Download America`s Explosive Park

America's Explosive Park
By Larry O'Hanlon
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 Yellowstone.
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, largescale 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: supervolcanoes.
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 supereruption, 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 Yellowstone.
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.
Analysis Questions: ANSWER ON YOUR OWN PAPER AND TURN IN FOR A GRADE!!!
1. Explain several (3) of the evidences that the volcano under Yellowstone National Park (YNP) is active.
2. How does the volcano under YNP compare to Mount Saint Helens?
3. What are “hot spots” and why are they unsual?
4. Why do they believe YNP is safe for tourists….why are they pretty sure it won’t erupt?
5. List and describe two uses of technology used to study YNP.
6. Summarize how the YNP caldera was formed.
7. What are some other famous “hot spots”?
8. What have geologist discvovered with their sensors?
9. What are the two theories about the cause of the upwelling and tilted pipe under YNP?
10. What is meant by “In Earth Science it’s interesting that nature does the experiments”?