Salt dome
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A salt dome is formed when a thick bed of evaporite minerals (mainly salt, or halite) found at depth intrudes vertically into surrounding rock strata, forming a diapir.
The salt that forms these deposits was laid down in prehistoric times, mainly in places where inland seas were periodically connected and disconnected from oceans. As these seas are cut off from the main body of water, the water evaporates, leaving immense salt pans. 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, sheets, pillars and other structures as it rises. In cross section, these large domes may be anywhere from 1 to 10 kilometers across and extend as far down as 6.5 kilometers.
One example of an island formed by a salt dome is Avery Island in Louisiana.
Commercial uses
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, 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.