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  • Dick writes about Earth and planetary science for Science magazine.
 

Oregon's Rising, an Eruption to Follow?

10 May 2001 7:00 pm
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Bull's-eye. Each Technicolor ring represents a 28-millimeter uplift.

Volcanologists have discovered a broad, 10-centimeter-high bulge on the flanks of the Three Sisters volcanoes in the Cascade mountain range of central Oregon. No one can say what, if anything, will happen next--the most dramatic possibility is that the uplift could grow and eventually erupt as a volcano. But researchers are thrilled to be in on the ground floor of what could become a classic case study in volcanology.

Usually, volcanologists arrive on the scene after the ground has begun to shake or even as gas and ash are spewing out. Around the Three Sisters--North Sister, Middle Sister, and South Sister--35 kilometers west of Bend, Oregon (population 52,000), there is no geologic sign of such activity in the past 1000 years or more. But as part of the U.S. Geological Survey's (USGS's) Cascades volcano monitoring duties, geophysicist Charles Wick and his colleagues at the USGS office in Menlo Park, California, were searching for any change in the shape of the Cascades.

The team used interferometric synthetic aperture radar to measure the distance from a satellite to the surface. Taking data from overflights of European Earth Resources Satellites in 1996 and 2000, Wick and his colleagues overlaid the two slightly out-of-phase radar signals to form an image of interference fringes. Each fringe would represent a rise or fall of the surface of 28 millimeters over the 4 years.

What the analysis produced was a stunning bull's-eye of interference fringes centered 5 kilometers west of the South Sister volcano. Fifteen to 20 kilometers across and 10 centimeters high at its center, the uplift could have formed as magma oozed up into a chamber 7 kilometers below the surface. "This came as a shock," says volcanologist C. Dan Miller of the USGS's Cascades Volcano Observatory (CVO) in Vancouver, Washington. "We may have caught an eruption in the very earliest stages."

Eruptions have certainly happened before around the Three Sisters, as recently as 1200 years ago. If the least explosive sort of magma reached the surface in the bull's-eye, the hazard would be largely limited to the immediate vicinity in the Three Sisters Wilderness, says William Scott, scientist-in-charge at CVO. Even so, USGS researchers are moving equipment into the sparsely instrumented region as the winter's snow recedes. They want to find out if the uplift is continuing and to be ready should any of the Three Sisters or their relations awaken.

News from the USGS's Three Sisters observatory

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