Castle Geyser. View of steam erupting from Castle Geyser in Yellowstone National Park, USA. The geyser's cone is 4 metres high. Geysers (Icelandic "geysir" = gusher) consist of deep wells in geothermal rock fissures. Somewhere near the base of the well is a large chamber. The water in the chamber is heated by the surrounding rocks until it is well over normal boiling point, but is prevented from boiling by hydrostatic pressure. The heating also causes the water to expand, spilling from the lip of the well. At a critical point, the loss of water allows the hydrostatic pressure to fall to a point where the superheated water boils spontaneously and erupts.
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