[Illustration: 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.] [Illustration: 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.] [Illustration: D, Renewed rise of molten rock domed the caldera floor above the magma chamber. A series of rhyolite lava flows poured out through fractures in the surrounding ring fracture zone and spread across the caldera floor.] [Illustration: ORIGINAL EXTENT OF THE YELLOWSTONE TUFF (ash-flow tuff) that covered most of Yellowstone National Park about 600,000 years ago. The tuff was erupted explosively from the ring fracture zones of the Yellowstone caldera. The outline of the caldera is shown by the dashed line. (Based on information supplied by R. L. Christiansen and H. R. Blank, Jr.) (Fig. 24)] [Illustration: YELLOWSTONE TUFF AT GOLDEN GATE. The rocks consist of layered ash-flow tuff; the height of the cliff is about 200 feet. (Fig. 25)] [Illustration: Closeup B shows typical characteristics of the tuff in most outcrop areas. Of the light-colored materials, the larger masses are compressed pumice fragments and the smaller masses are pumice, feldspar, and quartz. The dark grains are chiefly magnetite and pyroxene. Closeup A is of a coarse-grained specimen from Tuff Cliff. The large fragments are mostly crystallized pumice, and the light-colored matrix is composed of very fine particles of volcanic ash and dust.] [Illustration: GEOLOGIC CROSS SECTION showing generalized relationships along the north edge of the Yellowstone caldera in the Mount Washburn-Canyon area (line of section labeled D-D′ on pl. 1). The caldera subsided along normal faults in the ring fracture zone, and the Plateau Rhyolite (lava flows) poured out across the caldera floor between 600,000 and 500,000 years ago. The faults cut across the central intrusive igneous core of the 50-million-year-old (Eocene) Washburn volcano; the north half of the volcano is still preserved, but the south half subsided as part of the caldera and is now buried by lava flows. (Based on information supplied by H. J. Prostka and R. L. Christiansen.) (Fig. 26)]