PETER J. RAYMOND / SCIENCE PHOTO LIBRARY PETER J. RAYMOND / SCIENCE PHOTO LIBRARY
Geyser field. Tourists viewing the steam rising from hot pools in the Tatio geyser field, Chile. Geysers are deep natural wells in geothermal rock fissures. Their water lies in a large chamber at the well's base. The water is heated by surrounding rocks until it is well over its normal boiling point, but is prevented from boiling by hydrostatic pressure. At a critical temperature, the super-heated water spontaneously boils, causing an eruption.
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