There's more to an eruptive sequence than explosions. And there are times when a distinct lack of explosions are more troubling than endless ash columns. When earthquakes continue rattling the slopes, and one of those slopes is swelling outward several feet per day, concern and caution are the only reasonable responses.
In mid-April of 1980, Mount St. Helens had seemingly gone back to sleep. The frequent phreatic eruptions ceased. The volcano steamed quietly under stormy Pacific Northwest skies. Some residents thought the show might be over, but geologists knew better. The song wasn't done: the prima donna was taking a last deep breath. The only question was what sort of finale to expect.
One thing was certain. That bulge they had noticed in late March was going to come down. Its failure would be spectacular. That was beyond question. It was growing too fast, over-steepening itself too much, to stay put.
Pause for a moment to consider the enormity of the ground deformation on that north slope: an area 1.8 kilometers (about 1 mile) long and almost 1 kilometer (about .5 mile) wide was pushing out laterally. And it wasn't a slow change: it bulged at a rate of 1.5-2 meters (5-6.5 feet) per day. That would take a chunk of Seattle roughly from the waterfront to Interstate 5, Pike Place Market to Pioneer Square, and pop it up at least 91 meters (300 feet) in less than two months. Prop the city up vertically by around 30 degrees, add in dozens of earthquakes per day, not to mention the phreatic eruptions taking place intermittently for weeks at a time, and you have a recipe for downtown Seattle to end up heading south in a hurry. At speeds of up to 290 kilometers (180 miles) per hour, in fact. You can imagine why USGS geologist David Johnston said of the bulge on Mount St. Helens that the "only way it can stabilize is to come down."
When you look at the area of the mountain involved, you know why Dwight Crandell and other United States Geological Survey and Forest Service officials were seriously concerned about catastrophic landslides.
At the rate the bulge was growing, it didn't much matter whether Mount St. Helens erupted again or not. This episode wasn't going to end quietly.
By April 24th of 1980, the totality of the evidence gave Dr. Johnston the confidence to advise the media that it appeared magma was rising within the volcano. Studies on the bulge pointed toward a cryptodome, a thick, sticky mass of dacitic magma rising under the surface. If it had been free to squeeze out, it might have created a bump like Goat Rocks or Sugarbowl. But the old summit dome of cold, hard rock was in its way, so it remained hidden - cryptic. It pushed hard against the old dome, which stretched and split under the pressure. The summit graben - that long, linear fracture at the top - was produced this way. And the continued intrusion of the cryptodome caused the boundary between the graben and the bulge to speed northward at alarming speed: 28 feet (8.6 meters) each day. When geology moves that fast, something extreme is going on.
A huge amount of deformation was taking place, but it was remarkably sharply defined. Point surveying equipment at a target set up on the bulge, and you could measure displacement in meters. Aim at a target just 488 meters (1,600 feet) to the side, and you're down to millimeters. The south, west and east sides of the volcano hardly budged at all. The whole force of that rising magma seemed concentrated on one sweet spot.
You can imagine that hot magma pushing so close to the surface would heat things up a bit, and you'd not be wrong. The bulge abounded with infrared anomalies, which multiplied from early April onward. At the Boot, which registered the highest temperatures, the thermal area measured a surprisingly human 37°C (98.6°F) compared to the chilly -1°C (30°F) of unaffected rocks and ice nearby. It takes a lot of heat to keep cold rock that balmy in the cold of a Cascades spring.
And a May 2nd high-resolution thermal infrared survey undertaken by personnel from the United States Navel Academy, Whidbey Island, showed things heating up further. They found new warm areas in the middle, an anomaly on the south slope of the recently-created North Peak 2 and a much stronger one on its north slope, and several anomalies in the badly fractured upper end of the Forsyth Glacier. Something not far down was awfully hot, and getting hotter.
This, combined with other indications in that ominously quiet period between April 19th and May 7th, pointed toward a spectacular end to the show. The only questions, as the mountain swelled and earthquakes continued, were up, down or both?, how big? and when?
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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 19th, 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.