Mount St. Helens, Washington
May 18, 1980 Eruption

March 20 - May 18, 1980

This activity has been thoroughly documented and is familiar to most volcanologists. See especially papers in Lipman and Mullineaux (1981) ...

Seismicity began several days before March 20, 1980, when an earthquake (M=4.2) centered under the volcano commanded wide attention. The first of a series of small phreatic explosions occurred on March 27, accompanying the opening of a crater within a horseshoe-shaped graben concave northward at the summit of the cone. Strong seismicity continued, at times with bursts of deep volcanic tremor (Endo, et.al., 1981; Quamar, et.al., 1983); deep tremor was felt state-wide in early April but died away without returning. By mid-April a bulge was obvious on the north flank of the volcano; geodetic measurements began shortly thereafter and documented horizontal growth of the bulge at a steady maximum rate of >1.5 meters/day (Lipman, et.al., 1981a). The bulge was surface evidence of a cryptodome intruding the volcano. Seismicity continued into May, with fewer but larger earthquakes, and phreatic activity was intermittent. No magmatic gas was detected, although new fumaroles appeared in the crater and at the head of the bulge.

At 0832 on May 18, a complex earthquake (M=5.1) shook the volcano, probably causing (but possibly caused by) a huge, 2.7-cubic-kilometer-landslide that in three different blocks successively removed the bulge and upper 400 meters of the volcano (Voight, et.al., 1981, 1983), leaving a 600-meter-deep crater 2 kilometers wide rim-to-rim. The landslide quickly developed into a debris avalanche that sped at 110-240 kilometers/hour for 24 kilometers down the North Fork Toutle River; arms of the avalanche entered spirit Lake, 8 kilometers from the summit, and overtopped 300-380-meter high Johnston Ridge north of the Toutle. The avalanche buried the Toutle valley to a depth of nearly 50 meters. Its hummocky deposit is distinctive; similar morphology at other volcanoes has been reinterpreted in light of its observed origin (Siebert, et.al., 1987).

The landslide removed confining pressure on the cryptodome and its surrounding hydrothermal system. Juvenile gas was rapidly released from the cryptodome and superheated groundwater flashed to steam, causing a blast that exploded laterally from the collapsing north flank. The blast, called a "stone wind" by local journalists, knocked down most trees (the equivalent of about 150,000 houses) in a 600-square-kilometer area. Its maximum velocity may have been supersonic (Kieffer, 1981b). The degree to which juvenile gas or flashed groundwater drove the blast is debated (Eichelberger and Hayes, 1982; Kieffer, 1981a, b; see also Brugman, 1988), as is the question of whether the blast was, in volcanologic terms, a low-aspect-ratio ignimbrite (Walker and McBroome, 1983) or a surge (Hoblitt and Miller, 1984; Waitt, 1984b).

Soon after the blast, a lahar rushed down the South Fork Toutle River and several streams draining the south and east flanks of the volcano. Whether water for these lahars came from snowmelt or from groundwater ejected by the eruption is hotly argued. The largest lahar, down the North Fork Toutle, did not start until early afternoon; it was fed as the debris avalanche dewatered (Janda, et.al., 1981). In addition to causing havoc along the rivers themselves, the lahars fed so much debris into the columbia River that 31 ships were stranded in upstream ports until the 4-meter-deep channel was dredged to its pre-eruption depth of 12 meters -- the first in a series of similar dredgings to maintain Portland (Oregon) as a seaport.

Juvenile dacite pumice and ash mixed with lithic debris began erupting soon after the blast (Criswell, 1987), perhaps from the shallow root of the cryptodome. The flux increased about noon, apparently with arrival of pumice from a 7-10-kilometer-deep reservoir (Rutherford, et.al., 1985; Carey and Sigurdsson, 1985; Scandone and Malone, 1985). Experimental work suggests that just before eruption this reservoir was a a pressure of 220 +/- 30 MPO, P_water was 0.5-0.7 P_total, and the temperature was 930 +/- 10 degrees C (Rutherford, et.al., 19885). Pyroclastic flows fed by the eruption column covered the debris avalanche in the upper North Fork Toutle basin, forming the pumice plain. Hydroexplosions created phreatic pits on the pumice plain, possibly as the pyroclastic flows covered the dewatering debris avalanche (Moyer and Swanson, 1987).