Volcanism

Volcanism is the process where solids, liquids, gases, or mixtures of these materials erupt to the surface of an astronomical body. This can occur on planets or moons.

It is a fundamental way that celestial bodies transport energy and material from their interiors to their surfaces. This activity is driven by internal heat sources that mobilize material within a body. Once mobilized, this material can rise through the interior and break through the solid surface. Understanding volcanism helps scientists learn about the internal structures of worlds across our solar system.

For volcanism to occur, the temperature of a body's mantle must rise to about half its melting point. At this stage, the viscosity, or thickness, of the mantle drops significantly. When large-scale melting happens, the viscosity can fall from 10^21 Pascal-seconds to 10^3 Pascal-seconds or even less. This change increases the heat transport rate by a million-fold. The melted material is generally less dense and more mobile than the surrounding solid rock. This difference in density provides the buoyancy needed for the melt to rise toward the surface.

There are several ways to generate the heat required for these eruptions. One source is radiogenic heat, which comes from the radioactive decay of isotopes in minerals. Another source is primordial heat, which is the heat left over from a planet's formation. During formation, impacts from planetesimals provided massive amounts of heat. Larger bodies like Earth retain this heat longer than smaller bodies like the Moon. On moons in the outer solar system, tidal heating is often the primary power source. This happens when the gravitational attraction of a large planet deforms the moon's shape, generating internal heat.

Melting can happen through different physical mechanisms. Decompression melting occurs when solid material rises from deep within a body. As the material moves upward, the pressure decreases. This drop in pressure lowers the melting point, allowing the rock to turn into liquid even if the temperature stays the same. Flux melting is another method where the melting point is lowered by adding volatiles, such as water or carbon dioxide.

On Neptune's moon Triton, or possibly on Mars, cryogeyser activity occurs due to external heat from the Sun rather than internal sources.

Once melted, the material must ascend to the surface. In silicate volcanism, melt often collects in pockets or veins. It can flow through rivulets that join to form larger veins. One way it moves is through dikes, which are vertical, fluid-filled cracks. As magma rises in a dike, its lower density causes pressure to fall more slowly than in the surrounding rock. This allows the magma to push the top of the crack upward.

In cryovolcanism, the process is different because the liquid, or cryomagma, is often denser than the surrounding ice. To reach the surface, the liquid must become buoyant through gas bubbles or high pressure from a thickening ice shell.

Volcanism is categorized by the materials being erupted. Silicate volcanism involves silicate materials and occurs at very high temperatures.

When this magma reaches the surface, it is called lava. Silicate lava can be viscous or fluid. Fluid lavas create different textures, such as pillow lava, which forms when lava contacts water, or A'a lava, which has a spiny surface.

Mud volcanoes are another type, where fluids and gases under pressure bring mud to the surface. These often erupt quietly but can release flammable methane. Sulfur volcanism involves lava with a low melting point of about 120 degrees Celsius.

These processes connect the deep interiors of worlds to their visible surfaces. The study of volcanic features, such as impact basins or tidal plumes, provides clues about a moon's composition. For example, the presence of cryovolcanism on icy moons suggests the existence of liquid reservoirs beneath the ice. By observing how lava flows and how cracks form, scientists can map the thermal history of a planet. This helps us understand how heat moves through different types of planetary bodies.