In early Eocene time, between 55 and 50 million years ago, several large
volcanoes erupted in and near Yellowstone National Park. This volcanic
activity resulted in the accumulation of the vast pile of Absaroka
volcanic rocks (fig. 5) which now makes up most of the Absaroka and
Washburn Ranges and part of the Gallatin Range, and which covers several
other smaller areas in the Park (pl. 1).
What special geologic conditions would cause these spectacular eruptions
of molten rock at the earth’s surface? Measurements taken in deep mines
and oil wells show that the normal increase in the earth’s temperature
with depth is about 1°F per 100 feet. This heat is generated by the
decay of radioactive elements—chiefly uranium, thorium, and
potassium—which are present in at least small amounts in virtually all
rocks of the earth’s crust. Ordinarily, enough heat is conducted to the
earth’s surface so that the deeply buried rocks do not become hot enough
to melt. In some places, however, the heat is not carried off fast
enough, and the temperature rises slowly toward the melting point of the
rock. Such hot spots may develop (1) because the rocks in those places
contain more than an average amount of radioactive elements; (2) because
hotter material moves upward from still deeper levels in the earth; or
(3) because drastic changes in pressure are brought about by the
alternate squeezing and relaxing of mountain-building forces, which in
turn substantially affect the melting point of the rocks. Whatever the
cause, the eventual result is the accumulation of a huge body of molten
rock, called _magma_, enclosed in a deep underground chamber.
Magma, being a mixture of hot liquids and gases that is lighter in
weight than the solid rocks surrounding it, tends to rise toward the
earth’s surface. Forcing its way upward, some of the molten material
solidifies before reaching the surface and forms bodies of various kinds
of _intrusive igneous rocks_ (fig. 15). Some of the magma, however,
reaches the surface and either pours out as lava or is blown out
explosively as rock fragments, ash, and pumice to form _extrusive
igneous rocks_.
[Illustration: INTRUSIVE AND EXTRUSIVE IGNEOUS ROCK BODIES.
Extrusive rocks solidified above ground, and intrusive rocks
solidified below ground. All features shown occur in Yellowstone
National Park. Extrusive rocks are the predominant rock type seen
along the Park roads, and the table lists the three principal kinds
that are present. (Fig. 15)]
Rock name Principal rock-forming Color
minerals
Rhyolite Quartz,[a] feldspar[b] Light to medium shades of gray
(sanidine). and brown.
Andesite Feldspar[b] Medium to fairly dark shades of
(plagioclase), brown, red, purple, and gray.
pyroxene[c] (augite).
Basalt Feldspar (plagioclase), Nearly black.
pyroxene, olivine,[c]
magnetite.[d]
[a]Clear to light-colored silicon dioxide.
[b]Light-colored aluminum silicate minerals.
[c]Dark-colored iron and magnesium silicate minerals.
[d]Very dark colored iron oxide mineral.
The magmas which formed the Absaroka volcanoes erupted mainly through
large central vents (fig. 16). Most of the eruptions were fairly quiet,
with the molten rock welling up to the surface and cascading down the
sides of the volcanoes chiefly as viscous lava flows and breccias. Rain,
seeping into these porous rocks, caused huge landslides of mud and
broken rock to stream down the mountainsides. Hence, many of the rocks
seen today are volcanic breccias—jumbled but crudely layered deposits of
large and small angular blocks embedded in a sandy matrix, much like
man-made concrete except that the rock fragments are considerably
coarser (fig. 17). Viewed from a distance, however, most of the breccia
deposits have a distinct layered appearance (fig. 18). The predominant
extrusive igneous rock in the Absaroka volcanic sequence is andesite,
but basalt also occurs in places (fig. 15).
