Editing Stratovolcano (section)

==Formation==
[[Subduction|Subduction zone]] volcanoes form when [[hydrous]] [[mineral]]s are pulled down into the mantle on the slab. These hydrous minerals, such as [[chlorite]] and [[Serpentine subgroup|serpentine]], release their water into the [[Earth's mantle|mantle]] which decreases its [[melting point]] by 60 to 100&nbsp;°C. The release of water from [[hydrate]]d minerals is termed "[[dewatering]]", and occurs at specific pressures and temperatures for each mineral, as the plate descends to greater depths.<ref>{{Citation|last1=Schmidt|first1=A.|last2=Rüpke|first2=L. H.|last3=Morgan|first3=J. P.|last4=Hort|first4=M.|date=1 December 2001|title=How Large a Feedback Effect Does Slab Dewatering Have on Itself ?|journal=AGU Fall Meeting Abstracts |url=https://ui.adsabs.harvard.edu/abs/2001AGUFM.T41C0871S/abstract|volume=2001|pages=T41C–0871|bibcode=2001AGUFM.T41C0871S }}</ref> This allows the [[Earth's mantle|mantle]] to partially melt and generate [[magma]]. This is called [[flux melting]]. The magma then rises through the [[Crust (geology)|crust]], incorporating silica-rich crustal rock, leading to a final [[intermediate composition]]. When the magma nears the top surface, it pools in a [[magma chamber]] within the crust below the stratovolcano.<ref>{{Cite web|title=4 Igneous Processes and Volcanoes – An Introduction to Geology|url=https://opengeology.org/textbook/4-igneous-processes-and-volcanoes/|access-date=25 October 2024|language=en}}</ref>

The processes that trigger the final eruption remain a question for further research. Possible mechanisms include:<ref name=":3">{{cite journal|last1=Cañón-Tapia|first1=Edgardo|title=Volcanic eruption triggers: A hierarchical classification|journal=Earth-Science Reviews|date=February 2014|volume=129|pages=100–119|doi=10.1016/j.earscirev.2013.11.011|bibcode=2014ESRv..129..100C}}</ref>

* [[Igneous differentiation|Magma differentiation]], in which the lightest, most silica-rich magma and volatiles such as water, [[halogen]]s, and [[sulfur dioxide]] accumulate in the uppermost part of the magma chamber. This can dramatically increase pressures.<ref>{{Cite web|last=Nelson|first=Stephan|date=14 September 2015|title=Volcanoes, Magma, and Volcanic Eruptions|url=https://www2.tulane.edu/~sanelson/Natural_Disasters/volcan&magma.htm|publisher=Tulane University}}</ref>
* [[Fractional crystallization (geology)|Fractional crystallization]] of the magma. When [[anhydrous]] [[mineral]]s such as [[feldspar]] crystallize out of the magma, this concentrates volatiles in the remaining liquid, which can lead to a second boiling that causes a [[Phase (matter)|gas phase]] (carbon dioxide or water) to separate from the liquid magma and raise magma chamber pressures.<ref>{{cite journal|last1=Wech|first1=Aaron G.|last2=Thelen|first2=Weston A.|last3=Thomas|first3=Amanda M.|title=Deep long-period earthquakes generated by second boiling beneath Mauna Kea volcano|journal=Science|date=15 May 2020|volume=368|issue=6492|pages=775–779|doi=10.1126/science.aba4798|pmid=32409477|bibcode=2020Sci...368..775W|s2cid=218648557}}</ref><ref>{{Cite journal|last1=Garcia-Arias|first1=Marcos|last2=Stevens|first2=Gary|date=15 April 2017|title=Phase equilibrium modelling of granite magma petrogenesis: B. An evaluation of the magma compositions that result from fractional crystallization|url=https://www.sciencedirect.com/science/article/abs/pii/S002449371630319X|journal=Lithos|series=Eighth Hutton Symposium on Granites and Related Rocks|volume=277|pages=109–130|doi=10.1016/j.lithos.2016.09.027|bibcode=2017Litho.277..109G |issn=0024-4937}}</ref>
* Injection of fresh magma into the magma chamber, which mixes and heats the cooler magma already present. This could force volatiles out of solution and lower the density of the cooler magma, both of which increase pressure. There is considerable evidence for magma mixing just before many eruptions, including magnesium-rich [[olivine]] crystals in freshly erupted silicic [[lava]] that show no reaction rim. This is possible only if the [[lava]] erupted immediately after mixing since olivine rapidly reacts with silicic magma to form a rim of [[pyroxene]].<ref>{{Cite journal|last1=Scandone|first1=Roberto|last2=Cashman|first2=Katharine V.|last3=Malone|first3=Stephen D.|date=30 January 2007|title=Magma supply, magma ascent and the style of volcanic eruptions|url=https://www.sciencedirect.com/science/article/abs/pii/S0012821X06008181|journal=Earth and Planetary Science Letters|volume=253|issue=3|pages=513–529|doi=10.1016/j.epsl.2006.11.016|bibcode=2007E&PSL.253..513S |issn=0012-821X}}</ref>
* Progressive melting of the surrounding [[country rock (geology)|country rock]].<ref name=":3" />

These internal triggers may be modified by external triggers such as [[sector collapse]], [[earthquake]]s, or [[Phreatomagmatic eruption|interactions with groundwater]]. Some of these triggers operate only under limited conditions. For example, sector collapse (where part of the flank of a volcano collapses in a massive [[landslide]]) can only trigger the eruption of a very shallow magma chamber. Magma differentiation and [[thermal expansion]] also are ineffective as triggers for eruptions from deep magma chambers.<ref name=":3" />