Volcanism

Understanding Volcanism: The Phenomenon of Eruption

Volcanism, vulcanism, volcanicity, or volcanic activity—what is it exactly? It’s the fascinating process where solids, liquids, gases, and their mixtures erupt to the surface of a solid-surface astronomical body. But how does this happen?

The Heat Source Behind Volcanism

For volcanism to occur, the temperature in the mantle must rise to about half its melting point. This is like heating up a pot until it’s just shy of boiling—once that happens, the contents can start moving and escaping. The heat source could be internally generated, such as radioactive decay or tidal heating from nearby planets.

Types of Heat Generation

Imagine Earth during its formation—it was a fiery ball of molten rock and metal, with impacts from planetesimals causing intense heat. This initial heating could trigger the differentiation process, further warming the planet’s interior.

On outer solar system moons, tidal heating is the main driver. The gravitational pull between two bodies can stretch and squeeze a moon, generating internal heat. For icy bodies like Europa or Enceladus, radiogenic heat from radioactive decay plays a lesser role due to their lower density and lack of silicate material.

Ascending Magma: The Journey to the Surface

Melted materials are more mobile and less dense than solid ones. This means they can rise through the body, much like how hot air rises in a balloon. But what mechanisms allow this ascent?

• Convection currents and tidal heating create upward flows of heated material.
• Switching from vertical to horizontal propagation of fluid-filled cracks can also help magma move.
• The heating of ice due to lateral motion or large impacts can make it more permeable, allowing fluids to pass through.
• Diapirs are like pockets of melted material that accumulate and rise as they wet crystal faces.
• Dikes are vertical fluid-filled cracks where magma rises, pushing the crack upwards and squeezing it closed at its bottom.
• The standpipe model is a discredited theory but still interesting to consider. It suggests magma rises through a rigid open channel.
• Cryovolcanic melt ascent involves making water less dense, pressurizing the fluid, and using external stresses or freezing to break through ice shells.

Impact-Induced Volcanism: A Surprising Mechanism

Even impacts can create conditions that allow for enhanced magma ascent. An impact might remove the top few kilometers of crust, creating pressure differences that could lead to eruptions otherwise trapped beneath the surface.

A 2011 article showed that there would be zones of enhanced magma ascent at the margins of an impact basin. Not all mechanisms operate on a given body, and sometimes none do—nature is full of surprises!

Types of Volcanism: Silicate vs. Cryovolcanism

Silicate volcanism involves silicate materials erupting, while mud volcanoes form when fluids and gases under pressure erupt to the surface.

Cryovolcanism is the eruption of volatiles into an environment below their freezing point. Sulfur lavas have a different behavior with low melting points and rapid loss of viscosity after cooling. Lava types include viscous lava, pillow lava, ‘a’a lava, block lava, and pahoehoe lava.

Explosive Activity: The Force Behind Eruptions

Imagine a volcano as a pressure cooker. As the magma nears the surface, it loses its ability to hold dissolved gases, leading to explosive eruptions. This can release energy equivalent to a quarter of an equivalent mass of TNT.

The causes include exsolution of volatiles and the progression from gas-rich to gas-depleted material during an eruption. The magma at the top of a dike is enriched with gas when it breaches the surface, followed by lower-down magma that didn’t get enriched.

Volcanic Ash: The Result of Explosive Activity

The violently expanding gas disperses and breaks up magma, forming a colloid of gas and magma called volcanic ash. As this cools, it forms tiny glass shards from former liquid bubbles.

Gas-poor magmas cool into rocks with small cavities, becoming vesicular lava. Gas-rich magmas form pumice, which is less dense than water due to its porous nature.

Phreatic and Phreatomagmatic Eruptions

A phreatic eruption occurs when hot water under pressure suddenly boils and expands rapidly, shattering rock and hurling fragments. A phreatomagmatic eruption happens when hot magma meets water, creating an explosion.

Cryovolcanism: The Icy Version

Cryovolcanism can occur through the destabilization of clathrate hydrates or by the rapid boiling of water vapor in a vacuum. This is like seeing ice melt and then suddenly boil, creating an explosive release.

Volcanic Activity on Different Bodies

Volcanoes often form at plate boundaries where tectonic plates are diverging or converging. Volcanoes away from these boundaries may arise from upwelling diapirs near the core-mantle boundary, resulting in hotspot volcanism.

The Moon has volcanic features but no current activity, while Venus shows evidence of recent eruptions. Mars has four extinct shield volcanoes, and recent volcanic activity might have occurred on the planet. Jupiter’s moon Io is the most volcanically active object due to tidal interaction with its parent planet. Europa appears to have an active water-volcanic system due to cryovolcanism.

The moons of Saturn (Triton and Enceladus) and Neptune (Triton and Enceladus) show evidence of cryovolcanoes erupting frozen particles into space. A study suggests that exoplanet COROT-7b may exhibit intense volcanic activity due to tidal heating from its host star.

Volcanism is a complex and fascinating phenomenon, with many mechanisms driving its occurrence. From the heat generated by radioactive decay to the explosive force of eruptions, volcanoes continue to shape our understanding of planetary processes. Whether on Earth or in distant solar systems, these natural wonders remind us of the dynamic nature of our universe.