Volcanism

Volcanism is the process that creates volcanoes and makes special rocks called igneous rocks. It happens when hot, melted material and gases burst out to the surface of a planet or moon. This activity is powered by heat inside the body, often from radioactive decay or tidal heating.

The heat makes solid material melt or turn into gas. This melted or gaseous material then moves up through the inside of the planet or moon. Sometimes, it breaks through the solid surface, creating a volcano. This process helps shape the surfaces of many worlds in space.

Causes

For volcanism to happen, the deep inside parts of a planet or moon must get very hot. This heat can come from different places. One way is from the pull of gravity between objects in space, which can stretch and squeeze a planet or moon and create heat. Another way is from the leftover heat from when the planet or moon was first formed, long ago. Also, some rocks contain materials that naturally break down and give off heat over time.

When these hot places get hot enough, solid rock can melt into liquid. The melted liquid is lighter than the solid rock around it, so it rises up toward the surface. This is what causes volcanoes to erupt.

Heat source

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, 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. This heating could trigger differentiation, further heating the planet. The larger a body is, the slower it loses heat.

Another heat source is radiogenic heat, caused by radioactive decay. The decay of unstable isotopes in common minerals experience some of this heating.

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

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.

Flux melting

Flux melting occurs when the melting point is lowered by the addition of volatiles, for example, water or carbon dioxide. 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.

Ascent of melts

Diapirs

When material of a planetary body begins to melt, the melting first occurs in small pockets, 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. Under the influence of buoyancy, the melt rises.

Dikes

A dike is a vertical fluid-filled crack. 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.

Standpipe model

This model of volcanic eruption posits that magma rises through a rigid open channel, in the lithosphere and settles at the level of hydrostatic equilibrium.

Cryovolcanic melt ascent

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 or with the addition of exsolved gas bubbles in the cryomagma. Another is to pressurise the fluid to overcome negative buoyancy and make it reach the surface.

For a crack in the ice shell to propagate upwards, the fluid in it must have positive buoyancy or external stresses must be strong enough to break through the ice.

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.

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.

Not all of these mechanisms, and maybe even none, operate on a given body.

Types

Silicate volcanism

Silicate volcanism happens when materials made of silicate erupt. These lava flows harden when they cool down.

Mud volcanoes

A mud volcano forms when fluids and gases under pressure burst through the ground, bringing mud with them. This pressure can build up from the weight of dirt pressing down, or from fluids trapped deep underground. Mud volcanoes usually erupt quietly.

Cryovolcanism

Cryovolcanism is when special liquids, called volatiles, erupt in cold places. This is different from regular volcanism because the icy lava is heavier than the surrounding material.

Sulfur

Sulfur lava behaves differently from the lava we see on Earth. Sulfur melts at a much lower temperature. When it cools, it becomes runnier very quickly, unlike Earth’s lava.

Lava types

Main article: Lava

When hot, melted rock bursts out onto the surface of a planet, we call it lava. Thick lava makes short, stubby flows that look wavy once they cool. Thinner lava can look different when it cools. One type, called pillow lava, forms when lava touches water and cools quickly, making round piles. Another type, called A’a lava, has a rough, spiky surface. Block lava is similar but less spiky. The most common type is Pahoehoe lava, which has a smooth surface with small bumps and folds.

Gentle/explosive activity

A volcanic eruption can be a gentle flow of material, but it can also be very powerful and explosive.

Causes of explosive activity

Exsolution of volatiles

When volcanoes erupt on Earth, they often start by releasing gases and then change to less gassy material. This happens because gases build up in the magma as it moves toward the surface.

The gases in magma separate from it when the magma gets close to the surface because of changes in temperature and pressure. High pressure keeps gases dissolved, but when pressure drops, the gases escape. This is like opening a bottle of fizzy drink—when you open it, the pressure drops and bubbles form.

Fluid magmas erupt quietly because the gases escape easily. But in thick magmas, the gases get trapped and form bubbles. These bubbles grow as the magma nears the surface because the pressure drops, making the magma expand. This can cause volcanoes to erupt explosively.

