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. Much of the landscape may have resembled the rugged terrain now seen in the Absaroka Range along the east side of the Park. These mountains ([fig. 27]), and the Washburn Range in the interior of the Park ([fig. 4]), today represent but small remnants of the vast pile of Absaroka volcanic rocks that once covered all of Yellowstone and the surrounding regions.
Formation of the Yellowstone Caldera
We have now approached that point in geologic time—the beginning of the Quaternary Period between 2 million and 3 million years ago—when the stage was set for the triggering of those all-important events that culminated in the development of the 1,000-square-mile Yellowstone caldera and ultimately gave rise to the world-renowned hot-water and steam phenomena. Involved were some of the earth’s biggest explosions, which have had no apparent counterpart in recorded human history. A few extremely explosive eruptions have occurred historically, however, such as the one that took place on the uninhabited island of Krakatoa, between Java and Sumatra in the East Indies, during the latter part of August 1883. For several days this island had been shaken by a series of violent explosions. Then, on August 27, it was ripped by an explosion that was heard as far away as Australia, a distance of about 3,000 miles. Fifty-mile-high dust clouds became windborne around the globe, producing colorful sunrises and sunsets in all parts of the world for several years. When the air around Krakatoa finally cleared, it was found that two-thirds of the island, some 12 square miles, had collapsed and vanished into the sea. Though the Krakatoa eruption resulted in a caldera that is only a small fraction of the size of the one in Yellowstone, it provides a mental picture to help us understand what has been discovered about the great volcanic holocaust in Yellowstone National Park that was described briefly in an early part of this report.
OUTLINE OF THE YELLOWSTONE CALDERA produced by the enormous volcanic eruption 600,000 years ago. The two oval-shaped areas are resurgent domes that arched the caldera floor over twin magma chambers after the eruption. The margins of the resurgent domes are surrounded by ring fracture zones which extend outward toward the edge of the caldera. Numerous fractures in these zones provided escape routes through which lavas of the Plateau Rhyolite oozed to the surface and poured out across the caldera floor. Today these zones also provide underground channels for the circulation of hot water in the Yellowstone thermal system. The area outlined by the dotted line shows the smaller and younger inner caldera now occupied by the West Thumb of Yellowstone Lake. (Based on information supplied by R. L. Christiansen and H. R. Blank, Jr.; the existence of a caldera in Yellowstone National Park was first recognized by F. R. Boyd in the late 1950’s.) (Fig. 22)
GEOLOGIC MAP OF YELLOWSTONE NATIONAL PARK (PLATE 1)
Generalized from detailed mapping by R. L. Christiansen and H. R. Blank, Jr. (Quaternary volcanic rocks); H. W. Smedes and H. J. Prostka (Absaroka volcanic rocks); E. T. Ruppel (sedimentary and metamorphic rocks, northern part of park); and J. D. Love and W. R. Keefer (sedimentary rocks, southern part of park).