A cryovolcano is a location (on a "world") that erupts a substance that is liquid or gaseous at cold temperatures (cold relative to typical Earth temperatures). Example substances are water, methane, or ammonia. The eruption, for example, could be a water plume escaping through a gap in an outer ice layer. Cryovolcanism refers the presence of such locations and eruptions. The material erupted has been referred to as cryolava. Cryovolcanos are evident on some of the planets and moons that are at much lower temperature than Earth and that have deposits of such materials covered by a solid layer. Moons on which cryovolcanism has been observed include Triton, Enceladus, Europa and Ganymede.