Likely Site of Eruption at Mammoth Reassessed : Geology: New upwellings of carbon dioxide suggest that volcanic activity may be more probable west of town. But that could be 400 years from now.

It was a snowy day, what Fred Richter calls “a full Gore-Tex and goggle day,” when the U.S. Forest Service ranger stumbled across the latest geological development in this historically volcanic area.

While visiting a snow survey cabin beside Horseshoe Lake, below the Mammoth Mountain Ski Area and two miles west of town, Richter suddenly found himself unable to breathe and had to scramble outside for air.

It was not until this past summer, four years later, that scientists confirmed that Richter had fallen victim to a carbon dioxide concentration.

The odorless gas, more dense than air and collecting inside the floorless cabin, was welling up from a recent intrusion of magma, or molten rock, miles below the surface of this mountain that is popular with winter skiers and summer cyclists.

The discovery of the upwelling gas has caused scientists to re-evaluate the potential location of volcanic activity in the Mammoth area, and prompted mild official precautions, but Richter has more personal memories of the encounter.

“When I went inside . . . I suddenly found myself breathing hard,” Richter recalled last week. “I sat down and I was breathing real hard. I went back up top and outside, and in a few minutes I was breathing easy.”

After ventilating the cabin for 20 minutes, Richter went back in to retrieve some equipment he had left behind. This time he noticed that step by step as he descended a ladder, it became harder to breathe. “At the bottom, my respiration got very bad. I started to see stars,” he said.

A few others have had similar experiences as what scientists know as “degassing” from the magma slowly spreads in the vicinity. This past summer, Tim Russell, supervisor of snow-making crews at the Mammoth ski area, reported feeling lightheaded while inspecting a subterranean passage.

But the Forest Service says there have been no complaints from campers, skiers or hikers.

An extensive series of tests undertaken since July near Horseshoe Lake and on Mammoth Mountain has convinced scientists from the U.S. Geological Survey that it is time to reassess their view of the most likely sites of eventual volcanic activity nearby.

After four magnitude 6 earthquakes over three days in 1980, and a further moderately strong swarm of temblors in 1983, the scientists concluded that if an eruption were to occur it was most likely to be near the intersection of U.S. 395 and California 203 three miles east of Mammoth Lakes.

But a series of 1989 earthquakes under the southwest side of Mammoth Mountain, reflecting the new intrusion of molten rock deep below, has been followed by confirmation of the carbon dioxide rising from magma. The carbon dioxide has killed off trees in at least four areas around Mammoth.

Some radon has also been detected, and small amounts of hydrogen sulfide have long been evident at two fumaroles, or steam vents, on the mountain.

As a result, scientists are looking closer at the area above and west of the town of Mammoth Lakes and its permanent population of about 5,000. Any volcanic activity there, which would be expected to come with ample warning, would allow an easier evacuation than an eruption near the highway intersection.

There has been no volcanic eruption in the Mammoth area for about 500 years, and scientists caution that, in the words of Geological Survey hydrologist Michael L. Sorey, “it could come in the next 10 or 20 years, or in the next 300 to 400. We don’t know.”

“We have nothing that says it is about to erupt, but it has erupted every 500 or 600 years, so it’s about geologically due,” he said.

David Hill, the scientist in charge of monitoring seismic and volcanic activity around Mammoth, said, however, that another likely eventual site of an eruption is along the Inyo-Mono crater chain ranging 25 miles north from the Mammoth area to Mono Lake.

“We’re certainly looking more carefully at Mammoth Mountain now than we were,” Hill said. “I don’t feel it’s necessarily the most likely place for something to happen, but it’s one we want to watch.”

Given the population around Mammoth and the number of tourists who visit, Hill said, more measuring instruments have been concentrated there.

Mammoth Town Manager Glenn Thompson, who would be in charge in an emergency, said, “I feel much safer than I would in Los Angeles knowing that a big one (an earthquake) is coming.

“Take this in context,” he said. “We live in a beautiful place. There are no security problems. I don’t even lock my front door.”

But, he said, the Geological Survey has worked out a five-stage alert system in case a volcanic eruption seems impending, and even on the busiest ski weekend everyone could be removed a safe distance within hours, either in their own cars or in buses.

Thompson and other Mammoth officials note wryly that there was a lot of talk in the early 1980s of an eruption near the highway intersection, and a second paved road was built as an escape route from Mammoth in case such an eruption did take place. Real estate values were hurt for a while, and now it turns out scientists no longer seem to view that site as likely at all.

The dying trees, mostly lodgepole pine and red fir, have been spotted in four areas around the mountain. The largest die-off, besides Horseshoe Lake and its campground, covers 28 acres.

Scientists said the trees are not dying because of excess carbon dioxide in the air, but from concentrations of underground gas that affect their root systems. A person walking or skiing through the area would not be adversely affected, and the scientists are still trying to learn whether a camper occupying a low spot overnight, without wind, might be.

In September, Forest Service rangers and scientists took extensive depth readings and put out tents to see if carbon dioxide concentrations would rise inside them. While the authorities had been considering closing part of the campground at Horseshoe Lake, depending on these studies, Sorey said initial results showed no carbon dioxide buildup inside the tents.

“We measured a flux (outpouring) of carbon dioxide that’s coming out of the soil at a fairly rapid rate and that is clearly magmatic in origin,” Sorey said. “Normally, with the wind passing through, once it comes out, it blows away.

“There is probably no danger to people who are on the ground, with the exception of sleeping directly on the ground without a tent. . . . But it’s a little too early to say there’s no danger. I think we need to better evaluate the risk in the Horseshoe area.”

The testing also showed “very high levels of carbon dioxide in the soil” high on Mammoth Mountain, and there are large areas of the mountain, especially above the timber line, that have not yet been tested.

About two weeks ago, Sorey said, Geological Survey scientists advised the Mammoth Mountain Ski Area to buy a metering device costing about $1,600 for checking carbon dioxide concentrations in buildings and other closed areas.

A spokeswoman for the ski area, marketing supervisor Pam Murphy, said that the device will be purchased but that there should be no concern about any danger.

“We feel safer and more content than ever because the U.S. Geological Survey is so carefully monitoring the area, and Dave Hill has assured us any (volcanic) event would be detected in advance,” she said.

Mapping Mammoth Mountain

Areas of dead or dying trees on Mammoth Mountain show where gases are rising from molten rock deep below.

* BACKGROUND: Earthquakes under Mammoth Mountain in 1989 sparked a geologic process allowing carbon dioxide and radon gas to rise to the surface from a magma, or molten rock, intrusion beneath the mountain.

* WHAT HAPPENED: Trees began to die in at least four areas. In some places, carbon dioxide--a potentially dangerous gas when concentrated--collected in cabins, trenches or even hollows.

* ACTION: The U.S. Geological Survey and the U.S. Forest Service are mapping the affected areas, to provide notice of any potential danger spots. Also, although there have been no reports of campers or skiers being affected, the Mammoth Mountain Ski Area may use a carbon dioxide meter this winter as a precaution.