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).
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EXPLANATION
CENOZOIC QUATERNARY Stream sand and gravel, glacial and landslide debris, hot-spring deposits, and lake beds Basalt flows Plateau Rhyolite Yellowstone Tuff and related lava flows TERTIARY Absaroka volcanic rocks Intrusive igneous rocks Tertiary formations Mesozoic formations Paleozoic formations Precambrian gneiss and schist Contact
FAULT AND FOLD SYMBOLS Dotted where concealed beneath younger unfaulted rocks Reverse fault Sawteeth on side that moved up Normal fault Symbol on side that moved down Reverse fault, along which there was later normal-fault movement Anticlinal axis B B′ Line of cross section shown in [figure 14] (D-D’ is [figure 26])
Near the beginning of the Quaternary Period a vast quantity of molten rock had again accumulated deep within the earth beneath Yellowstone. This time, in contrast to Absaroka volcanism, the magma was charged with highly explosive materials which eventually caused two caldera-making eruptions, one 2,000,000 years ago and the other 600,000 years ago. Because both eruptions affected the central part of the Park, the features related to the older one were largely destroyed by the activity associated with the younger one. Thus, the outline of the volcanic caldera we now see in the Yellowstone landscape is chiefly the one that formed 600,000 years ago ([fig. 22]). The sequence of events described in the following pages, and illustrated diagrammatically in [figure 23], is based on studies of this later eruption; the pattern for the 2,000,000-year-old eruption probably was similar.