The mineral halite, also known as salt or sodium chloride, is formed by the evaporation of brine lakes or ancient seas. This mineral form of common table salt can be found across the world in massive sedimentary beds, or salt beds, which are the remains of dried up inland seas. In some areas, halite is found within salt domes, which are underground deposits of halite and other evaporite minerals that are pushed up to the surface by pressure from overlying rock.
The formation of halite usually occurs in hot and arid areas. About 90 percent of a volume of saltwater must evaporate before the resulting brine can begin to precipitate halite. Because of this, the loss of water through evaporation must be much greater than the inflow of additional water for halite to form. Halite is often found along with other mineral deposits that are created by evaporation, such as gypsum and calcite.
Well-known halite deposits in the United States include the growing deposits along Utah's Great Salt Lake and the 200-square-mile salt flats of Badwater Basin in Death Valley, California. The Death Valley salt flats are the result of a combination of dissolved salt entering an enclosed basin that doesn't drain well and water evaporation occurring quickly because of the hot and arid climate. Precipitation, which could wash a portion of the salt away, also takes place at a much lesser rate than the evaporation process.
An unusually large salt bed, more than a mile thick, exists under the Mediterranean Sea. It is believed to have been formed through evaporation about 8 million years ago when the Mediterranean Sea was an enclosed salt lake before the basin became connected to the Atlantic Ocean at the Straits of Gibraltar.