How Geysers Work


Geysers are named after a famous example, the great Geysir of Iceland, located northwest of Mt. Hekla, an active volcano in the south-central part of the island. The name means "roarer" in Icelandic. Geysers are distinguished by periodic forceful eruptions of water to heights of 100' to 200' (the record is 1500'). After the water comes a blow of steam, responsible for the roaring. The periodicity and vigor of these eruptions is what we want to understand. Geysers are always associated with hot springs and other hydrothermal activity, but are rather unusual phenomena. Iceland is located on the Mid-Atlantic ridge, a spreading centre noted for its basic igneous activity.

The image at the left shows an eruption of Old Faithful, the most famous geyser of Yellowstone National Park, shown in eruption in the image at the left, from the webcam listed in the References. Old Faithful is not the largest geyser in Yellowstone, but it erupts the most frequently and regularly. Presently, eruptions occur at intervals of 45-110 minutes, with an average of 76 minutes, and are more or less predictable. Most geysers, however, are quite irregular. Old Faithful ejects 14,000 to 32,000 litres of boiling water to heights of 30 to 55 m in each eruption. The hydrothermal area of Yellowstone is located in the Yellowstone Caldera, which was formed in the Pleistocene in a violent acidic eruption that has left thick welded tuffs, some earlier indentified as rhyolite, in the area, though most of the volcanism is basic. This is the most recent stage of Tertiary eruptions that left thick sheets of basalt in Washington and Oregon, and later filled the Snake River graben with basalt. The continental plate is apparently moving to the southwest over a "hot spot" in the mantle that provides copious carbon dioxide and water to make active fluids and magmas. The hot spot seems now to be under Yellowstone, and its active gases are responsible for the heat and hydrothermal activity. The Norris Geyser Basin is a notable hydrothermal area, as is Mammoth Hot Springs with its travertine terraces. Old Faithful is located in the southwest of the caldera, in a region of smaller geyser basins. The Yellowstone Park website has a excellent tours of the Old Faithful and Norris geyser basins that will give you an unsurpassed look at many hydrothermal features. With its approximately 300 geysers, Yellowstone has two-thirds of all the geysers in the world. Once upon a time, Minute Geyser in the Norris Geyser Basin erupted to 12-15 m every 60 seconds. Americans liked it so much they threw rocks in it and destroyed it. A number of Yellowstone features show similar visitor attention.

The North Island of New Zealand and the Kamchatka Peninsula also have remarkable geysers. The North Island is just to the west of the southernmost part of the Kermadec Trench, where the Pacific plate is subducting beneath the Australian plate. The geysers are mainly located in the hydrothermal area in the centre of the island, near Lake Taupo, surrounded by three active volcanoes. Kamchatka is just west of the northern end of the Kuril Trench, where it turns eastward as the Aleutian trench. The Pacific plate is moving northwestward relative to the North American plate in this area, and is being subducted beneath it. Kamchatka has many active volcanoes, including beautifully symmetric, snow-covered Klyuchevskoy, 15,600 ft. high with a wisp of vapor normally at the summit crater. The Dolina Geizerov, "Valley of the Geysers," in the southern part of the peninsula is a wonderful but inaccesible geothermal site, with some 200 geysers.

Of these four main areas of geysers--Iceland, Yellowstone, North Island and Kamchatka--one is located on a spreading centre, two are in subduction zones, and one is above a hot spot. A few geysers are found at other locations, perhaps 7 in East Africa, along the Rift Valley, 3 in the Andes of South America, 2 in southwestern Mexico, and one each in Japan, the Aleutians and the Azores. Geysers are rumored at Pauzhetsk, also in Kamchatka, and at Shaishkotan Island, in the Kuriles. These are the only places they occur, so they are a rare phenomenon in active areas of basaltic volcanism, requiring rather specific condtions, including percolating ground water and sufficiently hot heating fluids. Hot springs, with which geysers are always associated, are much more common, yielding flows of warm, often hot, water richly charged with gases such as carbon dioxide and hydrogen sulphide. Geysers are just hot springs in which certain conditions are satisfied, and there is a narrow line between an impressive geyser and a boiling pool.

The hot waters from geysers are usually charged with dissolved silica, which leaves a characteristic deposit of geyserite, a form of siliceous sinter, in a mound around the top of the vent or pipe, usually containing a shallow geyser pool. The mound is easily visible in the photograph above. Siliceous sinter is opaline, or hydrated, silica. The "rhyolite" rocks in the area are largely responsible for this. If the rocks were limestones, the deposit would be travertine, and if the rocks were not soluble at all, there would be nothing much. The crater in this mound is normally filled with clear, hot water of 75°C to 90°C. At irregular intervals, the surface begins to churn from bubbles arriving from below, and before long the contents of the geyser's conduit is expelled upwards with a blast of steam. Old Faithful was famous for its unusual regularity, erupting about once an hour, but the earthquake of 1959 disturbed its periodicity, and now it erupts every 37-93 minutes. Geysir erupts every 5 to 35 hours. Then, all is quiet until the next eruption. In the Norris Geyser Basin, subsurface water temperatures have been measured as high as 232°C. The reason for the siliceous sinter cones is that hydrothermal areas are often associated with acidic (silica) vulcanism. At Mammoth Hot Springs, at the northern edge of Yellowstone, the active fluids ascend through limestone, so the rock deposited there is travertine, not siliceous sinter.

