This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
What do you do when the volcano whose beauty you've admired for so long suddenly wakes with a shiver and blows a plume of steam and ash into the sky?
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Thanks to Dwight Crandell and Donal Mullineaux, officials had a reasonable idea of what to expect. On March 27th, as the first phreatic eruptions fractured the summit and sent pulverized rock billowing to 2,134 meters (7,000 feet), they moved quickly to mitigate the risks. Hundreds of people living within a 24 kilometer (15 mile) radius of Mount St. Helens found themselves bundled out to safety. Pacific Power and Light, working off hazards assessments, lowered their three reservoirs on the Lewis River to accommodate expected mudflows. And geologists flocked to the mountain for the opportunity of a lifetime. It's not often a volcano so close to major population centers gets frisky.
And frisky she was. Magma was definitely on the move within her, and as it worked its way up through her interior, its heat converted water to steam. Explosion after explosion - 13 of them on March 28th - tore through old rock, turning some to fragments blasted up to 4,877 meters (16,000 feet). Chunks up to 1 meter (3 feet) in diameter were hurled out of the newborn crater. Ash fell as far as 56 kilometers (35 miles away). And avalanches and mudflows cascaded down to the 1,951 meter (6,400 foot) level. As if all those pyrotechnics weren't clue enough, sulfur dioxide gas venting along with the steam and ash told geologists that high-temperature magma was rising.
By the morning of the 29th, a second crater had been excavated just to the west of the first, separated by a narrow septum. Activity shifted to the new crater. Throughout the day, phreatic eruptions followed a pattern: steam and ash to begin, steam only as their finale. They formed the backdrop to busy scientists performing gravity measurements on the north flank, while others measured sulfur dioxide emissions and analyzed streams for changes in their chemistry. Seismologists monitored the mountain's continued shaking. And during the night, David Johnston watched pale blue flames dance within and between the craters. He never did get a chance to positively identify their cause, but thought they were likely caused by flammable gas. Hydrogen sulfide was his number one suspect. It must have been mesmerizing, that pas de deux of those cerulean volcanic fires, glowing in the cold and dark of a wintery Cascades night.
The twin craters weren't long for this world. Ninety-three eruptions on March 30th sent ash drifting over Mounts Jefferson and Hood; some of the explosions were the largest yet. Photographs showed the north flank of the mountain bulging. And those eerie blue flames still danced.
On March 31st, the first harmonic tremor appeared on seismographs: it would be recognized the next day, and combined with all the other fuss, indicated that magma was definitely on the move. Harmonic tremors don't always indicate that a magmatic eruption is on its way, but combined with the vigorous phreatic blasts and the growing bulge on the north flank, they were a definite sign St. Helens wouldn't remain content just blowing off steam.
Those phreatic eruptions had a recognizable character to the geologists avidly studying them. They comprised three parts: a dark, spiky, fingerlike column of ash, with a grayish-brown ash cloud above, and a nice topping of billowy white steam.
Explosions continued into April, usually several per day, some with plumes that reached 6,096 meters (20,000 feet), dusting even Portland and Vancouver with ash if the wind was right. The north flank fractured as far down as 1,646 meters (5,400 feet). The Upper Wishbone Glacier succumbed to a series of long eruptions on April 8th, its top collapsing into the growing crater, while the bottom threatened to slide into the North Fork Toutle River and wreak all sorts of havoc. While sightseers flocked to watch the show, and local businesses took the opportunity to sell kitschy t-shirts and food with "volcano" appended to its name, officials scrambled to keep ahead of the dangers. On April 14th, Dwight Crandell, who understood St. Helens perhaps more intimately than anyone else, explained the potential for catastrophe to representatives from the United States Forest Service, Washington Department of Transportation, the Highway Department, and the State patrol. Road closures along the North Fork Toutle River, he advised, were important, because a landslide from that increasingly unstable north flank could quite possibly land in Spirit Lake and even extend right down the river valley. A landslide of that size, or a quite large earthquake, might trigger something very different from the phreatic throat-clearing St. Helens was doing now. Such a landslide or earthquake could release the magmatic eruption all those disparate signs were pointing to.
No one then had any way of knowing just how correct he would turn out to be.
But St. Helens continued delivering nothing but old news in the next four days. All of her ash was lithic, old stony stuff created from her own edifice. And her eruptions were becoming smaller, more fitful, almost laughably weak compared to her previous exertions. Locals and loggers began to believe she'd go back to sleep. Geologists weren't sure, but regardless of whether she fulfilled Crandell's expectations or just extruded a dome, that north flank was a concern. It kept growing, its ice and rock fracturing.
On April 18th, St. Helens emitted a few small steam plumes that achieved a paltry 152 meters (500 feet), barely enough to clear the summit crater. Earthquake activity calmed to a moderate hum. She seemed ready to settle back to rest.
The pale blue flames ceased their dancing, but the bulge didn't slow. David Johnston knew it was a symptom of "some type of dramatic change going on inside the mountain." This, then, was an intermission: no one knew what the next act would be. All eyes were on that dangerously distended north flank.
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References:
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 June 13th, 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.