[Illustration: ABSAROKA VOLCANOES and their rocks. Lava (mostly
andesite) poured from central vents and formed volcanoes, some steep
sided and others broad and relatively flat. As the lava spilled out,
much of it quickly solidified, broke up into large angular blocks
(breccia), and then either tumbled down the slopes of the volcanoes
as individual boulders or slid down in mudflows and landslides. Some
of the material was also explosively blown out as rock bombs,
cinders, and ash. The more fluid lava (mostly basalt), on the other
hand, flowed quietly down the volcanic slopes and onto the
surrounding lowlands. The rocks near the volcanic centers therefore
include thick crudely layered coarse breccias, thin fine ash and
dust falls, and thin to thick lava flows. The volcanoes were
repeatedly attacked by erosion, and the eroded material was
redeposited by streams and mudflows in widespread layers of volcanic
conglomerate and sandstone across the flat-floored valleys and
plains between the volcanoes. Forests, which grew luxuriantly in
these lowland areas, were repeatedly buried by volcanic eruptions
and are now preserved (see inset) as the fossil forests of
Yellowstone. (Based on information supplied by H. W. Smedes and H.
J. Prostka.) (Fig. 16)]
Chiefly lava flows of shield volcano
Chiefly volcanic sandstone and conglomerate of lowland areas
Chiefly volcanic breccias and thin lava flows of cone-type volcano
At times the Absaroka volcanic eruptions were violently explosive,
showering the countryside with rock bombs, cinders, and ash. The finer
debris that reached the lower slopes of the volcanoes was reworked and
carried by streams into the intervening valleys, where it was deposited
as sand and gravel (fig. 16). Eventually the entire Yellowstone region
was choked with volcanic debris, the material from one volcano mixing
with that from neighboring volcanoes. Even the mountain masses uplifted
during the preceding Laramide orogeny were covered by the vast
accumulation (fig. 18).
[Illustration: MASSIVE BEDS OF BRECCIA of the Absaroka volcanic
rocks along the road north of Dunraven Pass. This breccia formed
part of a steep-sided volcanic cone, of which Mount Washburn is a
remnant. (Fig. 17)]
[Illustration: Closeup view shows very coarse character of the
breccia, with large rock fragments imbedded in fine ash, dust, and
sand. Nearly all the rocks are of andesitic composition, consisting
chiefly of feldspar and pyroxene. Most common colors are medium to
fairly dark shades of brown, red, purple, and gray.]
Absaroka volcanism, however, was not a simple, continuous process—the
eruptions were intermittent, the many volcanoes were not always active
at the same time, and between eruptions there were long periods of
quiescence during which the erupted material was deeply eroded. The
repetitive nature of the eruptions is best illustrated by the famous
fossil forests of Yellowstone. Here is striking evidence that enough
time elapsed between eruptions for widespread forests to become
established on the lower slopes of the volcanoes and in the broad
valleys between them. Judged from the great size of some of the
now-petrified logs (fig. 19), several hundreds of years must have passed
before another volcanic outburst smothered the forest. Many different
forest layers have been recognized in the Specimen Ridge area as well as
in several other places throughout the Park.
As the Absaroka magma rose from deep underground, some of it squirted,
like toothpaste, into the layered Paleozoic and Mesozoic sedimentary
rocks through which it passed. These relatively small masses of molten
rock material slowly cooled and crystallized to form intrusive igneous
rocks such as diorite (fig. 20). The resulting intrusive bodies, called
_sills_, _dikes_, _stocks_, and _laccoliths_, depending on their form,
are most abundant in the Gallatin Range and in the vicinity of the East
Entrance (pl. 1). At the conclusion of volcanic activity, the last of
the rising magma solidified in the main conduits to form slender,
somewhat cylindrical bodies of rock called _volcanic necks_ that
probably conform closely to the shape of the original conduits. The
circular intrusive rock body at Bunsen Peak (fig. 21), now exposed to
view because erosion has stripped away the lava and volcanic breccia
that once completely buried it, represents either a volcanic neck or a
small stock that solidified directly beneath a volcano.
