William R. Keefer

“ Beyond the first stirring impressions derived from the grandeur of the vast Yellowstone wilderness and its myriad wildlife, assuredly shared by people of all ages and from all walks of life, the various aspects of the Park take on a very different meaning for different individuals. ”

“ 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. ”

“ Other similar anticlines, synclines, and reverse faults no doubt extend far into the interior of Yellowstone National Park, and perhaps entirely across it in places, but they lie buried beneath a thick capping of volcanic rocks. Nevertheless, it seems safe to conclude that none of the Park area escaped the effects of the great forces of the Laramide orogeny. These forces, regardless of how they originated deep within the earth, seem to have been compressional (fig. 13) , pushing the upper layers of the earth’s crust from the east and northeast toward the west and southwest. ”

“ The pronounced rise in elevation of the general ground surface and the chopping of the region into many mountainous fault blocks caused a profound increase in the rate of erosion. Once-sluggish streams turned into vigorous, fast-moving rivers that began to cut deeply into the Absaroka volcanic plateau. Huge quantities of rock debris were stripped off and carried out of the area, and at the end of the Pliocene, the Yellowstone region must have been very highly dissected mountains and table- and canyon-lands. ”

“ The principal requirement for the formation of glaciers is simple: more snow has to accumulate during the winter than is melted during the summer. If this condition continues for a long enough period of time (measured in centuries) , the snow compacts to ice, and extensive icefields grow until they finally begin to move under their own weight, thereby becoming glaciers. Records show that the average year-round temperature is 32°-33°F along Yellowstone Lake, 35°F at Old Faithful, and 39°F at Mammoth. Each winter, snow accumulates to depths of 5-10 feet throughout much of the Park. ”

“ For the Park is foremost a geological Park, created by an extraordinary sequence of natural processes and events that have combined to produce an immense outdoor laboratory for studies that have contributed to a fuller knowledge and a better understanding of the earth itself. ”

“ 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. ”

“ The gently rolling plateau surface of central Yellowstone, broken here and there by clusters of low-lying hills and ridges, is essentially the landscape that characterized the upper surfaces of the lava flows soon after they cooled and solidified. Natural valleys formed between some of the adjacent flows, and in places streams still follow these readymade channels. Rhyolite, in both lava flows and ash-flow tuffs, is by far the predominant rock type seen along the Park roads. ”

“ The boulder rests on rhyolite lava flows of Quaternary age, at least 15 miles from the nearest outcrops of the ancient gneiss to the north and northeast. Obviously, this seemingly immovable chunk of 54 rock was pushed or carried a long way by some very powerful transporting agent before it was finally dropped. A natural force of such magnitude could only have been exerted by moving ice; in fact, no further proof than this one boulder is needed for us to conclude beyond question that glaciers once existed in Yellowstone. ”

“ 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. ”

“ 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. ”

“ A significant effect of the Laramide orogeny was the uplift and contortion of many of the mountain ranges within what we today call the Rocky Mountains.
At the onset of the crustal disturbance, the gently rolling landscape of the Yellowstone region began to warp and flex into large upfolds (anticlines) and downfolds (synclines) (fig. 13) . Gradually the mountain-building pressures increased, finally reaching such magnitude that the limbs of the folds could bend and stretch no further; thereupon, the rock layers broke and were shoved over one another along extensive reverse faults. ”

“ THE ROCKS of Yellowstone National Park, separated into individual units or formations and arranged according to their geologic ages (see fig. 6) . A formation is a body of rock that contains certain identifying features (such as composition, color, and fossils) which set it apart from all other rock units. The identifying features of each formation provide valuable clues bearing on its origin. ”

“ MOUNT EVERTS, as viewed toward the northeast from the road south of Mammoth Hot Springs. The mountain, about 1,500 feet high above the plain, is formed by gently tilted sedimentary rocks of Cretaceous age, chiefly sandstone and shale of the Frontier, Cody, and Everts Formations (fig. 5) . The conspicuous rimrock at the top of the mountain to the right is composed of the Yellowstone Tuff. When the tuff was deposited (by explosive eruptions from the south) , there was no valley along the edge of the mountain. ”

“ An original horizontal rock layer may be upfolded into anticlines, downfolded into synclines, and broken by either reverse or normal faults. A fault is a fracture or a zone of fractures within the earth’s crust along which movement has taken place. ”

“ The existence of many waterfalls in Yellowstone today is due in large part to the fact that, because of recent volcanism and glaciation, much of the region’s topography is very young in terms of geologic time. Streams, even some of the largest ones, have not had enough time to wear away all the features that may produce waterfalls, cascades, or rapids along their channels. This is particularly true along the margins of lava flows, where there are sharp dropoffs between the tops of the flows and the lower ground beyond. ”

“ 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. ”

