Morgen Jahnke writes about geysers on the Interesting Thing of the Day web site. Americans who haven’t seen a geyser erupt should visit Yellowstone ASAP because they are threatened by environmental changes and nearby development.
It is estimated that there are only 1000 geysers in the world, half of which exist in the geyser-perfect environment of Yellowstone. Other countries with large numbers of geysers concentrated in a single area are Iceland, Russia, Chile, and New Zealand, and individual geysers exist in Peru, Bolivia, Mexico, Dominica, the Azores, Kenya, and Japan. One of the greatest threats to geysers is the diversion of their underground water sources, most often in connection with the creation of geothermal power plants. Such is the case in New Zealand, where the Wairakei geyser field on its North Island was devastated by the creation of a nearby power plant in 1958.
The life cycle of a geyser begins when water seeps into the ground from the surface (because of rain) or from underground reservoirs, eventually sinking deep enough to reach a layer of hot rock. This water is slowly heated and gathers at the bottom of the geyser channel, while colder water enters the channel from above, and sits on top of the warmer water. The pressure of the cold water prevents the warm water from boiling, although it continues to become super-heated. When the pressure becomes too great, the hot water turns to steam and pushes the colder water out of the channel. This reduces the pressure further, producing even more steam. This whole cycle can take 500 years, which means the water rushing from a geyser today may have fallen as rain during the 16th century.