In Yellowstone, a Subterranean Volcano Exerts Its Influence

YELLOWSTONE NATIONAL PARK, Wyo. — The rolling pine forests, snowcapped mountains and crisp fall evenings here tend to make people forget the fact that the park sits atop a huge simmering underground volcano. But new geologic events have served up reminders.

By JIM ROBBINS NY Times

In a few days in July, acidic ground water dissolved parts of the unpaved trails in the Norris Geyser Basin, and the ground temperature of the trails shot up to 200 degrees from the usual maximum of 80. Park officials closed nearly half of the basin’s trails, and they remain shut.

On Aug. 21, a magnitude 4.4 earthquake shook the southern boundary of the park and startled residents. Yellowstone is one of the most seismically active places on the planet, with hundreds of shakes and shimmers throughout the year. They reach magnitude 4 usually only every other year.

In the park, such events are no great surprise. "Change is what we expect in Yellowstone," said the park geologist, Hank Heasler.

Although there is no indication that any of the changes suggest an impending eruption, even that would not be so surprising.

Over last 630,000 years, Yellowstone has experienced 29 eruptions the size of the one on Mount Pinatubo in the Philippines in 1991. The average interval here has been 20,000 years, and 70,000 have passed since its last eruption.

But the volcano, with a caldera 45 by 28 miles, has the potential for far more catastrophic explosions. The last major eruption, estimated at a magnitude 1,000 times as great as the Mount St. Helens explosion of 1980, was 627,000 years ago. The ancient blast blew up miles of mountain range, and ash from it has been uncovered in 22 Western states. It was so thick 1,000 miles away in Kansas that it was mined in the 1930’s and used to make a cleanser.

Whether the caldera erupts or not, the stew of partly molten rock 5 to 10 miles below the park exerts a powerful and constant influence.

"The whole of the Yellowstone Plateau is going up and down from the magma," averaging one and a half centimeters a year, said Dr. Robert B. Smith, a professor of geophysics at the University of Utah and a member of the Yellowstone Volcano Observatory. "It’s like a living, breathing thing."

In light of the new activity, safety is a growing concern, and officials are writing a hazard plan in case the region grows more active. The ground warming could mean that heat is increasing water pressure, a possible cause of eruptions.

In 1989, Porkchop Geyser in Norris Basin became clogged with silica. It exploded and created a 12-foot-wide crater now called Porkchop Hot Spring.

A hydrothermal explosion at Mary Bay in Yellowstone Lake some 13,000 years ago blew out a crater more than three miles across.

Serious earthquakes are always a possibility. Even though the temblor on Aug. 21 caused no damage, it was widely felt.

The largest quake recorded in the West, 7.5 on the old Richter scale, was centered just outside the park in 1959. It dislodged a huge slice of a mountain west of the park, buried 25 campers as they slept in a national forest campground and dammed the Madison River to create Quake Lake.

One question that occupies geologists is how the caldera affects fault lines and vice versa. Five major faults terminate in the molten caldera, and even far-flung events can shake the earth here. In November 2002, a magnitude 7.9 quake in Denali National Park in Alaska rippled through the region, leading to more than 500 other quakes that Dr. Smith watched simultaneously on a computer in Utah.

"The whole of Yellowstone lit up like a Christmas tree," he said. "It was exciting. I had a ranger call me and say, `I’ve called you before about earthquakes, but these are coming at us from all directions.’ "

Except for the quake two months ago, Yellowstone has had far fewer quakes in recent years. "Seismically, its been deathly quiet," Dr. Smith said. "We average a half-dozen to 20 quakes" a day. "The last two years, we see a couple a day."

The energy of the quakes has been harnessed to shed light on the volcano. A measuring method, seismic tomography, which is similar to C.T. scanning, uses the shock waves that the quakes generate to map structures deep in the earth. Figures from 12,000 quakes gave Dr. Smith a picture of the size and shape of the magma chamber.

The magma also fuels geothermal features. All the geyser basins are similar, in that they sit over porous channelized rock layers that contain water under pressure. The water seeps toward the magma zone, where it is superheated. As the water is forced back toward the surface, the pressure is relieved and volume expands, causing geysers to erupt.

Even among the steaming, hissing and bubbling landscapes here, Norris Basin stands out. Steamboat Geyser is the tallest one in the park, at 380 feet, more than twice as high as Old Faithful. Test drilling in 1929 measured water temperatures 265 feet down at 400 degrees, and drilling equipment had to be withdrawn.

"The geothermal features of Norris are equally amazing to scientists who have been here for 30 years or someone on their first visit," said Mr. Heasler, the geologist.

Each year, a disturbance at Norris alters features and muddies the water. This year, the disturbance, on July 11, was more severe than usual. Because the "plumbing" is underground, the more precise mechanics of geyser basins are not well understood, and why Norris Basin has changed so markedly and suddenly is guesswork.

"The most common hypothesis is that snowmelt wanes and the water table lowers and weight on the system decreases and, as a result, the water boils more aggressively," said Dr. Jake Lowenstern, a geologist at the United States Geological Survey in Menlo Park, Calif., who is in charge of the Yellowstone Volcano Observatory.

Many features took longer than usual to return to base line, although some have not returned. Echinus Geyser once erupted frequently, every 35 to 75 minutes. In 1998 it switched to an irregular pattern. It had been erupting every two to nine hours before this season’s disturbance, which somehow made it blow on a schedule again, every 3 hours 30 minutes to 3 hours 40 minutes. The geyser has now reverted to irregularity.

Pearl Geyser, an erupting pool named for its opalescent blue color, usually has two-meter eruptions. After the disturbance, it changed color to crystal clear, then became a steam vent and later returned to an opalescent pool with one-meter eruptions.

At the northern end of the basin, a series of vents, or fumaroles, appeared and mud pots cropped up on the trail, splattering hot acidic mud, though it later disappeared.

"Norris," Mr. Heasler said, "is showing us something, and whether we can figure it out, we’ll see."