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. 23]C). 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.
CALDERA DEVELOPMENT. Schematic diagrams showing idealized stages in the development of the Yellowstone caldera 600,000 years ago. The scales shown in Diagram A are approximately the size of the features in Yellowstone. Although only one magma chamber is pictured in the diagrams, two chambers were involved in the Yellowstone eruption. (Based on information supplied by R. L. Christiansen and H. R. Blank, Jr.) (Fig. 23)
A, A large magma chamber formed deep within the earth, and the molten rock began to force its way slowly toward the surface. As it pushed upward, it arched the overlying rocks into a broad dome. The arching produced a series of concentric fractures, or a ring fracture zone, around the crest of the dome. The fractures extended downward toward the top of the magma chamber.
B, The ring fractures eventually tapped the magma chamber, the uppermost part of which contained a high proportion of dissolved gases. With the sudden release of pressure, tremendous amounts of hot gases and molten rock were erupted almost instantly. The liquid solidified into pumice, ash, and dust as it was blown out. Some of the dust and ash was blown high into the air and carried along by the wind, but much of the debris moved outward across the landscape as vast ash flows, covering thousands of square miles very rapidly.
C, The area overlying the blown-out part of the magma chamber collapsed to form a gigantic caldera. The collapse took place mostly along normal faults that developed from the fractures in the ring fracture zone. The depth of the collapse was probably several thousand feet.
