Volcanism

Volcanism, vulcanism, volcanicity, or volcanic activity is the phenomenon where solids, liquids, gases, and their mixtures erupt to the surface of a solid-surface astronomical body such as a planet or a moon. It is caused by the presence of a heat source, usually internally generated, inside the body; the heat is generated by various processes, such as radioactive decay or tidal heating. This heat partially melts solid material in the body or turns material into gas. The mobilized material rises through the body's interior and may break through the solid surface.

Causes

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 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> 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. 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 total heat.

During a planet's formation, 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, 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.

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. Like decompression melting, it is not caused by an increase in temperature, but rather by a decrease in melting point.

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 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 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.

Ascent of melts

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 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 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 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.

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.

Cryovolcanism

Cryovolcanism is the eruption of volatiles into an environment below their freezing point. The processes behind it are different to silicate volcanism because the cryomagma (which is usually water-based) is normally denser than its surroundings, meaning it cannot rise by its own buoyancy.

Lava types

When magma erupts onto a planet's surface, it is termed lava. Viscous lavas form short, stubby glass-rich flows. These usually have a wavy solidified surface texture. This splinters the surface of the lava, and the magma then collects into sacks that often pile up in front of the flow, forming a structure called a pillow. Block lava is another type of lava, with less jagged fragments than in a’a lava. Pahoehoe lava is by far the most common lava type, both on Earth and probably the other terrestrial planets. It has a smooth surface, with mounds, hollows and folds.

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.

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. Or it may happen when groundwater is suddenly heated, flashing to steam suddenly.

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.

Phreatomagmatic eruption

A phreatomagmatic eruption occurs when hot magma makes contact with water, creating an explosion.

Clathrate hydrates

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.

Large eruptions can affect atmospheric temperature as ash and droplets of sulfuric acid obscure the Sun and cool Earth's troposphere. Historically, large volcanic eruptions have been followed by volcanic winters which have caused catastrophic famines.

Moon

Earth's Moon has no large volcanoes and no current volcanic activity, although recent evidence suggests it may still possess a partially molten core. However, the Moon does have many volcanic features such as maria (the darker patches seen on the Moon), rilles and domes.

Venus

The planet Venus has a surface that is 90% basalt, indicating that volcanism played a major role in shaping its surface. The planet may have had a major global resurfacing event about 500 million years ago, from what scientists can tell from the density of impact craters on the surface. Lava flows are widespread and forms of volcanism not present on Earth occur as well. Changes in the planet's atmosphere and observations of lightning have been attributed to ongoing volcanic eruptions, although there is no confirmation of whether or not Venus is still volcanically active. However, radar sounding by the Magellan probe revealed evidence for comparatively recent volcanic activity at Venus's highest volcano Maat Mons, in the form of ash flows near the summit and on the northern flank. However, the interpretation of the flows as ash flows has been questioned.

Mars

thumb|upright|[[Olympus Mons (Latin, "Mount Olympus"), located on the planet Mars, is the tallest known mountain in the Solar System.]]

There are several extinct volcanoes on Mars, four of which are vast shield volcanoes far bigger than any on Earth. They include Arsia Mons, Ascraeus Mons, Hecates Tholus, Olympus Mons, and Pavonis Mons. These volcanoes have been extinct for many millions of years, but the European Mars Express spacecraft has found evidence that volcanic activity may have occurred on Mars in the recent past as well. Its lavas are the hottest known anywhere in the Solar System, with temperatures exceeding 1,800 K (1,500 °C). In February 2001, the largest recorded volcanic eruptions in the Solar System occurred on Io.

Europa

Europa, the smallest of Jupiter's Galilean moons, also appears to have an active volcanic system, except that its volcanic activity is entirely in the form of water, which freezes into ice on the frigid surface. This process is known as cryovolcanism, and is apparently most common on the moons of the outer planets of the Solar System.

Moons of Saturn and Neptune

In 1989, the Voyager 2 spacecraft observed cryovolcanoes (ice volcanoes) on Triton, a moon of Neptune, and in 2005 the Cassini–Huygens probe photographed fountains of frozen particles erupting from Enceladus, a moon of Saturn. The ejecta may be composed of water, liquid nitrogen, ammonia, dust, or methane compounds. Cassini–Huygens also found evidence of a methane-spewing cryovolcano on the Saturnian moon Titan, which is believed to be a significant source of the methane found in its atmosphere. It is theorized that cryovolcanism may also be present on the Kuiper Belt Object Quaoar.

Exoplanets

A 2010 study of the exoplanet COROT-7b, which was detected by transit in 2009, suggested that tidal heating from the host star very close to the planet and neighboring planets could generate intense volcanic activity similar to that found on Io.