The most powerful geyser in Yellowstone is Steamboat Geyser, which can send a plume up to 90 m high. Its major eruptions, however, are rare and irregular, though they may last between 3 and 40 minutes. Minor eruptions are only 3 to 12 m high, but are more frequent. The Biscuit Basin geyser was noted for the strange biscuit-shaped deposits around it. The Hebgen Lake earthquake of 1959 caused such a violet eruption that it blew away all the biscuits. The same earthquake rendered Giant Geyser dormant for a period, though it now erupts every 3 to 10 days to a considerable height, 55 to 80 m.

I have been informed that Giant Geyser is dormant again, and the timings of Old Faithful are different, which is to be expected.

Although most geysers are intermittent, alternately erupting and filling, sometimes a geyser may erupt constantly. Such a geyser appeared in April 2004 3 km from Lake Boringo in Kenya. This geyser erupts salt water to a height of 80 m and can be seen for 20 km. Since Lake Boringo is fresh, the salt water would seem to have another source. The stream of water is constantly heated, evaporated and expelled.

An animated illustration of the geyser cycle is shown at the right. The plumbing of the geyser is shown diagrammatically, with a central pipe and various fissures and openings entering into it. The total height shown is a few hundred feet. The cycle begins with the system largely empty of water after an eruption. Cooler ground water leaks into the system at higher levels, and hot, gas-charged water at lower levels. These waters are encouraged by the low pressure in the pipe. As the system fills, the hydrostatic pressure at the lower levels rises to its normal value. The waters are probably heated mostly by hot fluids from below. As they are heated, the carbon dioxide (we shall assume that this is the gas involved) becomes less soluble and forms bubbles that rise upwards because of their buoyancy. These bubbles expand as they rise, reducing the density of the water in the pipe, so that the hydrostatic pressure at the lower levels is reduced. When the reduced pressure reaches the vapor pressure of water at the temperature in the lower parts, the superheated water flashes into steam more or less simultaneously (since the pressure is the same everywhere at the same depth). This is a cooperative effect, since the resulting reduction in pressure stimulates more boiling. The liquid contents of the system are then largely ejected by the steam pressure, followed by the rest of the steam, and the cycle begins anew.

At a depth of 340', the hydrostatic pressure is 147 psi, and water will boil at 180°C. When the local temperature reaches, say, 175°C, the water will not yet boil, but if the pressure should be reduced to 120 psi, the superheated water will boil, since the boiling point at this pressure is 172°C. We note that this point has only to be just reached; once boiling starts, the reduction in pressure is rapid. Just before an eruption, the rise of a white volume with bubbles can be seen down the pipe. These bubbles are probably carbon dioxide, since steam would condense before it reached the surface. It is the release of pressure by the rising carbon dioxide bubbles that triggers an eruption. At 0°C, one volume of water dissolves 1.713 volumes of carbon dioxide at a pressure of 1 atm, but at 50°C only 0.436 volume. These are not the conditions in the superheated water of a geyser, but do show the large decrease in solubility with an increase in temperature. The carbon dioxide is probably released mainly from the ground water that percolates into the geyser, rather than from the hot, active heating water from below. The source of heat cannot be simply "hot rocks," because they would rapidly be cooled, and their heat conductivity is not large enough to replace the heat lost.

To illustrate the critical conditions that cause a geyser, there was a hot spring pool whose water was diverted and lowered to supply a swimming pool. This lowering of the pressure caused eruptions to start, about every 4 to 8 minutes, to a modest height of less than two metres, and the feature is now called Solitary Geyser. When the diversion was repaired, the eruptions continued in spite of the restoration of the normal height of the pool. The Plume Geyser was created by a steam explosion in 1922, and now erupts every 20 minutes with three to five bursts of steam rising to 8 m. The sudden variations in geyser activity with no obvious change in meteoric water conditions is excellent evidence that the cause of the activity is hot, active volcanic fluids, perhaps mainly water, from deep sources, and not simply "hot rocks."

When there are active fluids without water, fumaroles are the result, many emitting hydrogen sulphide and carbon dioxide as well as steam. The hydrogen sulphide most probably results from bacterial reduction of sulphur dioxide and sulphur trioxide. The next level of activity is the hot spring, often charged with minerals, and sometimes far from other volcanic activity. The hot springs of Thermopolis, Wyoming are more than a hundred miles southeast of Yellowstone, in an area completely devoid of other volcanic activity. There are mud pots where the active fluids have disintegrated rocks into a clayey mud, through which volcanic gases bubble. Sometimes they are called mud volcanoes, but features of this name also exist on the Caspian Sea, where the bubbles are made of natural gas, not volcanic gases. Finally, we have the rare and remarkable geyser, that has been the main subject of this article. Hydrothermal areas combine meteoric water from above and hot juvenile water from below to make a fascinating spectacle.

References

A. Holmes, Principles of Physical Geology, 2nd ed. (New York: Nelson, 1965). pp. 428-430.

A web search will turn up numerous articles on geysers, and many photos, including a live webcam of Old Faithful. This link will lead you into the excellent Yellowstone Park website, with many illustrations of geothermal activity. The virtual tours hit all the high spots, and are much easier and cheaper than walking.

A good site is Jonesy's Geyser Page, with many links to geyser sites. This is hosted by an American commercial ISP, so watch out.


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Composed by J. B. Calvert
Created 8 February 2003
Last revised 19 February 2008