Geologist reports on South Sister bulge

Larry Chitwood explained volcanic activity on South Sister to library patrons last week.

Lured by recent accounts of possible volcanic activity on the South Sister, a record number of people showed up at the Sisters Library on Monday evening, February 11, for geologist Larry Chitwood's talk about "the bulge."

Somewhere between 80 and 90 listeners crammed the library to hear and see details about the phenomenon.

"Central Oregon is on the move," said Chitwood, a geologist specializing in volcanology with the Forest Service.

Using an interferometry technique developed in 1992, two radar images were produced from satellite imagery and, after computer-intensive analysis, produced a rainbow colored pattern. Patterns over the years 1996-2000 measured an average bulge (or movement) of 1.4 inches per year.

Three to four miles below the surface is molten rock. Carbon dioxide, helium, water and hydrogen sulfide move towards the surface and cause bulging.

There are presently two seismographs in the area, one at the Husband (near South Sister but not at the middle of the bulge) and another at Three Creek Meadow. A third site is planned so that earth movements can be triangulated and better mapped as to location.

Carbon dioxide readings have fluctuated in the South Sister area and prove difficult to interpret, according to Chitwood.

Benchmarks posted in 1985 in the South Sister area and measured more recently indicate a tilting of the mountain eastward, the geologist noted.

The actual center of the bulge is about three miles west of the South Sister at Separation Creek Meadow.

Measurements at the Husband Mountain have measured a rise of one inch per year.

There have been two tiny earthquakes recently in the area. At 1.9 and 1.0 they would not be felt by local residents but would register on the seismographs, Chitwood said. There also was a recording of an avalanche on the South Sister on February 5.

The question most on the minds of audience members was obvious: whether the South Sister will erupt.

"We just don't know," Chitwood said. "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 flow and a cinder cone formed from basaltic flow," Chitwood said.

There have been numerous eruptions in Central Oregon over the past 4,000 years. Sand Mountain eruption was 3,000 years ago; Blue Lake and Collier Cone more recently, about 1,500 years ago.

"The Cascades are a major volcanic area of the world," said Chitwood. "The dome type eruptions (thousands of years ago) such as those that occurred at Sparks and Devils Lakes and rise tens of thousands of feet in the air are much more dangerous than the slow spreading lava flows seen in the examples on the McKenzie Pass (from 1,500 years ago)."

Chitwood finished his talk with a discussion about Carver Lake located east of the South Sister. This lake holds almost 900,000 cubic meters of water (about 750 acre feet). If an earthquake, South Sister eruption or break in the moraine wall occurs this water would flow down Squaw Creek towards Sisters 20 miles away.

Chitwood believes that this water would be dissipated before reaching Sisters but would cause some flooding of roads before reaching Sisters.

Those interested in following the seismograph activity at Husband Mountain can view it on the Internet at: http://www.ess.washington.edu/SEIS/PNSN/WEBICORDER/welcome.html.

The talk was presented by The Sisters Friends of the Library. Next month, Roger Riolo will present an account of the Barlow Trail.