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Scientists speculate on curious bulge near South Sister
It could be nothing, it could be a volcano in the making; geologists can do little more than keep tabs as part of the desert slowly rises
Monday, September 05, 2005
MATTHEW PREUSCH
BEND -- A group of surveyors on a bare patch of land in Central Oregon usually signals another golf course or subdivision, but the crew working its way across Wickiup Plain west of Bend was measuring a force more powerful than even real estate: a volcano.
As it has for the past four years, the government team trekked this summer to the Sisters bulge, a swelling in the Earth's crust that covers 100 square miles, an area roughly two-thirds the size of Portland.
This year, recent eruptions at nearby Mount St. Helens have rekindled interest in the Sisters survey and its findings. And a U.S. Geological Survey report earlier this year found only basic monitoring at about half of the nation's most active volcanoes.
Oregon has four of the 18 most threatening volcanoes -- Mount Hood, Crater Lake, Newberry and South Sister. The USGS says monitoring is inadequate at all of them.
The bulge, on the other hand, gets an extensive array of poking and prodding to track its growth. It's centered about three miles southwest of South Sister, about 25 miles from Bend.
The results of the late August trip won't be ready for weeks, but scientists have reached some conclusions about the bulge from past monitoring: 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. And this magma lake is rising 10 feet each year. The pooling magma is under tremendous pressure, and as it expands it deforms the Earth's surface above, causing the bulge.
Beyond that, the uplift 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 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 there are some intriguing alternatives for the bulge's future.
One has only to look at the volcanic variety of Central Oregon, with its surplus of complex volcanoes, obsidian flows and pumice plains. What today are well-visited tourist attractions and wilderness destinations could have started out as intrusions of magma not unlike the bulge, Chitwood said.
To see one possibility, drive Oregon 242 from Eugene over the McKenzie Pass. After winding through lush forest, the highway crosses a black moonscape of basalt that emerged from the series of cinder cones about 1,500 years ago and covered the region in a sheet of lava.
The basalt eruptions began when molten rock -- magma -- fizzed to the surface, releasing gases and throwing a hot froth of rock and ash into the air and building a cone from which lava poured.
Such cones are the most common volcanic features on Earth, Chitwood said. 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," he said. "The bus has arrived."
The good news is that such an eruption probably wouldn't seriously affect any population centers, Chitwood said. Basalt lava flows have limited range and their ash throws aren't cataclysmic. He suggests such an eruption could be viewed with relative safety from a lawn chair on Mount Bachelor.
Such wouldn't be the case if the magma building below the Earth's crust is of a different sort called rhyolite. Rhyolite is stiffer stuff, with much more silica than basalt. Consequently, its eruptions are far more explosive.
When a glob of highly pressurized molten rhyolite finds a surface outlet, it "just blows all to hell," Chitwood said. "It's the Mount St. Helens type of eruption that we're talking about now."
Large eruptions can trigger pyroclastic flows, essentially a tidal wave of superheated rock, ash and gas, moving at hurricane speeds. Such a flow covered Bend about 400,000 years ago and "made a royal mess of things," Chitwood said. But he's quick to point out that none of the evidence on the bulge suggests such an outcome.
Still, no one knows exactly how much magma, and of what type, is pooling below the Three Sisters. It's possible, even probable, that whatever type of lava it is, it may never reach the surface.
"My personal suspicion is that (the bulge) will just quit," Dzurisin said.
The summer expeditions cost the USGS $10,000 to $20,000 a year, said Michael Lisowski, an agency geophysicist who coordinates them. Two permanent monitoring stations on the bulge cost an additional $5,000 to operate and repair annually.
The early warning systems provide a safeguard in case an eruption does occur at the bulge and nearby South Sister, said Cynthia Gardner, scientist in charge of the Cascades Volcano Observatory in Vancouver.
"We have enough of a monitoring network that I don't think we would be surprised at all if we were heading towards an eruption," she said. "The question is, how good of a heads-up would we have?"
Matthew Preusch: 541-382-2006; [email protected]