Volcanism (or volcanic activity) is the eruption of magma onto the surface of the Earth. It is an endogenetic process caused by the movement of plate tectonics. Magma under the crust is under very great pressure. When folding and faulting occur, cracks or fractures appear. These are lines of weakness. When these lines of weakness develop downward in the crust and reach the magma, they will release the pressure in the magma. This allows magma to rise up along the lines of weakness and intrude into the crust. Some magma may even reach the Earth's surface.
Aspects[change | edit source]
Volcanoes[change | edit source]
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[change | edit source]
Intrusive volcanism is when magma is forced into the rocks that make up the Earth's crust. It occurs when there are lines of weakness such as faults, joints, or bedding planes in the crust. Then magma enters these lines of weakness. When it cools and become solid while still underground, different features called plutons are formed. The rock formed is intrusive igneous rock.
These plutons will be exposed at the surface of land when the overlying rocks are removed after a long time of denudation (laid bare by erosion).
Major features formed by intrusive volcanicity include: batholith, laccolith, dyke, pipe and sill.
- Batholiths: Have large scale magma that has been solidified at the base of the mountain.
- Dyke: Is a small scale magma cooled within the earths crust that stands vertically to the existing rocks.
- Sill: Is small scale magma cooled near the earths surface that lies horizontally to the existing rocks.
- Lacolith: A small scale magma that pushes the overlying layers of rocks to form a dome shaped structure.
Extrusions[change | edit source]
The magma under great pressure with molten materials forces its way through the fissure of the underground rocks and reaches the Earth's surface to form igneous extrusion. When the molten materials reaches the surface, it is called extruded materials and such volcanic activity is known as extrusive or surface vulcanicity.Some people classified extrusive vulcanicity into vent eruption and fissure eruption depending on how the magma rising to the ground.
- Gas - sulphur, hydrogen, carbon dioxide and hot steam (geysers). When uprising magma decreases in pressure suddenly, gases within magma explode to the Earth's surface and cause destruction.
- Solid - pyroclast. Mainly composed of: fragments of country-rock, solidified lava and fine materials of volcanic ash and dust. It can be classified in term of size into volcanic bombs, volcanic blocks, lapilli, volcanic ash and volcanic dust. This occurs inside the volcanic features under ground
- Liquid - lava, hot spring.
Features formed are:
- Caldera: Is a large depression in a former volcanic site. When it is filled with water it is called a 'Caldera Lake'.
- Composite cones: Is a large volcanic mountain formed by solidified lava on the surface.
- Geysers: Are eruption of hot water and steam shooting into the air at regular intervals.
- Hot springs: Are springs of hot water that flows out continuously without shooting up into the air.
Other websites[change | edit source]
|Wikimedia Commons has media related to: Volcanism|
- "Glossary of Volcanic Terms". G. J. Hudak, University of Wisconsin Oshkosh, 2001. http://www.d.umn.edu/~rmorton/ronshome/Volcanology/VolcanicGlossary.htm. Retrieved 2010-05-07.