Oregon's Rising, an Eruption to Follow·
On target. A bull's-eye of a bulge falls among major Oregon volcanoes
and hundreds of minor vents.
|Richard A. Kerr
When a parcel of land including a trio of volcanoes swells upward by a tenth of a meter over 4 years, volcanologists tend to get excited. That's exactly what's happened in the U.S. Pacific Northwest. By excruciatingly precise comparison of satellite radar data, they've discovered a broad, 10-centimeter-high uplift 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 continued doming and an eventual volcanic eruption. 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, rarely, 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 untoward activity in the past 1000 years or more. But as part of the U.S. Geological Survey's Cascades volcano monitoring duties, geophysicist Charles Wicks and his colleagues at the USGS office in Menlo Park, California, were using interferometric synthetic aperture radar (InSAR) to search for any change in the shape of the Cascades.
Like ordinary radar, a satellite-borne SAR measures the distance to the surface by clocking the travel time of a microwave signal bounced off the surface (Science, 28 June 1996, p. 1870 ). Taking data from overflights of European Earth Resources Satellites in 1996 and 2000, Wicks and his colleagues measured the change in the distance to the surface during the 4 years by letting the two slightly out-of-phase signals interfere with each other to form an image of interference fringes. Each rainbow fringe in an interferogram would represent a rise or fall of the surface of 28 millimeters over the 4 years, at least where snow, dense vegetation, and soil moisture variations didn't intervene.
What the InSAR 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 the crust within 7 kilometers of the surface; in fact, geophysicists are hard-pressed to think of any other explanation. "This came as a shock," says volcanologist C. Dan Miller of USGS's Cascades Volcano Observatory (CVO) in Vancouver, Washington. "We had no idea anything was going on in that part of the world. We may have caught an eruption in the very earliest stages."
Eruptions have certainly happened before near the Three Sisters. "Every bump around there is a volcano," says William Scott, scientist-in-charge at CVO. "It's what central Oregon is famous for." Beyond the Three Sisters, which last erupted with lots of ash about 2000 years ago, there are hundreds of volcanic vents and cones that have briefly spewed less explosive lavas as recently as 1200 years ago. If such magma reached the surface at the bull's-eye, which is in the Three Sisters Wilderness, the hazard would be largely limited to the immediate vicinity, says Scott. If the magma turned out to be the more explosive sort, ash could blow downwind toward Bend or flow down streams as searing ash clouds or muddy floods for many kilometers.
USGS researchers should have some answers by summer. They are moving equipment, including a seismometer and a telemetered Global Positioning System (GPS) receiver, into the sparsely instrumented region as the winter's snow recedes. GPS should tell them within a few months whether a rapid uplift is continuing. If it is, they'll want to be ready should any of the Three Sisters or their relations awaken.
Volume 292, Number 5520,
Issue of 18 May 2001, pp. 1281-1283.