A geyser is a special kind of hotunderground spring. It periodically ejects streams of hot water and steam intothe air. Geysers are very rare because they require a unique combination ofgeology that exists in only a few places earth. Only about 1,000 can be foundtoday, with half of them in Yellowstone National Park in the United States. Hotsprings are formed when surface water seeps down though the ground until itreaches rock that has been heated by magma, which is volcanic melted rock. Thehot water rises up to the surface through spaces between the underground rocklayer. The water becomes part of a geyser if the rising water is constrictedinto a narrow passageway that connects the underground water to the surface.
The water cools as it escapes to thesurface, causing it to exert downward pressure on the hotter water below. Thehot water has no place to go, being blocked by the solid layer of undergroundrock. As a result, the pressure and heat rises on the water, superheating it totemperatures around 95.6 degrees Celsius.
The boiling water surges through the ventleading to the surface. This escape reduces the pressure on the water below,causing the superheated water to burst out the vent into the air, resulting ina spray that we see as the end product of the geyser. The spray may continuefor several minutes.
Eventually, this release in pressure causesthe remaining water to cool below the boiling point, and the eruption ends,with the groundwater again seeping back into the ground. The cycle repeats itselfat predictable intervals. There are two type of geysers. Fountain geysers eruptfrom pools of water, usually in a series of violent bursts. Cone geysersemanate from mounds of mineral deposits known as geyserite. They form steadysprays lasting from a few seconds to several minutes. The most famous geyser isa cone geyser at Yellowstone National Park known as Old Faithful. It derivesits name from its regularity.
It erupts about every 74 minutes onaverage. Each eruption lasts between 1.5 and 5 minutes. The height of theeruption varies between 100 and 180 feet. The rarity of geysers is attributableto the unusual conditions necessary to produce them. The volcanic rock must bedissolved in order to form mineral deposits that line and strengthen the wallsof the passageway leading to the surface. Any interference with the vent candestroy the geyser. For example, many geysers have died from litter and debristhrown into them by people. And others have succumbed when nearby power plantshave diverted water away from them.