Volcanism

Volcanism—also known as volcanicity, volcanic activity, or vulcanism—is the set of geological processes through which molten rock, gases, liquids, or mixed materials erupt from the interior of a solid planetary body onto its surface. These processes occur not only on Earth but on a variety of astronomical objects, including moons and icy bodies. Volcanism requires an internal or external heat source capable of melting material or producing gases that migrate upward through the body’s crust until they escape at the surface.

Causes and Heat Sources

For volcanism to occur, mantle or interior temperatures must reach conditions where partial melting is possible. As temperature rises to around half the melting point of mantle rocks, viscosity decreases significantly, allowing material mobility. Once large-scale melting begins, viscosity can drop dramatically, speeding heat transfer and promoting ascent of molten material.
Melted material is typically less dense than the surrounding solid rock, enabling buoyant rise toward the surface. Several heat sources drive this process:

• Tidal heating: Deformation caused by gravitational interactions generates heat, particularly in outer Solar System moons such as Jupiter’s Io.
• Primordial heat: Heat retained from planetary formation remains significant in larger bodies like Earth but has largely dissipated in smaller bodies such as the Moon.
• Radiogenic heat: Ongoing radioactive decay in silicate minerals contributes to internal heating in terrestrial planets.
• Solar heating: In specific cases, such as cryogeysers on Triton or Mars, external heating from the Sun creates volatile-driven activity.

Melting Mechanisms

Decompression Melting

As mantle material rises, pressure decreases and the melting point drops. Rocks that are solid at great depth may partially melt without any temperature increase. An exception is water, whose melting point behaves differently under rising pressure until a critical threshold is reached.

Flux Melting

The addition of volatiles such as water or carbon dioxide lowers the melting point of solid rock. These volatiles promote melting at lower temperatures, commonly occurring in subduction-zone environments on Earth.

Cryovolcanic Reservoir Formation

Cryovolcanism on icy bodies may originate not from global subsurface oceans but from localised melt reservoirs. Such reservoirs can form from:

• Convective plumes of warm ice concentrating tidal heat.
• Fractures propagating horizontally rather than vertically.
• Stress-induced melting along faults.
• Heat released from impacts.

These mechanisms create pockets of liquid water or brine capable of feeding cryovolcanic eruptions.

Ascent of Melts

Diapirs

Melting begins in small pockets within parent rock, often at grain boundaries. If melt wets grain boundaries sufficiently, it collects into larger interconnected veins. Buoyancy then drives upward movement. Similar diapiric processes may occur in icy compositions on outer Solar System bodies.

Dikes

Dikes are vertical fractures filled with magma. Because magma is less dense than surrounding rock, pressure differences allow the crack to propagate upward while its base closes under elastic forces. Eventually the dike pinches off at its base and rises independently.

Standpipe Model

An older theory suggested magma rose through rigid, open conduits to a level of hydrostatic equilibrium. Although it explained some observed volcanic elevations, it is now discredited due to unrealistic assumptions about lithospheric rigidity.

Cryovolcanic Melt Ascent

Water-based cryomagma is often denser than overlying ice, making ascent difficult. Potential solutions include:

• Decreasing its density through dissolved volatiles or chemical composition.
• Increasing pressure through freezing-induced expansion in subsurface reservoirs.
• Fracture propagation aided by tidal stresses.
• Elastic trapping of gases in fractures, increasing buoyancy.

Cryovolcanic fractures may rise to near-surface levels before stalling. Tidal pumping can push fluids higher within the crack system. Impact events may also temporarily enhance magma ascent by thinning or removing surface material.

Types of Volcanism Across the Solar System

Volcanism varies widely among planetary bodies based on composition and heat sources:

• Silicate volcanism (Earth, Venus, Mars) involves molten rock, gas emissions and high-temperature eruptions.
• Cryovolcanism (Europa, Enceladus, Triton) involves water, ammonia or methane-based fluids erupting at extremely low temperatures.
• Geyser-like eruptions (Triton, Mars) may be driven primarily by solar heating rather than internal heat.