Volcano interest erupts in Sisters

Last week, approximately 80 Sisters area residents packed into the Sisters Fire Hall to find out if they are living on a volcanic powder keg.

The answer: not necessarily.

Since the discovery of a volcanic "bulge" three miles west of the South Sister, geologists have been flocking to the area to learn more about the phenomenon.

Local residents, in turn, were given the opportunity to hear what the U.S. Geological Service has to say about the hints of renewed volcanic activity in the area.

The presentation was extremely informative, often entertaining -- and always laced with equivocation.

Dr. Willie Scott, head of the USGS team studying the Sisters volcanoes, was pretty clear when he said, "We're not in a position where anybody needs to worry about this, but it is an exciting scientific opportunity."

Then he handed out emergency brochures on what to do in the event of a volcanic ashfall.

Scott presented an overview of the volcanic history and character of the Cascades, illustrating, with aerial photos, that Central Oregon is dotted with hundreds of mountains, hills, ridges and cinder cones.

Each of these bumps on the terrain, he said, resulted from the region's volcanic activity at some point in time.

He explained that the volcanism that characterizes the Cascades is due to heat generated by the movement of the geologic Pacific Plate many miles beneath the earth's surface.

This heat melts rock into "magma," and the "bulge" is apparently caused by an intrusion of that magma that has risen to within about four miles of the surface.

The bulge was discovered by satellite imagery that recorded a pattern of ground uplift centered on the area three miles west of the South Sister.

The effect of the uplift has been to essentially "jack up" the western edge of the South Sister by four inches and tilt it very slightly toward the east.

Scott said that he had received telephone calls from reporters back east asking where they could go to see the uplift.

He's had to explain that a four inch lift over many miles is not a very noticeable geographic feature.

Recent physical measurements taken on the mountain itself confirm that there has been movement of the mountain since earlier measurements were taken in 1985.

However, the evidence from the new instrumentation being employed would seem to indicate that most of the uplift has occurred since 1996.

According to Scott, the existing geologic scenario lends itself to one of three possibilities.

The least likely case is that the magmatic event has made its mark and is now over with.

More likely, he said, the uplift is simply an ongoing magmatic intrusion that adds mass and heat to the Cascade system -- the normal process by which the Cascade volcanoes "stay alive."

He suggested that such events have been going on for years, but it is only because of new technology that the process can now be observed so closely.

Also possible, he explained, is that the uplift is in fact a precursor to eruption, and whether this occurs in a matter of years, centuries or millennia is "anybody's guess."

He admitted that, "We're pretty good at detecting unrest, but not at predicting the time of eruptions."

When asked if residents were witnessing the birth of a potential "fourth Sister," he dodged the question by saying, "That's an interesting idea."

Scott said that initial eruptions tend to be of an explosive nature and added, "You're going to be glad you're in Sisters" -- referring to the fact that the town of Sisters is more than 20 miles away from center of the bulge and shielded by the mountains.

"The hazard here for Sisters is lahars in the event of a pyroclastic eruption in the Three Sisters," Scott said.

A lahar is a fast-moving slurry of rock and mud caused when an eruption melts snow and ice on the mountain.

He said that a lahar in this area would likely be confined to Squaw Creek Canyon until it reached terrain that fans out, such as the area surrounding Sisters.

Scott produced a map showing that all the Squaw Creek floodplain, including the lower Indian Ford basin, could be at risk.

In reference to the additional problem of ash hazards from an eruption, he said, "Volcanic ashfall is seldom life-threatening but is always a nuisance."

Data from the study are now being analyzed.

"I think, over the next few weeks, we'll be finding out more things," he said, promising that the information would be made public as soon as it was available.

Steve Otoupalik, Wilderness and Trails Manager for the Willamette National Forest is the "landlord" for the bulge and has been helping to coordinate the activities of the scientists in the wilderness.

Otoupalik said that it's been a balancing act to weigh the wilderness values against the intrusion of a scientific expedition. He noted, however, that scientific study is a legitimate use of wilderness and one of the original reasons for establishment of wilderness areas.

Rock slide question

By Craig F. Eisenbeis

Sometime during the middle of the last week in September, a rock slide tore down the eastern slopes of the South Sister, leaving Prouty Glacier littered with rock and debris.

USGS geologist Willie Scott wants to pinpoint the time of the rockfall and is seeking help from the area residents.

He's hoping that habitual mountain watchers can help the agency nail down the exact time of the slide.

The USGS is actively engaged in monitoring the dormant volcano (see related story, page 1) and has recently installed new seismographic equipment in the area. As a result, earthquakes of a relatively low magnitude are being detected that might previously have gone unnoticed.

One such tremor was recorded at 10:39 p.m. on Wednesday night, September 26. Since the seismic event was recorded at night and the mountain was shrouded in clouds at the time, the USGS is unsure whether the recorded event coincided with the rockslide.

At a recent meeting in Sisters, Scott was asked whether the tremor triggered the rockfall or if the rockfall itself was the seismic event. He indicated the latter, saying, "Rockfalls make earthquakes...."

Forest Service geologist Bob Jensen's opinion concurred, attributing the slide to dry conditions that have reduced the snowpack, making the mountain less stable.

Anyone with information about when the rock aslide was first observed is urged to contact the USGS at (360) 993-8900 or through the website vulcan.wr.usgs.gov.