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