Geological Survey Bulletin 1347 The Geologic Story of Yellowstone National Park

Although Yellowstone is geologically outstanding in many ways, the great abundance, diversity, and spectacular nature of its thermal (hot-water and steam) features were undoubtedly the primary reasons for its being set aside as our first National Park (fig. 43). The unusual concentration of geysers, hot springs, mudpots, and fumaroles provides that special drawing card which has, for the past century, made the Park one of the world's foremost natural attractions.

COMMON KINDS OF THERMAL FEATURES in Yellowstone National Park. A, Hot springs and terraces colored by algae at Mammoth Hot Springs; B, Castle Geyser erupting in Upper Geyser Basin; C, Fountain Paint Pots in Lower Geyser Basin; D, pool in Lower Geyser Basin. (Fig. 43)

To count all the individual thermal features in Yellowstone would be virtually impossible. Various estimates range from 2,500 to 10,000, depending on how many of the smaller features are included. They are scattered through many regions of the Park, but most are clustered in a few areas called geyser basins, where there are continuous displays of intense thermal activity. (See frontispiece.) The "steam" that can be seen in thermal areas is actually fog or water droplets condensed from steam; so the appearance of individual geyser basins depends largely on air temperature and humidity. On a warm, dry summer day, for example, the activity may seem very weak (fig. 44), except where individual geysers are erupting. On cold or very humid days, however, "steam" plumes are seen rising from every quarter.

NORRIS GEYSER BASIN, as viewed northward from the Norris Museum. This is one of the most active thermal areas in Yellowstone, but the photograph was taken on a warm dry summer day when little hot-water and steam activity was visible from a distance. Clouds of water droplets (the visible "steam" in thermal areas) normally form only when the air is cool and (or) moist. The floor of the basin is covered by a nearly solid layer of hot-spring deposits. (Fig. 44)

How a thermal system operates

An essential ingredient for thermal activity is heat. A body of buried molten rock, such as the one that produced volcanic eruptions in Yellowstone as late as 60,000 to 75,000 years ago, takes a long time to cool. During cooling, tremendous quantities of heat are transmitted by conduction into the solid rocks surrounding the magma chamber (fig. 45). Eventually the whole region becomes much hotter than non-volcanic areas (fig. 46). Normally, rock temperatures increase about 1°F per 100 feet of depth in the earth's crust, but in the thermally active areas of Yellowstone the rate of temperature increase is much greater. The amount of heat given off by the Upper Geyser Basin, for example, is 800 times the amount given off by normal (nonthermal) areas of the same size. This excess heat is enough to melt 1-1/2 tons of ice per second! And, contrary to popular opinion, the underground temperatures have not cooled measurably in the 100 years that records have been kept on the thermal activity in the Park. In fact, geologic studies indicate that very high heat flows have continued for at least the past 40,000 years.

HEAT FLOW AND SURFACE WATER. Diagram showing a thermal system, according to the explanation that water of surface origin circulates and is heated at great depths. (Based on information supplied by D. E. White, L. J. P. Muffler, R. O. Fournier, and A. H. Truesdell.) (Fig. 45)

INFRARED IMAGE of a part of Upper Geyser Basin. Infrared instruments, sensitive to heat, are able to detect "hot" spots in the landscape. Note especially the sharp "image" of Old Faithful. (Image courtesy of National Aeronautics and Space Administration.) (Fig 46)

A second, equally essential ingredient for thermal activity is water. Many thousands of gallons are discharged by the hot springs and geysers in Yellowstone every minute—where does all this water come from? Studies show that nearly all the water originates above ground as rain or snow (meteoric water; fig. 45), and that very little comes from the underlying magma (magmatic water).

The mechanism for heating the water, on the other hand, is a matter of some uncertainty. Until a few years ago the heating was assumed to occur near the ground surface and to be caused by hot magmatic gases (mostly steam) rising from the underlying magma chamber. Deep wells drilled recently in many thermal areas throughout the world (including research drill holes in Yellowstone), however, suggest a better explanation. According to this explanation, the surface water enters underground passages (fractures and faults) and circulates to great depths—as much as 5,000-10,000 feet in some areas (fig. 45)—there to become heated far above its surface boiling point. Research drill holes in Yellowstone, for example, have demonstrated that water of surface origin exists at all depths at least to the maximum drilled (1,088 feet), and that the water reaches temperatures up to at least 465°F. The increase in temperature with depth causes a corresponding decrease in the weight (density) of the water. Because of this, the hot, "lighter," water begins to rise again toward the ground surface, pushed upward by the colder, "heavier," near-surface water which sinks to keep the water channels filled. Thus is set into motion a giant convection current which operates continuously to supply very hot water to the thermal areas (fig. 45). Just how deep the waters circulate in Yellowstone no one really knows; as a guess, the depth probably is at least 1 or 2 miles.

The effect of pressure on the boiling temperature of water also plays a vital role in thermal activity. In a body of water, the pressure at the surface is that exerted by the weight of air above it (atmospheric pressure). Water under these conditions boils at 212°F at sea level and at about 199°F at the elevation of most of the geyser basins in Yellowstone. However, water at depth not only is subjected to atmospheric pressure but also bears the added weight of the overlying water. Under such additional pressures, water boils only when the temperature is raised above its surface boiling point. In a well 100 feet deep at sea level, for example, the water at the bottom would have to be heated to 288°F before it will boil. Thus it follows that in the underground "workings" of hot springs or geysers, (1) The deepest water is subjected to the greatest pressures, and (2) these deeper waters (in Yellowstone) must be heated well above 199°F before they can actually begin to boil. By this same reasoning but in reverse, if the pressure is released, which happens as the water rises toward the ground surface, the "hotter-than-boiling" water will then begin to boil. The boiling will be rather quiet if the pressure is released gradually, as in most hot springs. But if the pressure is released suddenly, boiling may become so violent that much of the water flashes explosively into steam, expanding to several hundred times its normal volume. This expansion provides the necessary energy for geyser eruptions.

Hot-spring deposits and algae

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.

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)

In all major thermal areas of the Park, with the exception of Mammoth Hot Springs, most of the material being deposited is sinter (the kind found around geysers is popularly called geyserite). Its chief constituent is silica (the same as in quartz and in ordinary window glass). At Mammoth, the deposit is travertine (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.

TERRACES OF TRAVERTINE at Opal Springs, Mammoth Hot Springs area. Closeup view shows layered and porous nature of the travertine. (Fig. 48)

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. 43 and 49) because they are coated by microscopic plants called algae. 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 and geysers

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 word geysir, 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 50 shows 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.) 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. (click on image for an enlargement in a new window) (Fig. 50)

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.

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)

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.

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)

Mudpots

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 form clay. 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).

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)

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

Fumaroles (from the Latin word fumus, 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.

Thermal explosions

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 contact with 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.

Faulting and its control of thermal activity

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.