By Joel Achenbach
Washington Post Staff Writer
Friday, November 9, 2007

Something is stirring deep below the legendary hot springs and geysers of Yellowstone, the first and most famous national park in America -- and home to a huge volcanic caldron.

Parts of the park have been rising the past three years at a rate never before observed by scientists. They believe that magma -- molten rock -- is filling pores in the Earth's crust and causing a large swath of Yellowstone to rise like a pie in the oven.

But that doesn't mean you should cancel any vacation plans. Scientists see no sign that Yellowstone is about to blow its top.

"There's no evidence of eruption," said Robert B. Smith, a University of Utah geophysicist and co-author of a new report on Yellowstone's unusual behavior, published today in the journal Science. The park's recent rise is "just part of the natural process."

That said, scientists are watching Yellowstone very closely. This latest glimpse of its unsettled nature offers a reminder that human-driven climate change is taking place on a planet that isn't an inert bystander.

Several volcanoes are currently rumbling in Indonesia, and one, Mount Kelud, in East Java, could be close to a major eruption. Climate scientists who try to understand global warming are trying to put volcanic eruptions into their models. Material blown high into the atmosphere by volcanoes can block sunlight and temporarily cool the planet, even though a volcano also produces prodigious amounts of greenhouse gases. In 1815, the eruption of Mount Tambora, a volcano in Indonesia, led to the famous "year without a summer," in which crops failed across the Northern Hemisphere.

Yellowstone's behavior of late doesn't match what scientists expect to see before an eruption, however. Seismic activity, for example, has actually been lower in the past three years. And there's nothing unusual happening with the hydrothermal system -- no strange geysers popping up, no weird explosions of steam, no odd gases spewing forth.

"We'd expect lots of earthquakes and deformation going hand in hand. And we're not seeing that at Yellowstone in this particular episode," Smith said.

In recent decades, Yellowstone has had its ups and downs, literally. Yellowstone rose about three feet between 1924 and 1985, then fell for a decade, then rose for a few years, then fell again, and finally in 2004 surged upward once more.

"It's truly breathing. I call it the living, breathing caldera," Smith said.

Yellowstone bears close monitoring, scientists say, because it is prone to hydrothermal explosions, volcanic eruptions (the most recent occurred 70,000 years ago) and, once in a very long while, a super-eruption, a continent-scorching explosion that makes your average volcanic event seem like a hiccup. The most recent super-eruption at Yellowstone, 640,000 years ago, launched 240 cubic miles of material into the atmosphere, burying much of the American West in a layer of hot ash. By comparison, Mount St. Helens in 1980 spewed forth less than a quarter of a cubic mile of material.

A caldera is essentially a collapsed volcano. The overlying material has been blown away or has sunk back into the emptied magma chamber. Although the cool crust of the Earth keeps the caldera's magma from reaching the surface, it heats groundwater that rises to form geysers and hot springs. The best explanation for the recent uplift, the new study concludes, is an infusion of new magma about six miles below Yellowstone's surface.

There are a couple of dozen active calderas around the world, but the last super-eruption may have been 26,000 years ago, in New Zealand. There has been speculation that the eruption of the Toba caldera on the Indonesian island of Sumatra 74,000 years ago may have caused global climate changes that led to a die-off of most human beings on the planet.

A report from the U.S. Geological Survey earlier this year tried to calibrate the volcanic hazards at Yellowstone.

"Depending on the nature and magnitude of a particular hazardous event and the particular time and season when it might occur, 70,000 to more than 100,000 persons could be affected; the most violent events could affect a broader region or even continent-wide areas," the USGS concluded.

Among the hazards are hydrothermal explosions, when steam breaks through the surface and forms a crater. In the 126 years since records have been kept at the park, 26 such explosions have been observed. But the same report argued that a super-eruption is essentially not worth worrying about. The magma chamber below Yellowstone is now "largely crystallized mush."

Yellowstone's hot spot is caused by a plume of magma rising through the Earth's mantle. Over the past 16 million years, the North American tectonic plate has been sliding to the west across the plume. The hot spot has, in turn, bubbled up ever farther to the east from a starting point in eastern Oregon. From a geological perspective, Yellowstone is not so much a permanent place on the map as it is a kind of migratory surface feature of the West.

Volcanologists will readily confess that their field is full of unknowns. The geological processes of the Earth are more chaotic than linear. Large calderas are a particular enigma, with no known major eruptions in recorded human history.

"I can't say we can predict specific events at this point," said USGS volcanologist Robert L. Christiansen. "We keep learning."

Jacob Lowenstern, the scientist in charge at Yellowstone Volcano Observatory, said that with new technology, "we're all kind of viewing the patient for the first time."