Betreff: Yellowstone's SuperVolcano
Datum: Tue, 12 Apr 2005 23:04:10 -0700

Airtime: Saturday, April 16, 2005

Could Yellowstone really erupt? Hear from the experts.


By Larry O'Hanlon
The Discovery Channel

Yellowstone National Park sits atop a subterranean chamber of molten rock
and gasses so vast that the region, known for its geysers and grizzlies, is
arguably one of the largest active volcanoes in the world.

Granted, it's not your typical volcano, either in scale (it's huge),
appearance (it's a vast depression, not a single mountain) or frequency of
eruption (at least hundreds of thousands of years apart).

But it is active, and the evidence is everywhere.

A relatively close-to-the-surface magma chamber ‹ as close as 5 miles
underground in some spots ‹ fuels thousands of spewing geysers, hissing
steam vents, gurgling mud pots and steaming hot springs that help make
Yellowstone such an otherworldly and popular tourist attraction, with 3
million summer visitors.

Molten rock and gas in a chamber near the Earth's surface is similarly
present below "traditional" cone-shaped active volcanoes, like Mount St.
Helens in Washington state.

But there are differences. Huge differences.

The crater atop Mount St. Helens is about 2 square miles. The Yellowstone
"caldera" -- a depression in the Earth equivalent to a crater top -- is some
1,500 square miles.

The 1980 Mount St. Helens eruption blew 1,300 vertical feet off the
mountain, sent an eruption column 80,000 feet high in 15 minutes, ejected
1.4 billion cubic yards of ash detectable over 22,000 square miles, and
killed 57 people.

But the last major eruption at Yellowstone, some 640,000 years ago, ejected
8,000 times the ash and lava of Mount St. Helens.

And that wasn't even the largest eruption in Yellowstone's prehistoric past.

"Yellowstone is much larger than any other volcanic feature in North
America," says geophysicist Bob Smith of the Yellowstone Volcano Observatory
and the University of Utah. "People don't realize this."

While many visitors to Yellowstone may be oblivious to the science at work,
geologists and volcano experts have long known about the region's explosive
prehistoric past.

An explorer identified the massive caldera in 1871. But satellite-based
global positioning systems (GPS), gravity mapping and a seismic network are
all helping scientists isolate more details of the area. And other new
information is being uncovered all the time.

In fact, the Yellowstone caldera is the place where Smith and other
geophysicists are beginning to finally pull aside the curtain that's been
hiding one of geology's most stubborn secrets: the strange workings of
Earth's "hot spots."

These "hot spots" are areas of volcanic activity not found in the usual
location, at the edges of Earth's tectonic plates. Why they exist is a
subject of scientific debate. And there are many around the world, not just

But at this hot spot's current position under Yellowstone there have been
three massive eruptions: 2.1 million, 1.3 million and 640,000 years ago.
While those eruptions have been spaced roughly 800,000 and 660,000 years
apart, three events are not enough statistically to declare this an eruption
pattern, explains Smith.

Though Yellowstone could erupt again someday, there is no evidence that the
caldera is readying for another massive blast, says Smith. That outlook is
shared by Jake Lowenstern, the U.S. Geological Survey's lead geologist at
the Yellowstone Volcano Observatory.

Volcanologists with the U.S. Geological Survey believe that supervolcanoes
are likely to give decades -- even centuries -- of warning signs before they
erupt. The scientists think those signs would include lots of earthquakes,
massive bulging of the land, an increase in small eruptions, "swarms" of
earthquakes in specific areas, changes in the chemical composition of lavas
from smaller eruptions, changes in gasses escaping the ground and, possibly,
large-scale cracking of the land.

None of those indicators are present at Yellowstone, says Smith.

There is no argument that a major eruption at Yellowstone in modern times
would be devastating. It would obliterate the national park and nearby
communities, spread ground-glass-like volcanic ash from the Pacific coast to
the Midwest, and cause worldwide weather changes from the airborne dust and
gases, according to Smith, who described the potential effects in detail in
his book Windows Into the Earth, published in 2000.

A modern full-force Yellowstone eruption could kill millions, directly and
indirectly, and would make every volcano in recorded human history look
minor by comparison. Fortunately, "super-eruptions" from supervolcanoes have
occurred on a geologic time scale so vast that a study by the Geological
Society of London declared an eruption on the magnitude of Yellowstone's
biggest (the Huckleberry Ridge eruption 2.1 million years ago) occurs
somewhere on the planet only about once every million years.

But there are several levels of eruptions smaller than Huckleberry Ridge and
yet still much larger and more destructive than any volcano ever witnessed
by modern man.

One way of looking at the power of volcanoes is what scientists call the
Volcano Explosivity Index (VEI) -- sort of a Richter scale for eruptions.
And like the Richter scale used to measure earthquakes, the power of an
eruption increases exponentially from number to number in the VEI index.

The VEI scale runs from zero to eight. The higher the VEI number, the bigger
-- and less frequent -- the eruptions. On one end there are the burbling,
rather gentle eruptions that happen on the big island of Hawaii. These
happen daily on Earth, and even with their occasional impressive fountains
of lava, they rate a zero on the VEI.

At the other extreme is the Yellowstone eruption of 2.1 million years ago,
which is described on the VEI as an eight: mega-colossal, with a towering
ash cloud 10 miles high that pours out at least a thousand cubic miles of
ash. That Yellowstone eruption had 10 times the ejected material as a VEI 7
volcano, which modern humans have never seen either.