Physics of a volatile-driven explosive eruption

Magmas rich in silica cool underground before they erupt. As they cool, bubbles form in the magma. When the magma gets close to the surface, the bubbles grow, and the magma expands. This pressure breaks through the surface, and the sudden release causes more gas to escape explosively. The gas can expand very fast, shooting the magma into the air.

Volcanic ash formation

The fast-expanding gas breaks up the magma, creating a mix of gas and magma called volcanic ash. As the gas cools, it chills the magma into tiny pieces, often forming glass shards. In less thick magmas, the bubbles can reform into droplets. The final rocks depend on how much gas there is. Less gas forms rocks with small holes, while lots of gas forms light rocks that can float, like pumice.

Pyroclastic flows

A mix of volcanic gas and magma can flow down slopes as a dense current called a pyroclastic flow. This flow can move over obstacles and can be very dangerous. Pyroclastic flows are common in explosive eruptions on Earth and have also been seen on other planets like Venus.

Phreatic eruption

A phreatic eruption happens when hot water under pressure suddenly loses its pressure. This causes the water to boil very quickly. Or it can happen when groundwater is heated suddenly, turning into steam. When water turns to steam, it expands very fast, which can break rocks apart.

Phreatomagmatic eruption

A phreatomagmatic eruption occurs when hot magma meets water, causing an explosion.

Clathrate hydrates

One way icy volcanoes can erupt is when warm magma touches special icy compounds called clathrate hydrates. These compounds break down when they get warm, releasing gas that can cause an explosion.

Water vapor in a vacuum

If a crack reaches the surface of an icy body and water rises into the empty space, it will start to boil because the air pressure is very low. The water and any gases in it will rise, creating a plume. This is thought to be one reason why the moon Enceladus has ice plumes.

Occurrence

Earth

On Earth, volcanoes are mostly found where tectonic plates move apart or come together. Because many of these plate edges are under the ocean, most volcanoes are underwater. For example, mid-ocean ridges like the Mid-Atlantic Ridge form when plates move apart, while the Pacific Ring of Fire forms when plates come together. Volcanoes can also appear where the Earth's crust stretches thin, like in the East African Rift and parts of North America. Some volcanoes form far from these edges due to hot spots deep inside Earth, like the Hawaiian hotspot.

Moon

Earth's Moon doesn't have active volcanoes today, but it does have many volcanic features like dark areas called maria, long narrow valleys called rilles, and rounded hills called domes.

Venus

The planet Venus has a surface covered mostly with a type of rock called basalt, showing that volcanoes shaped its surface. Scientists think Venus may have gone through a big change about 500 million years ago. There are wide streams of melted rock, and some types of volcanic activity happen there that we don't see on Earth.

Mars

Mars has several extinct volcanoes, including four huge ones that are even bigger than Earth's. These include Arsia Mons, Ascraeus Mons, Hecates Tholus, Olympus Mons, and Pavonis Mons. Though these volcanoes have been quiet for millions of years, a spacecraft called Mars Express found signs that Mars might have had volcanic activity more recently than we thought.

Moons of Jupiter

Io

Jupiter's moon Io is the most active volcanic world in our solar system. It has many volcanoes that shoot out sulfur, sulfur dioxide, and rock. Io is constantly changing its surface.

Europa

Europa, one of Jupiter's largest moons, seems to have volcanic activity too, but instead of lava, it shoots out water that freezes on its cold surface. This is called cryovolcanism.

Moons of Saturn and Neptune

In 1989, the Voyager 2 spacecraft saw ice volcanoes on Triton, a moon of Neptune. In 2005, the Cassini–Huygens probe saw fountains of frozen particles erupting from Enceladus, a moon of Saturn. These might be made of water, liquid nitrogen, ammonia, dust, or methane.

Exoplanets

A study from 2010 about the exoplanet COROT-7b suggested that close encounters with its star and neighboring planets could create intense volcanic activity.