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.
They say it probably began growing in 1997 and has been rising ever since at a rate of about 1.4 inches a year. It was first observed from space using a relatively new imaging technology known as radar interferometry that can measure changes in the Earth's surface.
The likely cause of the bulge is a pool of magma that, according to Deschutes National Forest geologist Larry Chitwood, is equal in size to a lake 1 mile across and 65 feet deep.
The magma lake is rising 10 feet each year, under tremendous pressure, and it deforms the Earth's surface as it expands, causing the bulge.
Other causes could be anything from the birth of a new volcano — a fourth Sister in the making — to a routine and anticlimactic pooling of liquid rock, researchers say.
"The honest and shortest answer is, we don't know," said Dan Dzurisin, a USGS geologist.
Dzurisin recently led a three-person leveling crew on a slow walk across the top of the bulge. They were hoping to detect any change in its surface using survey equipment accurate to one-sixteenth of an inch for every mile measured.
Dzurisin's survey data, in concert with space imaging and satellite positioning measurements from two dozen fixed points on the bulge, give scientists an idea of the bulge's depth and size.
Additional information from seismographs and chemical monitoring of area springs reveal movement of the magma underground. A swarm of 350 small earthquakes in March 2004 indicated magma was on the move, but the bulge has been quiet ever since.
Whether the magma will move again or ever reach the surface is a mystery. But if it did, geological history suggests it would result only in small cinder cones that spew ash and lava.
The good news is that such an eruption likely would not seriously affect any population centers, Chitwood said.
Such cones are the most common volcanic features on Earth, he added. Central Oregon has about 600. Basalt flows have occurred in the area of the bulge every 1,000 to 1,500 years for the past 4,000 years, he said. And the area is due for another.
"The bulge is on time," Chitwood said. "The bus has arrived."
This report includes information from .