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A geyser is a hot spring characterized by intermittent discharge of water ejected turbulently and accomplished by a vapor phase. The name geyser comes from Geysir, the name of an erupting spring at Haukadalur, Iceland; that name, in turn, comes from the Icelandic verb gjósa, “to gush”. The formation of geysers requires a favourable hydrogeology which exists in only a few places on Earth, and so they are fairly rare phenomena. There must be a volcanic heat source. Generally all geyser field sites are located near active volcanic areas. The surface water works its way down to an average depth of around 2134 m (7001 ft) where it meets up with the hot rocks. About 1,000 exist worldwide, with about half of these in Yellowstone National Park, U.S. A geyser's eruptive activity may change or cease due to ongoing mineral deposition within the geyser plumbing, exchange of functions with nearby hot springs, earthquake influences, and human intervention. Erupting fountains of liquefied nitrogen have been observed on Neptune's moon Triton, as have possible signs of carbon dioxide eruptions from Mars' south polar ice cap. These phenomena are also often referred to as geysers. Instead of being driven by geothermal energy, they seem to rely on solar heating aided by a kind of solid-state greenhouse effect. On Triton, the nitrogen may erupt to heights of 8 kilometres (5 mi).
