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.
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.
Intrusive volcanism is when magma is forced into the rocks that make up the Earth's crust. 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.
The molten magma under great pressure forces its way through the fissure of underground rocks and reaches the Earth's surface to form "igneous extrusion". Major extruded materials include gas, liquid and solid.
- 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: country rock, fragments of solidified lava and fine materials (volcanic ash and dust). It can be classified by size into volcanic bombs, volcanic blocks, lapilli, volcanic ash and volcanic dust.
- Liquid - lava, hot springs.
Features formed are:
- Caldera: a large depression in a former volcanic site. When it is filled with water it is called a 'caldera lake'.
- Composite cones: a large volcanic mountain formed by solidified lava on the surface.
- Geysers: eruption of hot water and steam shooting into the air at regular intervals.
- Hot springs: springs of hot water that flow out continuously without shooting up into the air.
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Volcanism Facts for Kids. Kiddle Encyclopedia.