May 23, 2012 | 1
We knew she was dangerous.
Her former perfection was a sign of the cataclysm to come. When we see a volcano so exquisitely formed, her flanks so full, her form ungouged by the glaciers draped over her, we know she’s young and full of zest. She’s been active recently; she’s likely to awaken again at any moment.
And we knew.
The Klickitat tribe called her Louwala-Clough, fire-mountain. Pioneers and explorers heard stories of her activities, and then witnessed the reason for her native name in 1835. She was in the midst of her Goat Rocks eruptive episode (1800-1857); she frequently put on a show. She showed painter Paul Kane what a volcano was all about, as she put the finishing touches on her cone.
Then, in 1857, she settled back to sleep. Cities grew up around her. People visited her clear streams and blue lakes, walked her forests, logged her flanks; when cameras became ubiquitous, so did photos of her symmetrical self. Geologists nosed around, fascinated by evidence of her recent eruptive history. They pieced together her past: 36,000 years of eruptions that had built a beauty with lava flows and tremendous explosions, ash clouds, pyroclastic flows, dome extrusions, and lahars. As volcanoes went, she was young. And by 1975, geologists suspected that her slumber was a mere nap. Dwight Crandell, Donal Mullineaux, and Myer Rubin published a paper in Science that year, “Mount St. Helens Volcano: Recent and future behavior.” Her prior antics suggested she “will erupt again – perhaps within the next few decades,” they wrote. “Potential volcanic hazards of several kinds should be considered in planning for land use near the volcano.”
Two of the authors, Crandell and Mullineaux, expanded on that theme in a 1978 USGS bulletin, “Potential hazards from future eruptions of Mount St. Helens, Washington.” They highlighted her youth: most of what we saw of her cone had been built over the last 1,000 years. Anyone reading this and remembering the size of her would have to pause here, and realize that’s a hell of a lot of activity over a very short period of time. Very recent time.
And, the authors said, she will erupt again. In a few terse, unflinching sentences, they told us, “Future eruptions of Mount St. Helens are a near certainty. It will not be possible to prevent them, or to stop them after they have begun.”
We were in for it, then. No doubt about that, and no way to avoid it. So what might we expect? The authors studied her eruptive history carefully, drawing on their own work and that of other geologists, to get a sense of her. They determined the types of lava St. Helens liked to erupt: dacite, andesite, and basalt. They compared her to other, similar volcanoes, and figured lava flows were more likely later in an eruption, appearing days or weeks later rather than right at the beginning. Based on the work of Hopson in 1971, they had her typical pattern nailed: St. Helens liked to start with a bang, gas-rich dacite exploding out in pumice-forming blasts. Then, once she’d got that out of her system, she settled in to some dacite dome building, with perhaps a few flows of andesite or basalt on the side. All of this activity would stretch from years up to decades. But, they warned, we couldn’t take that pattern as a given. “[N]ot all past eruptions at Mount St. Helens have followed this idealized succession,” they wrote, “and it is by no means certain that all future eruptions will.”
St. Helens had blown out laterally before. Though they didn’t dwell on it, they explored the possibility of lateral eruptions. “Explosions can also cause lateral blasts of great force which can carry steam and rock fragments from the dome outward at a high speed to distances of at least 10 km. Pyroclastic flows, rock-debris avalanches, and mudflows associated with the dome eruption could affect areas much farther away.” They noted that blasts of that sort were often associated with dome formation. Domes, they thought, would likely form on her flanks. The summit dome seemed to have things pretty well plugged up top.
Tephra eruptions – the kind that throw ash and pumic and other bits into the air – could issue from vents on the flanks or from the summit: no part of the volcano was safe, but people between 25-30km away would probably face no worse fate than a tough clean-up job. Pyroclastic flows could impact virtually any portion of the volcano within 6km (roughly 4 miles), and valley floors for an additional 10km (6.2 miles). Mudflows could seriously inconvenience valleys and reservoirs near the volcano. And, if a major eruption happened when snow pack and rains were heavy, floods could reach the Columbia River.
We don’t know what type of eruption we’ll be getting, the authors concluded, and there’s no way to tell when, but her behavior pattern suggested “the current quiet interval will not last as long as a thousand years; instead, an eruption is more likely to occur within the next hundred years, and perhaps even before the end of this century.” And when she did erupt, we could expect her to continue intermittent activity for decades.
They had no way of knowing that within two years, Mount St. Helens would not only prove them right, but exceed their expectations. But geologists had sounded the warning. Sleeping beauty would not sleep for long. We didn’t know what sort of mood she’d be in when she stirred. We needed to be ready for anything. And we needed to prepare now.
Crandell, D. R., and Mullineaux, D. R., 1978: Potential hazards from future eruptions of Mount St. Helens, Washington. U.S. Geological Survey Bulletin 1383-C.