Closeup B.
About 30 different flows have been recognized. Grouped within a major rock unit called the Plateau Rhyolite ([fig. 5]), they cover more than 1,000 square miles. 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.
Several basalt flows were erupted along with the more common rhyolite flows, and in the vicinity of Tower Falls they form some of the most unusual rock units in the whole Park area ([fig. 33]). As the flows cooled, contraction cracks broke the basalt into a series of upright many-sided columns; from a distance they appear as a solid row of fenceposts. They are now covered by younger rocks, but if one could see the upper flat surface of the basalt layers where just the ends of the columns are sticking out, the pattern would be like that seen in a honeycomb.
During the eruptions of the Plateau Rhyolite, at least one relatively small caldera-making event occurred in the central Yellowstone region. This “inner” caldera developed sometime between 125,000 and 200,000 years ago, forming the deep depression now filled by the West Thumb of Yellowstone Lake ([fig. 22]). Like the main Yellowstone caldera, but on a much smaller scale, it formed as a direct result of the explosive eruption of rhyolitic ash flows and subsequent collapse of an oval-shaped area approximately 4 miles wide and 6 miles long. West Thumb is nearly the same size as Crater Lake, Oregon, which occupies one of the world’s best-known calderas.
With the outpouring of the last lava flows 60,000-75,000 years ago, the forces of Quaternary volcanism finally died down. The hot-water and steam activity, however, still remains as a vivid reminder of Yellowstone’s volcanic past. But who can say even now that we are witnessing the final stage of volcanism? Someday, quite conceivably, there might be yet another outburst of molten rock—only time, of course, will tell.
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 1½ 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 brown rocks at the base of the cliff are Absaroka andesite breccias. (Fig. 33)
Pronounced columnar jointing of the basalt is seen at close range at the edge of the road on the opposite (west) side of the canyon. Inset shows the dense character of the black basalt, which consists of microscopic crystals of feldspar, pyroxene, olivine, and magnetite.