Editing Volcanism (section)

===Causes of explosive activity===

====Exsolution of volatiles====
Volcanic eruptions on Earth have been consistently observed to progress from erupting gas rich material to gas depleted material, although an eruption may alternate between erupting gas rich to gas depleted material and vice versa multiple times. This can be explained by the enrichment of magma at the top of a dike by gas which is released when the dike breaches the surface, followed by magma from lower down than did not get enriched with gas.<ref name=":2" />

The reason the dissolved gas in the magma separates from it when the magma nears the surface is due to the effects of temperature and pressure on gas [[solubility]]. Pressure increases gas solubility, and if a liquid with dissolved gas in it depressurises, the gas will tend to exsolve (or separate) from the liquid. An example of this is what happens when a bottle of carbonated drink is quickly opened: when the seal is opened, pressure decreases and bubbles of carbon dioxide gas appear throughout the liquid.<ref name=":2" />

Fluid magmas erupt quietly. Any gas that has exsolved from the magma easily escapes even before it reaches the surface. However, in [[Viscocity|viscous]] magmas, gases remain trapped in the magma even after they have exsolved, forming bubbles inside the magma. These bubbles enlarge as the magma nears the surface due to the dropping pressure, and the magma grows substantially. This fact gives volcanoes erupting such material a tendency to ‘explode’, although instead of the pressure increase associated with an explosion, pressure always decreases in a volcanic eruption.<ref name=":2" />

Generally, explosive cryovolcanism is driven by exsolution of volatiles that were previously dissolved into the cryomagma, similar to what happens in explosive silicate volcanism as seen on Earth, which is what is mainly covered below.<ref name=":4" />

====Physics of a volatile-driven explosive eruption====
Silica-rich magmas cool beneath the surface before they erupt. As they do this, bubbles exsolve from the magma. As the magma nears the surface, the bubbles and thus the magma increase in volume. The resulting pressure eventually breaks through the surface, and the release of pressure causes more gas to exsolve, doing so explosively. The gas may expand at hundreds of metres per second, expanding upward and outward. As the eruption progresses, a chain reaction causes the magma to be ejected at higher and higher speeds.<ref name=":2" />

=====Volcanic ash formation=====
The violently expanding gas disperses and breaks up magma, forming a [[colloid]] of gas and magma called [[volcanic ash]]. The cooling of the gas in the ash as it expands chills the magma fragments, often forming tiny glass shards recognisable as portions of the walls of former liquid bubbles. In more fluid magmas the bubble walls may have time to reform into spherical liquid droplets. The final state of the colloids depends strongly on the ratio of liquid to gas. Gas-poor magmas end up cooling into rocks with small cavities, becoming [[Vesicular texture|vesicular lava]]. Gas-rich magmas cool to form rocks with cavities that nearly touch, with an average density less than that of water, forming [[pumice]]. Meanwhile, other material can be accelerated with the gas, becoming [[volcanic bomb]]s. These can travel with so much energy that large ones can create craters when they hit the ground.<ref name=":2" />

=====Pyroclastic flows=====
A colloid of [[volcanic gas]] and magma can form as a density current called a [[pyroclastic flow]]. This occurs when erupted material falls back to the surface. The colloid is somewhat fluidised by the gas, allowing it to spread. Pyroclastic flows can often climb over obstacles, and devastate human life.<ref name=":2" /> Pyroclastic flows are a common feature at explosive volcanoes on Earth. Pyroclastic flows have been found on Venus, for example at the [[Dione Regio]] volcanoes.<ref name="Campbell_et_al_2017">{{cite journal | title=Pyroclastic flow deposits on Venus as indicators of renewed magmatic activity | first1=B.A. | last1=Campbell  | first2=G.A. | last2=Morgan | first3=J.L. | last3=Whitten | first4=L.M. | last4=Carter | first5=L.S. | last5=Glaze | first6=D.B. | last6=Campbell | journal=Journal of Geophysical Research: Planets | year=2017 | volume=122 | issue=7 | pages=1580–1596 | doi=10.1002/2017JE005299| pmid=31709132 | pmc=6839737 | bibcode=2017JGRE..122.1580C | hdl=10150/625517 | hdl-access=free }}</ref>

====Phreatic eruption====
A [[phreatic eruption]] can occur when hot water under pressure is depressurised. Depressurisation reduces the boiling point of the water, so when depressurised the water suddenly boils.<ref name="ESKP">{{Cite journal |title=Dangerous water vapor: phreatic eruptions |url=https://www.eskp.de/en/basic-knowledge/natural-hazards/dangerous-water-vapour-phreatic-eruptions-935881/ |last=Strehlow |first=K. |journal=Earth Science Knowledge Platform |date=2016-11-22 |doi=10.2312/eskp.051 |access-date=2024-03-17}}</ref> Or it may happen when groundwater is suddenly heated, flashing to steam suddenly.<ref name="USGS">{{cite web |title=VHP Photo Glossary: Phreatic eruption |url=http://volcanoes.usgs.gov/images/pglossary/HydroVolcEruption.php |access-date=13 November 2010 |work=Volcano Hazards Program |publisher=U.S. Geological Survey}}</ref>
When water turns into steam in a phreatic eruption, it expands at supersonic speeds, up to 1,700 times its original volume. This can be enough to shatter solid rock, and hurl rock fragments hundreds of metres.<ref>{{Cite web |last=Cronin |first=Shane |date=December 9, 2019 |title=Steam-driven volcanic eruptions difficult to predict, still poorly understood |url=https://www.upi.com/Science_News/2019/12/09/Steam-driven-volcanic-eruptions-difficult-to-predict-poorly-understood/5221575896208/ |website=UPI.com |access-date=2024-03-17}}</ref>

====Phreatomagmatic eruption====
A [[phreatomagmatic eruption]] occurs when hot magma makes contact with water, creating an explosion.<ref>{{Cite web |last=Mcnair |first=B. |date=January 10, 2024 |title=What Are Phreatomagmatic Eruptions and How They Form? |url=https://geologybase.com/phreatomagmatic-eruptions/ |website=Geology Base |access-date=2024-03-17}}</ref>

====Clathrate hydrates====
[[File:PIA19059-SaturnMoon-Enceladus-OceanMethane-20150311.png|thumb|Diagrammatic representation of a plume on Enceladus]]
One mechanism for explosive cryovolcanism is cryomagma making contact with [[clathrate hydrates]]. Clathrate hydrates, if exposed to warm temperatures, readily decompose. A 1982 article pointed out the possibility that the production of pressurised gas upon destabilisation of clathrate hydrates making contact with warm rising magma could produce an explosion that breaks through the surface, resulting in explosive cryovolcanism.<ref name=":3" />

====Water vapor in a vacuum====
If a fracture reaches the surface of an icy body and the column of rising water is exposed to the near-vacuum of the surface of most icy bodies, it will immediately start to boil, because its vapor pressure is much more than the ambient pressure. Not only that, but any volatiles in the water will exsolve. The combination of these processes will release droplets and vapor, which can rise up the fracture, creating a plume. This is thought to be partially responsible for [[Enceladus|Enceladus's]] ice plumes.<ref name=":3" />