Photoglossary

Geysers   Geysers are hot springs or fountains that periodically erupt steam and boiling water into the air. These columns of water can rise as high as 60 meters (about 200 ft). Three things are necessary for a geyser to develop. There must be hot rocks close to the surface, a system of fractures extending downward to an underground chamber, and a relatively large and constant supply of groundwater. When water collects in the underground chamber, it becomes heated by the hot rocks surrounding it. Because the water at the base of the chamber is under greater pressure from the weight of the overlying water, it does not boil at the normal surface temperature of 100 degrees Celsius (212 degrees Fahrenheit). Even though it does not boil, the temperature of the water continues to rise and this causes it to expand. The water is forced up through fractures in the rock until some of it spills out onto surface. This reduces the pressure on the water remaining in the chamber and lowers its boiling point. Once this occurs, the water quickly converts to steam and causes the geyser to erupt.     Illustration Credit: National Park Service Photo Credit: S. R. Brantley, USGS     Io Home Page Photoglossary Home Page