Rock salt melts ice by lowering the freezing point of the water. As the salt dissolves into the ice, it forms a solution with a much lower freezing point than pure water has. This causes the ice to melt unless the temperatures are about 15 F or lower, when the salt has little effect.
Geologists call rock salt halite. It is nearly the same chemical compound as table salt, but it occurs in a crystalline form. Usually, halite is created when an ocean or other salty body of water evaporates. Rock salt is rare on the surface of the Earth, and most of it used in commercial applications is mined from underground. Halite is a sedimentary rock, meaning that it is formed when layer after layer of sediment clump together and form a rock.
Rock salt has a number of uses besides melting ice. It is processed into a food-quality additive that is used to season some foods. Numerous industrial processes also require rock salt for chemical reactions. Salt is essential for most organisms, and it has been an important resource since prehistoric times. In ancient Rome, soldiers were often compensated with salt. This lead to the coining of the term “salary.”