a few years after Boyd's paper appeared, the U.S. Geological Survey
mounted an extensive investigation of Yellowstone's geology, assigning
some of its brightest young scientists to the task. Among them was Bob
Christiansen, who studied the young ash flow tuffs in great detail.
What follows is based on his research and that of his co-workers, including
geologists, chemists, and geophysicists, some of whom continue their
studies of Yellowstone today.
Christiansen and his team recognized
that not one but two welded tuffs rimmed the plateaulava flows; one
was 2.1 million years old (Huckleberry Ridge Tuff ) and the younger
0.65 million years (Lava Creek Tuff ). A third tuff, to the west in
Idaho, was 1.3 million years old (Mesa Falls Tuff). Together they form
the Yellowstone Group of tuffs.
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These tuffs demonstrated conclusively
that the volcanic events forming Yellowstone
were not the products of many million years of geologic change ending
many millions of years ago. Rather, their time scale was compressed
into only the last two million years. A long geologic history would
have allowed a more leisurely progression of events-a lava flow here,
then a million years later another flow there. A longer geologic history
would also have called for intermittent periods of magma (molten rock)
formation separated by periods of volcanic quiescence. Instead, this
short time scale compressed the sequence of explosions and flows and
required a heat source much larger and younger than ever before imagined.
Caldera's are large basin-shaped volcanic
depressions more or less circular in form. Caldera eruptions on the
Yellowstone scale have a world wide frequency of perhaps once every
hundred thousand years. Somewhat smaller eruptions, on the scale of
Crater Lake-Mount Mazama in Oregon, are more frequent, perhaps every
1,000 years or less. Such explosive eruptions were not isolated events.
Rather, they were climactic stages of magmatic processes that extended
over hundreds of thousands of years.
No one has ever seen a volcanic explosion
on the scale of the Yellowstone eruptions, but smaller explosions have
been observed and their activity described. Consider Mount Tambora,
on the island of Sumbawa, Indonesia to grasp some idea of what's involved
when a caldera forms during or just after an ash flow eruption. For
about three years the volcano rumbled and fumed before a moderate eruption
on April 5, 1815 produced thundering explosions heard 870 miles away.
Next morning volcanic ash began to fall and continued to fall though
the explosions became progressively weaker,
On the evening of April 10 the mountain
went wild. Eye witnesses 20 miles away described three columns of flame
rising from the crater and combining into one at a great height. The
whole mountain seemed to be covered with flowing liquid fire. Soon these
distant viewers were pelted with 8-inch
pumice stones hurled from the volcano. Clouds of ash, borne by violent
gaseous currents, blasted through nearby towns blowing away houses and
uprooting trees. The village of Tambora was destroyed by rolling masses
of incandescent, hot ash.
On April 16, booming explosions loud
enough to be heard on Sumatra, 1600 miles to the west, continued into
evening. Mount Tambora, still covered with clouds higher up, seemed
to be flaming on its lower slopes.
For a day or two, skies turned jet black and the air cold. When the
eruption ended, the ash cloud drifted west and settled on all islands
downwind. With the expulsion of so much magma, the mountain collapsed,
unsupported from within, forming a great caldera. Lombok, 124 miles
to the west, was covered by a blanket of ash two feet thick. Tidal waves
crashed on islands hundreds of miles away. Waves and ashfalls killed
more than 88,000 people.
Ash blasted into the stratosphere circled
the earth several times causing unusually beautiful sunsets in London
early that summer. In 1816, mean temperatures in the northern hemisphere
dropped by half to more than 1° E Farmers in Europe and America called
this the year without a summer.
Tambora's eruption was the largest and
deadliest volcanic event in recorded history. How does it compare with
the Yellowstone caldera eruptions? If we reduce all the ash from Tambora
to dense rock equivalents and include all ash flow tuffs that formed
at the same time, we come up with
about 36 cubic miles of rock. Quite a bit compared with the destructive
U.S. eruptions of Mount St. Helens in 1980 that produced about 1/4 cubic
Both of these shrink to insignificance
when compared with Yellowstone. The volume of volcanic rock produced
by the first Yellowstone caldera eruption was about 600 cubic miles-about
17 times more than Tambora, and 2,400 times as much as Mount St. Helen's,
an almost incomprehensible figure. One more statistic: Ash from Tambora
drifted downwind more than 800 miles; Yellowstone ash is found in Ventura,
California to the west and the Iowa to the east. It is likely the earth
has seldom in its long history experienced caldera explosions on the
scale of those that created Yellowstone.
