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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])

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 eruption

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. 23]A). 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. 23]B). 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. 23]B), 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]).

The collapse