This article, written by the American Ground Water Trust was originally published in THE AMERICAN WELL OWNER, 2002, Number 3]


We are used to ground water being described as slow moving. Geysers are dramatic exceptions! A geyser is a hot spring that periodically jets a stream of boiling water and steam out of the ground. The name is derived from the Icelandic word “geysir” that means “to rush forth” and the “Great Geysir” located in the town of Huakadalur in southwestern Iceland. They are associated with active volcanic regions around the world where ground water is heated as it infiltrates to hot zones in the earth’s crust.

To understand how a geyser works we must understand that the temperature at which water boils rises with increasing pressure. At sea level (“one atmosphere pressure” or 760 millimeters of mercury) water boils at 100 degrees Celsius (212° Fahrenheit). In Denver, Colorado, USA, also known as the “Mile High City,” atmospheric pressure is less than at sea level and water boils at about 95°Celsius (203°F). Just the opposite happens when water is at greater pressure in the earth’s crust. High pressures increase the temperature required to boil water deep underground.

Geysers are comprised of an intricate series of fractures and cracks. Narrow constrictions within the network of fractures act as pipes. When the fracture system is full of water, the pressure at the bottom of the geyser system is at a greater pressure, due to the weight of the overlying water column, than the water at the top of the geyser near the opening. The greater pressure at the bottom of the geyser keeps the deep water from boiling even though temperatures may reach 150°C (302°F). If the pressure of the overlying water column is reduced in some way, the effect is an immediate pressure drop throughout the water column in the geyser. This can initiate instantaneous high-temperature violent boiling at the bottom of the geyser that forces the water explosively out of the throat and mouth of the geyser. One mechanism that may start the process is for the water at the top to boil to the point where water (and pressure) is lost out of the geyser system allowing progressively deeper water to boil. The zone of boiling continues downward rapidly and increases in intensity forcing the water column out of the geyser.

“Cold water” artesian (free flowing) wells and springs should not be confused with geysers. Artesian wells flow because they are under pressure from a confined source of water that pushes the ground water out of the well without boiling.

Further reading: The physics of boiling water: http://hyperphysics.phy

[© American Ground Water Trust. This article may be reprinted for non-commercial educational purposes provided it is used in its entirety and that reference is made to its source as an article in THE AMERICAN WELL OWNER, 2002, Number 3]