Posted on 10/26/2001 11:45:11 AM PDT by holyscroller
Volcanic terrain in the central Oregon Cascades has quietly continued to swell over the past year, scientists say, and all signs point to an underground influx of molten rock just west of South Sister volcano near Bend.
There are no hints of an impending eruption of South Sister or of nearby vents that last spilled magma more than 1,200 years ago and lie in a region that has spewed more volcanic debris than any other in the Pacific Rim.
But federal researchers may seek U.S. Forest Service permission to install new seismic instruments in the remote wilderness area. That would allow them to detect earthquakes triggered if the subterranean magma -- now three to four miles deep -- surges toward the surface.
"The fact that it's still going on is scientifically very interesting, but it also has an element of public safety," said Daniel Dzurisin of the U.S. Geological Survey's Cascades Volcano Observatory in Vancouver, Wash. "If it continues, it could eventually culminate in an eruption, and we need to be prepared for that."
Satellite images earlier this year revealed that a region nine to 12 miles across had risen about 4 inches from 1996 to 2000. Geologists said it was the most striking geological change in the Cascade Range since the 1980 eruption of Mount St. Helens.
At the time, there were no current satellite images to show whether the swelling was continuing. In September, however, a European satellite circling about 500 miles above Earth snapped a new picture that includes precise measurements of the surface topography. Comparing it with earlier images, researchers found that the area centered three miles west of South Sister had continued bulging upward at about the same rate: a little more than an inch a year.
That brings its total swelling to about 5 inches in five years -- racetrack speed in geologic terms.
If magma is driving the rugged region up, researchers do not know how long the magma might take to find its way to the surface. Similar satellite images have revealed other volcanoes swelling and shrinking without erupting, but in other cases, small surges of molten rock have triggered eruptions.
An underground infusion of about 30 million cubic yards of molten rock could have caused the swelling, but that's less than one-tenth as much as Mount St. Helens ejected in 1980. Although that would be a small eruption as eruptions go, geologists said the incoming magma also could be adding to a larger reservoir of molten rock that has been sitting silently beneath the Cascades for centuries.
"We know from other places that there can be magma bodies present for some time, where a small increase causes the original body to erupt," Dzurisin said. "If the process were to continue, it would be like stretching a rubber band. Eventually it's going to break. It may take five years or it may take 10 or 20; it all depends on whether the process continues to that point."
Researchers have used other methods to confirm and build on the satellite findings. They installed a global positioning system receiver to keep continuous track of the elevation at sites near South Sister. They also surveyed geological benchmarks to detect any change in the topography.
The results verified the satellite results and suggested that the swelling had started about 1996, when the satellites first picked it up, Dzurisin said.
The new image, taken Sept. 2 by the European Space Agency's ERS-2 satellite, indicates that the bulge grew about 1.2 to 1.5 inches in the past year.
"It's in the same spot as it has been," said Chuck Wicks, a geophysicist with the Geological Survey in Menlo Park, Calif. "And it's about the same amount of material added to it as the previous year. It just continued to go up."
William E. Scott, scientist-in-charge of the Cascades Volcano Observatory, emphasized that more research would help unravel details of the uplift. "The suspicion is that there has been ongoing deformation for some time, but the issue is the rate," Scott said. "It would be good to know if things are accelerating, slowing down or about the same."
Scientists detected the bulge with Interferometric Synthetic Aperture Radar, which compares satellite images of the same 60-by-60-mile area over months or years to detect ground movements as small as four-tenths of an inch. The radar sees through clouds and darkness but not dense vegetation, ice or snow.
Injection of magma deep below the surface causes deformation -- bulging -- that can trigger earthquakes, although the area has been quiet. A new seismic-monitoring station was installed in April on The Husband, a peak west of South Sister. Only one small earthquake -- a magnitude 1.9 on Aug. 21 -- has been detected in the area.
"Except for the one event, the puzzling thing is that the seismicity is absolutely dead," Scott said. "When there's a certain amount of deformation, then you start getting adjustments within the crust and earthquakes are generated. Maybe we just haven't reached the point where that's been triggered."
If quakes start, he said, researchers could quickly deploy more seismic stations to gauge the depth and location of the tremors.
Scott said a permanent GPS station will be installed at Mount Bachelor to better track ground shifts. The implication of the satellite research is that many volcanoes may occasionally inflate with magma, without any outward signs.
In addition to monitoring ground movement, researchers have sampled springs in the area for volcanic gases that would be a signature of an underground magma reservoir. Those gases act as the driving force of eruptions.
Stephen E. Ingebritsen, a hydrologist with the Geological Survey, said carbon and helium isotopes in samples collected this summer "showed pretty conclusively" that magma is present.
A small amount of carbon dioxide also has been detected in the atmosphere above the region, which is consistent with the idea that magma is responsible for the uplift.
South Sister, the youngest of the three 10,000-foot volcanoes that make up the Three Sisters, erupted twice about 2,000 years ago. Subsequent eruptions occurred decades to centuries later from vents on or near the volcano's flanks.
You can reach Michael Milstein at 503-294-7689 or by e-mail at michaelmilstein@news.oregonian.com. You can reach Richard L. Hill at richardhill@news.oregonian.com or 503-221-8238.
If you get to the summit of Mt. Lassen, you'll gaze into a crater which has to be 20 miles across.
The plaque says that at that point the mountain used to extend another several miles upwards & did so until it blew its top several millinia ago.
That, along with the small quakes one felt living in that region made me anxious to move back to the relative safety of the good ol' midwest.
It would be much better if there were a number of small earthquakes occuring in the area. This is not a good sign.
This is one region that has profited from the creation of nearby wilderness areas and roadless areas. It's only about 50 miles east of Eugene, and by the late 1970's was being badly damaged by overuse.
Of course, like many conservation policies, things have been taken to an extreme. You need to get a permit from the U.S. Ranger Station to walk on any of the extensive network of trails, for example. All campgrounds and most of the parking has been eliminated.
Ah, well. Volcanic action is what created much of the beauty, I guess.
I didn't think Mt St Helens ejected magma, only pyroclastic materials.
Nope, not true at all..volcanic or magma-induced seismicity is a LOT different from tectonic seismicity.
Lots of small quakes (particularly "harmonic tremor") would be a sign that magma was REALLY moving and a ZILLION small harmonic tremors is a sign of an impending eruption.
No quakes is never an ominous sign when you're talking volcanic activity. It's not really a question of needing small quakes to relieve pressure. The absence of small quakes indicates there can't be all that much pressure.
Well any damage man did by overuse is going to look real puny compared to what will happen when the Sister blows her top! But as we learned with Mt. St. Helens, the Earth is very resilient and will heal itself rather quickly.
Aside from my lousy comprehension?
When I was a kid, 1915 probably seemed like a, "few millinium ago."
Thanks for the gentle correction; appreciate it.
Still: that's an awfully big hole in the ground up there.
Consider from where I lived (Susanville), it took an hour at 60 mph to ascend to where the mountain top was sheered-off by the explosion.
No matter how one slices or dices it?
Mt Lassen was one big mountain at one time.
Larger than anything we've around today on the NA continent, I'd have to think.
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