America's Explosive Park

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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."

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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, 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.

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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 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.

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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.


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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.

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YELLOWSTONE'S SUPER SISTERS
By Larry O'Hanlon


There are other supervolcanoes on Earth, some of which erupted in prehistoric times and could erupt again. At least one has had an eruption bigger than Yellowstone's largest and may have played a critical role in shaping human history.

The history — even the location — of others is less certain. Some are identifiable only by the deep layers of ash they left behind, such as the more than 1,000 cubic miles of tuff dumped in eastern Africa and the Red Sea by a mystery eruption somewhere in Ethiopia.

There are also calderas that have just not been well studied, a prime example being Ethiopia's 460-square-mile Awasa caldera and the 1,000-square-mile Pastos Grandes caldera of Bolivia, which rivals the latter, which in turn rivals the largest in the world: Lake Toba, Indonesia.

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LAKE TOBA, SUMATRA, INDONESIA
By Larry O'Hanlon


The 1,080-square-mile Toba caldera is the only supervolcano in existence that can be described as Yellowstone's "big" sister.

About 74,000 years ago, Toba erupted and ejected almost three times as much volcanic ash as the most recent major Yellowstone eruption (Lava Creek, 630,000 years ago) and about 12 percent more than Yellowstone's largest eruption (Huckleberry Ridge, 1.8 million years ago). That comes to several thousand times more material than erupted from Mount St. Helens in 1980.

Some researchers suspect that Toba's super eruption and the global cold spell it triggered might explain a mystery in the human genome. Our genes suggest we all come from a few thousand people just tens of thousands of years ago, instead of from a much older, bigger lineage — as the fossil evidence testifies. Both could be true if only a few small groups of humans survived the cold years following the Toba eruption.

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LONG VALLEY, CALIFORNIA
By Larry O'Hanlon


Second only to Yellowstone in North America is the Long Valley caldera, in east-central California.

The 200-square-mile caldera is just south of Mono Lake, near the Nevada state line. The biggest eruption from Long Valley was 760,000 years ago, which unleashed 2,000 to 3,000 times as much lava and ash as Mount St. Helens, after which the caldera floor dropped about a mile, according to the U.S. Geological Survey.

Some of the ash reached as far east as Nebraska.

Long Valley's most recent eruption was in Mono Lake just 250 years ago, but it was very small. More worrisome is a swarm of strong earthquakes in 1980 and the 10-inch rise of about 100 square miles of caldera floor. Those developments have geologists concerned that Long Valley is gearing up for another eruption of some sort.

In the early 1990s yet another subtle sign of trouble became evident: Large amounts of carbon dioxide gas from magma below had begun seeping up through the ground and killing trees in the Mammoth Mountain part of the caldera. When these sorts of signs are present at a "central vent" volcano like Mount St. Helens, trouble is on the way soon. At a caldera, which has many outlets, it could mean trouble is years, decades or even centuries away, say volcanologists.

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LAKE TAUPO, NEW ZEALAND
By Larry O'Hanlon


New Zealand's Taupo caldera has been filled by water, creating what many describe as one of the world's most beautiful landscapes.

Lake Taupo itself was created by a massive eruption 26,500 years ago. The caldera — the collapsed and subsided basin left after the huge eruption — became today's lake.

But Taupo did not die. The 485-square-mile caldera let loose again in the year A.D. 181, with estimates of ash and magma reaching as high as 22 cubic miles. That's on the order of a hundred times more than Mount St. Helens.

Today Lake Taupo still shows signs of life, which New Zealanders have put to good use. Ample hot springs and other hydrothermal activity enable New Zealand to generate about 8 percent of its electricity at a geothermal plant on the north side of Lake Taupo, at Wairakei.

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VALLES CALDERA, NEW MEXICO
By Larry O'Hanlon


There is a sleeping monster in the heart of New Mexico.
The 175-square-mile Valles caldera forms a large pock in the middle of northern New Mexico, west of Santa Fe. It last exploded 1.2 million and 1.6 million years ago, piling up 150 cubic miles of rock and blasting ash as far away as Iowa. That's equivalent to roughly 2,000 Mount St. Helens eruptions.

Around the fractured ring of Valles caldera, lava flows from after the major eruptions built up mountains and left obsidian flows as recently as 60,000 years ago. As with other calderas, there are still signs of heat below: hot springs are still active around Valles.

Geologists suspect the cause of Valles caldera has something to do with how the western United States' portion of the North American tectonic plate is being pulled apart. Will Valles erupt again? No one knows.

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AIRA, JAPAN
By Larry O'Hanlon


One of the most recently troubling calderas in the world is the 150-square-mile Aira caldera in southern Japan, on the edge of which sits the city of Kagoshima.

After a century of peace, the Sakura-jima volcano, which forms part of the Aira caldera, awoke on Jan. 10, 1914, and gave local residents two days' notice of its intentions by letting loose hundreds of earthquakes.

On Jan. 12, after 23,000 people and their farm animals living on its flanks were evacuated, Sakura-jima erupted with ash, steam and lava. It was not really a super eruption, but it taught people a lot about how volcanoes erupt.

There was another eruption in 1946, and since 1955 Sakura-jima has had hundreds of small eruptions every year. The biggest eruptions, however, took place 22,000 years ago when 14 cubic miles of material burped out of the ground and formed the Aira caldera, which is now largely Kagoshima Bay. That is equal to about 50 Mount St. Helens eruptions.

