As they have done for the last four years, United States Geological Survey (USGS) scientists were measuring an approximate 100 square mile bulge in central Oregon near the South Sister this past August. The bulge is located 25 miles outside of the city of Bend, Ore. and three miles from the South Sister.
The results of this years survey won't be available for some weeks, but geologists have come to some conclusions based on the past four years of monitoring. The intial discovery was made by using information from the European Space Agency's (ESA) Interferometric Satellite Aperture Radar satellite. Scientists believe the bulge is rising at a rate of 1.4 inches per year and is due to a large lake of fluid (likely magma) that is 4.5 miles below the surface. They also think the fluid covers an area about one mile across and extends to a depth of 65 feet. The pooling fluid could be shifting magma or the creation of a new volcano.   
Ground swells aren't an extraodinary occurrence in geology. Geologists suspect that these ground swellings occur in the Cascade Range and at other volcanoes, and the majority of them do not lead to eruptions. Using the same ESA satellite technology, geologists have seen lots of bulges in the Aleutian Islands that have not lead to eruptions.     
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Volcanic bulge found in Oregon