Eyeing a Sleeping Giant : Scientists are pursuing tantalizing--but inconclusive--clues to volcanic activity beneath Mammoth Mountain by monitoring seeping gases. : SCIENCE FILE: An exploration of issues and trends affecting science, medicine and the environment.

Since four magnitude 6 earthquakes occurred in a two-day period in the Long Valley caldera bordering Mammoth Mountain in the spring of 1980, there has been concern that future quake swarms might signal an eruption in this volcanically active region.

For that reason, geologists have been monitoring the area carefully, checking both seismicity and ground deformation with an array of recording instruments. Since 1989, when a series of deep, long-period quakes rumbled beneath the 11,053-foot mountain itself, attention has refocused from the area east of Mammoth Lakes to the mountain west of town.

Last year, scientists’ interest intensified when they confirmed that volcanic gases, particularly carbon dioxide, were spewing through the ground, and trees were dying on three sides of the mountain. Now, as the tree kills spread gradually, monitors of the U.S. Geological Survey are engaged in exhaustive tests to better understand the emissions, which they believe rise from underlying magma, or molten rock.

Some samplings taken in the past few weeks show as high as a 96% level of carbon dioxide in soil gases at depths of one meter or less, according to Chris D. Farrar of the U.S. Geological Survey, the scientist in charge of the testing. The toxic level for trees is 30% and soil readings normally are only in the tenths of 1%.

In an article appearing today in the journal Nature, several of the scientists involved in the tests say the ages of the dead trees show that there has not been a similar carbon dioxide emission over the past few hundred years, and possibly not since the last volcanic eruption in this area, 200 to 500 years ago.

Farrar and his colleagues estimate at least 1,200 tons of carbon dioxide are being released each day, about equal to the release at such active volcanoes as Hawaii’s Kilauea.

Unlike Mammoth, however, most active volcanoes have open crater vents to facilitate gas release, and the scientists also observe that Mammoth Mountain “seems unique among volcanoes exhibiting signs of magmatic unrest” in that there is no evidence of accompanying discharges of heat and, for the moment, little seismicity or deformation.

“An active magma source is sending out signals from deep beneath Mammoth Mountain, and we must give it much more attention,” concludes Stanley N. Williams, a volcanologist at Arizona State University, in an essay accompanying the Nature article. “Volcanic gases are useful as signals of reactivation of volcanoes because they are so mobile,” Williams adds.

“The gases are waving flags at scientists and, more importantly, the people who live or work near the activity.”

Yet the scientists who wrote the main Nature article are reluctant to reach conclusions about the likelihood of a new eruption soon, and even Williams stops short of predicting one. It is uncertain whether the magma intrusion of 1989, giving rise to the gases, continues today, and that is one reason why researchers want to find out whether the rate of emissions is rising. If it is, it makes a continuing intrusion seem more likely.

In his essay, Williams writes, “Events at Mammoth Mountain remind us that calderas such as Long Valley are not only big but complex, and that we don’t understand how the voluminous eruptions that cause them actually begin.”

Williams notes that the eruption that formed the caldera 760,000 years ago was 500 times larger than the 1980 eruption that blew the top off Mt. St. Helens.

Last week, Farrar and three colleagues collected and analyzed soil gas samples in a grid they traced at Horseshoe Lake, the scene of the biggest tree kill. Later, continuous automatic monitoring will be instituted.

Tests are being undertaken by NASA and the U.S. Department of Energy’s National Laboratories at Berkeley and Livermore.

One goal is to try to determine what the gas emissions tell about future volcanic prospects. Infrared pictures have also been taken to see if there are any hot spots showing up on the surface, which might indicate the presence of molten rock lurking in the ground; gamma ray detectors may soon be used to try to ascertain the exact date of past volcanic events, showing what frequency might be inferred for the future.

Whatever the prognosis, it is clear from the stark scenes at the foot of Mammoth Mountain that trees are dying at an alarming rate. Biologist George Koch from Northern Arizona University said he believes carbon dioxide from the fumes is displacing a lot of normal soil oxygen, damaging “the plant’s ability to absorb water and nutrients through its root system.”

Farrar noted that carbon dioxide can be toxic to humans beginning at a 3% concentration level, far below the toxic level for trees. But he said there is no danger to either human or plant life in the open air in the vicinity of the emissions.

However, he added, it is important to avoid closed places that may harbor the heavier-than-air gas, such as restrooms, cabins and utility vaults.

Farrar said he and USGS colleague Jim F. Howle noticed a peculiar smell and taste and a burning sensation in their eyes recently while digging in a six-foot pit on the north side of the mountain.

“We couldn’t finish the work,” Farrar said. “This could have been life-threatening in tens of seconds.”

Yet, he emphasized, all told, “I don’t think the evidence clearly points one way or another” as to whether an eruption will occur in the near future.

Last June, acting on the advice of the scientists, the U.S. Forest Service decided to close the Horseshoe Lake campground for this season to avoid any possible carbon dioxide danger to campers.

Last week, citing samplings of the gas level in closed tents placed on the site that showed no unhealthful surface concentrations, Forest Service ranger Fred Richter expressed hope that the campground could be reopened over the Labor Day weekend. However, economic factors later brought a decision to delay opening until next year.

Separate small tree kills east of Mammoth Lakes, near the intersection of U.S. 395 and California 203, may be related to a geothermal power plant there and do not reflect gas emissions in that area, according to Geological Survey scientists.

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Mountain of Trouble

Scientist are focusing attention on the western edge of the Long Valley Caldera at Mammoth Mountain where carbon dioxide emissions and tree kills are related to underlying volcanism.