An intrusion is liquid rock that forms under Earth's surface. Magma from under the surface is slowly pushed up from deep within the earth into any cracks or spaces it can find, sometimes pushing existing country rock out of the way, a process that can take millions of years. As the rock slowly cools into a solid, the different parts of the magma crystallize into minerals. Many mountain ranges, such as the Sierra Nevada in California, are formed mostly by intrusive rock, large granite (or related rock) formations.
Intrusions are one of the two ways igneous rock can form; the other is extrusive, that is, a volcanic eruption or similar event. Technically speaking, an intrusion is any formation of intrusive igneous rock; rock formed from magma that cools and solidifies within the crust of the planet. In contrast, an extrusion consists of extrusive rock; rock formed above the surface of the crust.
Intrusions vary widely, from mountain-range-sized batholiths to thin veinlike fracture fillings of aplite or pegmatite. When exposed by erosion, these cores called batholiths may occupy huge areas of Earth's surface. Large bodies of magma that solidify underground before they reach the surface of the crust are called plutons.
Devils Tower (Lakota: Matȟó Thípila ("Bear Lodge") or Ptehé Ǧí ("Brown Buffalo Horn") (Arapaho: Wox Niiinon ) is an igneous intrusion or laccolith in the Black Hills near Hulett and Sundance in Crook County, northeastern Wyoming, above the Belle Fourche River. It rises dramatically 1,267 feet (386 m) above the surrounding terrain and the summit is 5,114 feet (1,559 m) above sea level.
Devils Tower was the first declared United States National Monument, established on September 24, 1906, by President Theodore Roosevelt. The Monument's boundary encloses an area of 1,347 acres (545 ha).
In recent years, about 1% of the Monument's 400,000 annual visitors climb Devils Tower, mostly using traditional climbing techniques.