The Central Plateau covers an area approximately 50 by 40 miles, with a mean altitude of 8,000 feet. It is accidented by undulating basins of varied outline and scored by deep canyons and gorges. Strictly speaking, it is not a plateau; at least it is by no means a level area, but a rugged country, characterized by bold escarpments and abrupt edges of mesa-like ridges. But few large vents or centers of volcanic activity for the rhyolite have been recognized, the two principal sources being the volcano to which reference has already been made and Mount Sheridan in the southern end of the park. Mount Sheridan is the most commanding peak on the plateau, with an elevation 10,385 feet above sea level and 2,600 feet above Heart Lake. From the summit of the peak on a clear day one may overlook the entire plateau country and the mountains which shut it in, while almost at the base of the peak lie the magnificent lakes which add so much to the quiet beauty of the region, in contrast to the rugged scenery of the mountains. From no point is the magnitude and grandeur of the volcanic region so impressive. The lava flowsbounded on the east by the Absarokasextend westward not only across the park, but across the Madison Plateau, and out on to the great plains of Snake River, stretching far westward almost without a break in the continuity of eruptive flows. Over the central portion of the park, where the rhyolites are thickest, erosion has failed to penetrate to the underlying rock. Even such deep gorges as the Yellowstone, Gibbon, and Madison Canyons have nowhere worn through these rhyolite flows. In the Grand Canyon of the Yellowstone the andesitic breccias are found beneath the rhyolites, but the deepest cuts fail to reveal the underlying sedimentary beds. Although the rocks of the plateau for the most part belong to one group of acidic lavas, they by no means present the great uniformity and monotony in field appearance that might be expected. These 2,000 square miles offer as grand a field for the study of structural forms, development of crystallization, and mode of occurrence of acidic lavas as can be found anywhere in the world. They vary from a nearly holocrystalline rock to one of pure volcanic glass. Obsidian, pumice, pitchstone, ash, breccia, and an endless development of transition forms alternate with the more compact lithoidal lavas which make up the great mass of the rhyolite, and which in colors, texture, and structural developments present an equally varied aspect. In mineral composition these rocks are simple enough. The essential minerals are orthoclase and quartz, with more or less plagioclase. Sanidine is the prevailing feldspar, although in many cases plagioclase forms occur nearly as abundantly as orthoclase. Chemical analyses, whether we consider the rocks from the crater of Mount Sheridan, the summit of the plateau, or the volcanic glass of the world-renowned Obsidian Cliff, present comparatively slight differences in ultimate composition.
I have dwelt somewhat in detail upon the nature of these rocks for two reasons: First, because of the difficulty met with by the scientific traveler in recognizing the uniformity and simplicity of chemical composition of the rhyolite magma over the entire plateau, owing to its great diversity in superficial habit; second, on account of their geological importance in connection with the unrivaled display of the geysers and hot springs. That the energy of the steam and thermal waters dates well back into the period of volcanic action, there is in my opinion very little reason to doubt. As the energy of this underground heat is to-day one of the most impressive features of the country, I will defer commenting upon the geysers and hot springs until speaking of the present condition of the park.
Although the rhyolite eruptions were probably of long duration and died out slowly, there is, I think, evidence to show that they occupied a clearly and sharply defined period between the andesites and late basalt eruptions. Since the outpouring of this enormous mass of rhyolite and building up of the plateau, the region has undergone faulting and displacement; immense blocks of lava have been lifted bodily, and the surface features of the country have been modified. Following the rhyolite came the period of late basalt eruptions, which, in comparison with the andesite and rhyolite eras, was, so far as the park was concerned, insignificant, both as regards the area covered by the basalt and its influence in modifying the physical aspect of the region. The basalt occurs as thin sheets overlying the rhyolite and in some instances as dikes cutting the more acidic rocks. It has broken out near the edge of the rhyolite body and occurs most frequently along the Yellowstone Valley, along the western foothills of the Gallatin Range and Madison Plateau, and again south of the Falls River Basin.
After the greater part of the basalt had been poured out came the glacial ice, which widened and deepened the preexisting drainage channels, cut profound gorges through the rhyolite lavas and modeled the two volcanoes into their present form. Over the greater part of the Cordillera of the central and northern Rocky Mountains, wherever the peaks attain a sufficiently high altitude to attract the moisture-laden clouds, evidences of the former existence of local glaciers are to be found. In the Teton Range several well-defined characteristic glaciers still exist upon the abrupt slopes of Mount Hayden and Mount Moran. They are the remnants of a much larger system of glaciers. The park region presents so broad a mass of elevated country that the entire plateau was, in glacial times, covered with a heavy capping of ice. Evidences of glacial action are everywhere to be seen.
Over the Absaroka Range glaciers were forced down into the Lamar and Yellowstone Valleys, thence westward over the top of Mount Everts to the Mammoth Hot Springs Basin. On the opposite side of the park the ice from the summit of the Gallatin Range moved eastward across Swan Valley and passing over the top of Terrace Mountain joined the ice field coming from the east. The united ice sheet plowed its way northward down the valley of the Gardiner to the Lower Yellowstone, where the broad valley may be seen strewn with the material transported from both the east and west rims of the park.
