A Geyser is a hot spring, which erupts intermittently in a column of steam and hot water. Some geysers erupt at regular intervals, but most 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 (3 ft) to about 100 m (328 ft), and the amount of water ejected in a single eruption varies from a few litres to hundreds of thousands of litres.

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.

Almost all known geysers are located in three countries of the world—New Zealand, Iceland, and the United States. The most famous geyser in the world is Old Faithful in Yellowstone National Park in the United States, which expels about 38,000 to 45,000 litres (10,000 to 12,000 gallons) at each eruption. Old Faithful erupts at intervals of between 37 and 93 minutes, its column rising to a height of between 38 and 52 m (125 and 170 ft). The geyser gives warning of its impending activity by ejecting jets of water 3 to 7.6 m (10 to 25 ft) high.

Eruption intervals depend on such variables as the atmospheric pressure, the supply of heat, the amount and rate of inflow of subsurface water, and the nature of the geyser tube and its underground connections.