Appendix C
Geothermal Energy and Geysers

No two geysers behave in exactly the same way [255]. Studying the system of underground passages that supplies any one of them is a technological challenge. Geysers are distinct from other subterranean emissions. For example, while a volcano vents molten matter, solid rock and gases are released. On the other hand, fumaroles emit only gases or gases and water, but they do not spit water into the air as do geysers. During a late stage of volcanic activity, fumaroles occur along fissures or in apparently chaotic clusters or fields above hot spots in the mantle. Fumaroles are also sometimes described by the composition of their gases as, for example, in chlorine fumaroles. Although not violent, fumaroles can be dangerous to humans and livestock if hazardous or toxic fumes are inhaled.

Three components must be present for geysers to exist: an abundant supply of water, an intense source of heat, and specialized plumbing. Remote locations or locations legislatively protected from human activity are becoming increasingly important to maintaining geysers. The set of requirements make geysers a rare geological phenomenon. Water is sometimes not available, as in an arid part of the country. Heat may be close to the surface only along a fault line, where a tectonic plate is being subducted, from volcanic activity, or from a hard to explain hot spot. Even if both water and heat are present, the right plumbing is critical.

For water to be spit tens of feet into the air, geyser plumbing must be both water- and pressure-tight. Rhyolite, a volcanic rock high in silica, generally provides the seal. Rhyolite deposits a water-tight seal along the walls of the geyser plumbing. Most of the geyser fields in the world are found in rhyolite, but rhyolite fields are relatively uncommon. The right mixture of water, volcanic heat, and plumbing occurs best at Yellowstone National Park.

Yellowstone National Park, Wyoming

At Old Faithful in Yellowstone Park, the most famous geyser in the United States, the initial jet rises 10 to 26 feet (3 to 8 meters) into the air, erupting at intervals of from 30 to 90 minutes. With each eruption, about 10,000 to 12,000 gallons of water are discharged, some of which rises to heights of 115 to 165 feet (35 to 50 meters) in the geyser jet. The process is repeated, with a predictable time interval between ejections. Earthquakes and other geologic events (e.g., vulcanism, mudslides) can and have altered the conditions and change or destroy geyser activity.

Yellowstone National Park in northwestern Wyoming contains the world’s greatest number of geysers in nine geyser basins within the park. Steamboat Geyser, located in the Norris Basin, currently ejects the world’s tallest natural spouts of water. Major eruptions of Steamboat Geyser can be over 350 feet tall. The Upper Basin alone contains nearly 180 geysers. With more geysers than the rest of the world’s geyser basins combined, Yellowstone is by far the world’s largest geyser field.

At the heart of Yellowstone’s past, present, and future lies volcanism. About 2 million years ago, then 1.2 million years ago, and then again 600,000 years ago, catastrophic volcanic eruptions occurred here. The latest eruption spewed out nearly 240 cubic miles of debris. What is now the park’s central portion then collapsed, forming a 28- by 47-mile caldera, or basin. The magmatic heat powering those eruptions still powers the park’s famous geysers, hot springs, fumaroles, and mud pots. Earthquakes change eruption patterns whenever they occur, with the pattern change often preceding the earthquake.

If there are geysers nearby, geothermal electricity production depletes the geysers’ water and removes some of the available heat, reducing and sometimes destroying geyser activity. As a result, the second- and third-largest geyser fields in the United States have ceased spouting. Because of the loss of these geysers, the geothermal energy available in and around the Yellowstone National Park has been legislatively protected.

How Geysers Work

Throughout the world, various features exhibit geyser-like activity. Many perpetual spouters, hot springs, venting hot and cold wells, blow holes, karst water spouts, and fumaroles behave as to be interesting to the geyser researcher and enthusiast. These geysers increasingly must possess a fourth characteristic to exist: protection from human activities, either by legislation or by remoteness. Within the last twenty years, geothermal energy and abundant water have been increasingly harnessed to turn turbines for electricity production. Geothermal electricity can be produced at any site where sufficient heat and water are readily available, and geyser fields are ideal for this type of energy production.

As volcanic activity subsides, igneous rocks in the old magma chamber deep in the Earth remain hot for a million years or more. Descending groundwater mixes with perhaps either 25,000 year-old or even more antique connate water, and as one or the other comes into contact with the hot rock, the water becomes heated and tends to rise again toward the surface along a fault or some other fracture where it forms a thermal spring. Also called a hot or warm spring, thermal springs are at least 14 degrees Fahrenheit (8 degrees Centigrade) higher than the average temperature of the air. Water temperatures in thermal springs range all the way up to the boiling point. These hot springs are often unusually rich in mineral matter because dissolution is more rapid in warm water than in cold water. In some springs the mineral content is said to have medicinal properties. Of the thousands of hot springs in the United States, most are found in the Western mountains. While single geysers are rare, groups of geysers, called geyser fields, are rarer still. There were four geysers fields in 1980 and two in 1996—one in Yellowstone and one in the Aleutian Islands off the Alaska coast.

Warm water geysers and hot springs often dissolve some minerals readily so that in some regions certain characteristic rock deposits can be seen. When large amounts of silica dissolved in underground water are forced to the surface, the silica precipitates, or drops from solution, as the water cools. Initially a colloidal gel forms around the geyser mound, but it eventually consolidates into a noncrystalline form of quartz known as geyserite.

