We have now approached that point in geologic time—the beginning of the
Quaternary Period between 2 million and 3 million years ago—when the
stage was set for the triggering of those all-important events that
culminated in the development of the 1,000-square-mile Yellowstone
caldera and ultimately gave rise to the world-renowned hot-water and
steam phenomena. Involved were some of the earth’s biggest explosions,
which have had no apparent counterpart in recorded human history. A few
extremely explosive eruptions have occurred historically, however, such
as the one that took place on the uninhabited island of Krakatoa,
between Java and Sumatra in the East Indies, during the latter part of
August 1883. For several days this island had been shaken by a series of
violent explosions. Then, on August 27, it was ripped by an explosion
that was heard as far away as Australia, a distance of about 3,000
miles. Fifty-mile-high dust clouds became windborne around the globe,
producing colorful sunrises and sunsets in all parts of the world for
several years. When the air around Krakatoa finally cleared, it was
found that two-thirds of the island, some 12 square miles, had collapsed
and vanished into the sea. Though the Krakatoa eruption resulted in a
caldera that is only a small fraction of the size of the one in
Yellowstone, it provides a mental picture to help us understand what has
been discovered about the great volcanic holocaust in Yellowstone
National Park that was described briefly in an early part of this
report.
[Illustration: OUTLINE OF THE YELLOWSTONE CALDERA produced by the
enormous volcanic eruption 600,000 years ago. The two oval-shaped
areas are resurgent domes that arched the caldera floor over twin
magma chambers after the eruption. The margins of the resurgent
domes are surrounded by ring fracture zones which extend outward
toward the edge of the caldera. Numerous fractures in these zones
provided escape routes through which lavas of the Plateau Rhyolite
oozed to the surface and poured out across the caldera floor. Today
these zones also provide underground channels for the circulation of
hot water in the Yellowstone thermal system. The area outlined by
the dotted line shows the smaller and younger inner caldera now
occupied by the West Thumb of Yellowstone Lake. (Based on
information supplied by R. L. Christiansen and H. R. Blank, Jr.; the
existence of a caldera in Yellowstone National Park was first
recognized by F. R. Boyd in the late 1950’s.) (Fig. 22)]
[Illustration: GEOLOGIC MAP OF YELLOWSTONE NATIONAL PARK (PLATE 1)
Generalized from detailed mapping by R. L. Christiansen and H. R.
Blank, Jr. (Quaternary volcanic rocks); H. W. Smedes and H. J.
Prostka (Absaroka volcanic rocks); E. T. Ruppel (sedimentary and
metamorphic rocks, northern part of park); and J. D. Love and W. R.
Keefer (sedimentary rocks, southern part of park).]
[Illustration: 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.