Volcanic bulge has town excited
  An April 25, 2001 view of the South Sister range
 May 20, 2001 Posted: 7:25 PM EDT (2325 GMT)
SISTERS, Oregon (AP) -- At the Epicure Exchange Internet coffeehouse, it's hard not to get into a discussion about "The Bulge."

Anyone looking through the windows to the west gets a spectacular view of South Sister dominating the snowcapped skyline of the Cascade Range, rising above this mountain crossroad 100 miles southeast of Portland.

Inside, the latest U.S. Geological Survey reports on the curious bulge growing on the dormant volcano's flank can be called up in an instant on a computer.

"It's a big hoopla in town right now. Everybody's talking about it," said coffeehouse owner Jeannine Smith, who moved her family to this picturesque town of 850 people just to be closer to the majestic volcanic peaks that tower above it.

"It doesn't scare me," Smith said. But she added: "It is something to be considered."



Sister, Oregon


About a dozen USGS scientists are considering it very carefully.

Because the radar satellite images that spotted the bulge can only be taken once a year, a team of scientists plans to fly to The Bulge in a helicopter next week to set out instruments to see if it is still growing.

"It is clearly not a crisis we are responding to, as if we were having swarms of shallow earthquakes or anything of that nature," said USGS geologist Dan Dzurisin, referring to the telltale signs of an impending eruption.

But even though it shows no danger signs, the bulge "is scientifically the most interesting target we have in the Cascades right now," Dzurisin said.

Ever since Mount St. Helens, far to the north near the Washington border, erupted on May 18, 1980, the general public has been sensitive to any rumblings in the Cascades, a string of volcanoes stretching from British Columbia to Northern California.

And given the city of Sisters' dependence on tourism for the local economy, one thing a lot of people are wondering about is whether The Bulge becomes boon or bane, drawing more visitors or sending them away.

At the Epicure Exchange, employee Karen Mills isn't concerned. She has loved these mountains all her life. But she figures there is little to worry about since The Bulge is about 20 miles away, on the other side of the mountain, and pointed down into the Willamette Valley.

"It's very cool, and it's going to spark a lot of interest," she predicted.

About 20 years ago, there was a lot of worry over the possibility that Carver Lake, formed by a dam of loose rock pushed up by a glacier on South Sister, might burst out and flood the town, recalled Jeff Perin, owner of The Flyfishers' Place.

"People have pretty much forgotten about that," he said.



Sister, Oregon


When the USGS announcement of The Bulge hit the newspapers, Nate Turner, who works at The Flyfisher's Place, got a call from an East Coast flyfishing pal with a second home in Sisters who read about The Bulge on the train to New York and immediately called on his cell phone to see what was up.

"I told him I'd throw a sprinkler up on his roof if something was really going down," said Turner. "If The Bulge meant there would be better fishing, I might be interested in it."

Turner knows a little more about the local volcanic activity than most of his neighbors. In eighth grade he did a report on a USGS laser survey of South Sister to establish a network of benchmarks to monitor just this sort of thing.

The work was done by the USGS Cascade Volcano Observatory in 1985, shortly after it was established to keep a closer eye on the Cascades, said Dzurisin, who is leading the trip to set out the monitoring instruments.

When it was resurveyed in 1986, the bulge hadn't shown up yet. But when a new technology, radar satellite imagery, was trained on the South Sister, The Bulge came into startling view.

Volcanologists began using the European Space Agency's Interferometric Satellite Aperture Radar satellite, known as InSAR, in 1991 on Mount Etna in Italy, whose eruptions are so frequent an ice cream sundae was named for it.

Two months ago, USGS geophysicist Chuck Wicks was in his office in Menlo Park, Calif., going over some InSAR images of South Sister, when the bulge popped out at him in a bullseye ring of rainbow colors on his computer screen.

"I think I said, `Wow, this is great,' something like that," said Wicks, who had hiked the Sisters Wilderness and thought the mountain would be a good candidate for InSAR. The last eruption was small and occurred about 1,500 years ago.

Wicks compared a digital image of the west side of the volcano taken in 1996 with one taken in 2000, and the computer revealed a rise of four inches spread over a 10-mile diameter in the headwaters of Separation Creek. The area is about three miles west of the peak of South Sister, near a formation known as The Husband, and actually extends under Highway 242 as it climbs up to McKenzie Pass.

It is too soon to tell exactly what is going on, but the best guess is that a relatively small amount of magma is moving about 12 miles beneath the surface, said Wicks.

In volcanic terms, small is still pretty big: The magma would probably fill a swimming pool measuring a half mile on each side and 50 feet deep.

Wicks is hoping that the InSAR satellite holds out long enough to give another image this summer, when the snow is gone. It has already lived beyond its design life, and all but one of its aiming gyroscopes are out of commission. A replacement satellite isn't due to be launched until fall.

In the meantime, scientists had to get a special permit from the U.S. Forest Service to fly into a wilderness area to install a very sensitive global positioning system monitor and a seismometer.

