Hot springs and geysers

MOUND OF SINTER at Castle Geyser, Upper Geyser Basin. Lower part of mound has well-defined layers probably deposited by normal hot springs. The upper, irregular part resulted from the vigorous eruptions characteristic of geysers and marks a change in the local hot-spring activity. (Fig. 47)

Nearly all geysers and many hot springs build mounds or terraces of mineral deposits; some are so unusual in form that descriptive names have been given to them, such as Castle Geyser (fig. 47). These deposits are generally made up of many very thin layers of rock. Each layer represents a crust or film of rock-forming mineral which was originally dissolved in hot water as it flowed through the underground rocks, and which was then precipitated as the water spread out over the surrounding ground surface.

TERRACES OF TRAVERTINE at Opal Springs, Mammoth Hot Springs area. (Fig. 48)

Closeup view shows layered and porous nature of the travertine.

In all major thermal areas of the Park, with the exception of Mammoth Hot Springs, most of the material being deposited issinter(the kind found around geysers is popularly calledgeyserite). Its chief constituent is silica (the same as in quartz and in ordinary window glass). At Mammoth, the deposit istravertine(fig. 48), which consists almost entirely of calcium carbonate. The material deposited at any given place commonly reflects the predominant kind of rock through which the hot water has passed during its underground travels. At Mammoth Hot Springs the water passes through thick beds of limestone (which is calcium carbonate), but in other areas the main rock type through which the water percolates is rhyolite, a rock rich in silica.

Through centuries of intense activity, layers of sinter have built up on the floors of the geyser basins (fig. 44); these deposits are generally less than 10 feet thick. In one drill hole at Mammoth, deposits of travertine extend to a depth of 250 feet. Dead trees and other kinds of vegetation whose life processes have been choked off by the heat, water, and precipitated minerals of hot-spring activity are a common sight in many places (fig. 51).

Both travertine and sinter are white to gray. Around active hot springs, however, the terraces that are constantly under water may be brightly colored (figs.43and49) because they are coated by microscopic plants calledalgae. These organisms, which thrive in hot water at temperatures up to about 170°F, are green, yellow, and brown. Oxides of iron and manganese also contribute to the coloring in some parts of the thermal areas. The delicate blue color of many pools, however, results from the reflection of light off the pool walls and back through the deep clear water (fig. 43). Other pools are yellow because they contain sulfur, or are green from the combined influence of yellow sulfur and “blue” water.

ALGAL-COLORED TERRACES lining the west bank of the Firehole River at Midway Geyser Basin. Algae are microscopic plants that grow profusely on rocks covered by hot water at temperatures up to about 170°F. (Fig. 49)

Hot springs occur where the rising hot waters of a thermal system issue from the ground-level openings of the feeder conduits (fig. 45). By far the greatest numbers discharge water and steam in a relatively steady noneruptive manner, although they vary considerably in individual behavior. Depending upon pressure, water temperature, rate of upflow, heat supply, and arrangement and size of underground passages, some hot springs boil violently and emit dense clouds of vapor whereas in others the water quietly wells up with little agitation from escaping steam. In some hot springs, however, the underground channels are too narrow or the upflow of very hot water and steam is too great to permit a steady discharge; periodic eruptions then result. These special kinds of springs are called “geysers” (from the Icelandic wordgeysir, meaning to “gush” or “rage”). At least 200 geysers, of which about 60 play to a height of 10 feet or more, occur in Yellowstone National Park; this is more than in any other region of the world.

How does a geyser work? We cannot, of course, observe the inner plumbing of a geyser, except for that part which is seen by looking into its uppermost “well.” Deeper levels directly below the “well” can be probed by scientific instruments to some extent, and research drilling in some parts of the geyser basins also provides much useful information. The available information suggests that the plumbing system of a geyser (1) lies close to the ground surface, generally no deeper than a few hundred feet; (2) consists of a tube, commonly nearly vertical, that connects to chambers, side channels, or layers of porous rock, where substantial amounts of water can be stored; and (3) connects downward through the central tube and side channels to narrow conduits that rise from the deepwater source of the main thermal system.

Considering a geyser system as described above and applying what is known about the behavior of water and steam, we can understand what causes a natural thermal eruption.Figure 50shows diagrammatically the succession of events believed to occur during the typical eruptive cycle of a geyser such as Old Faithful.

