Editing Volcanism (section)

==Causes==
[[File:Magmatism and volcanism EN.svg|thumb|upright=1.75|[[Cross section (geology)|Cross section]] diagram of Earth showing some settings for volcanism on the planet]]
For volcanism to occur, the temperature of the [[Mantle (geology)|mantle]] must have risen to about half its melting point. At this point, the mantle's [[viscosity]] will have dropped to about 10<sup>21</sup> [[Viscosity|Pascal-seconds]]. When large scale melting occurs, the viscosity rapidly falls to 10<sup>3</sup> Pascal-seconds or even less, increasing the heat transport rate a million-fold.<ref name=":2" />

The occurrence of volcanism is partially due to the fact that melted material tends to be more mobile and less dense than the materials from which they were produced, which can cause it to rise to the surface.<ref name=":2" />

===Heat source===
There are multiple ways to generate the heat needed for volcanism. Volcanism on outer solar system [[Natural satellite|moons]] is powered mainly by [[tidal heating]].<ref name=":0" /> Tidal heating is caused by the deformation of a body's shape due to mutual gravitational attraction, which generates heat. Tidal heating is the cause of volcanism on [[Io (moon)|Io]],<ref name="Lopes_Williams_2015">{{cite book |chapter=Volcanism on Io |last1=Lopes |first1=R.M.C |last2=Williams |first2=D.A. |title=Encyclopedia of Volcanoes |year=2015 |edition=2 |editor-last=Sigurdsson |editor-first=H. |page=750 |isbn=978-0-12-385938-9 |publisher=Academic Press}}</ref> a moon of Jupiter. Earth experiences tidal heating from the [[Moon]], deforming by up to 1 metre (3 feet), but this does not make up a major portion of [[Earth's internal heat budget|Earth's total heat]].<ref name=":1">{{Cite book |first=Mike |last=Widdowson |editor1-first=David A. |editor1-last=Rothery | editor2-first=Neil |editor2-last=McBride |editor3-first=Iain |editor3-last=Gilmour |title=An Introduction to the Solar System |chapter=Origins of planets and planetary layering |chapter-url=https://books.google.com/books?id=SjdqDwAAQBAJ&pg=PA52 |date=2018 |isbn=978-1-108-43084-5 |pages=52–71 |edition=3rd |publisher=Cambridge University Press}}</ref>

During a [[Planet formation|planet's formation]], it would have experienced heating from [[Impact event|impacts]] from [[planetesimal]]s, which would have dwarfed even the [[Chicxulub crater|asteroid impact that caused the extinction of dinosaurs]]. This heating could trigger [[Planetary differentiation|differentiation]], further heating the planet. The larger a [[Astronomical object|body]] is, the slower it loses heat. In larger bodies, for example Earth, this heat, known as primordial heat, still makes up much of the body's internal heat, but the Moon, which is smaller than Earth, has lost most of this heat.<ref name=":1" />

Another heat source is radiogenic heat, caused by [[Radioactivity|radioactive decay]]. The decay of [[aluminium-26]] would have significantly heated planetary embryos, but due to its short [[Half life|half-life]] (less than a million years), any traces of it have long since vanished. There are small traces of [[Radionuclide|unstable isotopes]] in common minerals, and all the [[terrestrial planet]]s, and the Moon, experience some of this heating.<ref name=":1" /> The icy bodies of the outer solar system experience much less of this heat because they tend to not be very dense and not have much [[silicate]] material (radioactive elements concentrate in silicates).<ref name=":3" />

On Neptune's moon [[Triton (moon)|Triton]], and possibly on Mars, ''cryogeyser'' activity takes place. The source of heat is external (heat from the Sun) rather than internal.<ref name = "THEMIS">{{cite web | last = Burnham | first = Robert   | title = Gas jet plumes unveil mystery of 'spiders' on Mars   | website = [[Arizona State University]]  | date = 2006-08-16  | url = http://www.asu.edu/news/stories/200608/20060818_marsplumes.htm  | access-date = 2009-08-29 | url-status=dead | archive-url=https://web.archive.org/web/20071221171628/http://www.asu.edu/news/stories/200608/20060818_marsplumes.htm | archive-date=2007-12-21}}</ref><ref>{{Cite web |title=Planetary Volcanism |url=https://www2.mps.mpg.de/solar-system-school/lectures/planetary_interiors_surfaces/markiewicz.pdf |last=Markiewicz |first=W. |work=Solar System School |publisher=[[Max Planck Institute for Solar System Research|International Max Planck Research School for Solar System Science]], University of Göttingen|access-date=2024-03-17}}</ref>

