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Volcanism is the phenomenon where solids, liquids, gases, and their mixtures erupt to the surface of a body.^[1] It is caused by the presence of an internal heat source in a solid-surface planet or moon, which partially melts or turns into gas material that then breaks through the solid surface.^[2]

For volcanism to occur, the temperature of the mantle must have risen to about half its melting point. At this point, the mantle’s viscosity will have dropped to about 10²¹ Pascal-seconds. When large scale melting occurs, the viscosity rapidly falls to 10³ Pascal-seconds or even less, increasing the heat transport rate a million-fold.^[3]

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.^[3]

There are multiple ways to generate the heat needed for volcanism. Volcanism on outer solar system moons is powered mainly by tidal heating.^[1] Tidal heating caused by the deformation of a body’s shape due to mutual gravitational attraction, which generates heat. 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 total heat.^[4]

During the formative days of a planet, it would have experienced heating from impacts from planetesimals, which would have dwarfed even the asteroid impact that caused the extinction of dinosaurs. This heating could trigger differentiation, further heating the planet. The larger a body is, the slower it loses heat. In larger bodies like Earth, this heat, known as primordial heat, still makes up much of the body’s internal heat, but our Moon, which is smaller than Earth, has lost most of this heat.^[4]

Another heat source is radiogenic heat, caused by radioactive decay. The decay of Al-26 would have significantly heated planetary embryos, but due to its short half-life (less than a million years), any traces of it have long since vanished. There are small traces of unstable isotopes in common minerals, and all the terrestrial planets, and our Moon, experience some of this heating.^[4] 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).^[5]

On Neptune’s moon Triton and possibly on Mars, cryogeyser activity takes place, with the source of heat being external rather than internal (solar heating). ^[6][7]

This 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.^[8][3] However, in the case of water, increasing pressure decreases melting point until a pressure of 0.208 GPa is reached, after which the melting point increases with pressure.^[3]

This occurs when the melting point is lowered by the addition of volatiles, like water.^[3] Like decompression melting, it is not caused by an increase in temperature, but rather by a decrease in melting point.^[9]

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 model developed to investigate the effects of this on 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 propogation 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 reserviors.^[5]

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 intersections and where different crystals react to form eutectic liquid, that initially remain isolated from one another, trapped inside rock. If the contact angle of the melted material allows the melt to wet crystal faces and run along grain boundaries, the melted material will accumulate into larger quantities. On the other hand, if the angle is greater than about 60 degrees, much more melt must form before it can seperate 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 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 together to create larger veins. Under the influence of buoyancy, the melt rises.^[3] Diapirs may also form in non-silicate bodies, playing a similiar role in moving warm material towards the surface.^[5]

A 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.^[3]

This model of volcanic eruption, while now discredited, explains observations very well, which newer models cannot. The model 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, such as an apparent concordance of the elevation of volcanoes nearby each other, it cannot be correct, as the lithosphere thickness derived from it is too large for the assumption of a rigid open channel to hold.^[3]

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.^[5]

For a crack in the ice shell to propagate upwards, the fluid in it must either be positively buoyant 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.^[10]

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.^[5]

A 1988 article proposed a possibility for fractures propagating upwards from the subsurface ocean of Jupiter’s 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.^[5]

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.^[5]

Note that not all of these mechanisms, and maybe even none, operate on a given body.^[5]

A volcanic eruption could just be a simple outpouring of material onto the surface of a planet, but they usually involve a complex mixture of solids, liquids and gases which behave in equally complex ways.^[3]

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.^[11] 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.^[3]

The reason the dissolved gas in the magma seperates 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 seperate) 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.^[3]

Fluid magmas erupt quietly. Any dissolved gas trapped in the magma easily escapes once it reaches the surface. However, silica-rich magmas have such high viscosities that gases remain trapped in the magma even after they have exsolved, forming bubbles inside the magma. This fact gives silica-rich volcanoes a tendency to ‘explode’, although instead of the pressure increase associated with an explosion, pressure always decreases in a volcanic eruption.^[3]

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.

The violently expanding gas disperses and breaks up magma, forming an emulsion 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 emulsions depends strongly on the ratio of liquid to gas. Gas-poor magmas end up cooling into rocks with small cavities, becoming 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.^[3]

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.^[5]

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 process is thought to be partially responsible for Enceladus’s ice plumes.^[5]