[Illustration: MASSIVELY LAYERED BRECCIAS, conglomerates, and
sandstones of the Absaroka volcanic sequence at Barronette Peak, as
viewed from the road near the Northeast Entrance; the ridge is 3,000
feet high. These rocks, deposited as part of an alluvial plain
between volcanoes, once filled the Yellowstone region to a level
higher than the top of Barronette Peak, but erosion since late
Tertiary time has stripped the volcanics from much of the Park area.
The volcanic rocks (Eocene in age, fig. 5) rest directly on
Paleozoic sedimentary rocks along the line indicated. During the
Laramide orogeny, in Late Cretaceous and early Tertiary times, the
region was folded and uplifted into mountains. Thousands of feet of
Mesozoic and Paleozoic sedimentary rocks were then eroded off the
rising mountains before the Absaroka volcanic rocks were deposited,
(Fig. 18)]
[Illustration: GIANT PETRIFIED TREE TRUNKS in Yellowstone’s fossil
forest. The enclosing rocks, part of the Absaroka volcanic sequence
that forms Specimen Ridge, are approximately 50 million years old.
Many of the tree trunks are still upright, having been smothered and
buried in their original positions by breccia, ash, and dust from
nearby volcanoes. It is evident that more than one “forest” is
represented in this view. Prof. Erling Dorf, of Princeton
University, counted a total of 27 different forest layers in the
rocks now exposed at Specimen Ridge. He also determined that the
most common kinds of trees were sycamore, walnut, magnolia,
chestnut, oak, redwood, maple, and dogwood. The nearest living
relatives of many of these trees are now found in the warm temperate
to subtropical forests of the southeastern and southern United
States. (National Park Service photograph.) (Fig. 19)]
Mount Washburn is the north half of one of the ancient Absaroka
volcanoes (fig. 26), and many of the rocks and other features related to
this volcano, which characterized this great period of volcanism, can be
seen along the road between Canyon Village and Tower. In roadcuts just
south of Dunraven Pass several thin igneous dikes cut through volcanic
breccias. These dikes radiate outward from the nearby central core of
the volcano, which lies east of the highway in the vicinity of Washburn
Hot Springs. From Dunraven Pass northward for 2-3 miles, the road is
lined with lava flows and very coarse breccias that accumulated close to
the volcanic neck (fig. 17). Farther north toward Tower Falls, breccias
and conglomerates predominate, but the average size of individual rock
fragments decreases gradually northward away from the center of
eruption. Beds of sandstone then begin to appear in the sequence, having
been deposited mainly by streams that drained the north slope of the
volcano.
[Illustration: IGNEOUS ROCK. Closeup view of intrusive igneous rock
(diorite) from the Electric Peak stock in the Gallatin Range;
Electric Peak is pictured in figure 37. The rock is composed chiefly
of light-colored quartz and feldspar and dark-colored iron and
magnesium silicate minerals. (Fig. 20)]
At the end of Absaroka volcanism, approximately 40 million years ago
(fig. 6), all of Yellowstone lay buried beneath several thousand feet of
lavas, breccias, and ash (fig. 18). The landscape must have appeared as
a gently rolling plateau, drained by sluggish, meandering streams and
dotted here and there by volcanoes still rising above the general level
of the ground. This plateau surface, however, probably stood at a
maximum of only a few thousand feet above sea level, for animals and
plants now found as fossils in the Absaroka volcanic rocks indicate that
warm-temperature to even subtropical climates existed during the
volcanic period (fig. 19).
[Illustration: BUNSEN PEAK, a roughly circular body of intrusive
igneous rock, is the eroded remnant of either the “neck” of an
Absaroka volcano or a small stock that solidified directly beneath a
volcano. The peak rises approximately 1,200 feet above a flat plain
(foreground) that is covered by flows of younger basalt. The
Yellowstone Tuff, formed by volcanic ash and dust exploded from the
central Yellowstone region to the south, underlies the basalt. When
erupted, the volcanic debris (as well as the basalt lava) flowed
around this high-standing peak. (Fig. 21)]