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Volcanism (or vulcan activity) is the eruption of magma onto the surface of the Earth.
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 as lava.