On reaching the surface, calcite dissolved in hot water forms travertine, which is also known as dripstone. Geyser pools, which sometimes form below the mouth of geysers in the geyser crater at the top of the geyser pipe, may be quite beautiful because they reflect different colors associated with minerals dissolved in the heated water.

Geysers always indicate the presence of either a vapor-dominated or a water-dominated hydrothermal resource. This has allowed geysers to guide the identification of geothermal locations and the potential of the subterranean energy resource. For example, a total of 356 gravity stations were established in the Crump Geyser area, Oregon, in the mid-1970s. The gravity survey provided background geophysical information to assist in evaluating the area’s geothermal resource potential. Standard gravity reduction procedures were used to obtain values of the observed gravity, free air anomaly, and Bouguer anomalies [256].

However, some water-dominated hydrothermal resources have no obvious surface manifestations, with neither geysers, fumaroles nor hot springs (e.g., eastern side of the Cascade Mountain Range in Oregon). Hydrothermal resources are requisite for geothermal electric generating facilities.

Geysers are surface manifestations of hydrothermal resources, boiling hot springs with a natural system of plumbing and heating that causes intermittent eruptions of water and steam. The word geyser comes from the Icelandic word meaning “to gush.” Nearly all the world’s true geysers, also known as pulsating springs or gushers, are located in Iceland, New Zealand, and the United States. Though New Zealand and Iceland are known for their geysers, nowhere are there as many as in Yellowstone. Over 400 of the world’s 700 active geysers are in Yellowstone National Park, making geysers a very rare geological phenomenon.

Geysers are known for their often spectacular eruptions that throw water and steam high into the air. From an economic and public safety standpoint, some geysers have shown certain precursor activity prior to earthquakes. During the period 1973 to 1991 the interval between eruptions from a periodic geyser in Northern California exhibited precursory variations 1 to 3 days before the three largest earthquakes within a 250-kilometer radius of the geyser. These include the magnitude 7.1 Loma Prieta earthquake of October 18, 1989 for which a similar pre-seismic signal was recorded by a strain meter located halfway between the geyser and the earthquake [257].

The underground structure of a geyser consists of a crooked tube-like opening that leads from the interior to the ground surface. Several small caverns or chambers may be connected to the tube. Groundwater partially fills the tube and some of the connecting caverns. The heated water is trapped under pressure in the crooked tube [258]. Continued heating produces a water temperature above the boiling point, and the steam so produced develops enough pressure to eject a small amount of water to the surface. This expulsion of water in the initial upsurge reduces pressure on the superheated water in the tube. The reduction in pressure causes the remaining water to boil explosively to the point where it drives a column of water and steam, called the geyser jet, into the air. The eruption continues until water and steam are driven out of the tube and storage caverns.

The hot water, circulating up from great depth, flows into the plumbing system of a geyser. Because this water is many degrees above the boiling point, some of it turns to steam. Meanwhile, additional, cooler water is flowing into the geyser from the porous rocks nearer the surface. The two waters mix as the plumbing system fills.

The steam bubbles formed at depth rise and meet the cooler water. At first, they condense there, but as they do they gradually heat the water. Eventually, these steam bubbles rising from deep within the plumbing system manage to heat the surface water until it also reaches the boiling point. Now the geyser begins to function like a pressure cooker. The water within the plumbing system is hotter than boiling, but “stable” because of the pressure exerted by all the water lying above it.

The filling and heating process continues until the geyser is full or nearly full of water. A very small geyser may take but a few seconds to fill whereas some of the larger geysers take several days. Once the plumbing system is full, the geyser is about ready for an eruption. Often forgotten but of extreme importance is the heating that must occur along with the filling. Only if there is an adequate store of heat within the rocks lining the plumbing system can an eruption last for more than a few seconds. Again, each geyser is different from every other. Some are hot enough to erupt before they are completely full and do so without any preliminary indications of an eruption. Others may be completely full well before they are hot enough to erupt and so may overflow quietly for some time before an eruption occurs. But, eventually, the eruption will take place.

Because the water of the entire plumbing system has been heated to boiling, the rising steam bubbles no longer collapse near the surface. Instead, as more very hot water enters the geyser at great depth, even more and larger steam bubbles form and rise toward the surface. At first, they are able to make it all the way to the top of the plumbing system. But a time will come when there are so many steam bubbles that they can no longer simply float upwards. Somewhere they encounter some sort of constriction or bend in the plumbing. To get by they must squirt through the narrow spot. This forces some water ahead of them and up and out of the geyser.

This initial loss of water reduces the pressure at depth, lowering the boiling point of water already hot enough to boil. More water boils, forming more steam. Soon there is a virtual explosion as the steam expands to over 1,500 times its original, liquid volume. The boiling rapidly becomes violent and water is ejected so rapidly that it is thrown into the air.

The eruption will continue until either the water is used up or the temperature drops below boiling. Once an eruption has ended, the entire process of filling, heating, and boiling will be repeated, leading to another eruption.

Contact:

Mark Gielecki

Mark.Gielecki@eia.doe.gov

Phone: (202) 287-1729

Fax: (202) 426-1280