What is Volcanism?
[[Image:Tectonic plate boundaries.png|thumb|right|450px|Three types of plate boundary]] '''Volcanism''' (or '''vulcan activity''') is the [[Volcanic eruption|eruption]] of [[magma]] onto the surface of the Earth. Magma under the [[crust (geology)|crust]] is under very great pressure. When [[Fold (geology)|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]]. == Volcanoes == {{main|Volcano}} Volcanoes are the 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 == [[File:Devils Tower 18A.JPG|right|thumbnail|250px|The main feature of the [[Devils Tower National Monument]] was created via intrusion.]] {{main|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 [[pluton]]s 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 a small scale magma cooled near the earths surface that lies horizontally to the existing rocks. * Laccolith: A small scale magma that pushes the overlying layers of rocks to form a dome shaped structure. == Extrusions == The molten magma under great pressure forces its way through the fissu