Geyser at sunset. A steaming geyser at sunset. 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 reduces the hydrostatic pressure to a level where the super- heated water boils spontaneously and erupts. Photographed at Yellowstone National Park, USA.

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