Vulcanicity
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Vulcanicity (also known as volcanic activity or igneous activity) is one of the endogenetic processes.
Magma under the crust is under very great pressure. When folding and faulting occur, cracks or fractures which 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.
There are two types of vulcanicity: intrusive and extrusive.
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[edit] Intrusive vulcanicity
Intrusive vulcanicity refers to magma (molten rock) being 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 vulcanicity include: batholith, laccolith, dike, pipe and sill.
[edit] Extrusive vulcanicity
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The molten matrial forces its way through the underground rocks and readhes the earth's surface to form igneous extrusion. When the molten rocks reaches the surface, it is called lava and such flow is known as extrusive or surface vulcanicity.
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