DOUGLAS FAULKNER / SCIENCE PHOTO LIBRARY DOUGLAS FAULKNER / SCIENCE PHOTO LIBRARY
Aerial view of a geyser. The scale of the geyser can be gauged by the people on the pathway that runs along the bottom of the frame. The brown colour of the rim is due to iron minerals. Geysers are features of high geothermal activity. They consist of a tube several hundred metres deep, near the base of which are one or more large chambers. Water in these chambers is heated by the rocks, but hydrostatic pressure prevents it from boiling. At a critical temperature, enough water is spilled out of the geyser to allow the water to boil. It does so explosively, throwing a jet of water high into the air. The vent then fills with fresh water and the cycle restarts.
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