Last June a team of French researchers showed that they could measure the shape of a volcano to within a fraction of an inch and perhaps help predict an eruption--from space. The researchers used images made by the European satellite ERS-1, which sweeps a radar beam over the ground and collects the echoes as it orbits Earth. If a feature on the ground is stationary, one radar picture of it looks like the next. But if the feature has moved even slightly--as a volcano does, for instance, when it inflates with magma just before erupting--the second set of echoes is different. Combining both sets of echoes produces an image called an interferogram, like the one shown here of Mount Etna in Sicily, that reveals with extraordinary sensitivity just how much the ground has moved.
The French team, led by radar image specialist Didier Massonnet of the National Center of Space Studies, didn’t predict an eruption this time; they monitored one in progress. Mount Etna erupted from December 1991 to March 1993, without doing much damage. The researchers found that during the last seven months of the eruption, the volcano deflated around four- fifths of an inch per month as it lost magma. It was quite a surprise, says Massonnet. Such a large-scale deflation was not expected, even though this volcano is one of the most surveyed in the world.
Presumably the technique will be just as good at catching volcano inflation. Massonnet is already watching other volcanoes, including Mount Vesuvius and California’s Long Valley Caldera. The future of the method is as a preliminary warning, he says. We can actually monitor most of the volcanoes of the world without moving from our office. Then, if one of the volcanoes gives signs of awakening, we can direct specialized instrumentation to it.