MENLO PARK, Calif., May 17 (UPI) -- Geologists are convinced bulging ground at the foot of a dormant volcano in the Cascade Range of central Oregon signals molten rock oozing toward the surface, but they are at a loss to explain the absence of other key indicators an explosive giant may be stirring from a 1,500-year nap.
Monitoring activities around South Sister, the youngest, tallest and best preserved of the Three Sisters peaks, which dominates the picture-perfect landscape of verdant pines and sapphire lakes 22 miles from Bend, Ore., have heated up since sensitive, state-of-the-art technology gave first hint nearly a year ago of the suspicious swell over a 60-square-mile area 3 miles west of the volcano's summit.
Even if the five-inch rise since 1996 emanates from molten rock crawling up the 10,358-foot volcanic vent mountain toward a fiery escape, as they think, it does not portend an imminent eruption, geologists told United Press International.
The results of their watch over South Sister have incited increased vigilance in a wilderness area dotted with mushrooming residential communities and holiday resorts.
Using high-precision radar data from satellites, Charles Wicks of the Earthquake and Volcano Hazards Division of the U.S. Geological Survey in Menlo Park, Calif., and his colleagues detected the uprising in Earth's crust near the base of South Sister, which is capped by a well-preserved, snow-filled summit crater whose summer runoff forms the highest lake in Oregon.
The lift continues to expand "at a fairly constant rate" of 1 inch a year, Wicks told UPI.
"Geologists discovered this rising of the ground on the west side of South Sister volcano about a year ago with a very sophisticated modern technique for using satellite measures to determine ground movements," said Terrence Gerlach of the USGS Cascades Volcano Observatory in Vancouver, Wash.
The swelling could be caused by the slow accumulation of molten rock 4 miles underground, and the data suggest the speed of the sizzling pile-up may be faster than in the past, scientists speculated.
"We are not 100 percent certain it's magma. It could be a pressured geothermal system, such as you see happening at Yellowstone (National Park) or down in Death Valley," Wicks said in a telephone interview. "The problem with that scenario is that you would have surface manifestations such as geysers or hot springs and you would have seismicity, but you don't have either at South Sister."
"It seems pretty clear there's magma behind this event" at the 25,000-year-old volcano, which has remained dormant since about 500 A.D., geologist William Scott of Vancouver, Wash., told UPI.
Intrigued by the findings, geoscientists set out to seek other key signs of an impending volcanic awakening. Some groups started measuring the gas composition of the air and the springs surrounding the bulge. Others began monitoring in earnest the area's earthquake activity.
"Gauging ground inflation is one of the techniques used these days to identify or detect what we call volcano unrest. The other methods are earthquakes, or seismicity, and gas emissions," Gerlach said in a telephone interview.
"When the USGS found evidence for this rising ground, they put in a number of seismic stations in the area and started tracking earthquakes and started looking at the amount of gas coming out of this area and checking for emissions of gases that come out of magma when it gets into shallower depths. When we were notified of the bulge, we proposed gas studies to validate this observation."
As magma rises to the surface, it spews forth gases, most of which are dissolved in overlying groundwater and some of which, notably carbon dioxide, escape into the air.
In addition to released gases and bulged earth, precursors to an eruption would include earthquakes, typically swarms of small shakers generated by rock fracturing as magma slithers up the volcanic vent.
Other than finding small amounts of gas from molten rock are showing up in some of the neighborhood's spring waters, however, the scientists came up empty. For example, air samples collected with instruments aboard a specially equipped helicopter and fixed-wing aircraft showed no tell-tale signs of any upcoming catastrophe.
Because it is not easily masked by water, which is plentiful in the rainy Pacific Northwest, carbon dioxide was considered a front-line indicator of volcanic brewings in their studies, scientists said.
"The other gas that's nice to use is hydrogen sulfide, or the rotten egg gas as it is more commonly known. This gas also has a tendency to be released early from magma and not be terribly soluble in groundwater," Gerlach told UPI. "If a volcano is heating up, CO2 and H2S would be seen first."
Neither was detected beyond the normal levels.
"Our gas measurements show nothing," Gerlach said. "We're not able to detect any significant amount of gas above just background atmospheric gases at South Sister. Seismicity is also very low and doesn't seem to be suggesting anything unusual."
William Evans and his coworkers at USGS in Menlo Park and at the Lawrence Berkeley National Laboratory in Berkeley, Calif., who analyzed the nearby spring waters for their chemical content, also found nothing out of the ordinary.
"Comparing spring chemistry in the area of uplift of 10 years ago, our re-survey of springs last summer didn't show any detectable changes," Evans said in a telephone interview.
In contrast, the scientists have tracked measurable emissions of carbon dioxide at Mammoth Mountain, part of Southern California's rumbling Mono-Inyo Craters volcanic chain, which blew its top as recently as 250 years ago and remains volcanically active.
We're in a little bit of a quandary about the scenario we laid out here," Wicks told UPI.
"It's pretty surprising that one indicator shows something consistently, but the other two show nothing," agreed Gerlach. "Generally, you see all three when volcanoes become restless, so this is rather peculiar."
It may be that the process of awakening is at too early a stage for all the indicators to manifest themselves, scientists proposed.
Or, the magma may be beneath the "brittle zone" where heat and pressure are so intense, the rock behaves more like plastic, failing to incite the ground to move with any noticeable seismic activity, geoscientists suggested.
Perhaps, the previously unavailable instruments may be detecting a common but previously unidentified phenomenon, they added.
It is premature to determine whether the swelling indicates magma movement presaging a volcanic eruption, they agreed.
Nevertheless, the unique observations are prompting scientists to increase their vigil at the mountain's base, adding ground instruments to fill in the gaps left by satellite images which cannot penetrate snow and are, therefore, confined to the late summer-early fall months.
"They give us a year-to-year snapshot. In-ground instruments could fill in the void and give us certain types of data not available from space," Wicks said.
The research is providing invaluable clues into the life of a volcano.
"Only now, with this technology are we seeing that volcanoes can inflate like this. We're not sure of the significance of this," Wicks said. "Normally, people don't look at volcanoes unless they're seismically active and doing something. Here, we have a chance to study the dormant cycle, which we've never studied before."
"These findings show us that even though a volcano is not making any noise, whether it's surface eruptions or seismicity, it may still be active and doing things we should take note of," Scott told UPI.
The results were overviewed at a Geological Society of America meeting in Corvallis, Ore.
