Caldera development - first of four schematic diagrams showing idealized stages in the development of the Yellowstone caldera 600,000 years ago. Yellowstone National Park, Wyoming. 1970.

Album caption and index card: 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. Yellowstone National Park, Wyoming. 1970. Published as Figure 23-A in U.S. Geological Survey. Bulletin 1347. 1971.