Gas Emissions Provide Clue to Volcanic Activity

Mammoth Mountain, a mecca for winter skiers and summer hikers in the eastern Sierras, recently has attracted the attention of a number of scientists. Part of an ancient but by no means inactive volcanic basin, the mountain has been sending out signals of resurgent activity.

Since 1989, when unusual seismic activity began being detected at Mammoth Mountain, Mack Kennedy of Berkeley Lab's Earth Sciences Division has been part of a team of scientists attempting to understand what is brewing deep beneath the surface of this recreational area. The effort is aimed not only at anticipating events at Mammoth Mountain, but at enhancing our limited ability to discern and interpret signs of unrest in volcanic areas.

Kennedy's research reveals a hitherto unknown sign of volcanic activity. Trees that have been dying on the flank of the mountain are not the victims of drought, as had been thought. Instead, they are being killed by carbon dioxide that is being emitted as a result of molten rock intruding into the mountain.

The tree kills, which involve four distinct areas totaling some 75 acres, were first reported in 1990. Over several years, foresters realized that the pattern did not match that attributable to drought. In the areas where trees were dying, all trees regardless of age or species were affected, and insect infestations were not involved.

A year ago, a team that includes Kennedy and researchers at the U.S. Geological Survey and Lawrence Livermore National Laboratory surveyed soil gases in these areas. They found carbon dioxide concentrations ranging from about one percent in healthy forest to as high as 90 percent within tree-kill areas. Where concentrations were above 30 percent, most trees were dead.

Kennedy's research indicates that the carbon dioxide soil gas in the tree-kill zones is mostly of volcanic origin. No other lethal gases were detected. Researchers believe high concentrations of carbon dioxide kill plants by inhibiting root function.

Mammoth Mountain, which is about 30 miles south of Mono Lake, is part of the Long Valley caldera that was formed about 760,000 years ago during an eruption 500 times larger than Mount St. Helens. Smaller eruptions have occurred in the area as recently as 500 years ago.

Researchers report that the total discharge of carbon dioxide from the 75 acres involved in the vegetation die-off is at least 1,200 tons per day. This is comparable to that seen during low-level eruptions of a number of volcanoes, including Kilauea, Augustine, and Mount St. Helens. Scientists say there are no signs of an imminent eruption at Mammoth Mountain, however the mountain should continue to be closely monitored.

Although researchers can't directly observe the underground flow of magma, they can deduce a great deal through the monitoring of the gases it emits. One effective tracer of fresh magmatic volatiles is a particular helium isotope, known as helium 3. Kennedy, a member of the Berkeley Center for Isotope Geochemistry, has led a helium 3 monitoring effort at Mammoth. He reports that concurrent with the increase in unusual seismic activity, helium 3 levels rose in 1989 and have remained high ever since.

Typically, the atmosphere includes five parts per million of helium. This helium consists of two isotopes, helium 3 and helium 4. While helium 4 is produced constantly in the earth's crust due to the decay of uranium and thorium, almost no new helium 3 is produced. However, primordial sources of the isotope remain, almost all buried deep in the mantle. Provided this helium 3 remained locked in the mantle, the ratio of helium in the air ought to be one hundred million parts of helium 4 to one part of helium 3. Kennedy reports that at a Mammoth Mountain fumarole, the ratio of helium 3 recently has ranged from five to 6.7 times that found in air, indicating that magma flowing up from the mantle is carrying helium 3 with it. Prior to the swarm of seismic activity that began in 1989, that ratio had averaged 3.8.

The ratio of helium 3 in the tree-kill areas is also as high as 6.7 times the air ratio and the ratio of helium 3 to carbon dioxide is identical to that in the Mammoth Mountain fumarole, providing evidence that the carbon dioxide responsible for the tree kills has a magmatic source.

In addition to indicating a resurgence in volcanic activity, Kennedy notes that monitoring, combined with mapping of high carbon dioxide in soil gas, may provide an indirect measure of the size, depth, and duration of the molten material deep inside Mammoth, and help to provide a clearer picture of what lies ahead for the mountain.