Diapir

illustrates Rayleigh–Taylor instability-type diapirism in which the tectonic stresses are low. Differential loading causes salt deposits covered by overburden (sediment) to rise upward toward the surface and pierce the overburden, forming diapirs (including salt domes), pillars, sheets, or other geological structures. In addition to Earth-based observations, diapirism is thought to occur on Neptune's moon Triton, Jupiter's moon Europa, Saturn's moon Enceladus, and Uranus's moon Miranda. ==Formation==

Diapirs commonly intrude buoyantly upward along fractures or zones of structural weakness through denser overlying rocks. This 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 many of their original properties, e.g. pressure; their pressure can be significantly different from the pressure of the shallower strata they get pushed into. Such overpressured "floaters" pose a significant risk when trying to drill through them. There is an analogy to a Galilean thermometer.{{cite book |title=Plates, plumes, and planetary processes; Volume 430 of Special Papers |chapter=The eclogite engine: Chemical geodynamics as a Galileo thermometer |chapter-url=https://books.google.com/books?id=l9a7tIJBYbkC&pg=PA47 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==

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 (arch-like shape folds), salt domes (mushroom/dome-shaped diapirs), and other structures capable of trapping hydrocarbons such as petroleum and natural gas. Igneous intrusions themselves are typically too hot to allow the preservation of preexisting hydrocarbons. == Occurrences ==

featuring the Jashak salt dome (white spot in center). Erosion revealed the uplifted tan and brown rock layers surrounding the salt dome to the northwest and southeast (center of image). Radial drainage patterns indicate another salt dome is located to the southwest (image left center). There are many salt domes and salt glaciers in the Zagros mountains, formed by the collision of two tectonic plates, the Eurasian Plate and the Arabian Plate. There are underwater salt domes in the Gulf of Mexico. . Gravity has caused the salt to flow like glaciers into adjacent valleys. The resulting tongue-shaped bodies are more than 5 kilometers long. The darker tones are due to clays brought up with the salt, as well as the probable accumulation of airborne dust. ==See also==