Since the dying out of the rhyolite eruptions erosion has greatly modified the entire surface features of the park. Some idea of the extent of this action may be realized when it is recalled that the deep canyons of the Yellowstone, Gibbon, and Madison Riverscanyons in the strictest use of the wordhave all been carved out since that time. To-day these gorges measure several miles in length and from 1,000 to 1,500 feet in depth.
To the geologist one of the most impressive objects on the park plateau is a transported bowlder of granite which rests directly upon the rhyolite near the brink of the Grand Canyon, about 3 miles below the falls of the Yellowstone. It stands alone in the forest, a long way from the nearest glacial bowlder. Glacial detritus carrying granitic material may be traced upon both sides of the canyon wall. This massive block, although irregular in shape and somewhat pointed toward the top, measures 24 feet in length by 20 feet in breadth and stands 18 feet above the base. The nearest point from which it could have been transported is distant 30 or 40 miles. Coming upon it in the solitude of the forest with all its strange surroundings it tells a most impressive story. In no place are the evidences of frost and fire brought so forcibly together as in the Yellowstone National Park.
Since the close of the ice period no geological events of any moment have brought about any changes in the physical history of the region other than those produced by the direct action of steam and thermal waters. A few insignificant eruptions have probably occurred, but they failed to modify the broad outlines of topographical structure and present but little of general interest beyond the evidence of the continuance of volcanic action into quaternary times. Volcanic activity in the park may be considered as long since extinct. At all events indications of fresh lava flows within historical times are wholly wanting. This is not without interest, as evidence of underground heat may be observed everywhere throughout the park in the waters of the geysers and hot springs. All our observations point in one direction and lead to the theory that the cause of the high temperatures of these waters must be found in the heated rocks below, and that the origin of the heat is in some way associated with the source of volcanic energy. It by no means follows that the waters themselves are derived from any deep-seated source; on the contrary, investigation tends to show that the waters brought up by the geysers and hot springs are mainly surface waters whlch have percolated downward a sufficient distance to become heated by large volumes of steam ascending through fissures and vents from much greater depths. If this theory is correct it is but fair to demand that evidence of long-continued action of hot waters and superheated steam should be apparent upon the rocks through which they passed on their way to the surface. This is precisely what one sees in innumerable places on the Central Plateau. Indeed, the decomposition of the lavas of the rhyolite plateau has proceeded, on a most gigantic scale, and could only have taken place after the lapse of an enormous period of time and the giving off of vast quantities of heat, if we are to judge at all by what we see going on around us to-day. The ascending currents of steam and hot water have been powerful geological agents, and have left an indelible impression upon the surface of the country. The most striking example of this action is found in the Grand Canyon of the Yellowstone. From the Lower Falls for 3 miles down the river abrupt walls upon both sides of the canyon, a thousand feet in depth, present a brilliancy and mingling of color beyond the power of description. From the brink of the canyon to the water's edge the walls are sheer bodies of decomposed rhyolite. Varied hues of orange, red, purple, and sulphur-yellow are irregularly blended in one confused mass. There is scarcely a piece of unaltered rock in place. Much of it is changed into kaolin; but from rhyolite, still easily recognized, occur transition products of every possible kind to good porcelain clay. This is the result of the long-continued action of steam and vapors upon the rhyolite lavas. Through this mass of decomposed rhyolite the course of ancient steam vents in their upward passage may still be traced, while at the bottom of the canyon hot springs, fumaroles, and steam vents are still more or less active, but probably with diminished power.
Still other areas are quite as convincing, if not on so grand a scale, as the Yellowstone Canyon. Josephs Coat Basin, on the east side of the canyon, and Brimstone Hills, on the east side of the Yellowstone Lake, an extensive area on the slopes of the Absaroka Range, both present evidences of the same chemical processes brought about in the same manner. It is not too strong a statement to make to say that the plateau on the east side of the Grand Canyon, from Broad Creek to Pelican Creek, is completely undermined by the action of superheated steam and alkaline waters on the rhyolite lava. Similar processes may be seen going on to-day in all the geyser basins. A long period of time must have been necessary to accomplish these changes. The study of comparatively fresh vents shows almost no change from year to year, although careful scrutiny during a period of five years detects a certain amount of disintegration, but infinitely small in comparison with the great bodies of altered rock. This is well shown in a locality like the Monarch Geyser in the Norris Geyser Basin, where the water is thrown out at regular intervals through a narrow fissure in the rock.
The Grand Canyon of the Yellowstone offers one of the most impressive examples of erosion on a grand scale within recent geological times. It is self-evident that the deep canyon must be of much later origin than the rock through which it has been worn, and it seems quite clear that the course and outlines of the canyon were in great part determined by the easily eroded decomposed material forming the canyon walls, and this in turn was brought about by the slow processes just described.
Last Updated: 02-Apr-2007