The final violent eruption 600,000 years ago, although releasing much of
the explosive energy of the gases contained in the magma, did not quell
all potential volcanic activity in the twin chambers. Molten rock again
rose in both of them, and in a few hundreds or thousands of years the
overlying caldera floor was domed over the two chambers. One of these
prominent domes lies near Old Faithful and the other east of Hayden
Valley (figs. 22 and 23D). Soon, too, the magma found its way upward
through the wide ring fracture zones encircling the caldera. Pouring out
rather quietly from many openings (fig. 23D), the lavas flooded the
caldera floor and began to fill the still-smoldering pit. The first
lavas appeared soon after the collapse 600,000 years ago, and the latest
ones only 60,000-75,000 years ago. The flows were confined chiefly to
the caldera proper, but here and there they spilled out across the rim,
particularly toward the southwestern part of the Park (fig. 28). Some
flows also erupted along fractures outside the caldera, the most
prominent flow being the very famous one at Obsidian Cliff (fig. 29).
[Illustration: YELLOWSTONE LAKE. View southeast across Yellowstone
Lake toward the western foothills and crest of the Absaroka Range.
The Absaroka Range is an erosional remnant of a vast pile of
volcanic lavas and breccias (Absaroka volcanic rocks) that once
covered all of Yellowstone; the lake occupies part of the
Yellowstone caldera. (Fig. 27)]
The chief rock type in the lava flows is rhyolite, similar in
composition to the welded tufts erupted earlier but different in other
major characteristics. The rock, for example, shows much contorted
layering as evidence of having flowed as a thick liquid across the
ground (fig. 30). A coarse brecciated texture is also a common feature,
well shown by lavas along the Firehole Canyon drive (fig. 31). Locally,
some parts of the flows cooled so rapidly that few crystals formed, and
the lava solidified mainly into a natural glass (fig. 32).
[Illustration: RADAR IMAGE of a part of southwestern Yellowstone
National Park. The lobate landforms are the edges of a lava flow of
the Plateau Rhyolite that forms the Pitchstone Plateau (fig. 1). The
low concentric ridges that parallel the toe of the flow are pressure
ridges produced by the wrinkling of the nearly solidified crust of
lava along the edge of the flow. (Image courtesy of National
Aeronautics and Space Administration.) (Fig. 28)]
[Illustration: OBSIDIAN CLIFF, Jim Bridger’s famous “mountain of
glass.” The rock is rhyolite lava which contains a high proportion
of obsidian, a kind of black volcanic glass. Note columnar jointing
along the sides of the cliff, similar to that shown by the basalt
flows at Tower (fig. 33). The cliff is approximately 200 feet high.
(Fig. 29)]
[Illustration: THICK RHYOLITE LAVA FLOW along west bank of Firehole
River. (Fig. 30)]
[Illustration: Closeup view is of a cut surface of rhyolite, showing
the striking banding that results from the flowage of viscous molten
rock. The dark bands are chiefly concentrations of volcanic glass
(also some cavities), and the light bands are concentrations of tiny
crystals of feldspar and quartz.]
[Illustration: BRECCIATED RHYOLITE LAVA FLOWS along the Firehole
Canyon drive. As a lava flow moves outward from its center of
eruption, a chilled crust develops along its upper surface and outer
edges because of the cooler temperatures in those parts of the flow.
Continued movement of the still-molten rock in the interior of the
flow causes this crust to break up (brecciate) into angular blocks.
The blocks are then tumbled along until the whole mass finally
solidifies. (Fig. 31)]
[Illustration: OUTCROP OF GLASSY RHYOLITE LAVA along the road
between Canyon Village and Norris Junction. The conspicuous lines in
the face of the rock outline different layers produced by lava
flowage. The feldspar crystals are alined parallel to the direction
of flow. (Fig. 32)]
[Illustration: In closeup A, dark parts of the rock are volcanic
glass (closeup B shows glassy fracture) and light-colored crystals
are quartz (blocky) and feldspar (tabular).]
[Illustration: Closeup B.]
About 30 different flows have been recognized. Grouped within a major
rock unit called the Plateau Rhyolite (fig. 5), they cover more than
1,000 square miles. The gently rolling plateau surface of central
Yellowstone, broken here and there by clusters of low-lying hills and
ridges, is essentially the landscape that characterized the upper
surfaces of the lava flows soon after they cooled and solidified.
Natural valleys formed between some of the adjacent flows, and in places
streams still follow these readymade channels. Rhyolite, in both lava
flows and ash-flow tuffs, is by far the predominant rock type seen along
the Park roads.
Several basalt flows were erupted along with the more common rhyolite
flows, and in the vicinity of Tower Falls they form some of the most
unusual rock units in the whole Park area (fig. 33). As the flows
cooled, contraction cracks broke the basalt into a series of upright
many-sided columns; from a distance they appear as a solid row of
fenceposts. They are now covered by younger rocks, but if one could see
the upper flat surface of the basalt layers where just the ends of the
columns are sticking out, the pattern would be like that seen in a
honeycomb.
During the eruptions of the Plateau Rhyolite, at least one relatively
small caldera-making event occurred in the central Yellowstone region.
This “inner” caldera developed sometime between 125,000 and 200,000
years ago, forming the deep depression now filled by the West Thumb of
Yellowstone Lake (fig. 22). Like the main Yellowstone caldera, but on a
much smaller scale, it formed as a direct result of the explosive
eruption of rhyolitic ash flows and subsequent collapse of an
oval-shaped area approximately 4 miles wide and 6 miles long. West Thumb
is nearly the same size as Crater Lake, Oregon, which occupies one of
the world’s best-known calderas.
With the outpouring of the last lava flows 60,000-75,000 years ago, the
forces of Quaternary volcanism finally died down. The hot-water and
steam activity, however, still remains as a vivid reminder of
Yellowstone’s volcanic past. But who can say even now that we are
witnessing the final stage of volcanism? Someday, quite conceivably,
there might be yet another outburst of molten rock—only time, of course,
will tell.
[Illustration: TWO LEDGES OF BASALT spectacularly exposed in the
east wall of the Grand Canyon of the Yellowstone at The Narrows near
Tower Falls. The light-colored rocks between the basalt flows are
ancient stream gravels deposited about 1½ million years ago, when
the channel of the Yellowstone River was farther east and not as
deep as it is today. The hill is capped by lake sediments, sand, and
gravel deposited when the Yellowstone River was blocked by a glacial
dam farther downstream (to the left). The brown rocks at the base of
the cliff are Absaroka andesite breccias. (Fig. 33)]
[Illustration: Pronounced columnar jointing of the basalt is seen at
close range at the edge of the road on the opposite (west) side of
the canyon. Inset shows the dense character of the black basalt,
which consists of microscopic crystals of feldspar, pyroxene,
olivine, and magnetite.]