The mountain boomed. Steam and ash soared to 3,962 meters (13,000 feet), announcing the end to a two-week lull. At the top of Shoestring Glacier, an opening steamed. It was May 7th, 1980, and Mount St. Helens was letting everyone know she wasn’t ready to sleep yet.
The next day, a muddy rain splattered Timberline, where the USGS had been taking measurements of the mountain as that ominous bulge grew. As the first harmonic tremors since April 12 traced their unique pattern on seismometers, new avalanches cascaded up to 305 meters (1000 feet) down Wishbone Glacier. And all the while, the crater steamed.
Geologists abandoned Timberline on May 9th. The danger from avalanches had grown too great. They’d have to rely on other observation posts, such as Coldwater II; a new tiltmeter installed at Ape Cave would help provide data on the continued swelling as the suspected cryptodome continued pushing out the north flank by up to 1.5 meters (5 feet) per day.
None of the renewed phreatic eruptions were particularly large: most of them after May 7th were small, spitting steam and ash and putting on a decent show for the locals and observing scientists on clear days, but not exactly spectacular. Still, they signaled that things were definitely hot within St. Helens. And their message was backed by the fumarole at Shoestring Glacier, which steamed ever harder.
Geologists had been worried for some time about the unstable north flank. On May 12, while small explosions continued and observers watched a cluster of fumaroles on the crater’s western rim steam vigorously, everyone got a taste of what might come. An earthquake registering a robust 5.0 shook the mountain, sending an avalanche 244 meters (800 feet ) wide cascading from the lower end of the humped-up Forsyth and Leschi glaciers. The surveying target on Sugar Bowl perished. Seismologists from the University of Washington hiked their portable seismometer from Spirit Lake to the top of a nearby ridge, figuring the valley floor right below the bulge would eventually get hit by an avalanche.
Everyone knew it was just a matter of time. The whole area around Goat Rocks had swollen 106 meters (348 feet) northward. And the volcano continued to swell, crack and steam.
By May 13th, the fumarole at the head of Shoestring Glacier had enlarged considerably; new steam vents appeared the following day on the north crater rim. Older ash on the upper southern slopes was encrusted with yellow-green deposits, most likely sulfur. Measurements of sulfur dioxide gas showed a dramatic increase: from .3 tons per day on March 30th to 1 ton per day now (skyrocketing to 10-20 tons per day during explosions). All of those steam vents, all of that sulfur, warned that an extremely hot body of magma was rising, and might be nearing the surface. The volcano had been bulging: now, it began inflating. Something was coming.
Explosions ceased on May 15th, but the bulge continued its outward journey unabated, and steam hissed from its upper reaches. The summit radiated so much heat that the thermal energy equaled around 3 megawatts – if it could have been captured and used, it would have powered about 1,800 homes. Geologists found pits that appeared bottomless in the hottest areas. Two clusters of thermal infrared anomalies marked areas of weakness in the bulge. In two days, the bulge would come apart at those seams.
But things seemed calm by May 17th. The bulge slowed its growth. Even the earthquakes went down: there were only 6 larger than a 4.0, a lower rate than previous days. USGS geologist David Johnston sampled gasses at the fumarole up on the Boot, clinging to the fractured side of the mountain as a fellow geologist snapped his picture from the helicopter waiting to collect him.
He stashed those samples at Coldwater II, and prepared for a long night with the volcano. His assistant, geology student Harry Glicken, had been scheduled to monitor the mountain, but had an interview at the University of California-Santa Barbara. Don Swanson would be taking over his shifts, but not that night – he had a student of his own to see safely onto a plane. So Dave agreed to take over until Sunday afternoon.
Harry snapped a last photo of his mentor before leaving.
And the last quiet night passed.
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.