===Melting methods===
====Decompression melting====
[[Decompression melting]] happens when solid material from deep beneath the body rises upwards. Pressure decreases as the material rises upwards, and so does the melting point. So, a rock that is solid at a given pressure and temperature can become liquid if the pressure, and thus melting point, decreases even if the temperature stays constant.<ref>{{Cite book |chapter=3.2 Magma and Magma Formation | date=September 2015 |title=Physical Geology|url=https://opentextbc.ca/geology/chapter/3-2-magma-and-magma-formation/#:~:text=Decompression%20melting%20takes%20place%20within,of%20a%20mantle%20convection%20cell |last1=Earle |first1=Steven |publisher=BCcampus Open Education |access-date=17 March 2024}}</ref><ref name=":2" /> However, in the case of water, increasing pressure decreases melting point until a pressure of 0.208 [[Pascal (unit)|GPa]] is reached, after which the melting point increases with pressure.<ref name=":2" />

====Flux melting====
[[Flux melting]] occurs when the melting point is lowered by the addition of volatiles, for example, water or carbon dioxide.<ref name=":2" /><ref name="NatGeo">{{cite web | url=https://education.nationalgeographic.org/resource/magma-role-rock-cycle/ | title=Magma's Role in the Rock Cycle | publisher=[[National Geographic Society]] | date=19 October 2023 | access-date=17 April 2024 | editor1-first=Jeannie |editor1-last=Evers |editor2-last=Emdash Editing}}</ref> Like decompression melting, it is not caused by an increase in temperature, but rather by a decrease in melting point.<ref>{{Cite web |title=3.2 Magma and Magma Formation | date=September 2015 |url=https://opentextbc.ca/geology/chapter/3-2-magma-and-magma-formation/#:~:text=Decompression%20melting%20takes%20place%20within,of%20a%20mantle%20convection%20cell. | last1=Earle | first1=Steven }}</ref>

====Formation of cryomagma reservoirs====
[[Cryovolcanism]], instead of originating in a uniform subsurface ocean, may instead take place from discrete liquid reservoirs. The first way these can form is a plume of warm ice welling up and then sinking back down, forming a convection current. A [[Scientific modelling|model]] developed to investigate the effects of this on [[Europa (moon)|Europa]] found that energy from tidal heating became focused in these plumes, allowing melting to occur in these shallow depths as the plume spreads laterally (horizontally). The next is a switch from vertical to horizontal propagation of a fluid filled crack. Another mechanism is heating of ice from release of stress through lateral motion of fractures in the ice shell penetrating it from the surface, and even heating from large impacts can create such reservoirs.<ref name=":3">{{Cite book |chapter=Cryovolcanism |title=Planetary Volcanism Across the Solar System |chapter-url=https://ntrs.nasa.gov/api/citations/20210026013/downloads/Quick_%20Chapter5_Cryovolcanism_13Jun2021.docx.pdf |first1=Sarah A. |last1=Fagents |first2=Rosaly M.C. |last2=Lopes |first3=Lynnae C. |last3=Quick |first4=Tracy K.P. |last4=Gregg |editor1-first=Tracy K.P. |editor1-last=Gregg |editor2-first=Rosaly M.C. |editor2-last=Lopes |editor2-link=Rosaly Lopes |editor3-first=Sarah A. |editor3-last=Fagents |date=2021 |publisher=Elsevier |isbn=978-0-12-813987-5 |pages=161–234}}</ref>

===Ascent of melts===
[[File:Volcanosed.svg|thumb|upright=1.75|Some features of volcanism found in Earth's crust]]

====Diapirs====
When material of a planetary body begins to melt, the melting first occurs in small pockets in certain high energy locations, for example [[Grain boundary |grain boundary intersections]] and where different crystals react to form [[Eutectic system|eutectic liquid]], that initially remain isolated from one another, trapped inside rock. If the [[contact angle]] of the melted material allows the melt to [[Wetting|wet]] crystal faces and run along [[Grain boundary|grain boundaries]], the melted material will accumulate into larger quantities. On the other hand, if the contact angle is greater than about 60 degrees, much more melt must form before it can separate from its parental rock. Studies of rocks on Earth suggest that melt in hot rocks quickly collects into pockets and veins that are much larger than the [[Crystallite|grain]] size, in contrast to the model of rigid melt [[percolation]]. Melt, instead of uniformly flowing out of source rock, flows out through rivulets which join to create larger veins. Under the influence of [[buoyancy]], the melt rises.<ref name=":2" /> Diapirs may also form in non-silicate bodies, playing a similar role in moving warm material towards the surface.<ref name=":3" />

