Editing Volcanism (section)

===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" />