In a paper appearing in the July 11 issue of the journal Nature, California Institute of Technology geophysics graduate student Matt Pritchard and his faculty adviser, Mark Simons, unveil their analysis of eight years of radar interferometry data taken on 900 volcanoes in the Andes. The data were gathered from 1992 to 2000 by the European Space Agency's two remote-sensing satellites, ERS 1 and ERS 2.

Of the four centers of activity, Hualca Hualca volcano in southern Peru is especially worth close observation because of the population density in the area and because it is just a few miles from the active Sabancaya volcano.

A second volcano now shown to be active, Uturuncu in Bolivia, is bulging vertically about 1-to-2 centimeters per year, according to the satellite data, while a third, Robledo caldera in Argentina, is actually deflating for unknown reasons.

A fourth region of surface deformation, on the border between Chile and Argentina, was unknown prior to the study, so the authors christened it "Lazufre" because it lies between the two volcanoes Lastarria and Cordon del Azufre.

While the study provides important new information about volcanic hazards in its own right, Pritchard, the lead author, says it also proves the mettle of a new means of studying ground deformation that should turn out to be vastly superior to field studies.

The fact that none of the four volcanoes were known to be active -- and thus probably wouldn't have been of interest to geophysicists conducting studies using conventional methods -- shows the promise of the technique, he says.

"Achieving this synoptic perspective would have been an impractical undertaking with ground-based methods, like the GPS system," Pritchard says.

The sensitive data is superior to ground-based results in that a huge amount of subtle information can be accumulated about a large number of geological features. The satellites bounce a radar signal off the ground, and then accurately measure the time it takes the signal to return. On a later pass, when the satellite is again in approximately the same spot, it sends another signal to the ground.

If the two signals are out of phase, then the distance from the satellite to the ground is either increasing or decreasing, and if the features are volcanic, then the motion can be assumed to have been caused by movement of magma in the subsurface or by hydrothermal activity.

"You can think of a magma chamber as a balloon beneath the surface inflating and deflating. So if the magma is building up underground, you expect a swelling upward, and this is what we can detect with the satellite data."

Given the appropriate satellite mission, all the world's subaerial volcanoes could be easily monitored for active deformation on a weekly basis. Such a capability would have a profound impact on minimizing volcanic hazards in regions lacking necessary infrastructure for regular geophysical monitoring.

Another unusual finding from the study that shows its promise in better understanding volcanism is the Lascar volcano's lack of motion. Lascar has had three major eruptions since 1993, as well as several minor ones, and many volcanologists assume there should have been some ground swelling over the years of the study, Pritchard says.

"But we find no deformation at the volcano," he explains. "Some people find it curious, others think it's not unexpected. But it's a new result, and regardless of what's going on, it could tell us interesting things about magma plumbing."

There are several possible explanations to account for the lack of vertical motion at the Lascar volcano, Pritchard says. The first and most obvious is that the satellite passes took place at times between inflations and subsequent deflations, so that no net ground motion was recorded.

It could also be that magma is somehow able to get from within Earth to the atmosphere without deforming surfaces at all; or that a magma chamber might be deep enough to allow an eruption without surface deformations being visible, even though deformation is occurring at depth.

The study is also noteworthy in that Simons and Pritchard were able to do their work without leaving their offices on the Caltech campus. The data analysis was done with software developed at Caltech and the Jet Propulsion Laboratory, and the authors say this software was critical to the study's success.

Simons, an assistant professor of geophysics at Caltech, and Pritchard are scheduled to attend a geophysics conference in Chile in October, and will try to see some or all of the four volcanoes at that time.

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