A GEYSER IN ACTION. Photographs of successive stages in the eruption of Old Faithful illustrate what probably happens during a natural geyser eruption. The underground plumbing is diagrammatic and does not reflect any specific knowledge of Old Faithful’s system. Direction of flow of water is shown by arrows. (Based on information supplied by D. E. White, L. J. P. Muffler, R. O. Fournier, and A. H. Truesdell.) (Fig. 50)Stage 1 (Recovery or recharge stage). After an eruption, the partly emptied geyser tubes and chambers fill again with water. Hot water enters through a feeder conduit from below, and cooler water percolates in from side channels nearer the surface. Steam bubbles (with some other gases such as carbon dioxide and hydrogen sulfide) start to form in upflowing currents, as a decrease in pressure causes a corresponding decrease in boiling temperature. At first the bubbles condense in the cooler, near-surface water that is not yet at boiling temperature, but eventually all water is heated enough that the bubbles will no longer condense or “dissolve.”

Stage 1 (Recovery or recharge stage). After an eruption, the partly emptied geyser tubes and chambers fill again with water. Hot water enters through a feeder conduit from below, and cooler water percolates in from side channels nearer the surface. Steam bubbles (with some other gases such as carbon dioxide and hydrogen sulfide) start to form in upflowing currents, as a decrease in pressure causes a corresponding decrease in boiling temperature. At first the bubbles condense in the cooler, near-surface water that is not yet at boiling temperature, but eventually all water is heated enough that the bubbles will no longer condense or “dissolve.”

Stage 2 (Preliminary eruption stage). As the rising gas bubbles grow in size and number, they tend to clog certain parts of the geyser tube, perhaps at some narrow or constricted point such as at A. When this happens, the expanding steam abruptly forces its way upward through the system and causes some of the water to discharge from the surface vent in preliminary spurts. The deeper part of the system, however, is not yet quite hot enough for “triggering.”

Stage 3 (Full eruption stage). Finally, a preliminary spurt “unloads” enough water (with resulting reduction in pressure) to start a chain reaction deeper in the system. Larger amounts of water in the side chambers and pore spaces begin to flash into steam, and the geyser rapidly surges into full eruption.

Stage 4 (Steam stage). When most of the extra energy is spent, and the geyser tubes and chambers are nearly empty, the eruption ceases. Some water remains in local pockets and pore spaces, continuing to make steam for a short while. Thereafter the system begins to fill again, and the eruptive cycle starts anew.

No two geysers have the same size, shape, and arrangement of tubes and chambers. Also, some geysers, such as Great Fountain, have large surface pools not present in cone-type geysers such as Old Faithful. Hence, each geyser behaves differently from all others in frequency of eruption, length of individual eruptions, and amount of water discharged. Geysers may also vary in their own behavior as their plumbing features change through the years. The great amount of energy that builds up in some of them from time to time creates enough explosive force to shatter parts of the plumbing system, thereby causing a change in their eruptive behavior. In fact, some geyser eruptions have been so violent that large chunks of rock have been exploded out of the ground and scattered around the surrounding area (fig. 51). With time, the precipitation of minerals may partly seal a tube or chamber, gradually altering the eruptive mechanism.

Despite all the variable factors involved in geyser eruptions, and all the changes that can take place from time to time to alter the pattern of those eruptions, several of the Yellowstone geysers function regularly, day after day, week after week, and year after year. Within this group of regulars is the most famous feature of all—Old Faithful—which has not missed an eruption in all the many decades that it has been under close observation (fig. 52). We can only conclude that nature has provided this incredible geyser with a stable plumbing system that is just right to trigger delightfully graceful eruptions at close-enough time intervals to suit the convenience of all Park visitors.

Mudpots are among the most fascinating and interesting of the Yellowstone thermal features. They are also a type of hot spring, but one for which water is in short supply. Whatever water is available becomes thoroughly mixed with clay and other fine undissolved mineral matter. The mud is generally gray, black, white, or cream colored, but some is tinted pale pink and red by iron compounds (fig. 43); hence, the picturesque term “paint pots” is commonly used.

