Pluton redirects here. For the ancient Roman god, see Pluto. For the French nuclear missile system, see Pluton missile.

For the physical act of trepass with a possible intent to steal, see intruder.

In geology an intrusion is usually a body of igneous rock that has crystallized from a molten magma below the surface of the Earth. Intrusive rocks include all varieties of igneous rocks from coarse-grained, phaneritic granites of large batholiths to very fine grained, aphanitic, rhyolites in volcanic necks or feeder pipes. In composition, intrusive rocks also include the entire sequence of igneous rock types from the dense and dark ultramafic peridotites to the very light-colored and low-density alkali granites and syenites. Bodies of magma that solidify underground before they reach the surface of the earth are called plutons, named for Pluto, the Greek god of the underworld.

Intrusive rocks also exist in a wide range of forms from mountain range sized batholiths to thin vein-like fracture fillings of aplite. Structural types include:

• batholith: large irregular intrusions.
• stock: smaller irregular disordant intrusions.
• dike: a relatively narrow tabular discordant body with near vertical attitude.
• sill: a relatively thin tabular concordant body intruded along bedding planes, horizontal attitude.
• pipe or volcanic neck: circular or tube shaped nearly vertical body which may have been a feeder vent for a volcano.
• laccolith: concorant body with essentially flat base and dome shaped upper surface, usually has a feeder pipe below.

Diapir is the more general term for intrusions that includes both igneous and non-igneous structures, such as salt domes. A diapir is any relatively mobile mass that intrudes into preexisting strata. Diapirs commonly intrude vertically through more dense rocks because of buoyancy forces associated with relatively low-density rock types, such as salt, shale and hot magma, all of which form diapirs. The process is known as diapirism.

Piercement structures may be created by movement through strata.

By pushing upward and piercing overlying rock layers, diapirs can form anticlines, salt domes and other structures capable of trapping petroleum and natural gas. Igneous intrusions themselves are typically too hot to allow the preservation of preexisting hydrocarbons.

A mud volcano may be created by a pressurized mud diapir which breaches the Earth's surface or ocean bottom.