A geyser is essentially an underground hot spring that, owing to pressure exerted against its constricted plumbing toward the surface of the ground, issues a stream of steam and boiling water from time to time. The term "geyser" is derived from the Icelandic word "geysir," which means to rush forth. A geyser stops erupting once its reservoir is empty, or the water below the surface cools.
Yellowstone National Park, which features more geysers than any other place in the world, is home to the most famous geyser in the world, Old Faithful. This predictable geyser emits a jet of water and steam roughly every 78 minutes.
As with other geysers, Old Faithful is heated by magma or molten rock. Its supply of underground water, which is a collection of melted snow and rain, begins to heat up, rising through the same cracks that gave it entry in the first place.
As the water and steam reaches the surface, it is forced into natural nozzles formed by mineral deposits. The pressure, combined with constricted spaces, forces the water and steam to jet outward.
Geysers tend to form in geological regions in which magma is particularly close to the surface of the earth's crust.