[Illustration: 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)] [Illustration: 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).] [Illustration: 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)