====Dikes====
A [[Dike (geology)|dike]] is a vertical fluid-filled crack, from a mechanical standpoint it is a water filled crevasse turned upside down. As magma rises into the vertical crack, the low density of the magma compared to the wall rock means that the pressure falls less rapidly than in the surrounding denser rock. If the average pressure of the magma and the surrounding rock are equal, the pressure in the dike exceeds that of the enclosing rock at the top of the dike, and the pressure of the rock is greater than that of the dike at its bottom. So the magma thus pushes the crack upwards at its top, but the crack is squeezed closed at its bottom due to an elastic reaction (similar to the bulge next to a person sitting down on a springy sofa). Eventually, the tail gets so narrow it nearly pinches off, and no more new magma will rise into the crack. The crack continues to ascend as an independent pod of magma.<ref name=":2" />

====Standpipe model====
This model of volcanic eruption posits that magma rises through a rigid open channel, in the lithosphere and settles at the level of [[hydrostatic equilibrium]]. Despite how it explains observations well (which newer models cannot), such as an apparent concordance of the elevation of [[volcano]]es near each other, it cannot be correct and is now discredited, because the lithosphere thickness derived from it is too large for the assumption of a rigid open channel to hold.<ref name=":2" />

====Cryovolcanic melt ascent====
Unlike silicate volcanism, where melt can rise by its own buoyancy until it reaches the shallow crust, in cryovolcanism, the water (cryomagmas tend to be water based) is denser than the ice above it. One way to allow cryomagma to reach the surface is to make the water buoyant, by making the water less dense, either through the presence of other compounds that reverse negative buoyancy, or with the addition of exsolved gas bubbles in the cryomagma that were previously dissolved into it (that makes the cryomagma less dense), or with the presence of a densifying agent in the ice shell. Another is to pressurise the fluid to overcome negative buoyancy and make it reach the surface. When the ice shell above a subsurface ocean thickens, it can pressurise the entire ocean (in cryovolcanism, frozen water or brine is less dense than in liquid form). When a reservoir of liquid partially freezes, the remaining liquid is pressurised in the same way.<ref name=":3" />

For a crack in the ice shell to propagate upwards, the fluid in it must have positive buoyancy or external stresses must be strong enough to break through the ice. External stresses could include those from tides or from overpressure due to freezing as explained above.<ref name=":4">{{Cite journal |first1=M. |first2=S.J. |first3=E.L. |first4=C.R. |last1=Neveu |last2=Desch |last3=Shock |last4=Glein |title=Prerequisites for explosive cryovolcanism on dwarf planet-class Kuiper Belt objects |doi=10.1016/j.icarus.2014.03.043 |journal=Icarus |year=2015 |volume=246 |pages=48–64|bibcode=2015Icar..246...48N |hdl=2286/R.I.28139 |hdl-access=free }}</ref>

There is yet another possible mechanism for ascent of cryovolcanic melts. If a fracture with water in it reaches an ocean or subsurface fluid reservoir, the water would rise to its level of hydrostatic equilibrium, at about nine-tenths of the way to the surface. Tides which induce compression and tension in the ice shell may pump the water farther up.<ref name=":3" />

A 1988 article proposed a possibility for fractures propagating upwards from the subsurface ocean of Jupiter's [[Natural satellite|moon]] Europa. It proposed that a fracture propagating upwards would possess a low pressure zone at its tip, allowing volatiles dissolved within the water to exsolve into gas. The elastic nature of the ice shell would likely prevent the fracture reaching the surface, and the crack would instead pinch off, enclosing the gas and liquid. The gas would increase buoyancy and could allow the crack to reach the surface.<ref name=":3" />

Even impacts can create conditions that allow for enhanced ascent of magma. An impact may remove the top few kilometres of crust, and pressure differences caused by the difference in height between the basin and the height of the surrounding terrain could allow eruption of magma which otherwise would have stayed beneath the surface. A 2011 article showed that there would be zones of enhanced magma ascent at the margins of an impact basin.<ref name=":3" />

Not all of these mechanisms, and maybe even none, operate on a given [[Astronomical object|body]].<ref name=":3" />