Geyser, hot spring that erupts intermittently in a column of steam and hot water. Some geysers erupt at regular intervals, but the majority erupt irregularly, the intervals ranging from a matter of minutes to years. The length of time of the eruption varies with the geyser, from seconds to hours. The height of the column ranges from about 1 m (about 3 ft) to about 100 m (about 328 ft), and the amount of water ejected in a single eruption varies from a few liters to hundreds of thousands of liters.

A geyser erupts when the base of a column of water resting in the earth is vaporized by hot volcanic rock. The force with which the water column is expelled depends on its depth. The weight of the water column increases with its depth. The weight, in turn, increases the pressure exerted on the base of the column, thereby increasing the boiling point of the water there. When the water finally boils, it expands, driving some water out into the air. With the weight of the column reduced, the pressure correspondingly drops, and the boiling point of the water remaining in the column falls below its actual temperature. Thereupon, the entire column instantly vaporizes, causing the geyser to erupt. Eruption intervals depend on such variables as the supply of heat, the amount and rate of inflow of subsurface water, and the nature of the geyser tube and its underground connections.

Almost all known geysers are located in three countries of the world—New Zealand, Iceland, and the United States. A famous geyser is Old Faithful in Yellowstone National Park, which expels about 38,000 to 45,000 liters (about 10,000 to 12,000 gallons) at each eruption. Old Faithful usually erupts at intervals of between 30 and 90 minutes, its column rising as high as 52 m (170 ft), but natural changes in the underground water system have recently made the timing of the eruptions less reliable. The geyser gives warning of its impending activity by ejecting jets of water 3 to 8 m (10 to 25 ft) high.

See Geothermics; Volcano.