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Satellite-based radar images can reveal otherwise-imperceptible heavings of magma beneath Earth's surface. One group of researchers is using the technique to search for baby volcanoes in Oregon (see "A Volcano Is Born" by Karen Wright, Discover, December 2001), with uncertain results. But geophysicist Mark Simons of the California Institute of Technology has uncovered clear radar signatures showing that some allegedly dormant volcanoes in South America's Andes Mountains are alive and possibly dangerous.

Simons and graduate student Matthew Pritchard scanned the Andes using the European Space Agency's ERS1 and ERS2 radar satellites. "In a few orbits, we could survey hundreds of volcanoes," Simons says. He and Pritchard measured the distance between satellite and volcano by bouncing a radar pulse off Earth's surface; then they repeated the process on a later pass. Some volcanic regions thought to be dormant showed a distinct change in distance (seen as color fringes in the images at right), indicating that something—most likely a buildup of magma—had moved the ground from below. Simons is most excited about the results for Lazufre, a 20-mile-wide circular deformation on the border between Chile and Argentina, which is sinking about one inch a year. Lazufre may be a caldera, a cavern that feeds volcanoes with magma. "Calderas can collapse and produce enormous eruptions," says Simons. He and Pritchard plan to conduct a hands-on seismic survey at Lazufre to confirm that something big is brewing.

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Photograph courtesy of Matthew Pritchard/ESA (3).

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