Published: August 2009
When Yellowstone Explodes
By Joel Achenbach




No supervolcano has erupted in recorded human history, but geologists have pieced together what an explosion must have been like. First, a plume of heat wells up from deep within the planet and melts rock just beneath the crust of the Earth, creating a vast chamber filled with a pressurized mix of magma, semisolid rock, and dissolved water vapor, carbon dioxide, and other gases. As additional magma accumulates in the chamber over thousands of years, the land above begins to dome upward by inches. Fractures open along the dome's edges, as if burglars were sawing a hole from beneath a wooden floor. When the pressure in the magma chamber is released through the fractures, the dissolved gases suddenly explode in a massive, runaway reaction. It's like "opening the Coke bottle after you've shaken it," says Bob Christiansen, a U.S. Geological Survey scientist who pioneered research on the Yellowstone volcano in the 1960s. With the magma chamber emptied, the surface collapses. The entire domed region simply falls into the planet, as though the Earth were consuming itself. Left behind is a giant caldera, from the Spanish word for "cauldron."

The "hot spot" responsible for the Yellowstone caldera has erupted dozens of times in the past, going back some 18 million years. Since the hot spot is rooted deep in the Earth, and the tectonic plate above it is moving southwest, ghostly calderas from the more ancient explosions are strung out like a series of gigantic beads across southern Idaho and into Oregon and Nevada, the subsequent lava flows forming the eerie moonscapes of the Snake River Plain.


Each time, the whole planet would have felt the effects. Gases rising high into the stratosphere would have mixed with water vapor to create a thin haze of sulfate aerosols that dimmed sunlight, potentially plunging the Earth into years of "volcanic winter." According to some researchers, the DNA of our own species may pay witness to such a catastrophe around 74,000 years ago, when a supervolcano called Toba erupted in Indonesia. The ensuing volcanic winter may have contributed to a period of global cooling that reduced the entire human population to a few thousand individuals—a close shave for the human race.

For all their violence, the supervolcanoes have left little behind beyond a faintly perceptible sense of absence. The Yellowstone caldera has been eroded, filled in with lava flows and ash from smaller eruptions (the most recent was 70,000 years ago) and smoothed by glaciers. Peaceful forests cover any lingering scars. The combined effect makes it almost impossible to detect, unless you've got a good eye, like Doane had, or a geologist whispering in your ear.

"You're seeing two-thirds of the entire caldera," says Bob Smith. "The size is so immense that people don't appreciate it." Smith is a University of Utah geophysicist and a prominent expert on the supervolcano at Yellowstone. We're standing atop Lake Butte, an overlook at the east end of Yellowstone Lake, one of the best places to see the caldera. But I don't see it. I can see the lake spread out for miles beneath us and a few little hills to the north—old lava domes. But I can't follow the caldera rim visually because much of it is beneath the lake and because of the sheer scale of the thing—roughly 45 miles across. Like Doane atop Mount Washburn, I see only distant mountains on the hori­zon on either side and between them, to the west, the "unmountains," the emptiness where the land swallowed itself in the course of a few days.

The effects of the past eruptions are nevertheless profoundly felt in the present. The lodgepole pines that dominate the park's forests are adapted to growing in nutrient-poor soils, like those in the Yellowstone caldera. So too are the white­bark pines, whose nuts sustain grizzlies and black bears.







In the years since, Smith and his colleagues have used every trick they can devise to "see" beneath the park. Gradually, the proportions and potential of the subterranean volcanic system have emerged. At the shallowest level, surface water percolates several miles into the crust, is heated, and boils back up, supplying the geysers and fumaroles. About five to seven miles deep is the top of the magma chamber, a reservoir of partially melted rock roughly 30 miles wide. Basaltic magma is trapped inside the chamber by denser, overlying rhyolitic magma, which floats on top of the liquid basalt like cream on milk. By looking at the way sound waves created by earthquakes propagate through subsurface rock of varying densities, the scientists have discovered that the magma chamber is fed by a gigantic plume of hot rock, rising from the Earth's upper mantle, tilted downward to the northwest by 60 degrees, its base per­haps 400 miles below the surface. When the plume pumps more heat into the chamber, the land heaves upward. Small earthquakes allow hydro­thermal fluids to escape to the surface, easing the pressure inside the chamber, which causes the ground to subside again. After the 1985 earthquake swarm, Yellowstone fell eight inches over the course of a decade or so. Then it rose again, faster this time. Since 2004, portions of the caldera have surged upward at a rate of nearly three inches a year, much faster than any uplift since close observations began in the 1970s. The surface continues to rise despite an 11-day earthquake swarm that began late in 2008, causing a flurry of apocalyptic rumors on the Internet.

"We call this a caldera at unrest," Smith says. "The net effect over many cycles is to finally get enough magma to erupt. And we don't know what those cycles are."

So, the colossal question: Is it going to blow again? Some kind of eruption—perhaps a modest one like Mount Pinatubo's in the Philippines, which killed 800 people in 1991—is highly likely at some point. The odds of a full, caldera-forming eruption—a cataclysm that could kill untold thousands of people and plunge the Earth into a volcanic winter—are anyone's guess; it could happen in our lifetimes, or 100,000 years or more from now, or perhaps never. Bob Christian­sen, now retired, suspects the supervolcano may be safely bottled up. For most of its history, the Yellowstone hot spot has formed calderas in the thin crust of the Basin and Range area of the American West. Now the hot spot is lodged beneath a much thicker crust at the crest of the Rockies.

"I think that the system has more or less equilibrated itself," says Christian­sen. Then he quickly adds, "But that's an interpretation that would not stand up in court."