GRAND CANYON AND LOWER FALLS of the Yellowstone River, as viewed upstream (southwest) from Artists Point on the south rim. The yellow-hued rocks lining the canyon walls are soft, hydrothermally altered rhyolite lavas. The rocks at the brink of the falls consist of less altered and therefore more resistant rhyolites. The falls, 309 feet high, formed at the contact between the hard and soft rhyolite units. (Photograph courtesy of Sgt. James E. Jensen, U.S. Air Force.) (Fig. 41)
DEVELOPMENT OF GRAND CANYON. Profiles along the floor of the Grand Canyon of the Yellowstone as it appears today (C) and as it appeared at two older stages in its development (A and B). Note particularly the various kinds of rocks through which the canyon has been cut, and how rock differences have influenced the location of the two falls. Diagonal lines indicate unaltered rhyolite; large dots, rhyolite with much volcanic glass; small dots, hydrothermally altered rhyolite; and circles and dots, Absaroka volcanic rocks. (Based on information furnished by R. L. Christiansen and G. M. Richmond; vertical scale is exaggerated about 10 times.) (Fig. 42)
C: As it appears today Yellowstone Lake Hayden Valley Grand Canyon of the Yellowstone Upper Falls Lower Falls Inspiration Point North edge of Yellowstone caldera Confluence of Lamar and Yellowstone Rivers B: A stage somewhat before 300,000 years ago Profile of Yellowstone River today A: A stage somewhat before 600,000 years ago Profile of Yellowstone River today
At first glance, the canyon may appear to be a giant crack which suddenly opened up and into which the Yellowstone River then plunged headlong over high waterfalls at its southwest end. This, of course, is not the way the canyon formed. Nevertheless, it is apparent that certain unusual conditions caused the river, after winding slowly through flat-floored Hayden Valley for about 13 miles, to cut a precipitous gorge 1,000-1,500 feet deep and 20 miles long ([fig. 42]C). A full explanation must be based on all the many events surrounding the eruption of the Yellowstone Tuff, the collapse of the Yellowstone caldera, the outpouring of the Plateau Rhyolite, and the various episodes of glaciation. Geologic studies show that all these events took place while the canyon was being cut, and that each one played an important role in its development. Hot-water and steam activity likewise was a significant factor. However, despite its many complexities, the history of the Grand Canyon can be divided into a few major stages, as outlined below:
1. From more than 2,000,000 years ago to about 600,000 years ago, a shallow canyon was gradually being cut into the Absaroka volcanic sequence by the ancestral Yellowstone River as it eroded headward from a point near the present confluence of the Yellowstone and Lamar Rivers ([fig. 33]). By the time of the climactic volcanic eruption in central Yellowstone 600,000 years ago, the head of the “old” canyon probably had been eroded southward nearly to the place where the north rim of the Yellowstone caldera was to form later ([fig. 42]A). This point now lies about 5 miles below Lower Falls.
2. Ash-flow tuffs that were erupted 600,000 years ago filled the “old” canyon, and the river recarved its channel, chiefly along its previous course.
3. A large lake formed behind (south of) the north rim of the caldera, the damming resulting in part from lava flows of Plateau Rhyolite that poured out across the caldera floor in this area between 600,000 and 500,000 years ago. Eventually the lake rose and spilled northward into the head of the “old” canyon, causing additional downcutting in what is now the lower 15-mile stretch of the canyon.
4. As the lake emptied, the river began to erode upstream into the thick rhyolite lava flows toward the present site of Lower Falls; the process was very similar to that of a common stream gully eroding headward into a hillside. At a stage somewhat more than 300,000 years ago, the head of the canyon probably lay near the falls, and the river had cut a channel 400-600 feet deep along this upper 5-mile stretch ([fig. 42]B).
5. Approximately 300,000 years ago the canyon area was covered by ice during pre-Bull Lake glaciation. During and after the retreat of this ice, sediments accumulated in a lake that occupied the upper reaches of the canyon between the present site of Upper Falls and Inspiration Point. Subsequently, very little downcutting was accomplished until about 150,000-125,000 years ago, when the canyon was eroded nearly to its present depth.
6. Canyon development was further interrupted by the advance and retreat of glaciers during Bull Lake and Pinedale Glaciations. During and since the melting of the Pinedale glaciers about 12,000 years ago, the canyon has attained its present depth, and its walls have acquired much of their picturesque erosional form. The Yellowstone River now maintains a fairly uniform gradient (60-80 feet per mile) throughout the 20-mile-long gorge, even though different segments of the canyon were cut at different times and through different kinds of rocks ([fig. 42]C).
The spectacular erosional development in the upper 5-mile segment of the Grand Canyon, which is the only part seen by most Park visitors, except for the very lower end near Tower Falls ([fig. 33]), has taken place mostly within the past 150,000-125,000 years. One reason for such a rapid rate of erosion stems from the fact that this part of the canyon overlies one of the wide ring fracture zones of the Yellowstone caldera ([fig. 22]). The fracture zone extends to great depth, providing a ready avenue of travel for the upflow of hot water and steam rising in the Yellowstone thermal system, as described in the following chapter. Through many thousands of years, the upward percolation of the hot fluids has caused severe chemical and physical changes (known as hydrothermal alteration) in the rhyolite lava flows. One spectacular result of the alteration has been the change from the normal brown and gray color of the rhyolites to the bright yellow and other colorful hues now seen in the canyon walls (as well as in many other places throughout the Park). Another significant result of alteration has been the weakening of the rocks; that is, the altered rocks are softer and less resistant to erosion than unaltered rocks. Hence, the river has been able to erode these softer rocks, upstream to Lower Falls, at a very rapid rate.
The position of Lower Falls, as might be expected, coincides with a change from highly altered to less altered rhyolite; the difference in the erosion rates of the two kinds of rocks here is self-evident (figs. [41] and [42]C). The position of Upper Falls is likewise closely controlled by differences in rock hardnesses. The rhyolites on the upstream side are hard and dense, whereas those on the downstream side contain a high proportion of volcanic glass which causes them to be more easily eroded ([fig. 42]C).
