A Geyser is a periodically erupting hot spring. The term comes from a spring at Haukadalur, Iceland, which itself comes from the Icelandic verb meaning "to gush." Geyser eruptions occur when water collects in an area heated by subterranean magma, and is temporarily trapped by cooler water from above. Once the pressure of the superheated water below surpasses the force of the cooler water's weight, the surface water is breached and the heated water bursts out in the form of water and steam.
Geysers require specific geological conditions in order to manifest. There are only about 1,000 in the world, of which about half can be found at Yellowstone National Park. The regularity of a geyser's eruptions can vary, but one of the most famous geysers, Old Faithful, has such uniform timing that it has been said a visitor could set his watch by it.
- Geysers and How They Work from the National Park Service
- Geyser Observation and Study Association