FORMATION OF THE YELLOWSTONE CALDERA
We have now approached that point in geologic timethe beginning of the Quaternary Period between 2 million and 3 million years agowhen 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.
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.
The giant reservoir of molten rock that built up beneath the Park area fed two large magma chambers that rose to within a few thousand feet of the surface. As the pressures increased, the overlying ground arched, stretched, and cracked (fig. 23A). Small amounts of lava began to flow out through the cracks in places, but finally, in a great surge of rapid, violently explosive eruptions, first from one chamber and then the other, mountains of hot pumice, ash, and rock debris spewed from the earth (fig. 23B). The dense, swirling masses of erupted material spread out across the countryside in extremely fast moving ash flows, swept along by hot expanding gases trapped within them. Large quantities of ash and dust were also blown high into the air and dispersed by the wind. Thin layers of airborne volcanic ash from Yellowstone are now found throughout much of the central and western United States.
The ash flows (fig. 23B), as they sped across the Yellowstone countryside, first filled the old canyons and valleys that had been eroded into the Absaroka volcanic pile and older rocks during Pliocene time. Eventually much of this older landscape was buried by ash. Some of the larger highlands, such as Mount Washburn and adjacent ridges and Bunsen Peak, however, stood well above the level of the sweeping ash flows; so the debris flowed around them rather than across them (fig. 21). Finally coming to rest, the hot pumice, ash, and rock particles settled down in vast horizontal sheets (fig. 24). Upon cooling and crystallizing, the particles welded together to form a series of compact rocks with the composition of rhyolite (figs. 15 and 25). The term "ash-flow tuff" (also, the term "welded tuff") is commonly used to describe these rocks, which now make up the Yellowstone Tuff (fig. 5).
With the sudden removal of hundreds of cubic miles of molten rock from underground, the roofs of the twin magma chambers collapsed. Enormous blocks of rock fell in above each of the chambers, and a great crater, or caldera, broke the ground surface in central Yellowstone (fig. 23C). The exact depth to which the original surface collapsed is unknown, but it must have been several thousand feet. The subsidence took place chiefly along large vertical, or normal, faults in the ring fracture zones above the margins of the magma chambers (fig. 22). Abundant, though less extensive, normal faults also formed outside the caldera proper, as the surrounding areas adjusted to the staggering impact of the explosive eruptions and subsequent collapse.
Because the Yellowstone caldera now lies partly buried by thick lava flows, the appearance of the caldera today is not nearly as impressive as it must have been when the caldera was first formed. Many of the important features, however, are particularly well exposed in the vicinity of Canyon Village (fig. 26). The steep south slope of the nearby Washburn Range (fig. 4) marks the north edge of the caldera, and the range itself stands high because it was not involved in the collapse. Canyon Village, on the other hand, lies at a much lower elevation within the caldera proper. Turnouts on the road just south of Dunraven Pass provide especially fine views of the northern part of the caldera, and on a clear day Flat Mountain and the Red Mountains, which mark the south edge of the caldera, south of Yellowstone Lake, can be seen 50 miles away. As might be expected, the large basin occupied by Yellowstone Lake owes its existence in part to caldera collapse. The south edge of the caldera cuts across the south-central part of the lake, along Flat Mountain Arm and the north tip of the Promontory; the east edge coincides approximately with the east edge of the lake north of Southeast Arm (fig. 27). Also, the prominent bluffs north of the Madison River near Madison Junction mark part of the north rim of the caldera.
The final violent eruption 600,000 years ago, although releasing much of the explosive energy of the gases contained in the magma, did not quell all potential volcanic activity in the twin chambers. Molten rock again rose in both of them, and in a few hundreds or thousands of years the overlying caldera floor was domed over the two chambers. One of these prominent domes lies near Old Faithful and the other east of Hayden Valley (figs. 22 and 23D). Soon, too, the magma found its way upward through the wide ring fracture zones encircling the caldera. Pouring out rather quietly from many openings (fig. 23D), the lavas flooded the caldera floor and began to fill the still-smoldering pit. The first lavas appeared soon after the collapse 600,000 years ago, and the latest ones only 60,000-75,000 years ago. The flows were confined chiefly to the caldera proper, but here and there they spilled out across the rim, particularly toward the southwestern part of the Park (fig. 28). Some flows also erupted along fractures outside the caldera, the most prominent flow being the very famous one at Obsidian Cliff (fig. 29).
The chief rock type in the lava flows is rhyolite, similar in composition to the welded tuffs erupted earlier but different in other major characteristics. The rock, for example, shows much contorted layering as evidence of having flowed as a thick liquid across the ground (fig. 30). A coarse brecciated texture is also a common feature, well shown by lavas along the Firehole Canyon drive (fig. 31). Locally, some parts of the flows cooled so rapidly that few crystals formed, and the lava solidified mainly into a natural glass (fig. 32).
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 rockonly time, of course, will tell.
Last Updated: 18-Jan-2007