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.
[change] Intrusive volcanicity
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 volcanicity include: batholith, laccolith, dike, pipe and sill.
[change] Extrusive volcanicity
- This section needs expansion.
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.
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: 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.
Liquid - lava,hot spring.
