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.
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.
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 1½ 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.
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)
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)
Water enters at ground surface and sinks in conduit formed by fault or fracture Surface (meteoric) water sinks to levels perhaps as much as 10,000 feet below ground. Heated far above its normal boiling point, it begins to rise toward the surface Descending cool surface water Permeable zone allows water to flow through it Cooling magma chamber Water begins to boil near ground surface because of greatly reduced pressures Rising hot water Hot spring or geyser
