Magma under the crust is under very great pressure. When folding and faulting occur, cracks or fractures which are lines of weakness are created. When ase lines of weakness develop downward in the crust and reach the magma, ay 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.
Intrusive vulcanicity refers to magma (molten rock) being forced into the rocks that make up the Earth's crust.
It occurs when are are lines of weakness such as faults, joints, or bedding planes in the crust. Then magma enters ase 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 the long time of denudation (laid bare by erosion).
Major features formed by intrusive vulcanicity include: batholith, laccolith, dike, pipe and sill.
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The molten material forces its way through the underground rocks and reaches 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.
and then this lava flows over the surface of the which leads the formation of different features on the earths surface
such as volcanic mountains , larva dammed lakes, volxcanic lakes among others.