Sunday, August 13, 2006

Getting to know volcanoes, in and out!

Predicting volcano eruptions has always been more black art than science. No one really knows what's happening beneath the surface--until it's too late. That could soon change. Armed with an extensive network of sensors, a team of researchers has used computed tomography (CT) scanning to probe the interior of Mount Etna before, during, and after an eruption. The technique might someday give observers a picture of the threat before it breaks the surface.

In medicine, CT scanning uses x-rays to create detailed 3-dimensional pictures of the human body. The x-rays pass through porous material such as lung tissue more easily than through denser stuff such as bones.

CT scanning a volcano applies the same idea, but instead of using x-rays to judge density, geophysicists use waves from earthquakes. Earthquakes generate two kinds of waves: pressure waves, which alternately compress and stretch the rock they pass through, and shear waves, which stress the rock up and down. Pressure waves slow down as they travel through less-dense rock, so researchers can track the density of the rock by comparing the relative speed of pressure and shear waves. If the ratio drops, the density of the rock has probably dropped, too.

In today's issue of Science, Domenico Patane and colleagues at the National Institute for Geophysics and Volcanology in Italy illustrate the first attempt to CT scan a volcano before, during, and after an eruption. They used an extensive network of seismographs to listen for earthquakes around Mount Etna. After two and a half years of tracking, Mount Etna erupted.

Following the eruption, the researchers divided the data into three periods: long before the eruption, just before, and then during and shortly after; they then plotted their interpretation of the density variations within the volcano for each time period. The finest details resolved by the images are a kilometer in size, so no one is claiming they can see fingers of magma rising; instead, the density differences probably correspond to rock cracking and filling up with volcanic gas as magma rises nearby.

The technique is winning fans. "I'm impressed," says Steve Malone, a seismologist at the University of Washington in Seattle. Still, he cautions that there's a long way to go before this can be used for real-time volcano monitoring. Few volcanoes are as well monitored as Etna, and the most dramatic changes in the images happened when the eruption was already underway. "But it's certainly the type of thing to shoot for."

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