Volcanic activity has quieted

Back in 2001, folks in the Sisters Country were reminded that South Sister, one of the beauties of our horizon, is, in fact, a volcano.

Scientists from the USGS (U.S. Geological Survey) were doing an analysis of satellite radar data and discovered an area about 10 miles in diameter had bulged about four inches at its center. The uplift, often referred to as the "South Sister Bulge," grew at an average rate of about one inch per year through 2004.

Preliminary work suggested magma - molten volcanic rock beneath the Cascades - was rising toward the surface, creating the bulge and emitting gases into the surrounding streams and lakes.

Earlier studies, begun in 1985 and measured more recently, suggested that South Sister, which is made up of flow after flow of ash, lava, cinders, and more lava had been tilted eastward just a tad.

That set the USGS geologists into looking at the area roundabout the Three Sisters, and they found more evidence of uplift. Husband Mountain has been going up about an inch a year as well. That information stimulated enough funding to place tilt stations, additional GPS measuring equipment, and seismographs around the Three Sisters area to better understand what was going on beneath Wickiup Plains and the surrounding area near South Sister.

The late Larry Chitwood, a Forest Service geologist, gave a great talk about the South Sister Bulge at the Sisters Library not long after the activity was reported,

"Central Oregon is on the move..." he said. "Three to four miles below the surface is molten rock, containing carbon dioxide, helium, water and hydrogen sulfide; it's moving toward the surface and causing bulging."

One summer day, sitting on the side of Mount Bachelor, Chitwood pointed to a spot about three miles southwest of South Sister, "...out there near Separation Creek Meadow, a cinder cone may blast its way to the surface, and maybe, like what took place at Lava Butte, we'll see flows of pahoehoe lava sliding past us." Then he laughed and said, "We can drag our lawn chairs up here and toast marshmallows on the lava as it goes

past."

There are presently several seismographs and GPS stations located near South Sister so that earth movements can be triangulated and better mapped when earthquakes occur. And they do occur frequently, albeit tiny jigglings that we humans can't feel, but seismographs record as 1.0 to 1.9 on the Richter scale.

So what is going on with the bulge?

The activity beneath the surface southwest of South Sister has ceased - for now. Quiet has once again settled over the Cascades; that is, as quiet as a chain of dormant volcanoes can be. Will it erupt any time soon?

Chitwood put the whole phenomenon in perspective that time he gave his talk in the Sisters Library: "We just don't know. You expect one eruption per lifetime in the Cascade chain as a ballpark average. We've already had Mt. St. Helens, but there are no certainties in these averages. If it does erupt, you can expect slow lava flows and a cinder cone formed from a basaltic flow."

Bart Wills, the present Deschutes National Forest geologist, reminds us, "The mechanism that's driving the forces that created the bulge are still there: the subduction of the Pacific Plate beneath the North American Plate. It's been going on for millions of years and it will be going on long after we're gone - there's no reason to think it will just go away. The last earthquake of 0.1 magnitude under the bulge reminds us that something is still going on four miles or so beneath our feet."