Largely subsurface geologic structure that consists of a vertical cylinder of salt embedded in horizontal or inclined strata. In the broadest sense, the term includes both the core of salt and the strata that surround and are “domed” by the core. Major accumulations of oil and natural gas are associated with salt domes in the U.S., Mexico, the North Sea, Germany, and Romania; domes along the Gulf Coast contain large quantities of sulfur. Salt domes are also major sources of salt and potash on the Gulf Coast and in Germany, and they have been used for underground storage of liquefied propane gas. Storage “bottles,” made by drilling into the salt and then forming a cavity by subsequent solution, have been considered as sites for disposal of radioactive wastes.
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The salt that forms these domes was deposited within restricted marine basins. Due to restricted flow of water into a basin, evaporation occurs resulting in the precipitation of salts from solution, depositing evaporites. It is recognised that a single evaporation event is rarely enough to produce the vast quantities of salt found in evaporite deposits indicating that a sustained period of episodic flooding and evaporation of the basin must occur, as can be seen from the example of the Mediterranean Messinian salinity crisis. At the present day, evaporite deposits can be seen accumulating in basins that merely have restricted access but do not completely dry out; they provide an analogue to some deposits recognised in the Geological record, such as the Garabogazköl basin in Turkmenistan.
Over time, the salt is covered with sediment and becomes buried. Since the density of salt is generally less than that of surrounding material, it has a tendency to move upward toward the surface, forming large bulbous domes, diapirs , sheets, pillars and other structures as it rises. If the rising salt diapir breaches the surface, it can become a flowing salt glacier. In cross section, these large domes may be anywhere from 1 to 10 kilometers across and extend as far down as 6.5 kilometers.
Another example of an emergent salt dome is at Onion Creek / Fisher Towers near Moab, UT, U.S. These two images show a Cretaceous age salt body that has risen as a ridge through several hundred meters of overburden, predominately sandstone. As the salt body rose, the overburden formed an anticline (arching upward along its centerline) which fractured and eroded to expose the salt body.
The term "salt dome" is also sometimes inaccurately used to refer to dome-shaped silos used to store rock salt for melting snow on highways. These domes are actually called monolithic domes and are used to store a variety of bulk goods.
The rock salt that is found in salt domes is mostly impermeable. As the salt moves up towards the surface, it can penetrate and/or bend strata of existing rock with it. As these strata are penetrated, they are generally bent slightly upwards at the point of contact with the dome, and can form pockets where petroleum and natural gas can collect between impermeable strata of rock and the salt. The strata immediately above the dome that are not penetrated are pushed upward, creating a dome-like reservoir above the salt where petroleum can also gather. These oil pools can eventually be extracted, and indeed form a major source of the petroleum produced along the coast of the Gulf of Mexico.
Other uses include storing oil, gas, or even hazardous waste in large caverns formed after salt mining, as well as excavating the domes themselves for uses in everything from table salt to the granular material used to prevent roadways from icing over.