Long Valley Caldera is on the rise, as it has been since 1980, and a new study confirms that it is magma, not water, that is forcing it upward. This finding reinforces scientists' concern that a volcanic eruption could occur in the region some time in the future.
The caldera is located on the east side of the Sierra, with Mammoth Mountain perched on its western rim and the town of Mammoth Lakes inside the caldera itself.
The new study, reported Dec. 8 at the American Geophysical Union Meeting in San Francisco, compared the strength of gravity measured in the caldera in 1983 with new measurements obtained in 1998. The study shows a mass gain beneath the dome in the middle of the caldera -- the so-called resurgent dome -- probably from intrusion of magma into the shallow subsurface during the intervening 15 years.
The gravity study was conducted by Maurizio Battaglia, doctoral student in geophysics; Paul Segall, professor of geophysics, both of Stanford; and Carter Roberts of the U.S. Geological Survey in Menlo Park, Calif.
The results add credence to the prevailing hypothesis that a magmatic intrusion is at the root of the Long Valley Caldera's recent and ongoing episode of unrest.
"We've had independent evidence for a magmatic intrusion under the Long Valley Caldera -- uplift of the resurgent dome, long-period earthquakes, CO2 outgassing and seismic tomography studies," Segall says. "The gravity data confirm that the magma is there."
Long Valley Caldera is a 17-by-32 kilometer hole in the ground, created 750,000 years ago by a volcanic eruption of such gargantuan proportions that it blew 500 cubic kilometers of rock into the air (compared to 1 cubic kilometer discharged from Mount Saint Helens) and sent airborne ash all the way to Nebraska. The caldera spewed out a few subsequent, milder eruptions -- the last one 500 years ago -- then settled down for a long nap that lasted until 1980.
That year the caldera re-awoke with a series of earthquakes -- four events of magnitude 6 -- and smaller temblors have been occurring there ever since on a daily basis. Since 1980 the resurgent dome in the middle of the caldera has risen by about 70 centimeters, or a little over 2 feet, and in recent years trees on Mammoth Mountain have begun to die from heightened fluxes of carbon dioxide gas.
Is the recent activity anything to worry about?
That depends, in part, on what is making the caldera rise, Battaglia says. If hot water circulating underground is the cause, as in the case of the Yellowstone Caldera, then it is likely that more hot springs and geysers will develop, but probably not an eruption. If a shallow magma body is the cause, as was the case at Rabaul Caldera in Papua New Guinea -- which blew its top in 1994 -- an eruption is more likely.
So Battaglia and Segall set out to distinguish magma from water, with a series of very precise gravity measurements taken at about 15 sites in Long Valley Caldera. They reoccupied sites where gravity was measured in 1983 and remeasured it in 1998. Some of the gravity stations are along Highway 395, the scenic "eastside highway" that transects the caldera from rim to rim.
Battaglia adjusted the 1983 and 1998 gravity measurements to account for changes in the elevation of the land surface and the water table. The adjusted, or "residual," gravity shows changes in mass beneath the caldera. And the scientists saw a systematic increase in mass over the 15 years between measurements in the area beneath the resurgent dome.
Gravity may seem like a constant commodity, but on a dynamic planet like Earth, it's always changing -- not enough to make a person fall down, but enough to measure with precise instruments. When magma moves through a volcano, or when rocks are forcibly relocated by faulting, the shifting of those masses creates changes in the strength of the gravity field in the affected area.
So the scientists think the mass increase in Long Valley Caldera reflects the addition of magma in the area beneath the resurgent dome, precisely under the area where ground uplift has been greatest.
"The gravity results indicate that magma injection accompanied, and probably caused, the ground uplift," Battaglia says.
That does not mean that an eruption is imminent. But the presence of magma does increase the likelihood of an eruption.
Battaglia is quick to point out that caldera systems are very complex and that scientists are a long way from being able to forecast eruptions. "These types of studies can help improve our understanding of the mechanisms that drive volcanic eruptions," he says. "In the meantime, it is very important to be prepared."
The research was supported by the Volcano Hazard and Earthquake Hazard Programs of the U.S. Geological Survey and by funding from Stanford's School of Earth Sciences.
RELATED MATERIAL: Maurizio Battaglia's web site on the Long Valley Caldera experiment is http://pangea.stanford.edu/~maurizio/maurizio.html.