Mudpots form in places where the upflowing thermal fluids have chemically decomposed the surface rocks to formclay. Such small amounts of water are involved, however, that the surface discharge is not great enough to flush the clay out of the spring. Caldrons of mud of all consistencies result, from the very thin soupy material in many mudpots to the almost hard-baked material in the less active features. Some mudpots expel pellets of very thick viscous mud which build up circular cones or mounds; this type is commonly called a “mud volcano” (fig. 53).

SEISMIC GEYSER, showing rock rubble blown out during an explosive thermal eruption. Note the trees that have been killed by the heat and eruptive activity. According to George D. Marler of the National Park Service, this geyser developed from cracks caused by the Hebgen Lake Earthquake of August 17, 1959. (Fig. 51)

OLD FAITHFUL IN FULL ERUPTION. The interval between eruptions averages about 65 minutes, but it varies from 33 to 96 minutes. The time lapse between eruptions can be predicted rather closely, mainly on the basis of the length of time involved in the previous eruption. If an eruption lasted 4 minutes, for example, this means that a certain amount of water emptied from the geyser’s chambers and that a certain length of time will be necessary to recharge the system for the next eruption. But if the previous eruption lasted only 3 minutes, less time will be needed for recharge, and the next eruption will occur sooner. (The above discussion is based primarily on many years of observation and study of Old Faithful by George D. Marler and other observers of the National Park Service; photograph courtesy of Sgt. James E. Jensen, U.S. Air Force.) (Fig. 52)

Mudpot activity differs from season to season throughout the year because of the varying amounts of rain and snow that fall upon the surface to further moisten the mud. Accordingly, mudpots are commonly drier in late summer and early fall than they are from winter through early summer.

Fumaroles (from the Latin wordfumus, meaning “smoke”) are those features that discharge only steam and other gases such as carbon dioxide and hydrogen sulfide; hence, they are commonly called “steam vents.” Usually these features are perched on a hillside or other high ground above the level of the flowing springs. In many fumaroles, however, water can be heard boiling violently at some lower, unseen level.

A few features present in the Yellowstone thermal areas display evidence that extremely violent thermal explosions occurred in the past, particularly during Pinedale Glaciation, about 15,000 years ago. Such explosion features, of which Pocket Basin in Lower Geyser Basin is a good example, appear as craterlike depressions a few tens of feet to as much as 5,000 feet across surrounded by rims of rock fragments that were blown out of the craters. The underground mechanism causing the explosions was similar to that of geysers, but in these special cases the energy remained bottled-up until a very critical explosive stage was reached.

The best explanation for Pocket Basin and related features is that the ground above the sites of the explosions was weighted down by the water of small lakes which had formed in melted-out pockets of glacial ice. Such localized melting of the glaciers would occur where the ice was in direct contactwith underlying thermal features. A rapid draining of the lake waters would then produce a sudden release of pressure over the hot area, resulting in an unusually violent thermal eruption.

MUD VOLCANO near Pocket Basin in the Lower Geyser Basin. The mud is formed by chemical decomposition of the rocks chiefly by the action of carbon dioxide and sulfuric acid. The splatter, 5-6 feet high, is caused by the escaping gases. (Fig. 53)

Most of the major thermal areas of Yellowstone are related to the ring fracture zones of the Yellowstone caldera (fig. 22). Many deep-seated faults and fractures in these zones are presumably situated above the main source of heat of the thermal system. Thus, they provide convenient avenues of travel for underground waters to circulate to great depths, there to become heated and then rise to the earth’s surface (fig. 45). A few areas like Mammoth Hot Springs and Norris Geyser Basin, on the other hand, are not within the ring fracture zones of the caldera. In these areas, the thermal activity is commonly related to other prominent zones of faulting which also afford readymade channelways for the circulation of hot water and steam.

Earthquakes occur frequently in areas of active faulting and volcanism; they are caused by sudden movements between adjacent blocks of the earth’s crust as the crust adjusts to new conditions and pressures. Because of its volcanic history and the fact that very recent fault movements have occurred there, it is not surprising that Yellowstone is an especially active earthquake area. Sensitive instruments (seismographs) record an average of about five earth tremors daily in and around the Park, and on rare occasions they may record 100 tremors or more in a single day. Nearly all these tremors are so slight that they cannot be felt by man, but at times, perhaps only once in a human lifetime, one is triggered with high enough intensity to sharply draw our attention to the very real earthquake potential that exists constantly in this geologically active area. Such a high-intensity quake occurred in the Yellowstone region near midnight on August 17, 1959.

