A geyser is a spring that intermittently discharges water and steam turbulently. The most powerful geyser has been observed ejecting water to 460 m (1,500 ft) (Waimangu geyser, now extinct). For a geyser to occur, three geologic conditions are required.
The heat needed to form a geyser comes from magma that is close to the Earth’s surface. For this reason, geysers are associated with volcanic areas.
An water source must travel underground in deep pressurized fissures, where it can be exposed to the hot crust
A Plumbing System
Fractures, faults, porous spaces and cavities must provide a natural reservoir to hold the heating water. Constrictions in the system is needed to build up pressure before an eruption.
As the underground reservoir fills, the heated water rises by convection. As it rises, the water at the top of the column cools, but due to restricted space, it is unable to cool the whole system by convective cooling. This lid of cold water presses down against the hot water below, increasing the pressure in the reservoir.
The high pressure allows for the water to become superheated. The water at the bottom eventually becomes steam and rise up the column. As they exit the vent, the water lid at the top of the column is spilled out, releasing the pressure under it. With this change in pressure, the superheated water rapidly boils and is ejected out of the vent.
(Geysir, Iceland depicted)
Howard Ignatius on Flickr