A geyser is a rare kind of hot spring that is under pressure and erupts, sending jets of water and steam into the air. They are characterized by an intermittent discharge of water ejected turbulently and accompanied by steam. Geysers are nonpermanent geological features and are generally associated with volcanic areas. The formation of geysers requires the combination of three geologic conditions that are usually found in volcanic terrain: intense heat, water, and a plumbing system. There are two types of geysers: fountain geysers which erupt from pools of water, typically in a series of intense, even violent, bursts; and cone geysers which erupt from cones or mounds of siliceous sinter (including geyserite), usually in steady jets that last anywhere from a few seconds to several minutes. Old Faithful, located in Yellowstone National Park, is the worlds best-known geyser and is an example of a cone geyser.

To understand how a geyser works, you must first understand the relationship between water and steam. Steam is a gaseous form of water. Steam is produced when water is heated to its boiling point. When water converts into steam at surface conditions, it undergoes an enormous expansion because steam occupies 1600 times as much space as the original volume of water. The eruption of a geyser is powered by a "steam explosion" when boiling-hot water suddenly expands into the much more voluminous steam. A geyser erupts when superheated groundwater, confined at depth, becomes hot enough to blast its way to the surface.

Geysers are fragile phenomena and if conditions change, they may go dormant or extinct. Many have been destroyed simply by people throwing debris into them while others have ceased to erupt due to dewatering by geothermal power plants.