony|last6=Barros|first6=Louis De|date=2016-11-25|title=Mega-earthquakes rupture flat megathrusts|journal=Science|language=en|volume=354|issue=6315|pages=1027–1031|doi=10.1126/science.aag0482|issn=0036-8075|pmid=27885027|bibcode=2016Sci...354.1027B|doi-access=free}}</ref> By examining subduction zone geometry and comparing the degree of lower plate curvature of the subducting plate in great historical earthquakes such as the 2004 Sumatra-Andaman and the 2011 Tōhoku earthquake, it was determined that the magnitude of earthquakes in subduction zones is inversely proportional to the angle of subduction near the trench, meaning that "the flatter the contact between the two plates, the more likely it is that mega-earthquakes will occur".<ref>{{Cite news|url=https://www.sciencedaily.com/releases/2016/11/161124150207.htm|title=Subduction zone geometry: Mega-earthquake risk indicator|work=ScienceDaily|access-date=2017-06-21|language=en}}</ref>

[[Outer trench swell|Outer rise]] earthquakes on the lower plate occur when normal faults oceanward of the subduction zone are activated by flexure of the plate as it bends into the subduction zone.<ref>{{cite journal
|url=https://sites.google.com/site/daniggcc/research-interests/former-research/tonga-kermadech-subduction
| title=Slab pull effects from a flexural analysis of the Tonga and Kermadec Trenches (Pacific Plate)
|author=Garcia-Castellanos, D. |author2=M. Torné |author3=M. Fernàndez
| journal=Geophys. J. Int.
| volume=141
| issue=2
| pages=479–485
| doi=10.1046/j.1365-246x.2000.00096.x
| year=2000|bibcode = 2000GeoJI.141..479G | doi-access=free
| hdl=10261/237992
| hdl-access=free
}}</ref> The [[2009 Samoa earthquake and tsunami|2009 Samoa earthquake]] is an example of this type of event. Displacement of the sea floor caused by this event generated a six-meter tsunami in nearby Samoa.

Seismic tomography has helped detect subducted lithospheric slabs deep in the mantle where no earthquakes occur.<ref name=":0" /> About one hundred slabs have been described in terms of depth and their timing and location of subduction.<ref name="Van der Meer-2017">{{cite web|url=http://www.atlas-of-the-underworld.org/|title=Atlas of the Underworld {{!}} Van der Meer, D.G., van Hinsbergen, D.J.J., and Spakman, W., 2017, Atlas of the Underworld: slab remnants in the mantle, their sinking history, and a new outlook on lower mantle viscosity, Tectonophysics|website=atlas-of-the-underworld.org|language=en-GB|access-date=2017-12-02}}</ref> The great seismic discontinuities in the mantle, at {{Convert|410|km|mi|abbr=on}} depth and {{Convert|670|km|mi|abbr=on}}, are disrupted by the descent of cold slabs in deep subduction zones. Some subducted slabs seem to have difficulty penetrating the major [[Transition zone (Earth)|discontinuity]] that marks the boundary between the upper mantle and lower mantle at a depth of about 670&nbsp;kilometers. Other subducted oceanic plates have sunk to the [[core–mantle boundary]] at 2890&nbsp;km depth. Generally, slabs decelerate during their descent into the mantle, from typically several cm/yr (up to ~10&nbsp;cm/yr in some cases) at the subduction zone and in the uppermost mantle, to ~1&nbsp;cm/yr in the lower mantle.<ref name="Van der Meer-2017" /> This leads to either folding or stacking of slabs at those depths, visible as thickened slabs in seismic tomography. Below ~1700&nbsp;km, there might be a limited acceleration of slabs due to lower viscosity as a result of inferred mineral phase changes until they approach and finally stall at the [[core–mantle boundary]].<ref name="Van der Meer-2017" /> Here the slabs are heated up by the ambient heat and are not detected anymore ~300 Myr after subduction.<ref name="Van der Meer-2017" />

===Orogeny===
{{Main|Orogeny}}
Orogeny is the process of mountain building. Subducting plates can lead to orogeny by bringing oceanic islands, oceanic plateaus, sediments and passive continental margins to convergent margins. The material often does not subduct with the rest of the plate but instead is accreted to (scraped off) the continent, resulting in [[exotic terrane]]s. The collision of this oceanic material causes crustal thickening and mountain-building. The accreted material is often referred to as an [[accretionary wedge]] or prism. These accretionary wedges can be associated with [[ophiolites]] (uplifted ocean crust consisting of sediments, pillow basalts, sheeted dykes, gabbro, and peridotite).<ref>{{cite book|title=Encyclopedia of Environmental Change|editor-first=John A.|editor-last=Matthews|volume=1|location=Los Angeles|publisher=SAGE Reference|year=2014}}</ref>

Subduction may also cause orogeny without bringing in oceanic material that accretes to the overriding continent. When the lower plate subducts at a shallow angle underneath a continent (something called "flat-slab subduction"), the subducting plate may have enough traction on the bottom of the continental plate to cause the upper plate to contract by folding, faulting, crustal thickening, and mountain building. Flat-slab subduction causes mountain building and volcanism moving into the continent, away from the trench, and has been described in western North America (i.e. Laramide orogeny, and currently in Alaska, South America, and East Asia.<ref name="Van der Meer-2017"/>

The processes described above allow subduction to continue while mountain building happens concurrently, which is in contrast to continent-continent collision orogeny, which often leads to the termination of subduction.