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SIBERIAN TRAPS
By Larry O'Hanlon


While a worst-case-scenario supervolcano eruption sometime in the future would be catastrophic for large parts of the world, that destruction would be minor compared with what scientists believe could be the largest lava flow in Earth's history: the Siberian Traps of 251 million years ago.
The gigantic lava flow in Siberia lasted upward of a million years and flooded an area about the size of the lower 48 United States with layer upon layer of dark basalt lava — thousands of feet thick.


Some geologists suspect the eruption was caused by an extra-large plume of hot material welling up from the edge of the Earth's core. But what makes it especially important is that the Siberian Traps is the prime suspect in wiping out 90 percent of all living species 251 million years ago — the most severe extinction event in Earth's history.


"This is the numero uno candidate for a mass extinction caused by volcanism," said paleontologist Spencer Lucas, a curator at the New Mexico Museum of Natural History. "There's good reason to believe it had something to do with that extinction."


Not all scientists agree that the Siberian Traps were the main reason for the mass extinctions. But the timing of the Siberian eruption is perfect, for those looking for a culprit: It crosses a boundary in geologic time that marks the great die-off at the end of the Permian period and the beginning of the Triassic period (the P-T extinction).

And unlike the more famous dinosaur-killing Cretaceous-Tertiary mass extinction (aka the K-T extinction) 65 million years ago, the Permo-Triassic extinctions have not been linked to giant asteroids.


Since the Siberian flood basalts, as they are called, poured out during the time of the mass extinction, it's reasonable to think they might have played a role, said paleontologist Gerta Keller of Princeton University.


There are recent examples, after all, of the global impact of even relatively minor volcanic eruptions. The 1991 eruption of Mount Pinatubo in the Philippines, for example, sent millions of tons of sulfur dioxide into the atmosphere. One of the largest volcanic eruptions of the 20th century, it caused a recordable drop in global temperatures (a few tenths of a degree) for several years. And it was at least thousands of times smaller than the Siberian Traps eruption.

There is even reason to believe that an eruption of the largest super volcano in the recent history of the Earth — the Toba caldera on Sumatra, Indonesia — caused enough climate change to almost wipe out humanity 74,000 years ago.


As for exactly how the Siberian eruption could wipe out most life worldwide, it's probably not simply by burying the Earth in lava or ash, says Lucas. Instead, it was likely a complicated series of events, involving dust, volcanic gases and how they conspired to wreak havoc on the global climate — perhaps even causing the oceans to become oxygen deprived ("anoxic").


"I still think that right now greenhouse warming and anoxia is the strongest interpretation" for why most plant and animal species died across the globe, said Keller.

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JUPITER'S IO
By Larry O'Hanlon


Because Earth's last supervolcano erupted more than 70,000 years ago, scientists studying the phenomenon rely largely on studying geological evidence of past eruptions, or monitoring seismic and other activity underground.

But they do have another option: looking to Jupiter.

More specifically, looking to Jupiter's moon, Io, the most volcanic place in the solar system.

Chief volcano of Io's fiery fiefdom is a Connecticut-sized volcano called Loki. When scientists got a fresh look at Loki with the Galileo spacecraft on Thanksgiving Day 2002, they found a 125-mile-wide crater brimming with molten material, a lava expanse as wide as Lake Michigan.

"That one volcano is putting out as much heat as the entire planet Earth," said planetary astronomer John Spencer of the Southwest Research Institute.

Galileo's imager also caught many other volcanoes, including another monstrous eruption whose heat overwhelmed the instrument's sensor. It was dubbed Tvashtar, with a mile-high curtain of 1,800-degree-F lava gushing out of a 15-mile fissure down its center.

Volcanologists studying the science of "super eruptions" can learn a lot from Io, says Spencer. He explains that the ongoing eruptions on Io are somewhat like "super volcano" eruptions that have happened in Earth's past — like the 15 million-year-old Columbia River Flood Basalts in the Pacific Northwest, or the more recent (640,000-year-old) Yellowstone eruptions — only more extreme.

The reason for Io's endless eruptions is simple: Jupiter and the other jovian satellites won't let it rest. Io orbits closest to Jupiter and feels the tug of the outer satellites as well. Their vying gravities are constantly kneading the moon-sized world with tidal forces, keeping its innards roiling and hot.

"Humans have never witnessed Columbia River Basalts or Yellowstone eruptions," said Spencer. "Io has them frequently, so we can see them."

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These views of Jupiter's moon Io in the eclipse of the large planet's shadow are color coded so that blue to yellow to red represents increasing brightness. The bright spots indicate the locations of volcanic vents on Io, which are spewing hot lava.


This image and other data from NASA's Galileo spacecraft indicate that the lava at Pillan Patera (the brightest spot) exceeded 1,700 degrees Kelvin (2,600 degrees Fahrenheit) and may have reached 2,000 degrees Kelvin (3,140 degrees Fahrenheit). The hottest eruptions on Earth today reach temperatures of about 1,500 Kelvin (2,240 degrees Fahrenheit), but hotter lava erupted billions of years ago.

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This pair of images taken by NASA's Galileo spacecraft captures a dynamic eruption at Tvashtar Catena, a chain of volcanic bowls on Jupiter's moon Io. They show a change in the location of hot lava over a period of a few months in 1999 and early 2000.The image on the left uses data obtained on Nov. 26 and July 3, 1999, at resolutions of 183 meters (600 feet) and 1.3 kilometers (0.8 miles) per pixel, respectively. The red and yellow lava flow itself is an illustration based upon imaging data. The image on the right is a composite using a five-color observation made on Feb. 22, 2000, at 315 meters (1030 feet) per pixel.