DESCRIPTION:
May 18, 1980 Eruption of Mount St. Helens

MSH80_eruption_mount_st_helens_05-18-80.jpg
Eruption of Mount St. Helens.
USGS Photograph taken on May 18, 1980, by Austin Post.
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This activity has been thoroughly documented and is familiar to most volcanologists. See especially papers in Lipman and Mullineaux (1981), a series of nine papers in Science (v.221,no.4618, 1983), and Swanson, et.al. (1987) for general summaries. Only a synopsis is given here; other specifics are mentioned in the road log.

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. Hydorexplosions created phreatic pits on the pumice plain, possibly as the pyroclastic flows covered the dewatering debris avalanche (Moyer and Swanson, 1987).

The 1980 activity of Mount St. Helens is summarized in U.S. Geological Survey (USGS) Professional Papers 1249 (Foxworthy and Hill, 1982) and 1250 (Lipman and Mullineaux, 1981). The activity began on March 15 with an increasing number of earthquakes beneath the volcano. The first phreatic eruption occurred on March 27, coincident with a high level of seismic activity. A summit crater formed and continued to enlarge for 2 months as phreatic activity continued. Tephra erupted during this time was composed of pulverized old rock, no new magma; however, viscous magma was intruding high into the cone, forming a cryptodome whose surface manifestation was the famous "bulge" on the north flank. This bulge grew outward at a maximum rate of 8.2 feet per day (2.5 meters per day) with no acceleration or other significant change until the climactic eruption.

The eruption at 8:32 a.m. P.D.T. May 18 was apparently triggered by a magnitude 5.1 earthquake that caused the unstable north flank to fail as three great retrogressive landslide blocks. The landslides developed into a complex debris avalanche that sped down the valley of the North Fork Toutle river, reaching its termination 16 miles west of the volcano in about 10 minutes. Unloading of the volcano by these landslides relieved pressure on the cryptodome and its associated hydrothermal system; the depressurized gases violently expanded and generated a northward-directed lateral explosion or blast. A pyroclastic surge (Moore and Sisson, 1981) or flow (Walker and McBroome, 1983) developed from the blast and fanned outward from the volcano, felling trees and killing most wildlife in a 212-square-mile (550-square-kilometer) area. Two columns convectively rose from the devastated zone and joined to reach a maximum height of 16 miles (25 kilometers) by 9:00 a.m.

The landslides and blast removed the upper 1,312 feet (400 meters) of the cone and left a crater 2,050 feet (625 meters) deep, 1.7 miles (2.7 kilometers) long, and 1.3 miles (2.0 kilometers) wide. About 30 minutes after the blast, debris falling from the unstable crater wall and lesser vesiculating dacitic magma from the roots of the cryptodome were explosively ejected in an eruption column that ranged from 9 to 10 miles (14-16 kilometers) in height throughout the morning. Dark-gray ash, consisting largely of lithic debris from this column, fell more than 930 miles (1,500 kilometers) away. The eruption column lightened in color and became more energetic at about noon, possibly as a fresh supply of gas-rich magma reached the surface; pumiceous pyroclastic flows spilled northward from the crater and covered part of the debris avalanche, forming the Pumice Plain. Light-gray magmatic ash from this 9 to 12 mile (14-19 kilometer) column fell on earlier, dark-gray lithic ash in eastern Washington and northern Idaho. The eruptive activity declined and ended that night.

Many mudflows formed on May 18, mostly by melting of snow and glacial ice. The largest mudflow, down the North Fork Toutle River, formed as the debris avalanche dewatered (Janda and others, 1981; Voight and others, 1981, 1983). This flow destroyed or heavily damaged 200 homes and deposited more than 95 million cubic yards (72 million cubic meters) of sediment in the Cowlitz and Columbia Rivers, where clogged shipping channels required costly dredging (Schuster, 1981).

Mount St. Helens, Washington: The catastrophic eruption on May 18, 1980, was preceded by 2 months of intense activity that included more than 10,000 earthquakes, hundreds of small phreatic (steam-blast) explosions, and the outward growth of the volcano's entire north flank by more than 80 meters. A magnitude 5.1 earthquake struck beneath the volcano at 08:32 on May 18, setting in motion the devastating eruption.

Within seconds of the earthquake, the volcano's bulging north flank slid away in the largest landslide in recorded history, triggering a destructive, lethal lateral blast of hot gas, steam, and rock debris that swept across the landscape as fast as 1,100 kilometers per hour. Temperatures within the blast reached as high as 300 degrees Celsius. Snow and ice on the volcano melted, forming torrents of water and rock debris that swept down river valleys leading from the volcano. Within minutes, a massive plume of ash thrust 19 kilometers into the sky, where the prevailing wind carried about 540 million tons of ash across 57,000 square kilometers of the Western United States.

The well-documented landslide at Mount St. Helens has helped geologists to recognize more than 200 similar deposits at other volcanoes in the world, including several other Cascade peaks. Geologists now realize that large landslides from volcanoes are far more common than previously thought--seventeen such volcanic landslides have occurred worldwide in the past 400 years. Consequently, when scientists evaluate the types of volcanic activity that may endanger people, giant landslides are now included, in addition to other types of volcanic activity such as lava flows, pyroclastic flows, lahars, and falling ash.

Following the 1980 explosive eruption, more than a dozen extrusions of thick, pasty lava built a mound-shaped lava dome in the new crater. The dome is about 1,100 meters in diameter and 250 meters tall.

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