Three gigantic caldera eruptions rocked
the Greater Yellowstone Ecosystem. The first and largest, Huckleberry
Ridge caldera, blew up about 2.1 million years ago. Its center was in
western Yellowstone National Park, but it extended into Island Park,
Idaho. Welded tuff from this cycle is called the Huckleberry Ridge Tuff.
The yellow rocks along the road in Golden Gate between Mammoth Hot Springs
and Swan Lake Flats are Huckleberry Ridge Tuff. So are the tuffs that
hold up much of Signal Mountain in Grand Teton National Park, and that
crop out along the west side of the Teton Range, in Idaho.
The second great explosion formed the
Island Park caldera 1.3 million years ago. This caldera, the smallest
of the three, lies just west of Yellowstone in Idaho, within the western
part of the Huckleberry Ridge caldera.
The youngest caldera, Lava Creek, erupted
the Lava Creek Tuff, 0.65 million years old. It overlaps the Huckleberry
Ridge caldera, but its eastern margin is about 10 miles farther east.
Because it is the youngest, its tuffs and associated lava flows are
best exposed and its history best
known. Its eruption may have destroyed the south part of the Washburn
Although the Lava Creek Tuff is 0.65
million years old, its caldera began to evolve about 1.2 million years
ago when rhyolite lavas flowed intermittently onto the surface of the
Yellowstone Plateau from slowly forming, crescentic fractures. Over
a period of 600,000 years these ring fractures grew and coalesced to
form a system of fractures enclosing the part of the plateau that later
collapsed into the Lava Creek caldera.
The ring fractures were a surface expression
of a huge body of magma or molten rock, forming in upper levels of the
earth's crust. As the magma chamber grew in volume, it stretched and
bulged the crust above it. The upper crust was rigid and brittle; it
fractured more easily than it bent; thus fractures, or faults, developed
around the bulge. As the bulge rose higher, the ring fractures propagated
downward toward the magma chamber.
In the magma chamber itself, the molten
material was evolving chemically. Less dense materials were concentrating
in the upper part of the chamber, including the more silica-rich magma,
various gases, and water. Then, with maximum segregation in the magma
chamber, volatiles concentrating in its upper part, and ring fractures
propagating downward, the gun was loaded and cocked. What actually triggered
the caldera-forming explosions is hard to say, but the pressures in
the magma chamber must have exceeded the gravitational pressures of
the overlying rocks.
Imagine a bottle of carbonated water
lying in the sun. Pick it up, shake it vigorously, maybe tap the cap...boom,
it blows off. Instantly the pressure in the bottle drops, the dissolved
carbon dioxide exsolves into bubbles and an expanding mass of bubbles
and water jets into the sky. In a few seconds, the event is over. Wipe
off your face and check the bottle;
some of the water remains, but most of the gas is gone. This simple
scenario is a scaled-down analogy of what happened 600,000 years ago
in Yellowstone when the volatile-rich upper part of the magma chamber
vented and erupted the Lava Creek Tuff. The exolv-ing gas expanded in
the magma, making a much larger volume of frothy fluid. This expanding,
low-density hot gas and magma mixture rose rapidly. It vented at the
surface as a sustained explosion of white-hot froth. A scene from the
depths of Dante's Inferno.
Driven by hot vapors, giant fountains
of incandescent ash at temperatures near 1,800° F burst from the ring
fractures. Plumes of ash jetted into the stratosphere where planetary
winds carried it around the world blanketing tens of thousands of square
miles with thin coverlets of volcanic dust. Nearer the vents, fiery
clouds of dense ash, fluidized by the expanding gas, boiled over crater
rims and rushed across the countryside at speeds over one hundred miles
per hour, vaporizing forests, animals, birds, and streams into varicolored
puffs of steam. Gaping ring fractures extended downward into the magma chamber providing conduits for continuing
foaming ash flows.
More and more vapor-driven ash poured
from the ring fractures, creating a crescendo of fury. As the magma
chamber emptied, large sections of the foundering magma chamber roof
collapsed along the ring fractures, triggering a chain reaction that
produced a caldera 45 miles long and 28 miles wide.
Hot ash flows are fascinating. Driven
by expanding gas, they are really clouds of hot glass shards and pumice
plus expanding gas whose turbulence keeps everything flowing like water.
