In geology, an igneous intrusion (or intrusive body or simply intrusion) is a body of intrusive igneous rock that forms by crystallization of magma slowly cooling below the surface of the Earth. Intrusions have a wide variety of forms and compositions, illustrated by examples like the Palisades Sill of New York and New Jersey; the Henry Mountains of Utah; the Bushveld Igneous Complex of South Africa; Shiprock in New Mexico; the Ardnamurchan intrusion in Scotland; and the Sierra Nevada Batholith of California.
Because the solid country rock into which magma intrudes is an excellent insulator, cooling of the magma is extremely slow, and intrusive igneous rock is coarse-grained (phaneritic). Intrusive igneous rocks are classified separately from extrusive igneous rocks, generally on the basis of their mineral content. The relative amounts of quartz, alkali feldspar, plagioclase, and feldspathoid is particularly important in classifying intrusive igneous rocks.
Intrusions must displace existing country rock to make room for themselves. The question of how this takes place is called the room problem, and it remains a subject of active investigation for many kinds of intrusions.
The term pluton is poorly defined, but has been used to describe an intrusion emplaced at great depth; as a synonym for all igneous intrusions; as a dustbin category for intrusions whose size or character are not well determined; or as a name for a very large intrusion or for a crystallized magma chamber.