In fact, the last VEI 7 eruption was in Toba, Indonesia, 74,000 years ago,
and it caused such global cooling that some scientists think it nearly drove
humans to extinction.

The largest known eruption in the last several thousand years is believed to
be that of Tambora, Indonesia, in 1815. It was tens of times more massive an
eruption than Mount St. Helens in 1980. Despite pouring out 7 cubic miles of
ash and causing short-term global cooling, Tambora was small fry compared
with any of Yellowstone's big eruptions, or even the eruption of Toba 74,000
years ago.

No eruptions of this magnitude have happened since the dawn of civilization,
about 10,000 years ago -- which is lucky for us, and perhaps one reason
civilization has been able to develop.

As with Yellowstone, none of the other caldera-based supervolcanoes around
the globe fit the classic volcano image.

Calderas are broad, sunken areas often filled with lakes, ringed with hot
springs and landscaped with domes of lava. They are something like
cauldrons, after which they were named, and tend to be the largest volcanoes
on the planet -- hence the less formal name they are sometimes given:

Like other calderas worldwide, the Yellowstone caldera landscape was created
by the "roof collapse" on a subterranean chamber after molten rock -- called
magma -- was ejected in massive prehistoric eruptions. It's almost as if
there was a giant magma balloon under the surface that suddenly deflated.
The deflation itself is the super-eruption, and the sunken land left behind
is the caldera.

Also, as with many other calderas, there is still hot material not far
underground at Yellowstone, which is why there are so many hot springs and
geysers today.

As for what caused the land to inflate with magma and explode in the first
place, it was a powerful "hot spot" welling up from deep in the Earth and
melting rock closer to the surface into magma, says Smith.

But what caused the hot spot? And what can explain it today? Scientists are
still learning answers to these questions.

"A hot spot is a long-lived point spot of magmatism," explains geologist
Paul Ihinger of the University of Wisconsin at Eau Claire. Among the most
famous places made by hot spots are the Hawaiian Islands, Iceland and, of
course, Yellowstone.

In the case of Yellowstone, the hot spot has left a 350-mile trail marked by
several generations of ever-older and deader calderas marching away to the
southwest of Yellowstone.

The oldest is a 15- to 16.5-million-year-old dead caldera straddling the
Oregon-Nevada state line near McDermitt, Nev. The trail of dead calderas is
evidence that the hot spot has remained in place while the North American
continent has moved southwest over it.

Yellowstone gives geologists an opportunity to look inside the Earth at an
active hot spot.

Taking advantage of the park's size and location on dry land, Smith and
other geophysicists have built a network of sensors that pick up the seismic
waves from earthquakes as they pass under and are altered by the structures
beneath Yellowstone.

The seismic network has recently revealed a 3,600-cubic-mile banana-shaped
body located a half-mile to several miles underneath part of Yellowstone.
This chamber slows down seismic waves, which means it could contain 10 to 20
percent melted rock, Smith says.

And a broader seismic array has just recently revealed an even deeper
feature: a tilted pipe rising up through the Earth from the northwest, from
400 miles down.

"It's this pipe that's bending over in the wind," says Ihinger. "That is at
the heart of the matter." The "pipe" appears to be the track of a slow,
viscous upwelling of hot rock from below the crust. The upwelling could
cause rocks in the crust to melt, creating the magma chamber below

But what causes the upwelling, and why is the pipe tilted under Yellowstone?
There are currently two schools of thought, says Smith. One argues that
something happens down at the boundary between the Earth's core and the
mantle to create a narrow, upwelling plume of hot material.

"One of the main controversies is the plume idea," says geophysicist Eugene
Humphreys of the University of Oregon. "In the last 10 years, it's been
under quite a bit of attack."

The second school of thought says the upwelling is caused by spreading of
the Earth's crust.

Scientists know that a good portion of the western United States is
spreading wider, at a rate of at least a couple of centimeters every year.
The Yellowstone Volcano Observatory's GPS network has revealed a spreading
rate at the park that's even faster, says Smith.

So much extending of the land means something has got to give somewhere. The
crust gets thinner and weaker in some places. When that happens, rocks
deeper down get a load lifted off them and decompress -- which is another
way rocks can shift from solid to a liquid, magma phase, says Ihinger. "And
when it becomes liquid, it has to come up," he says.

Most geologists studying Yellowstone aren't convinced either scenario alone
describes what is happening under Yellowstone. It may be, for instance, that
the hot spot is caused by melting of the long-lost Farallon Plate, which was
driven under North America's western edge 80 million years ago. Perhaps that
has something to do with the odd tilt to the "pipe" under Yellowstone,
Ihinger suggests.

"Often times we're stuck in this complicated middle ground," said Humphreys.
It's the nature of a science where it's hard enough to observe what's
happening, much less conduct experiments. "In Earth science it's interesting
that nature does the experiments."

And that's exactly what makes Yellowstone one of the best laboratories in
the world.


Larry O'Hanlon is a science writer, geologist and former National Park
Service ranger based in New Mexico. Besides being a long-time science
correspondent for Discovery News, Larry has reported on Earth science
research for numerous outlets and organizations, including Nature, New
Scientist and the Geological Society of America. He also currently serves as
the science writer for the Explora Science Center in Albuquerque.