Volcanism is the phenomenon of eruption of molten rock (magma) onto the surface of the Earth or a solid-surface planet or moon, where lava, pyroclastics and volcanic gases erupt through a break in the surface called a vent. It includes all phenomena resulting from and causing magma within the crust or mantle of the body to rise through the crust and form volcanic rocks on the surface.
Volcanic processes 
Magma from the mantle or lower crust rises through its crust to the surface. If magma reaches the surface, its behavior depends 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.
Driving forces of volcanism 
Movement of molten rock in the 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 post-glacial rebound) drive plate tectonic motion and ultimately volcanism.
Aspects of volcanism 
|Cross-section through a stratovolcano (vertical scale is exaggerated):|
|1. Large magma chamber
3. Conduit (pipe)
7. Layers of ash emitted by the volcano
|9. Layers of lava emitted by the volcano
11. Parasitic cone
12. Lava flow
15. Ash cloud
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.
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 
Volcanic winter 
Forming rocks 
When the magma cools it solidifies 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. Some of this magma may cool extremely rapidly and will form volcanic glass (rocks without crystals) such as obsidian. 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 may be melted and assimilated into the magma. Other rocks adjacent to the magma may be altered by contact metamorphism or metasomatism as they are affected by the heat and escaping or externally circulating hydrothermal fluids.
Volcanism on other bodies 
Volcanism is not confined only to Earth, but is thought to be found on any body having a solid crust and fluid mantle. Evidence of volcanism should still be found on any body 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.
See also 
- "Cooling Planets: Some Background: What is volcanism?". The Lunar and Planetary Institute, Department of Education and Public Outreach. 2006. p. 4. Retrieved 2012-10-14.
- Watson, John; Watson, Kathie (January 7, 1998). "Volcanoes and Earthquakes". United States Geological Survey. Retrieved May 9, 2009.
- Nemzer, J. "Geothermal heating and cooling".
- Robock, Alan (2000). "Volcanic eruptions and climate". Reviews of geophysics 38 (2): 191-219. doi:10.1029/1998RG000054
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- "Glossary of Volcanic Terms". G. J. Hudak, University of Wisconsin Oshkosh, 2001. Retrieved 2010-05-07.
- Crumpler, L. S., and Lucas, S. G. (2001). "Volcanoes of New Mexico: An Abbreviated Guide For Non-Specialists". Volcanology in New Mexico. New Mexico Museum of Natural History and Science Bulletin 18: 5–15. Archived from the original on 2007-03-21. Retrieved 2010-04-28.