The GPS will detect changes in the elevation of the ground as small as a couple of millimeters, and the seismometer will detect any earthquakes, which are the most reliable predictor of an impending eruption, said Dzurisin.

Special efforts will be made to hide the instruments, which will be connected to the Internet by radio telemetry modems. Scientists will be able to read them daily in their offices.

If the bulge continues to grow, there will be more GPS monitors and seismometers, a new survey, and ground and air samples to check for volcanic gases.

"I suspect these kinds of ground swellings probably go on in the Cascades and at other volcanoes, and most of them -- maybe even the vast majority -- don't lead to eruptions," said Dzurisin. "If we had not had the radar technology, we might never have noticed. This is what volcanoes do."

InSAR has detected lots of bulges on volcanoes in the Aleutian Islands in Alaska, and none of them has led to an eruption, said Dzurisin.

That would be fine with Jim Wills, who has a view of South Sister over his shoulder when he is setting out tubs of strawberries and blackberries at Richard's Produce stand.

"I remember Mount St. Helens," he said. "I don't want to see that around here."
 

 DESCRIPTION:
 Three Sisters Volcanoes, Oregon 
Aerial view, Three Sisters Volcanoes, Oregon, showing (left to right) South Sister, Middle Sister, and North Sister.
-- USGS Photo by Lyn Topinka, September 1985

Three Sisters is one of three potentially active volcanic centers that lie close to rapidly growing communities and resort areas in Central Oregon. Two types of volcanoes exist in the Three Sisters region and each poses distinct hazards to people and property. South Sister, Middle Sister, and Broken Top, major composite volcanoes clustered near the center of the region, have erupted repeatedly over tens of thousands of years and may erupt explosively in the future. In contrast, mafic volcanoes, which range from small cinder cones to large shield volcanoes like North Sister and Belknap Crater, are typically short-lived (weeks to centuries) and erupt less explosively than do composite volcanoes. Hundreds of mafic volcanoes scattered through the Three Sisters region are part of a much longer zone along the High Cascades of Oregon  in which birth of new mafic volcanoes is possible.

From: Scott, et.al., 2001, Volcano Hazards in the Three Sisters Region, Oregon: USGS Open-File Report 99-437



The Three Sisters area contains 5 large cones of Quaternary age-- North Sister, Middle Sister, South Sister, Broken Top, and Mount Bachelor. North Sister and Broken Top  are deeply dissected and probably have been inactive for at least 100,000 years. Middle Sister is younger than North Sister, and was active in late Pleistocene but not postglacial time. South Sister is the least dissected; its basaltic andesite summit cone has a well preserved crater. Most of South Sister predates late Wisconsin glaciation and is therefore older than 25,000 years; however, eruptions of rhyolite from flank vents have occurred as recently as 2000 years ago.

 From: Hoblitt, Miller, and Scott, 1987, Volcanic Hazards with Regard to Siting Nuclear-Power Plants in the Pacific Northwest: USGS Open-File Report 87-297



The Three Sisters-Broken Top area is a long-lived center of basaltic to rhyolitic volcanism. The clustering of large composite cones sets the area apart from others in the High Cascades, although the Mount Mazama area prior to the formation of Crater Lake caldera was also a cluster of composite cones.

The ages of most volcanoes in the Three Sisters area are not precisely known. North Sister, a basaltic andesite pyroclastic and lava cone that rests on a shield volcano, is the oldest of the Three Sisters and postdates the approximately 0.3-million-year-old Shevlin Park Tuff. Middle Sister is intermediate in age between North and South Sister and, like South Sister, is compositionally diverse. Broken Top volcano is also younger than Shevlin Park Tuff and is older than South Sister, but its age relation to Middle and North Sister is not known. The relative degree of erosion of Broken Top is a complex complex composite cone of dominantly basaltic andesite that intermittently erupted andesite, dacite, and rhyolite as lava flows, pyroclastic flows, and pyroclastic falls. Cayuse Crater, which is located between Broken Top and the Cascades Lakes Highway, and two nearby vents on the southwest flank of Broken Top erupted during the earliest Holocene or lates Pleistocene time, but these events were probably unrelated to the long-inactive Broken Top system.

South Sister is the youngest composite volcano of the Three Sisters-Broken Top center and has erupted lavas ranging from basaltic andesite through rhyolite. Although not dated directly, most, if not all, of South Sister is probably of late Pleistocene age. ... The cone of basaltic andesite that forms the summit of South Sister is probably of latest Pleistocene age; its crater is still closed and is filled with 60 meters of ice and snow. Le Conte Crater a basaltic andesite scoria cone on the south flank, is between about 15,000 and 6,850 years old. The youngest eruptions recognized on the volcano occurred at a series of vents on the south and northeast flanks that erupted rhyolite tephra and lava flows and domes between about 2,200 and 2,000 years before the present.

From: Scott and Gardner, 1990, Field trip guide to the central Oregon High Cascades, Part 1: Mount Bachelor-South Sister area: Oregon Geology, September 1990, v.42, n.5.