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Three types of plate boundary

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.

Volcanoes[change | change source]

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.

Intrusions[change | change source]

The main feature of the Devils Tower National Monument was created via intrusion.

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.

Extrusions[change | change source]

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.

Features formed are:

  • Caldera: Is a large depression in a former volcanic site. When it is filled with water it is called a 'Caldera Lake'.
  • Composite cones: Is a large volcanic mountain formed by solidified lava on the surface.
  • Geysers: Are eruption of hot water and steam shooting into the air at regular intervals.
  • Hot springs: Are springs of hot water that flows out continuously without shooting up into the air.

Other websites[change | change source]