===Subduction of continental lithosphere===
Continents are pulled into subduction zones by the sinking oceanic plate they are attached to. Where continents are attached to oceanic plates with no subduction, there is a deep basin that accumulates thick suites of sedimentary and volcanic rocks known as a passive margin. Some passive margins have up to 10&nbsp;km of sedimentary and volcanic rocks covering the continental crust. As a passive margin is pulled into a subduction zone by the attached and negatively buoyant oceanic lithosphere, the sedimentary and volcanic cover is mostly scraped off to form an orogenic wedge. An orogenic wedge is larger than most accretionary wedges due to the volume of material there is to accrete. The continental basement rocks beneath the weak cover suites are strong and mostly cold, and can be underlain by a >200&nbsp;km thick layer of dense mantle. After shedding the low density cover units, the continental plate, especially if it is old, goes down the subduction zone. As this happens, metamorphic reactions increase the density of the continental crustal rocks, which leads to less buoyancy.

One study of the active Banda arc-continent collision claims that by unstacking the layers of rock that once covered the continental basement, but are now thrust over one another in the orogenic wedge, and measuring how long they are, can provide a minimum estimate of how far the continent has subducted.<ref>{{cite journal
| title=Australia going down under: Quantifying continental subduction during arc-continent accretion in Timor-Leste
|author=Garrett W. Tate|author2=Nadine McQuarrie |author3=Douwe J.J. van Hinsbergen |author4=Richard R. Bakker| author5=Ron Harris|author6=Haishui Jiang
| journal=Geosphere
| volume=11
| issue=6
| pages=1860–1883
| doi=10.1130/GES01144.1
| year=2015| bibcode=2015Geosp..11.1860T
| doi-access=free
}}</ref> The results show at least a minimum of 229 kilometers of subduction of the northern Australian continental plate. Another example may be the continued northward motion of India, which is subducting beneath Asia. The collision between the two continents initiated around 50 my ago, but is still active.

=== Intra-oceanic: ocean/ocean plate subduction ===
Oceanic-Oceanic plate subduction zones comprise roughly 40% of all subduction zone margins on the planet. The ocean-ocean plate relationship can lead to subduction zones between oceanic and continental plates, therefore highlighting how important it is to understand this subduction setting. Although it is not fully understood what causes the initiation of subduction of an oceanic plate under another oceanic plate, there are three main models put forth by Baitsch-Ghirardello et al. that explain the different regimes present in this setting.<ref>{{Cite journal |last=Baitsch-Ghirardello, Gerya, Burg |first=Bettina, Taras, Jean-Pierre |date=March 2014 |title=Geodynamic regimes of intra-oceanic subduction: Implications for arc extension vs. shortening processes |journal=Gondwana Research |volume=25 |issue=2 |pages=546–560 |doi=10.1016/j.gr.2012.11.003 |bibcode=2014GondR..25..546B |url=https://www.sciencedirect.com/science/article/pii/S1342937X12003528#f0025|url-access=subscription }}</ref>

The models are as follows: 
# ''retreating subduction:'' caused by weak coupling between the lower and upper plate which leads to the opening of a back arc basin and the subduction zone being moved by slab rollback.
# ''stable subduction:'' caused by intermediate coupling between the lower and upper plate. The subduction zone generally stays in the same place and the subduction plate subducts at a consistent angle. 
# ''advancing subduction:'' caused by strong coupling between the upper and lower plate. The subducting sediments thicken causing partially molten plumes to be on top of subducting plate.

=== Arc-continent collision and global climate ===
In their 2019 study, Macdonald et al. proposed that arc-continent collision zones and the subsequent [[obduction]] of oceanic lithosphere was at least partially responsible for controlling global climate. Their model relies on arc-continent collision in tropical zones, where exposed [[ophiolite]]s composed mainly of mafic material increase "global weatherability" and result in the storage of carbon through silicate weathering processes. This storage represents a [[carbon sink]], removing carbon from the atmosphere and resulting in global cooling. Their study correlates several [[Phanerozoic]] ophiolite complexes, including active arc-continent subduction, with known global cooling and glaciation periods.<ref>{{Cite journal |last1=Macdonald |first1=Francis A. |last2=Swanson-Hysell |first2=Nicholas L. |last3=Park |first3=Yuem |last4=Lisiecki |first4=Lorraine |last5=Jagoutz |first5=Oliver |date=2019 |title=Arc-continent collisions in the tropics set Earth's climate state |journal=Science |volume=364 |issue=6436 |pages=181–184 |doi=10.1126/science.aav5300 |pmid=30872536 |bibcode=2019Sci...364..181M |s2cid=78094267 |issn=0036-8075|doi-access=free }}</ref> This study does not discuss [[Milankovitch cycles]] as a driver of global climate cyclicity.