Sisters: under the volcano

Information provided by the U.S. Geological Survey

Scientists from the U.S. Geological Survey have detected a slight swelling of the ground surface over a broad area centered three miles west of South Sister volcano in the Three Sisters region.

The uplift, which occurred between 1996 and 2000, covers an area about nine to 12 miles in diameter and the maximum amount of uplift at its center is about four inches.

According to information from USGS, the bulge is too broad and low to be noticed from the ground, and several close aerial inspections of the area have revealed no unusual surface features.

The relatively new technique used to detect the uplift is called Satellite Radar Interferometry (InSAR).

InSAR uses satellite data to make radar images of a portion of the Earth's surface. Images acquired at different times, but from the same location in space, can be used to detect even minor (a few centimeters or one inch) changes in the elevation of the ground.

The images that reveal the 10-centimeter uplift near South Sister were obtained in 1996 and 2000. The exact timing of the uplift, or whether it is continuing at present, is unknown, but is being studied further, according to USGS.

The specific cause of the uplift is uncertain. Because the Three Sisters region is a volcanic area, the uplift may reflect intrusion of a small volume of magma (molten rock) deep under the surface -- probably at a depth of about four miles.

Such a process, which keeps volcanic areas "alive" and prepares them for future eruptions, is a common occurrence under volcanoes, but until development of techniques such as InSAR, it has been difficult to detect.

If intrusion of magma were to continue, it could eventually lead to a volcanic eruption; however, according to USGS, an eruption is unlikely without significant activity leading up to an event.

In addition to continued or accelerating uplift, precursors to an eruption would include earthquakes and large emissions of volcanic gases, such as carbon dioxide, which are released from the magma.

At present, earthquake activity appears to be at or near "background" level and gas emissions are low.

USGS plans to enhance the existing monitoring network to better detect any activity on the mountain.

Installation of one or more additional seismometers and a Global Positioning System (GPS) receiver, resurvey of existing benchmarks and installation of new benchmarks, and periodic airborne and ground-based sampling of gases are currently being considered in consultation with managers of the Willamette and Deschutes National Forests.