But as the gas escapes, the viscosity increases, motion ceases, and
the ash settles into a layer more than one hundred feet thick. This
deposit is still extremely hot, and as it compresses under its own weight,
the sticky glass shards fuse into a welded tuff. The upper part of the
ash cools too rapidly to weld and is either unconsolidated or weakly
cemented by vapors of escaping gas.
The engine of destruction didn't take
long to run down, just a few hours or, at most, a few days. Hours? Days?
Yes, incredible as it may seem. Evidence for the astonishing rapidity
of this eruption is found in detailed study of the tuff. Eruptions that
are separated by any significant period of time have discernible boundary
effects that clearly separate one tuff from another. Runoff water, for
example, would erode small channels in the surface of a flow or the
chilled tops of separate flows would mark the emplacements of separate
cooling units. No evidence exists to suggest such a cooling history
in Yellowstone. Rather, the caldera venting appears to have developed
in two separate parts of the magma chamber simultaneously and been continuous
over a very short time. In a period of time reasonably inferred to be
hours, more than 240 cubic miles of Lava Creek Tuff was emplaced around
the caldera rim and within the caldera itself.
The explosions died away. A complex ecosystem
was snuffed out and replaced by a sterile, steaming moonscape where
hardly a living thing survived. The Yellowstone Plateau, the Teton Range,
and thousands of surrounding square miles
of Wyoming, Montana, and Idaho were barren and nearly lifeless for the
third time in two million years.
The caldera-forming magma chamber, however,
like our fizzed-out soda bottle, was far from empty. In fact, it may
have contained 90 percent of its original magma volume. No sooner did
the magma chamber roof collapse, than it began to rise again owing to
pressures from underlying magma. Two resurgent domes soon began to form
near the center of the elliptical caldera, one near Le Hardy Rapids
on the Yellowstone River, and another east of Old Faithful.
The rejuvenated magma chamber also sent
rhyolite to the surface where its eruption formed lava flows that buried
part of the western resurgent dome and completely buried the caldera's
western rim. Three such eruptive pulses about 150,000, 110,000, and
70,000 years ago produced about 240 cubic miles of rhyolite.
Because rhyolite lavas are rich in silica
and poor in water, they tend to be quite viscous. Instead of flowing
easily and rapidly as does Hawaiian basalt, rhyolite lava form piles
of taffy-like incandescent rock whose margins will advance so slowly
that observers will have to watch closely to see them moving.
Young rhyolite flows provide much of
central Yellowstone's beauty; its lakes, waterfalls, and stream courses.
For example, Yellowstone Lake fills a basin in the southeast part of
the 600,000 year-old caldera between the east rim of the caldera and
rhyolite flows on the west. Shoshone and Lewis lakes fill basins formed
between adjacent flows. The Upper and Lower falls of the Yellowstone
River tumble over resistant layers in caldera-filling flows. Nez Perce
Creek, from its headwaters to its junction with the Firehole River,
flows along a seam between lava flows. So does the Firehole River itself
to its junction with the Madison River. The Gibbon River is pinched
between younger flows and the Lava Creek Tuff through much of its course.
Driving west from Canyon Village you
climb the steep eastern front of the Solfatara flow, drive miles across
its rolling top, then descend its western slope to Gibbon River. Similarly,
the drive from West Thumb to Old Faithful crosses several young rhyolite
Silica, the primary constituent of rhyolite,
provides a relatively sterile soil environment that is unfriendly to
most living things. But not lodgepole pine. These hardy trees, pine
grass, and fire-weed love such inhospitable sites. Their adaptability
is why you see so many miles of boring lodgepole forest along Yellowstone
In summary, three caldera eruptions and
associated lava flows produced about 1,600 cubic miles of rhyolite in
the last 2.1 million years. This staggering figure requires rates of
magma production comparable to the most active volcanic regions on earth,
such as Iceland and Hawaii. As we shall see, the processes that produced
this enormous amount of magma also uplifted significant portions of
northwestern Wyoming, southwestern Montana, and southern Idaho.
