|All too frequently do we fail to recognise ice as a mineral because it does not behave as we believe conventional rocks and minerals should. We are brought up from an early age with an almost magical awe of this substance, which forms sparkly icicles and glittering snowflakes, and is great fun to slide on. Yet water ice is among the most important rock forming minerals in the solar system. Its behaviour as a crystalline solid and as polycrystalline aggregates (i.e., rocks) are indistinguishable from the materials which we, as geologists, are familiar with; it is simply the case that, on Earth, it exists closer to its melting point than, for example, silicates. In the frigid outer solar system, however, ice finds its true home as a substance from which mountains and canyons are built and, perhaps, from which dunes, beaches, and deltas are constructed (e.g., Whalley, 1985).
By direct analogy with terrestrial igneous processes we would therefore refer to a melt of the native rocks as a magma. On icy satellites this magma will consist mainly of water, with admixtures of ammonia. Like terrestrial vents these fluids will construct volcanic edifices such as shields and domes, complete with flow fields, and also form intrusive bodies such as sills and dykes. The accepted term for these fluids is cryomagma and the eruptive process, cryovolcanism.