“ Measurements taken in deep mines and oil wells show that the normal increase in the earth’s temperature with depth is about 1°F per 100 feet. This heat is generated by the decay of radioactive elements—chiefly uranium, thorium, and potassium—which are present in at least small amounts in virtually all rocks of the earth’s crust. Ordinarily, enough heat is conducted to the earth’s surface so that the deeply buried rocks do not become hot enough to melt. ”

“ The principal events in the geologic history of Yellowstone National Park are listed to the right of column C, opposite the time intervals in which they occurred. The ages, in years, are based on radiometric dating. Many rocks contain radioactive elements which begin to decay at a very slow but measurable rate as soon as the parent rock is formed. ”

“ 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) . ”

“ Waterfalls, features for which Yellowstone is also justly famous (fig. 40) , generally result from abrupt differences in rock hardness. If a stream flows over rocks that are more resistant to erosion than the rocks immediately downstream, a ledge or bench will form across the streambed at that place 64 because the less resistant rocks are worn away faster. And, as the ledge becomes higher, the softer downstream rocks will erode even faster. A true waterfall is one in which there is a free, vertical fall of water. ”

“ During the latter part of the Tertiary Period, erosion, too, had begun to make its own deep marks. But only in the last 100,000 years or so have the powerful exterior forces of the earth—chiefly running water and moving ice—had a virtually free hand in shaping the Park’s landscape. Nevertheless, in this short period of time they have wrought profound changes.
Glaciation
A giant boulder of Precambrian gneiss lies among the trees beside the road leading to Inspiration Point on the north rim of the Grand Canyon of the Yellowstone (fig. 34) . ”

“ 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. ”

“ The effects of the Pinedale glaciers, on the other hand, are obvious in many parts of the Park, and the history of this youngest glacial cycle (described below) is known in much greater detail than that of the two older ones.
In the early stages of Pinedale Glaciation, an enormous icefield built up in the high Absaroka Range southeast of the Park area. A glacier, fed by this icefield, flowed northward 56 down the upper Yellowstone valley and into the basin now occupied by Yellowstone Lake. ”

“ Mount Washburn is the north half of one of the ancient Absaroka volcanoes (fig. 26) , and many of the rocks and other features related to this volcano, which characterized this great period of volcanism, can be seen along the road between Canyon Village and Tower. In roadcuts just south of Dunraven Pass several thin igneous dikes cut through volcanic breccias. These dikes radiate outward from the nearby central core of the volcano, which lies east of the highway in the vicinity of Washburn Hot Springs. ”

“ Whatever the cause, the eventual result is the accumulation of a huge body of molten rock, called magma, enclosed in a deep underground chamber.
Magma, being a mixture of hot liquids and gases that is lighter in weight than the solid rocks surrounding it, tends to rise toward the earth’s surface. ”

“ The rock is composed chiefly of light-colored quartz and feldspar and dark-colored iron and magnesium silicate minerals. (Fig. 20)
At the end of Absaroka volcanism, approximately 40 million years ago (fig. 6) , all of Yellowstone lay buried beneath several thousand feet of lavas, breccias, and ash (fig. 18) . The landscape must have appeared as a gently rolling 31 plateau, drained by sluggish, meandering streams and dotted here and there by volcanoes still rising above the general level of the ground. ”

“ 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 vicinity 88 of the Rock Creek campground about 25 miles downstream on the Madison River from the west boundary of the Park. ”

“ GIANT PETRIFIED TREE TRUNKS in Yellowstone’s fossil forest. The enclosing rocks, part of the Absaroka volcanic sequence that forms Specimen Ridge, are approximately 50 million years old. Many of the tree trunks are still upright, having been smothered and buried in their original positions by breccia, ash, and dust from nearby volcanoes. It is evident that more than one “forest” is represented in this view. Prof. Erling Dorf, of Princeton University, counted a total of 27 different forest layers in the rocks now exposed at Specimen Ridge. ”

“ TWO LEDGES OF BASALT spectacularly exposed in the east wall of the Grand Canyon of the Yellowstone at The Narrows near Tower Falls. The light-colored rocks between the basalt flows are ancient stream gravels deposited about 11⁄2 million years ago, when the channel of the Yellowstone River was farther east and not as deep as it is today. The hill is capped by lake sediments, sand, and gravel deposited when the Yellowstone River was blocked by a glacial dam farther downstream (to the left) . ”

“ The artist sees grand vistas to be painted, the naturalist delights in the flower-laden meadows and the native habitats of many kinds of birds and animals, the engineer visualizes the amount of energy stored in the waterfalls and steaming geysers, and so 4 on. To the geologist, in particular, who studies rocks and fossils and all of the natural processes involved in shaping the surface of the land, and to all those who would share such interests, Yellowstone takes on a very special meaning. ”