IGNEOUS ROCK. Closeup view of intrusive igneous rock (diorite) from the Electric Peak stock in the Gallatin Range; Electric Peak is pictured in [figure 37]. 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 plateau, drained by sluggish, meandering streams and dotted here and there by volcanoes still rising above the general level of the ground. This plateau surface, however, probably stood at a maximum of only a few thousand feet above sea level, for animals and plants now found as fossils in the Absaroka volcanic rocks indicate that warm-temperature to even subtropical climates existed during the volcanic period ([fig. 19]).
BUNSEN PEAK, a roughly circular body of intrusive igneous rock, is the eroded remnant of either the “neck” of an Absaroka volcano or a small stock that solidified directly beneath a volcano. The peak rises approximately 1,200 feet above a flat plain (foreground) that is covered by flows of younger basalt. The Yellowstone Tuff, formed by volcanic ash and dust exploded from the central Yellowstone region to the south, underlies the basalt. When erupted, the volcanic debris (as well as the basalt lava) flowed around this high-standing peak. (Fig. 21)
A quiet period
Little is known in detail of the geologic events in Yellowstone during Oligocene and Miocene times. Rocks of these ages have not been recognized within the Park; if ever deposited there, they have since been removed by erosion or buried by younger volcanic rocks. Thus, we can only speculate as to what events took place during this 25-million-year period. No doubt the broad Absaroka volcanic plateau was eroded, but not deeply, because the topographic relief and stream gradients of the region remained low. There are also hints that some volcanic activity took place, for volcanic rocks representing parts of this time interval occur south of the Park, and some of these rocks may have originated within the Park area. Little transpired, however, to significantly alter the existing geological makeup of the Park; it was indeed a quiet time, particularly when compared with the extremely dynamic periods which immediately preceded and followed it.
More mountain building and deep erosion
Many features of the present-day landscape of Yellowstone stem from Pliocene time, about 10 million years ago. At that time the entire region—in fact, much of the Rocky Mountain chain—was being uplifted by giant earth movements to heights several thousand feet above its previous level. This episode of regional uplift accounts in large measure for the present high average elevation of the Yellowstone country. Although the precise cause of the uplift is unknown, the uplift assuredly reflects profound changes that were taking place deep within or beneath the earth’s crust.
Great tensional forces, operating during Pliocene time, pulled the Yellowstone region apart and partially broke it into large steep-sided blocks bounded by normal faults ([fig. 13]). Some blocks sank while others rose, commonly on the order of several thousand feet. The Gallatin Range, in the northwest corner of the Park, for example, was lifted as a rectangular mountain block along north-trending 20-mile-long normal faults that border it on each side ([fig. 14], section A-A′; [pl. 1]). In the south-central part of the Park, the differential movements between several adjacent fault blocks totaled more than 15,000 feet ([fig. 14], section C-C′). Farther south, the Teton Range moved up and the floor of Jackson Hole moved down along a normal-fault zone that stretches along the east foot of the range. An enormous offset of about 30,000 feet developed between the two crustal blocks, accounting in large part for the now incredibly steep and rugged east face of the Teton Range.