Prelude to a Catastrophe: "Something Dramatic"

The earthquake activity at Mount St. Helens had built to a crescendo. When a volcano shakes this hard, it almost always spells trouble: magma rising, an eruption imminent. You can?t know exactly what they are going to do, and when, and to what degree. But you suspect. You prepare as best you can.

On March 27th, the USGS issued a Hazards Watch, informing public officials of the dangers St. Helens might pose. After a week of increasing shakes, there was little doubt in any scientist?s mind that ?something dramatic? was about to happen. By 11:20am Pacific Standard Time, something dramatic had.

Summit area of Mount St. Helens. Aerial view on the afternoon of March 27 looking east, showing newly formed crater, swath of dark new ash mainly to southeast of new crater, an east-west fault across middle of summit area, and an uplift or bulge on upper north flank of the volcano. Photo by David Frank. Skamania County, Washington. March 27, 1980. Portion of Figure 6, U.S. Geological Survey Professional paper 1250.

It must have been eerie for the observer in the Army National Guard plane who first saw the gaping hole in her formerly-pristine summit. A dark gray streak of ash stained the snow, following the winds to the southeast. The deep snow had cracked. She was awake. And it wouldn?t be long before she let folks know she was feeling feisty.

At 12:36pm, a boom echoed through the wilderness near the volcano. Portland reporter Mike Beard, flying above her, saw ash roiling ?like smoke out of a chimney? through the clouds.

Phreatic explosion from Mount St. Helens. This image is from a later eruption, but is similar to what Mike Beard would have seen on March 27th. USGS photo courtesy of Dan Miller.

When volcanoes erupt, many people think lava. Lots of lava. And that is true for some volcanoes, like the lovely shields that make Hawaii such an interesting place to live. But a volcano like St. Helens doesn?t start with lava. She begins by clearing her throat, so to speak, gargling a bit, putting on a small performance before the main event. In her case, thick, sticky dacite, a type of magma rather rich in silica, was making its laborious way up toward the surface. Dacite has a tendency to form domes. It also has a distressing tendency to blow up. But the rising dacite hadn?t reached the surface just yet: it had just gotten close enough that its heat flashed water to steam. The result was a phreatic (steam) eruption, strong enough to break and pulverize summit rock and blow a hole in the top. Steam and ash gave Mike Beard a jolly good show, and made a racket. But no lava. Not yet.

Still. The phreatic eruptions caused some pretty dramatic changes.

Oblique aerial photographs of the Mount St. Helens summit area showing historic thermal areas. Distance from False Summit to summit about 600 m. Viewed from north, thermal area A, near The Boot, is covered by snow, but its location corresponds to shallow dimple in snow surface. Photo by D. Frank. Skamania County, Washington. March 24, 1980. Portion of Figure 149-A, U.S. Geological Survey Professional paper 1250.

Three days before, all had been serene snowfields. Cascades volcanoes tend to look like ice cream cones, with their steep rounded shapes and their deep white coats. Mount St. Helens, for all the seismic excitement, was still a perfect example of the type.

Nothing would ever be the same again.

Oblique aerial photograph of summit of Mount St. Helens, looking south. Location of thermal area A indicated. Note that the new fractures cross ice and rock areas with no apparent change of style, indicating the fractures are deep seated. Photo by D. Frank. Skamania County, Washington. March 27, 1980. Portion of Figure 150, U.S. Geological Survey Professional paper 1250.

Compare the two photos above. They?re the same area. They hardly look it.

The afternoon that had started with a bang continued on a sustained dramatic note. At 2:00pm, seismologists at the University of Washington watched their equipment register a hefty magnitude 4.7 earthquake, the second-strongest of the swarm. They couldn?t see events on the mountain, but others witnessed a gigantic black plume unfurling to a respectable 7,000 feet (2,133 meters). Earthquakes during the eruption happened at such a frequency that seismologists had trouble telling them apart on the graphs. David Gibney, an aerial spotter for the United States Forest Service, saw fractures open and close, the north flank break and rise, as he flew over in the hours after that first eruption. Think of that: a mountain, hurling ash and steam, cracking and breaking beneath your gaze. Something dramatic, indeed.

At the end of that opening sequence, Mount St. Helens sported a crater 200-250 feet (60-75 meters) across nestled within the old, ice-choked summit crater. The summit had been cracked like an egg, split open by an eastward-trending fracture that slashed her flanks from northwest to northeast, a gash measuring nearly 5,000 feet (1500 meters) in length. Other fractures ran alongside it. They defined the south side of an uplifted block, possibly already pushing outward, on her north flank.

The Bulge had been born.

View from the NW, March 30, 1980, showing the summit graben, the north-side bulge, and an on-going steam eruption. Note the dark gray pulverized rock falling on the south side of the mountain. Annotations: north and west sides of the mountain are marked; a curved arrow shows the continuing movement of the the north side bulge; dotted lines show the approximate edges of the summit graben; double-headed arrow illustrates extension across the graben. Image courtesy USGS, annotations and caption by Lockwood DeWitt.

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.

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