BERNHARD EDMAIER / SCIENCE PHOTO LIBRARY BERNHARD EDMAIER / SCIENCE PHOTO LIBRARY
Grotto Geyser. Eruption of steam and hot water from Grotto Geyser. Geysers consist of deep natural wells in geothermal rock fissures. Their water lies in a large chamber at the base of the well. The water is heated by the surrounding rocks until it is well over its normal boiling point. However, it is prevented from boiling by hydrostatic pressure. At a critical temperature, the superheated water spontaneously boils, causing an eruption. Grotto Geyser is in Yellowstone National Park, Wyoming, USA.
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