BY FRANCES COLE
A new study confirms that a rising mass of molten rock is the cause of recent uplift of Long Valley Caldera, a finding that reinforces scientists' concern that the caldera a prehistoric volcano near Mammoth Lakes in eastern California may erupt again, although such an event is far from certain.
The study found an increase in the strength of gravity in the central part of the caldera, in an actively uplifting region known as the resurgent dome. The researchers say the gravity change corresponds to an underground magma body that is rising upward and forcing the ground to rise with it.
"We've had independent evidence for a magma intrusion under the Long Valley Caldera uplift of the resurgent dome, long-period earthquakes, carbon dioxide outgassing and seismic imaging studies," says Paul Segall, professor of geophysics. "The gravity data confirm that the magma is there."
Precise measurements of the strength of gravity in Long’s Caldera have shown that it has grown slightly stronger at the center of the valley in the last 15 years. Lighter areas on the map show areas of increased gravitational forcew while darker areas indicate a decrease.
The caldera is what's left of a volcano that literally blew itself away 750,000 years ago, leaving an elliptical hole in the ground, 17 by 32 kilometers across.
It sits on the east side of the Sierra, with Mammoth Mountain perched on its southwestern rim and the town of Mammoth Lakes nestled inside the caldera itself. The scenic Eastside Highway traverses the western part of the caldera, from rim to rim.
The new gravity study was conducted by Maurizio Battaglia, doctoral student in geophysics; Segall; and Carter Roberts of the U.S. Geological Survey in Menlo Park. The results were presented Dec. 8, 1998, at the American Geophysical Union meeting in San Francisco.
In 1998 the geophysicists made precise gravity measurements at 15 stations inside the caldera. They compared the 1998 measurements with those obtained by the U.S. Geological Survey in 1983 and found an increase in the residual gravity that is, the gravity adjusted for vertical shifts of the land and the water table beneath the resurgent dome in the middle of the caldera.
They found that the residual gravity grew by a small but significant amount (60 billionths of Earth's gravity) during the 15 years that elapsed between measurements, and they say that the increase signals intrusion of magma into the underground part of the caldera.
Gravity may seem like a constant commodity, but on a dynamic planet like Earth, it's always changing not enough for a person to notice, but enough to deflect the reading on a precise gravimeter. Masses of solid rock move up and down and side to side when they are displaced by faulting and erosion, and masses of molten rock jostle their way upward through the dikes and vents inside volcanoes. Gravity reflects those mass movements, growing stronger as masses move closer and weaker as masses move away from a fixed reference point, in this case a gravimeter station on the ground.
The gravity increase detected in Long Valley Caldera probably was caused by movement of magma from a deep reservoir tens of kilometers underground to a region about seven kilometers beneath the caldera's resurgent dome, the scientists said.
The resurgent dome is also the area where ground uplift has been greatest over the past 18 years. It has risen more than two feet since 1980, and the scientists say that rising magma explains the uplift.
"The gravity results indicate that magma injection accompanied, and probably caused, the ground uplift," Battaglia said.
The gigantic eruption that formed Long Valley Caldera 750,000 years ago blew 500-plus cubic kilometers of rock into the air (compared to 1 cubic kilometer discharged from Mount Saint Helens), deposited the widespread ash-flow known as the Bishop Tuff, and sent airborne ash all the way to Nebraska. The caldera underwent a few subsequent, milder eruptions the last one 500 years ago then settled down for a long nap.
In 1980 it re-awoke with a series of earthquakes that culminated in four events of magnitude 6 just one week after the eruption of Mount Saint Helens. Smaller temblors have been shaking the caldera ever since. The resurgent dome continues to bulge a few centimeters higher each year, and in recent years pine trees on Mammoth Mountain have begun to die from heightened fluxes of carbon dioxide gas on the west side of the caldera.
Those are all signs that something is brewing under the caldera. Most scientists think that "something" is magma, and the new gravity study adds credence to that hypothesis.
A similar magma intrusion was implicated in the 1994 eruption at Rabaul Caldera in Papua New Guinea. Rabaul blew its top after a 20-year growth spurt that ended with intense ground uplift nearly 6 meters in some places in the final hours before eruption.
The current activity at Long Valley Caldera is not so frenetic, so scientists are not predicting an eruption within days, weeks or months. However, the presence of magma does increase the chances of an eruption some time in the future, and the gravity study points to the resurgent dome as a likely location for such an event.
Calderas are very complex systems, and scientists' present understanding of them just is not good enough to make long-term predictions, Battaglia said.
The U.S. Geological Survey is monitoring seismic activity, ground movements and gas fluxes in and around Long Valley Caldera, waiting and watching for any sign that an eruption is imminent. The hope is that such signs will provide fair warning in the event of an eruption, as they did at Rabaul.
The gravity study 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