diapir n : a domed rock formation where a core of rock has moved upward and pierced through the more brittle overlying strata

A diapir ( http://www.bartleby.com/61/93/D0199300.html) (French, from Greek diapeirein, to pierce through) is an intrusion caused by buoyancy and pressure differentials. The term was introduced by the Romanian geologist Ludovic Mrazek, who was the first to understand the principle of salt intrusion and plasticity. The term "diapir" may be applied to igneous structures, but it is more commonly applied to non-igneous, relatively cold materials, such as salt domes and mud diapirs. Diapirism is thought to occur on Enceladus. A diapir is any relatively mobile mass that intrudes into preexisting strata. A lava lamp is an example of a diapir.

Diapirs commonly intrude vertically upward along fractures or zones of structural weakness through more dense overlying rocks because of density contrast between a less dense, lower rock mass and overlying denser rocks. The density contrast manifests as a force of buoyancy. The process is known as diapirism. The resulting structures are also referred to as piercement structures.

In the process, segments of the existing strata can be disconnected and pushed upwards. While moving higher, they retain much of their original properties such as pressure, which can be significantly different from that of the shallower strata they get pushed into. Such overpressured Floaters pose a significant risk when trying to drill through them.

Rock types such as evaporitic salt deposits, and gas charged muds are potential sources of diapirs. Diapirs also form in the earth's mantle when a sufficient mass of hot, less dense magma assembles. Diapirism in the mantle is thought to be associated with the development of large igneous provinces and some mantle plumes.

Explosive, hot volatile rich magma or volcanic eruptions are referred to generally as diatremes. Diatremes are not usually associated with diapirs, as they are small-volume magmas which ascend by volatile plumes, not by density contrast with the surrounding mantle.

Economic importance of diapirs

Diapirs or piercement structures are structures resulting from the penetration of overlaying material. 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.

See also

• Geyser
• Hydrothermal vent
• Mud volcano
• Granite
• Salt tectonics