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.

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.

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.

There are multiple ways to generate the heat needed for volcanism. Volcanism on outer solar system moons is powered mainly by tidal heating. 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, 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.

Another heat source is radiogenic heat, caused by radioactive decay. The decay of aluminium-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 the Moon, experience some of this heating. 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).

On Neptune's moon Triton, and possibly on Mars, cryogeyser activity takes place. The source of heat is external (heat from the Sun) rather than internal.

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. 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.