Rock salt is the name of a sedimentary rock that consists almost entirely of halite, a mineral composed of sodium chloride, NaCl. It forms where large volumes of sea water or salty lake water evaporate from an arid-climate basin — where there is a replenishing flow of salt water and a restricted input of other water.

Deposits of rock salt occur as laterally extensive rock units in many parts of the world. These water-soluble rocks are rarely found at Earth’s surface, except in arid countries. The most geographically extensive salt layers in the United States underlie thousands of square miles and reach thicknesses of over 100 feet.

People have mined rock salt or produced salt by evaporation for thousands of years. Most of the ancient salt production was used in cooking and food preservation.

Today in the United States, highway deicing is the most important use of rock salt. Rock salt is also used in chemical manufacturing, food processing, agriculture, plastics, paint, textiles, leather processing, water treatment, and many other industries.