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A geyser is a spring characterized by intermittent discharge of water ejected turbulently and accompanied by a vapour phase (steam). The word geyser comes from Geysir, the name of an erupting spring at Haukadalur, Iceland; that name, in turn, comes from the Icelandic verb geysa, "to gush", the verb itself from Old Norse.

The formation of geysers is due to particular hydrogeological conditions, which exist in only a few places on Earth, so they are a fairly rare phenomenon. Generally all geyser field sites are located near active volcanic areas, and the geyser effect is due to the proximity of magma. Generally, surface water works its way down to an average depth of around 2,000 metres (6,600 ft) where it contacts hot rocks. The resultant boiling of the pressurized water results in the geyser effect of hot water and steam spraying out of the geyser's surface vent.

About a thousand known geysers exist worldwide, roughly half of which are in Yellowstone National Park, Wyoming, United States. A geyser's eruptive activity may change or cease due to ongoing mineral deposition within the geyser plumbing, exchange of functions with nearby hot springs, earthquake influences, and human intervention.[1]

Jet-like eruptions, often referred to as geysers, have been observed on several of the moons of the outer solar system. Due to the low ambient pressures, these eruptions consist of vapor without liquid; they are made more easily visible by particles of dust and ice carried aloft by the gas. Water vapor jets have been observed near the south pole of Saturn's moon Enceladus, while nitrogen eruptions have been observed on Neptune's moon Triton. There are also signs of carbon dioxide eruptions from the southern polar ice cap of Mars. In the latter two cases, instead of being driven by geothermal energy, the eruptions seem to rely on solar heating via a solid-state greenhouse effect.

Steamboat Geyser in Yellowstone National ParkGeysers are temporary geological features. The life span of a geyser is, at the most, only a few thousand years. Geysers are generally associated with volcanic areas.[2] As the water boils, the resulting pressure forces a superheated column of steam and water to the surface through the geyser's internal plumbing. The formation of geysers specifically requires the combination of three geologic conditions that are usually found in volcanic terrain.[2]

Intense heat
The heat needed for geyser formation comes from magma that needs to be near the surface of the earth. The fact that geysers need heat much higher than normally found near the earth's surface is the reason they are associated with volcanoes or volcanic areas. The pressures encountered at the areas where the water is heated makes the boiling point of the water much higher than at normal atmospheric pressures.
Water
The water that is ejected from a geyser must travel underground through deep, pressurized fissures in the earth's crust.
A plumbing system
In order for the heated water to form a geyser, a plumbing system is required. This includes a reservoir to hold the water while it is being heated. Geysers are generally aligned along faults.[2] The plumbing system is made up of a system of fractures, fissures, porous spaces and sometimes cavities. Constrictions in the system are essential to the building up of pressure before an eruption.
[edit] Eruptions
Geyser activity, like all hot spring activity, is caused by surface water gradually seeping down through the ground until it meets rock heated by magma. The geothermally heated water then rises back toward the surface by convection through porous and fractured rocks. Geysers differ from non-eruptive hot springs in their subterranean structure; many consist of a small vent at the surface connected to one or more narrow tubes that lead to underground reservoirs of water.[3]


1. Steam rises from heated water 2. Pulses of water swell upward 3. Surface is broken 4. Ejected water spouts upward and falls back
As the geyser fills, the water at the top of the column cools off, but because of the narrowness of the channel, convective cooling of the water in the reservoir is impossible. The cooler water above presses down on the hotter water beneath, not unlike the lid of a pressure cooker, allowing the water in the reservoir to become superheated, i.e. to remain liquid at temperatures well above the standard-pressure boiling point.[3]

Ultimately, the temperatures near the bottom of the geyser rise to a point where boiling begins; steam bubbles rise to the top of the column. As they burst through the geyser's vent, some water overflows or splashes out, reducing the weight of the column and thus the pressure on the water underneath. With this release of pressure, the superheated water flashes into steam, boiling violently throughout the column. The resulting froth of expanding steam and hot water then sprays out of the geyser vent.[2]

The rocks in the nearby region produce a material called geyserite. Geyserite—mostly silicon dioxide (SiO2), is dissolved from the rocks and gets deposited on the walls of the geyser's plumbing system and on the surface. The deposits make the channels carrying the water up to the surface pressure-tight. This allows the pressure to be carried all the way to the top and not be leaked out into the loose gravel or soil that are normally under the geyser fields.[3]

Eventually the water remaining in the geyser cools back to below the boiling point and the eruption ends; heated groundwater begins seeping back into the reservoir, and the whole cycle begins again. The duration of eruptions and time between successive eruptions vary greatly from geyser to geyser; Strokkur in Iceland erupts for a few seconds every few minutes, while Grand Geyser in the United States erupts for up to 10 minutes every 8–12 hours.[3]

[edit] General categorization
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 (also known as geyserite), usually in steady jets that last anywhere from a few seconds to several minutes. Old Faithful, perhaps the best-known geyser at Yellowstone National Park, is an example of a cone geyser. Grand Geyser, the tallest predictable geyser on earth, (although Geysir in Iceland is taller, it is not predictable), also at Yellowstone National Park, is an example of a fountain geyser.[4]


1. Fountain geyser (erupting from the pool) 2. Old Faithful geyser (cone geyser having mound of siliceous sinter) in Yellowstone National Park erupts approximately every 91 minutes
The intense transient forces inside erupting geysers are the main reason for their rarity. There are many volcanic areas in the world that have hot springs, mud pots and fumaroles, but very few with geysers. This is because in most places, even where other necessary conditions for geyser activity exist, the rock structure is loose, and eruptions will erode the channels and rapidly destroy any nascent geysers.

Most geysers form in places where there is volcanic rhyolite rock which dissolves in hot water and forms mineral deposits called siliceous sinter, or geyserite, along the inside of the plumbing systems which are very slender. Over time these deposits cement the rock together tightly, strengthening the channel walls and enabling the geyser to persist; as mentioned in the previous section.

Geysers are fragile phenomena and if conditions change, they can "die". Many geysers have been destroyed by people throwing litter and debris into them; others have ceased to erupt due to dewatering by geothermal power plants. The Great Geysir of Iceland has had periods of activity and dormancy. During its long dormant periods, eruptions were sometimes humanly-induced—often on special occasions—by the addition of surfactants to the water.

[edit] Biology of geysers
Further information: Thermophile and Hyperthermophile