Volcanoes are one of the most interesting parts of the geologic story of
the Teton region. Although ash from distant volcanoes had settled in
northwestern Wyoming at least as far back in time as Jurassic, the first
nearby active volcanoes (since the Precambrian) erupted in the
Yellowstone-Absaroka region during the early Eocene, about 50 million
years ago. From then on, the volcanic area grew in size and the violence
of eruptions and volume of debris increased until Pliocene time. This
debris had a profound influence on the color and composition of the
sediments and on the environment and types of plants and animals.
The color of the volcanic rocks and the sediments derived from them
varies significantly from one epoch to another. For example, the middle
Eocene rocks are white to light-green, red, and purple, upper Eocene are
dark-green, Oligocene are light-gray, white, and brown, Miocene are
dark-green, brown, and gray, and Pliocene are white to red-brown.
[Illustration: Figure 53. _Air oblique view south, showing the north
end of the Teton Range disappearing beneath Pleistocene lava flows.
Light-colored bare area at lower left is vertical Paleozoic
limestone surrounded on three sides by nearly horizontal rhyolite
lava flows. Bare slope at lower right is west-dipping Pinyon
Conglomerate, also overlapped by lava. Grand Teton is on right
skyline and Mt. Moran is rounded summit on middle skyline._]
As mentioned earlier, it is probable that the vast outpouring of
volcanic rocks during late Tertiary time in the Teton region and to the
north and northeast is directly related to the subsidence of Jackson
Hole and the rise of the Tetons.
The spectacular banded cliffs of the Wiggins Formation on both sides of
Togwotee Pass (fig. 52) and farther north in the Absaroka Range are
remnants of Oligocene volcanic conglomerate and tuff that once spread as
a blanket several thousand feet thick across eastern Jackson Hole and
partially or completely buried the nearby older folded mountain ranges.
[Illustration: Figure 54. _Obsidian, a volcanic glass less than 10
million years old, especially prized by Indians who used it for
spear and arrow points and for tools._]
About 25 million years ago, with the start of the Miocene Epoch,
volcanic vents opened up within, and along the borders of, Grand Teton
National Park. Major centers of eruption were at the north end of the
Teton Range, east of Jackson Lake, and south of Spread Creek. They
emitted a prodigious amount of volcanic ash and fragments of congealed
lava. For example, adjacent to one vent a mile in diameter, about 4
miles north-northeast of Jackson Lake Lodge, is a continuous section,
7,000 feet thick, of waterlaid strata derived in large part from this
volcanic source. These sedimentary rocks comprise the Colter Formation
which is darker colored and contains more iron and magnesium than the
Wiggins Formation. The site of deposition at this locality was a
north-trending trough that represented an early stage in the downwarping
of Jackson Hole.
Pliocene volcanoes erupted in southern and central Yellowstone Park. The
volcanoes emitted viscous, frothy, pinkish-gray and brown lava called
_rhyolite_. This is an extrusive igneous rock that has the same
composition as granite, but is much finer grained. In several places,
lava apparently flowed into the north end of Teewinot Lake, chilled
suddenly, and solidified into a black volcanic glass called _obsidian_.
Because it chips easily into thin flakes having a smooth surface,
obsidian was prized by the Indians, who used it for spear and arrow
points (fig. 54). Some of this obsidian has a potassium-argon date of 9
million years.
[Illustration: Figure 55. _East face of Signal Mountain showing
Bivouac Formation (upper Pliocene or Pleistocene). Tilted ledge is
rhyolitic welded tuff 2.5 million years old, and slopes above and
below it are conglomerate. National Park Service photo by W. E.
Dilley._]
After Teewinot Lake was filled with sediment, the floor of Jackson Hole
became a flat boulder-covered surface. Nearby vents erupted heavy fiery
clouds of gaseous molten rock that rolled across this plain and then
congealed into hard layers with the general appearance of lava flows.
Under a microscope, however, the rock is seen to be made up of
compressed fragments of glass that matted down and solidified when the
clouds stopped moving. This kind of rock is called a _welded tuff_. One
of these forms the conspicuous ledge in the Bivouac Formation on the
north and east sides of Signal Mountain (fig. 55), and is especially
important because it has a potassium-argon date of 2.5 million years.
More of this _welded tuff_ flowed southward from Yellowstone National
Park, engulfed the north end of the Teton Range (fig. 53), and continued
southward along the west side of the mountains for 35 miles and along
the east side for 25 miles.
[Illustration: Figure 56. _The final 3 million years on our
yardstick of time, enlarged to show approximate dates of major
events._]
THE LAST HUNDREDTHS ABSOLUTE TIME IMPORTANT EVENTS
OF AN INCH OF THE (Years ago)
YARDSTICK
0 0 Last glaciation followed by
faulting
¹/₁₀₀₀ 50000 Second glaciation
²/₁₀₀₀ 100000 ?—First glaciation
⁶/₁₀₀₀ 700000 ?—Second Quaternary lake
⁸/₁₀₀₀ 1 million ?—Tilting and faulting of
southern part of Jackson Hole
¹¹/₁₀₀₀ 1.3 million ?—First Quaternary lake
¹²/₁₀₀₀ 1.5 million } Complex series of volcanic
eruptions in southern Jackson
Hole
¹⁵/₁₀₀₀ 1.9 million }
¹⁶/₁₀₀₀ 2 million ?—Development of Hoback
normal fault
²/₁₀₀ 2.5 million Eruption of welded tuff in
Bivouac Formation
²⁴/₁₀₀₀ 3 million