With the sudden removal of hundreds of cubic miles of molten rock from
underground, the roofs of the twin magma chambers collapsed. Enormous
blocks of rock fell in above each of the chambers, and a great crater,
or _caldera_, broke the ground surface in central Yellowstone (fig.
23C). The exact depth to which the original surface collapsed is
unknown, but it must have been several thousand feet. The subsidence
took place chiefly along large vertical, or normal, faults in the ring
fracture zones above the margins of the magma chambers (fig. 22).
Abundant, though less extensive, normal faults also formed outside the
caldera proper, as the surrounding areas adjusted to the staggering
impact of the explosive eruptions and subsequent collapse.
Because the Yellowstone caldera now lies partly buried by thick lava
flows, the appearance of the caldera today is not nearly as impressive
as it must have been when the caldera was first formed. Many of the
important features, however, are particularly well exposed in the
vicinity of Canyon Village (fig. 26). The steep south slope of the
nearby Washburn Range (fig. 4) marks the north edge of the caldera, and
the range itself stands high because it was not involved in the
collapse. Canyon Village, on the other hand, lies at a much lower
elevation within the caldera proper. Turnouts on the road just south of
Dunraven Pass provide especially fine views of the northern part of the
caldera, and on a clear day Flat Mountain and the Red Mountains, which
mark the south edge of the caldera, south of Yellowstone Lake, can be
seen 50 miles away. As might be expected, the large basin occupied by
Yellowstone Lake owes its existence in part to caldera collapse. The
south edge of the caldera cuts across the south-central part of the
lake, along Flat Mountain Arm and the north tip of the Promontory; the
east edge coincides approximately with the east edge of the lake north
of Southeast Arm (fig. 27). Also, the prominent bluffs north of the
Madison River near Madison Junction mark part of the north rim of the
caldera.
[Illustration: CALDERA DEVELOPMENT. Schematic diagrams showing
idealized stages in the development of the Yellowstone caldera
600,000 years ago. The scales shown in Diagram A are approximately
the size of the features in Yellowstone. Although only one magma
chamber is pictured in the diagrams, two chambers were involved in
the Yellowstone eruption. (Based on information supplied by R. L.
Christiansen and H. R. Blank, Jr.) (Fig. 23)
A, 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.]
[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)]
Grand Canyon
Plateau Rhyolite (lava flows)
Edge of caldera
Intrusive igneous rocks of Washburn volcano
Mount Washburn
Absaroka volcanic breccias