The Caldera Today
Is Yellowstone's history of volcanic
activity at an end? Has time tamed its explosive violence, leaving only
a heritage of aging geysers and eroding lava flows? Has the magma chamber
beneath Yellowstone exhausted its supply of molten rock? Is it now incapable
of producing more lava flows or explosions? Well, let's consider these
questions; questions that have intrigued scientists ever since Yellowstone
Anyone who has seen a geyser or hot spring
immediately thinks of heat. Early geologists speculated that the heat
in geyser waters came from the cooling of young lava flows beneath the
geyser basins. They speculated
that rain and snow meltwater percolated into gravels and sands of the
basins and into the young lava flows where it was heated before rising
to the surface via geysers and hot springs. The lava flows were thought
to be young, but even the most daring geologist tucked them well back
in time. As we learned in Chapter 4, however, U. S. Geological Survey
studies that dated the lava flows found some of them to be rather young,
Given that the youngest lava flows are
only 70,000 years old, yesterday in geologic time, might not there still
be molten magma beneath Yellowstone today?
Direct methods, such as deep drilling, have not been employed to test
this possibility, but other methods suggest magma exists beneath Yellowstone.
The earth's interior is warmer than its
surface causing heat flow outward to the surface. The flow of heat in
geyser basins is hundreds of times greater than normal heat flows. If
the total conductive heat flow of major hydrothermal basins is averaged
over the 965 sq. miles of the Yellowstone Caldera, we find flow levels
that are 60 times greater than mean global rates.
Geophysical studies monitor the caldera
and its magma body indirectly. From seismic studies we learn that shock
waves from earthquakes and man-caused explosions traveling through the
earth's crust are slowed significantly as they pass beneath the caldera.
Material with a seismic velocity that is slower than normal underlies
the caldera at depths as shallow as I mile. This may indicate
local zones of molten magma in the upper crust. Near the northeast part
of the caldera, seismic velocities are even lower to within about 2
miles of the surface; this may indicate a more continuous magma body
that extends from the northeastern part of the caldera to about 10 miles
beyond it. Down below the crust and in the mantle at depths of 100 miles,
lower than normal local seismic velocities may indicate thin rising
columns of magma.
Earthquake data also suggest that soft
or molten rock is close to the surface of Yellowstone. Minor earthquakes
jiggle Yellowstone hundreds of times each year, but above the caldera
the foci of these quakes are extremely shallow, less than three miles
below the surface. These clues suggest that the material underlying
Yellowstone is still very hot and ductile, as would be expected if a
magma chamber still exists.
Gravity studies back up conclusions drawn
from seismic data. We know that gravity values across the Yellowstone
Plateau are much lower than normal, and low gravity values are associated
with low rock densities. In Yellowstone the low densities imply molten,
thermally expanded material. As you might expect, the lowest gravity
anomalies are found in the same place where seismic velocities are slowest-under
the northeast caldera rim and beyond.
Local uplift and subsidence within Yellowstone
are fast enough to be measured by surveying techniques. Benchmarks,
points of precisely measured altitude, were established along the road
systems of Yellowstone in 1923. One center of uplift on these surveys
is at Le Hardys Rapids in the central part of the Yellowstone caldera
and 3 miles down the Yellowstone River from its outlet from Yellowstone
Lake. Until 1985, these surveys showed uplift at a rate of about V^
inch a year centered on Le Hardys Rapids, with total uplift since 1923
of about 3 feet. The profile of the Yellowstone River on both sides of Le Hardys Rapids suggests this
uplift has been going on for a much longer time. Upstream from Le Hardys
Rapids, the Yellowstone River is remarkably tranquil with a low gradient,
whereas downstream it is many times steeper. Carbon dating of muds in
the drowned channel of the Yellowstone River upstream from Le Hardys
Rapids shows this overall uplift cycle had started by 3,000 years ago.
Surveys in 1986 and later show this pattern of uplift has changed to
subsidence, also at a rate of about 1/2 inch a year. We do not know
if the change after 1985 represents the start of a major interval of
subsidence or a minor reversal in a longer interval of uplift, but surveys
as recent as 1993 show subsidence.
These various investigations of hydrothermal
features, heat flow, seis-micity, earthquakes, gravity, and historic
altitude change give us an interesting picture of what underlies the
Yellowstone Plateau. These conditions are consistent with a large, partly
molten magma body at shallow depth that extends northeast of the caldera
rim. Although rocks underlying the rest of the caldera have low densities
and low seismic velocities, the variations are less extreme, so the
rocks there may be very hot but not necessarily contain much molten
Thus we see that Yellowstone's fires
are only banked, not out. Geologists don't expect another caldera explosion
any time soon, but sometime new lava flows quite likely will once again
consume lodgepole forests, and a new generation of geysers will burst
forth, perhaps in Hot Springs Basin.