Geyser, a spring from which hot water and steam erupt periodically. The word is derived from the Icelandic Geysir (“gusher”), the name of a famous geyser in Iceland. Geysers can erupt at regular or irregular intervals. They are found only in areas that have experienced volcanic activity. Most of the geysers in the world occur in Iceland, the United States, and New Zealand.

Some geysers erupt less than 3 feet (1 m) into the air; others reach great heights. The largest known geyser was Waimangu. in New Zealand. It was active from 1901 until 1903 and during its greatest activity sent up a black, boiling jet of mud, stones, and water as high as 1,500 feet (450 m).

The most famous geysers are in Yellowstone National Park, Wyoming. There are 200 in the park. Old Faithful is noted for the regularity of its eruptions. It sends a jet more than 100 feet (30 m) into the air, even 77 minutes on the average. Giant Geyser spouts every week or two, sending up a jet about 250 feet (75 m).

The exact causes of geysers are not definitely known, but the most widely accepted theory is that they occur where a tubelike channel extends from the surface to a network of smaller channels in masses of very hot rock below the earth's surface. The channels fill with water that seeps into the ground from the surface. As water in contact with the hot rock is heated, the water's temperature rises above its boiling point in open air. The water does not immediately boil because of the pressure of the column of water above it. However, as the water continues to be heated, some of the superheated water turns to steam. The steam expands, causing some of the water in the main channel to overflow at the surface. The overflow reduces the pressure on the water below, which then flashes into steam and blows any remaining water into the air.