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 volcano 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 a 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 vein-like 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.
Coarse grained intrusive igneous rocks that form at depth within the earth are called abyssal while those that form near the surface are called hypabyssal. Intrusive structures are often classified according to whether or not they are parallel to the bedding planes or foliation of the country rock: if the intrusion is parallel the body is concordant, otherwise it is discordant.
A well-known example of an intrusion is Devil's Tower.