The Hebgen Lake Earthquake, as it is known, was centered in the Madison Valley along the west boundary of Yellowstone National Park about 12 miles north of the town of West Yellowstone, Montana (fig. 1). As a result of the quake, a 200-square-mile area, occupied in part by the Hebgen Lake reservoir, subsided a foot or more; maximum subsidence was 20 feet. Movements of several feet along old faults in the highlands along the north side of the valley produced fresh scars several miles long (fig. 54). Moreover, the severe vibrations that rocked the surrounding countryside caused the loose silt, sand, and gravel of the valley floor to slip and become “faulted” in many places. By far the most drastic result was the shaking loose of a huge landslide in the vicinityof the Rock Creek campground about 25 miles downstream on the Madison River from the west boundary of the Park.

EARTHQUAKE DAMAGE. Severe damage caused by reactivation of a fault during the Hebgen Lake Earthquake of August 17, 1959. The building is on the Blarneystone Ranch, about 10 miles north of West Yellowstone, Montana, and 1½ miles west of the west boundary of Yellowstone National Park. (Fig. 54)

Within Yellowstone National Park, the quake caused only slight damage to buildings at Old Faithful, Mammoth Hot Springs, and a few other places. Small landslides also occurred in various places, for example, at Tuff Cliff near Madison Junction. The earthquake affected many thermal features, particularly those in the main geyser basins near the west side of the Park. In several places the intensity of the thermal activity increased markedly, in fewer places the activity decreased. Some geysers, long dormant, erupted immediately after the earthquake; others erupted with muchgreater force and frequency than usual; still others became dormant and have remained so. A general, widespread effect was a noticeable increase in the muddiness of many pools and springs, as if the quake had produced a giant surge of water coursing through the underground channels of the geyser basins. Of immediate concern to everyone, of course, was the earthquake’s effect on Old Faithful. Fortunately, the only measurable effect was a slight lengthening of time between eruptions. After several months the time interval stabilized at about 65 minutes.

Detailed scientific studies bearing on the Hebgen Lake Earthquake in the weeks that followed showed that it was felt over more than 600,000 square miles of the Western United States, and that it was the strongest shock ever recorded in this part of the Rocky Mountains. In 12 years’ time many traces of the quake have disappeared, but its frightful aspects will not soon be forgotten. It serves as a vivid reminder, once again, of the great restlessness that through the ages has been, and continues to be, the very special trademark of the Yellowstone country.

A hundred years ago another powerful force entered the Yellowstone scene. Man, arriving in ever increasing numbers, came armed with the power to choose between preserving or destroying the wonders that nature has taken more than 2.5 billion years to create. Sensing this grave responsibility, he took the necessary steps to insure that these irreplaceable natural features would be preserved and protected. Today, Yellowstone National Park indeed exists “for the benefit and enjoyment of the people,” a fitting and lasting symbol of a great national heritage that now includes more than 275 places of natural and historical interest. On the eve of the 100th anniversary of our first National Park, we are again reminded of that continuing responsibility we all share in preserving these unique places for the benefit and enjoyment of all future generations of visitors.

The subject matter of this bulletin is based chiefly on the results of a systematic program of geological investigations in Yellowstone National Park, conducted by the U.S. Geological Survey during the years 1965-71. The program, ably organized and directed by A. B. Campbell, required the special skills and knowledge of many individuals to make a comprehensive study of all the varied and complex features of the Park area. Without their invaluable cooperation, assistance, and interest, this endeavor to summarize the geologic story of Yellowstone would not have been possible. I therefore express my sincere thanks to the colleagues listed below, all of whom furnished unpublished information bearing on different aspects of that story: R. L. Christiansen and H. R. Blank, Jr. (Quaternary volcanism); H. W. Smedes and H. J. Prostka (Absaroka volcanism); D. E. White, L. J. P. Muffler, R. O. Fournier, and A. H. Truesdell (thermal activity); G. M. Richmond, K. L. Pierce, and H. A. Waldrop (glaciation); E. T. Ruppel and J. D. Love (sedimentary rocks and geologic structure); J. D. Obradovich and Meyer Rubin (radiometric dating). W. L. Newman provided many helpful suggestions regarding the preparation of the manuscript.

