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The Geology of Geysers

Geysers are among the most spectacular and rare of all geological phenomena. A geyser is a type of hot spring that periodically erupts, forcefully ejecting a column of hot water and steam into the air. While hot springs are found in many parts of the world, geysers are far less common because their formation requires a very specific combination of underground conditions. Only a few locations on Earth, such as Yellowstone National Park in the United States and parts of Iceland, have the necessary geology to support geyser activity.

For a geyser to exist, three essential ingredients must be present: an abundant source of water, a potent source of heat, and a special type of plumbing system. The water typically comes from rain and snow that seeps deep into the ground. The heat is supplied by a body of magma, or molten rock, located relatively close to the Earth's surface. The most critical element, however, is the underground plumbing, which is composed of a series of fissures and cavities with narrow constrictions. These constrictions are vital because they impede the free circulation of water.

The eruption cycle of a geyser is a process of pressure buildup. As cool groundwater fills the plumbing system, it is rapidly heated by the nearby magma to a temperature well above the normal boiling point. However, the pressure from the weight of the water column above and the narrow passages prevent it from boiling. Eventually, the water becomes superheated, and some of it flashes into steam. This steam expands dramatically, violently pushing the entire column of water above it out of the ground in a powerful eruption, releasing the pressure and starting the cycle anew.