Some 600,000 years ago the rumblings of an impending volcanic eruption
sounded ominously across the Yellowstone country. Suddenly, in a mighty
crescendo of deafening explosions, tremendous quantities of hot volcanic
ash and pumice spewed from giant cracks at the earth’s surface. Towering
dust clouds blackened the sky, and vast sheets of volcanic debris spread
out rapidly across the countryside in all directions, covering thousands
of square miles in a matter of minutes with a blanket of utter
devastation. Abruptly, a great smoldering pit—a caldera 30 miles across,
45 miles long, and several thousand feet deep—appeared in the central
Yellowstone region, the ground having fallen into the huge underground
cavern that was left by the earth-shaking eruptions. Lava then began
oozing from the cracks to fill the still-smoking caldera.
Thus, in one brief “moment” of geologic time there was launched that
incredible chain of events which led to the creation of many of the
natural wonders of Yellowstone National Park. Heat from the enormous
reservoir of molten rock which produced the massive eruption still
remains deep within the earth beneath Yellowstone, sustaining the
spectacular hot-water and steam phenomena for which the Park is so
justly famous. The formation of the caldera and the eruption of lavas
profoundly influenced the shape of the present-day landscape. Once a
land covered almost entirely by mountains, the part that
collapsed—nearly one-third of the total Park area—is now characterized
by low rolling plateaus formed from the thick lava flows that filled the
caldera (figs. 1 and 2; see fig. 22 for the outline of the Yellowstone
caldera). Moreover, the carving of the spectacular Grand Canyon of the
Yellowstone (fig. 41) and the fashioning of the large interior basin now
occupied by beautiful Yellowstone Lake (fig. 27) were closely related to
this mighty volcanic event.
North, east, and south of the central plateaus are extensive mountain
ranges and other highlands which provide much of the Park’s scenic
beauty (figs. 3 and 4). Formed by many episodes of intense mountain
building and ancient volcanism, these uplands bear the lasting imprints
of a wide variety of geological activities that date back approximately
2.7 billion years. Indeed, as we study all the features of the
Yellowstone landscape, we find in them a most impressive and fascinating
story of that ageless conflict between the internal forces of nature
that raise the land through the upheaval of mountains and the eruption
of volcanoes, and the external forces of erosion that wear the land
down. It is this vast relentless interplay of giant forces that
determines the appearance of any given place upon the earth’s surface.
And, in few other places around the globe can the processes of both
building up and tearing down the landscape be illustrated more
dramatically than in Yellowstone National Park.
[Illustration: SKYLINE OF THE GALLATIN RANGE in northwestern
Yellowstone National Park, as viewed from a point on the road
between Canyon Village and Norris Junction. The range consists
chiefly of Paleozoic and Mesozoic sedimentary rocks and Precambrian
metamorphic rocks that were uplifted by folding and faulting of the
earth’s crust. The dark-gray rocks along the roadcut in the left
foreground are rhyolite lava flows of the Solfatara Plateau. (Fig.
3)]
[Illustration: HAYDEN VALLEY. View north along the Yellowstone River
and Hayden Valley toward the Washburn Range. Mount Washburn, part of
an ancient Absaroka volcano, is the highest prominence (elevation,
10,293 feet) on the skyline to the right, and Dunraven Pass is in
the notch in the center of the skyline. The foot of the range marks
the north edge of the Yellowstone caldera. Hayden Valley is cut in
glacial lake sediments that overlie thick lava flows covering the
caldera floor. (Fig. 4)]