The geological studies in Yellowstone received the full support and cooperation of former Park Superintendent J. S. McLaughlin, Superintendent J. K. Anderson, and other personnel of the U.S. National Park Service. In particular, the helpful advice, interest, and enthusiasm of the entire Park Naturalist staff, especially J. M. Good and W. W. Dunmire, former and present Chief Park Naturalists, respectively, greatly facilitated the work in all phases of the program.

[1]The specific area about which the early-day Indians first used the term that is now translated as “Yellowstone” is unknown. The name may have referred to the yellowish rocks that line the banks of the Yellowstone River near its confluence with the Missouri River in eastern Montana and western North Dakota. However, in the opinion of H. M. Chittenden, who studied the question in considerable detail, there is little doubt that the name was taken from the striking yellow-hued walls of the gorge now known as the Grand Canyon of the Yellowstone.

Allen, E. T., and Day, A. L., 1935, Hot springs of the Yellowstone National Park: Carnegie Institution of Washington Publication 466, 525 pages.

Boyd, F. R., 1961, Welded tuffs and flows in the rhyolite plateau of Yellowstone Park, Wyoming: Geological Society of America Bulletin, volume 72, number 3, pages 387-426.

Christiansen, R. L., and Blank, H. R., Jr., 1972, Volcanic stratigraphy of the Quaternary rhyolite plateau in Yellowstone National Park: U.S. Geological Survey, Professional Paper 729-B (in press).

Dorf, Erling, 1960, Tertiary fossil forests of Yellowstone National Park, Wyoming,inWest Yellowstone—Earthquake area, Billings Geological Society Guidebook 11th Annual Field Conference, 1960: pages 253-260.

Hague, Arnold, Iddings, J. P., Weed, W. H., Walcott, C. D., Girty, G. H., Stanton, T. W., and Knowlton, F. H., 1899, Geology of the Yellowstone National Park: U.S. Geological Survey Monograph 32, part 2, 893 pages and atlas of 27 sheets folio.

Hague, Arnold, Weed, W. H., and Iddings, J. P., 1896, Description of the Yellowstone National Park quadrangle [Wyoming]: U.S. Geological Survey Geologic Atlas, Folio 30.

Hayden, F. V., 1872, Preliminary report of the United States Geological Survey of Montana and portions of adjacent Territories, being a fifth annual report of progress—Part I: Washington, U.S. Government Printing Office, pages 13-204.

Howard, A. D., 1937, History of the Grand Canyon of the Yellowstone: Geological Society of America Special Paper 6, 159 pages.

Marler, G. D., 1969, The story of Old Faithful: Yellowstone Library and Museum Association, 49 pages.

Richmond, G. M., Pierce, K. L., and Waldrop, H. A., 1972, Surficial geologic map of Yellowstone National Park: U.S. Geological Survey Miscellaneous Geologic Investigations Map I-710 (in press).

Ruppel, E. T., 1972, Geology of pre-Tertiary rocks in the northern part of Yellowstone National Park, Wyoming: U.S. Geological Survey Professional Paper 729-A (in press).

Smedes, H. W., and Prostka, H. J., 1972, Absaroka Volcanic Supergroup in the Yellowstone National Park region: U.S. Geological Survey Professional Paper 729-C (in press).

Smith, R. L., 1960, Ash flows: Geological Society of America Bulletin, volume 71, number 6, pages 795-841.

U.S. Geological Survey, 1964, The Hebgen Lake, Montana, earthquake of August 17, 1959: US Geological Survey Professional Paper 435, 242 pages.

U.S. Geological Survey, 1972, Geologic map of Yellowstone National Park: U.S. Geological Survey Miscellaneous Geologic Investigations Map I-711 (in press).

White, D. E., 1967, Some principles of geyser activity, mainly from Steamboat Springs, Nevada: American Journal of Science, volume 265, number 8, pages 641-684.

★ U. S. GOVERNMENT PRINTING OFFICE: 1972 O-467-725

U. S. DEPARTMENT OF THE INTERIOR · March 3, 1849

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