The whole point of volcano monitoring is risk.
Well, there's also the sheer joy of scientific discovery for its own sake - volcanoes are fascinating in their own right. They're windows on what goes on inside this planet, they can tell us things about how the earth works in ways no other natural features can, and so we'd study them even if not a single human being would ever be inconvenienced by one. But a volcano like Mount St. Helens threatens to harm the people who built lives near it in quieter times. It presents hazards to homes and businesses, infrastructure and commerce, aviation and recreation, and it puts lives in mortal jeopardy. It can kill those close and seriously inconvenience those who might have believed they're too far away to worry. So when a volcano like St. Helens wakes up feeling rather indisposed, prudent officials give geologists a call and then provide them with the money, equipment and resources needed to assess the threat.
And you don't stop just because the volcano falls into a fitful, feverish nap.
The ominous and rapidly-growing bulge aside, there was plenty to keep geologists on their toes during that two and a half week lull beginning in mid-April 1980. The mountain may have stopped exploding, but on April 20th, seismicity hit a two-week high. Steam still vented from the summit, with small bursts at midday on the 22nd showing things were still hot, and two new vents opened by the 23rd, confirming that things weren't cooling down. Tilt measurements revealed a pattern that told geologists that magma might be putting increased pressure on the mountain, inflating it from within. If you were living in areas close to the volcano, USGS geologist Dwight Crandell told folks via the Columbian, plan for evacuation. Have a route ready.
People depended on the scientists getting it right. They needed information: what was she up to? What was she likely to do next? What were the current and future hazards? Could they give a warning before the big bang came?
Remote monitoring of an active volcano can only tell you so much. Seismometers, tiltmeters, and other tools to assess from a distance can give some indication of what the volcano's up to, but only so much. So as soon as St. Helens quieted down enough to be relatively safe, geologists took some calculated risks, not knowing for certain what the odds were. They collected samples of the gasses in the steam vents near the summit, looking for the kinds of gasses that could tell them more about magma. Sulfur dioxide levels turned out to be high - an indication magma was close to the surface. As magma rises, there's less pressure on it, and gasses come out of solution, escaping through cracks and crevices, forcing vents, and giving scientists a look at what might be going on. Studies of sulfur dioxide emissions and their relation to potential eruptive activity at active volcanoes were pretty new then - they'd only begun to be closely studied in the 60s and 70s, but scientists had already noticed that increased levels of SO2 preceded eruptions at other active volcanoes such as Etna and Asama. If St. Helens followed the same program, SO2 might be one of the critical indicators things were about to blow.
So David Johnston, who was one of the USGS experts in volcanic gasses, grabbed some ash samples from the summit crater on April 23rd. Fumaroles deposited films of chemicals on the ash grains: they could be leached, and studied, and a little more about what St. Helens was up to discovered. That was a risk. The volcano had only stopped exploding a few days before: it could resume at any time, and climbing around by the crater had to be a bit nerve-wracking, even with helicopter support.
But David Johnston was a dedicated geologist who knew that more samples collected as close to the scene of the action as possible would give vital information, and so he climbed down in to the crater on April 30th to sample the brand-new crater lake.
Explosions, avalanches, rock falls, earthquakes, sudden bursts of steam, poisonous gasses, probably about a billion other hazards I haven't thought of: these are the things geologists will risk to understand a volcano. They do it for love of science, and they do it to save lives. Keep that in mind next time you're out for a drink, and raise a glass their way.
While Dr. Johnston risked his life in the maw of the mountain, Dwight Crandell discussed matters with the USFS, who, under pressure from folks who were being kept from homes and jobs by closures on the mountain, and in light of the lull, were considering opening bits for restricted use. Dr. Crandell painted them a rather stark picture of risk. That bulging north flank could send avalanches plunging into Spirit Lake or across the North Fork Toutle River, causing sudden floods people couldn't outrace.
The Forsyth Glacier, he said on April 25th, was "humped up into one big black rock mass." Jiggle it with one big earthquake or steam blast, and it would go careening down the slopes at 290 kilometers (180 miles) an hour an hour. Two minutes later, the whole mass would plunge into Spirit Lake. "There will be no way to warn people," Dr. Crandell said. "If they are in the path, they won't be able to get out of the way." And with the area flooded and Spirit Lake Highway blocked by avalanche debris, any survivors would be stuck.
This was serious stuff. Any notion of easing restrictions needed to be considered carefully, and due caution employed.
New cracks appeared in the bulge on April 27th. On April 29th, the terrible risk it posed caused officials to ask Governor Dixie Lee Ray to close more of the mountain. Their plan sketched two distinct areas: a Red Zone ranging from 5 to 11 kilometers (3-7 miles), which would allow no public access at all, and a Blue Zone, where a few authorized civilians could, with special permission, gain access for work during daylight hours. Gov. Ray acceded to their request the next day.
On May 1st, geologists watched a survey target on the north flank zoom out over half a meter (2 feet) in less than twelve hours.
One can imagine no one watching that questioned the wisdom of establishing stringent restrictions.
A new observation post was set up at Coldwater II, five miles north of the summit. Geologists staffed it round the clock, with only a small travel trailer for comfort. From there, with the time-lapse cameras and monitoring equipment, they could watch the mountain change and be there to capture its further explosive antics, should they happen.
They had only a week to wait.
Klimasauskas, E. and Topinka, L. (2000-2010): Mount St. Helens, Washington, Precursors to the May 18, 1980 Eruption. Cascades Volcano Observatory website, USGS (last accessed July 26th, 2012).
Korsec, M.A., Rigby, J.G., and Stoffel, K.L. (1980): The 1980 Eruption of Mount St. Helens, Washington. Department of Natural Resources Information Circular 71. (PDF)
Lipman, Peter W., and Mullineaux, Donal R., Editors (1981): The 1980 Eruptions of Mount St. Helens, Washington. U.S. Geological Survey Professional Paper 1250.