1. Water which has frozen into a crystal lattice. Pure water freezes at 0°C at 10 MPa (megapascals) pressure. The presence of salts in solution depresses the freezing point of water. Liquid water has its maximum density at 4 °C, in consequence of which ice floats on water. With increasing pressure, a series of denser polymorphs of ice forms, each designated by a Roman numeral, ordinary ice being ice I.
2. Several properties and varieties of ice are important in geomorphological processes. Expansion on freezing (9.05% in specific volume) generates very high pressures. In an enclosed space in the laboratory the pressure reaches 216 MPa at −22°C but reaches only about 10% of this when unenclosed, as in nature. Such ice I converts into the denser ice III at lower temperatures, but the pressure exerted by it changes little. ‘Ground ice’ forms when interstitial water freezes, and this may bring about heaving as well as frost wedging. ‘Glacier ice’ is a relatively opaque mass of interlocking crystals, and has a density of 0.85–0.91 g/cm3. ‘Regelation ice’ is relatively clear and is formed by the freezing of meltwater beneath a temperate glacier.
3. In planetary geology other ices are important. Water ice condenses at 160 K at solar nebular pressures and appears in abundance forming the surfaces of the Galilean satellites Europa, Ganymede, and Callisto. The satellites of the jovian planets are mostly mixtures of water ice and rock. Water ice will exist in high-pressure polymorphs (e.g. ice VIII, density 1670 kg/m3) above about 1.5–2.0 MPa in satellite interiors. Other possible ices important in satellites (e.g. Titan) include NH3.H2O, CH4.nH2O, and H2O.CO2.