Volcanism

Volcanism is the phenomenon connected with volcanoes and volcanic activity. It includes all phenomenon resulting from and causing magma (from the mantle within a planet) to rise through the crust to form volcanic rocks.^[1]

Volcanic processes

Magma from the planet's mantle rises through the planet's crust, if the magma from the mantle reaches the planet's surface it behaves differently depending on the viscosity of the molten constituent rock. Viscous (thick) magma produces volcanoes characterised by explosive eruptions, while non viscous (runny) magma produce volcanoes characterised by an effusive eruptions pouring large amounts of Lava onto the surface.
In some cases, rising magma can cool and solidify without reaching the surface of a planet. Instead, the cooled and solidified igneous mass freezes within the crust of a planet to form an igneous intrusion.

Driving forces of volcanism

Movement of molten rock in the planet's mantle, caused by thermal convection currents, coupled with gravitational effects of changes on the earth's surface (erosion, deposition, even asteroid impact and patterns of snow fall and melt) drive plate tectonic motion and ultimately volcanism.

Aspects of volcanism

Volcanoes

Volcanoes are places where magma reaches the earth's surface. The type of volcano depends on the location of the eruption and the consistency of the magma.

Intrusions

These are formed where magma pushes between existing rock, intrusions can be in the form of batholiths, dikes, sills and layered intrusions.

 Earthquakes

Main article: Earthquake

Earthquakes are generally associated with plate tectonic activity, but some earthquakes are generated as a result of volcanic activity (though that itself is ultimately driven by the same forces).

 Hydrothermal vents

Main article: Hydrothermal vents

These are formed where water interacts with volcanism. These include Geysers, Fumaroles, Hotsprings and Mudpots, they are often used as a source of geothermal energy.

 Volcanic winter

Main article: Volcanic winter

The amount of gas and ash emitted by volcanic eruptions has a significant effect on the planet's climate. Large eruptions correlate well with some significant climate change events.

 Forming rocks

Main articles: Igneous rock and Metamorphic rock

When the magma cools it crystallises and forms rocks, the type of rock formed depends on the chemical composition of the magma and how rapidly the magma cools. Magma that reaches the surface to become lava cools rapidly resulting in rocks with small crystals such as basalt. Magma that remains trapped below ground in thin intrusions cools slower than magma exposed to the surface and produces rocks with medium sized crystals. Magma that remains trapped in large quantities below ground cools most slowly resulting in rocks with larger crystals - such as granite and gabbro
Existing rocks that come into contact with Magma are often melted by the heat, those that do not come into direct contact with the magma do not melt fully but retain some of their original structure - this process results in changes to the structure of the original rocks changing them so much they become a new type of rock - metamorphic rocks.

Volcanism on other planets

Volcanism is not confined only to Earth, but is thought to be found on any planet which has a solid crust and molten (liquid) mantle. And so its effects should still be found on any planet that has had volcanism at some point in its history. Volcanoes have indeed been clearly observed on other bodies in the solar system; on some, such as Mars, in the shape of mountains that are unmistakably old volcanoes (most notably Olympus Mons), but on Io actual ongoing eruptions have been observed. It can be surmised that volcanism exists on planets and moons of this type in other solar systems as well.