A recent survey of a bulge that covers about 100 square miles near the South Sister indicates the area is still growing, suggesting it could be another volcano in the making or a major shift of molten rock under the center of the Cascade Range.
Recent eruptions at nearby Mount St. Helens in Washington state have rekindled interest in the annual Sisters survey and its findings.
Oregon has four of the 18 most active volcanoes in the nation — Mount Hood, Crater Lake, Newberry and South Sister. A recent U.S. Geological Survey report said monitoring is inadequate at all of them, with only basic monitoring at about half of the active volcanoes.
Unlike the volcanoes, the bulge gets an extensive annual survey to track its growth. Spread out across an area nearly as big as the city of Portland, It’s centered about three miles southwest of the South Sister, about 25 miles from Bend.
The results of the late August survey won’t be ready for weeks, but scientists have reached some conclusions about the bulge from past monitoring.