Chemical study zeros in on deep magma.

On the south side of the Long Valley caldera in California lies a patch of land that looks geologically innocuous. Unlike the Inyo craters to the northwest, which were the sites of the last Long Valley eruption 550 years ago, this southern region is not laced with obvious surface faults. And without geothermal vents steaming with gases from below, it is not the kind of region in which geochemists go hunting for chemical clues of magmatic rumblings underfoot. But results of a new study show that this is precisely the region that is shrouding underlying magma movement, making it the Long Valley site with the Greatest potential of volcanic activity.

What makes this grochemical study different from most others conducted at the Long Valley caldera--a basin-shaped volcanic depression -- is that it measured radon levels in soil gas at 600 sites spread over the entire caldera. "By sampling the whole caldera, we didn't presuppose that we knew where the area of interest would be," says Stanley Williams, who conducted the study.

Moreover, by measuring mercury as well as radon, Williams, a volcanologist at Louisiana State University in Baton Rouge, was able to distinguish areas permeated by gas flow today from those still saturated with chemicals from past gas movement: High levels of radon, an inert and very short-lived gas, should reflect ongoing geotermal convection, whereas mercury, which is reactive and can accululate on soil grains for hundreds of years, marks regions where heat from buried magma may have been rising for a long time.

Williams identified three areas with pronounced mercury levels, all of which seem, on the basis of seismic data and other considerations, to overlie large magma bodies. He also reports in the Aug. 9 SCIENCE a few regions with elevated radon concentrations. One area coincides with a mercury high over the "resurgent dome" into which magma was thought to begin to move in 1980.

But the area containing the highest radon concentration by far -- measuring more than nine times more than background values -- lies in the southern moat of this dome, a "monotonous flat piece of this dome, a "monotonous flat piece of land" devoid of any apparent geothermal activity or faults, according to Williams. Strangely, Williams found that mercury concentrations in this region were abnormally low compared with both background levels and concentrations measured there 10 years ago.

Most significantly, this radon-high and mercury-low zone is situated just where geophysicists had noted a number of seismic swarms and a high rate of deformation in 1982 and 1983 -- around the time when Williams was in the field. Two of these geophysicists subsequently postulated that a dike of magma had moved up to within 3 kilometers of the southern moat floor.

Williams believes the intrusion of the hot dike upset the caldera's geothermal system, sending steam and gases up to the southern moat. Radon, carried by the upwelling steam, makes it to the surface but for some reason the mercury does not. Williams suspects that the mercury is precipitated out as mercury sulfide when the hot plume encounters cold or strongly oxidized water near the surface.

The agreement between the geophysical data and his study, Williams believes, demonstrates that geochemistry is as important as geophysics in volcano hazard evaluation. "This shows that the radon and mercury, chemicals I'm measuring at the very surface of the earth, are responding not to very shallow phenomena--the hydrology of the caldera or local surface faulting -- but they're apparently relfecting deep-seated events," he says.