By Patrick Pringle
From Washington Geologic Newsletter
Vol. 18, No. 2, May 1990
Quarterly publication of the Washington State Department of Natural Resources
Ten years ago, on May 18, 1980, Mount St. Helens volcano erupted cataclysmically, producing a huge debris avalanche, an explosive, laterally direction "blast", lahars, and a Plinian eruption column. This powerful eruption had a profound impact on the Pacific Northwest – and on volcano studies as well. This article briefly reviews the effects of that eruption and subsequent eruptive activity and discusses some of the implications of the eruption for volcanology. For those seeking greater details, the 1980 volcanic activity and many of its impacts are richly documented (see Manson and others, 1987).
Studies by the U.S. Geological Survey (Crandell and others, 1975) described Mount St. Helens as the youngest and most active volcano in the Cascade Range and documented the eruptive history of this 40,000-year-old volcano. Although the mountain had been quiet since about 1857, the authors warned of the likelihood of future eruptions on the basis of the frequency of its past eruptions. The mountain erupted only 2 years after publication of their report (Crandell and Mullineaux, 1978).
Earthquake-swarm precursors to the May 18 eruption began under the volcano on March 20, 1980. On March 27 a phreatic (steam) eruption created a small summit crater, making Mount St. Helens the first Cascade Range volcano to erupt since Lassen Peak in 1921. The phreatic activity occurred intermittently until May 18, nevertheless, earthquakes, selling, and disruption of the mountain continued.
On May 18, at 08:32 PDT the catastrophic eruption began, apparently triggered by a magnitude 5.1 earthquake. A large bulge that had formed on the north flank of the mountain in response to the intrusion of magma failed retrogressively in a series of three huge block slides. This debris avalanche, the largest landslide in recorded history (about 2.5 km3 or 0.67 mi3), traveled northward into Spirit Lake, over 350-m (1,150-ft) ridge into the Coldwater Creek drainage, and westward 25 km (15.5 mi) down the North Fork Toutle River (Voight and others, 1981).
The sudden removal of this immense volume of material from the mountain unloaded the hydrothermal and matmatic system and released the laterally directed blast, which leveled all vegetation within 18 km (11.2 mi) in a 1800 sector north of the volcano (about 600 km2 or 230 mi2). Major lahars, generated by snowmelt caused by the explosion, flowed down the South Fork Toutle and Muddy Rivers, carrying logs, trucks, and even bridges with them. The largest and most destructive lahar occurred in the North Fork Toutle River later in the day when the saturated debris-avalanche deposit began to dewater. The lahar flowed slowly to the Columbia River and deposited as much as 30 million m3 (39 million yd3) of sediment, blocking the shipping channel to ocean-going vessels.
A Plinian column (vertical eruption of ash) rose to more than 20 km (12.4 mi) within 10 minutes of the eruption’s onset. An eruption column was maintained for more than 9 hours (Christiansen and Peterson, 1981). Fallout, including ash-sized particles, from the eruption amounted to more than 1 km3 (0.24 mi3) (Sarna-Wojcicki and others, 1981) and spread across Washington and Idaho and into Montana, disrupting human activities and transportation facilities and damaging civil works such as sewage- and water-treatment facilities.
In the aftermath, about 60 people had died as a result of the eruption, most from ash asphyxiation. More than 200 miles (320 km) of roads, 15 miles (25 km) of railways, at least 43 bridges, and about 200 homes were destroyed or severely damaged (Schuster, 1981). Mount St. Helens was reduced volumetrically by about 2.5 km3 (0.67 mi3).
Five smaller explosive eruptions occureed during 1980, and were accompanied by pyroclastic flows and tephra. Small dacite lava domes were emplaced at the end of three of these episodes. From October 1980 until 1990, 19 episodes of dome growth constructed a composite lava dome 267 m (876 ft) high (Fig. 1). Minor explosions accompanied several of these episodes, generating lahars that flowed at least 15 km (~9 mi) from the crater on two occasions (March 19, 1982, and May 14, 1984). The most recent lahar was generated by a hot avalanche on May 8, 1986 (Cameron and Pringle, 1990). After the most recent dome-growth event (October 1986), the volume of the dome had reached 74 million m3 (~97 million) (Swanson and Holcomb, 1990), more than 40 times the volume of the Seattle Kingdome. Although this figure seems impressive, it amounts to only about 3 percent of what the mountain lost in the May 18 eruption.
Volcanic hazards: preparedness and mitigation
The scope and philosophy of volcanic hazards analysis have evolved worldwide as a result of the scale of 1980 events at Mount St. Helens. In particular, those events produced the general recognition that steep-sided volcanoes are hazardous features and that unusual events like the Mount St. Helens blast are possible and must be included in a volcanic-hazards inventory.
In New Zealand, within five months of the Mount St. Helens’ eruption, the National Civil Defense Planning Committee on Volcanic Hazards was formed. Citing the increasing importance (and cost) of problems created by volcanic hazards in the industrialized society (as exemplified at Mount St. Helens), the committee solicited reports, risk analysis, and precautionary measures for the volcanic areas of the North Island, New Zealand (Dibble and others, 1985).
Similar investigations were already in progress in the United States and resulted in general hazards assessments for most Washington volcanoes and neighboring Mount Hood by 1982 (Beget, 1982; Crandell, 1973, 1980; Hyde and Crandell, 1978; and Hoblitt and others, 1987).
The three most important aspects of preparedness and mitigation with respect to volcanic hazards are: (1) communication of volcano monitoring and volcanic hazards information by geoscientists to the public, the media, and responsible agencies and officials, (2) emergency preparedness by responsible agencies and officials, and (3) the status of community and regional planning and land-use designations. All three aspects are interrelated insofar as they depend on the communication of understandable scientific information about, first, the current state and expected behavior of a volcano, and, second, the nature, extent, implications, and likelihood of impacts from a variety of volcanic processes near that volcano.
The communication of scientific information about the status of a volcano has improved mainly because geoscientists have an improved ability to predict eruptions at Mount St. Helens. Public demand for prompt and more understandable technical information and, for scientists, the experience of working with an accessible volcano such as Mount St. Helens, have helped to "fine tune" the communication process. Improvements in communications have taken place both in electronic media and newspapers and in geoscience literature (such as explanatory journal articles, books, and training manuals; see Tilling, 1989).
As an impetus for the transformation in communications, Swanson and others (1985) defined factual statements, predictions, and forecasts as these terms are used in public statements about volcanic activity at Mount St. Helens. Factual statements provide information but do not anticipate future events. Predictions are relatively precise statements about the time, place, nature and size of impending activity (usually on the basis of measurements at the volcano), whereas forecasts are comparatively imprecise statements about the nature of expected activity (typically based on the past history and potential of a volcano and geologic mapping). These and other terms have been incorporated into the lexicon of public statements about Mount St. Helens and have been accepted by the media and the public because they have provided a means to define and translate scientific information and to clarify public expectations and understanding of volcanic events and hazards.
Newhall (1982) devised a method for quantifying long-term hazards and risks at a volcano on the basis of sizes and types of eruptions, time intervals between eruptive episodes, and the effects at various distances from the volcano. He used a similar technique for short-term hazards based on work at Mount St. Helens (1984). In a more site-specific study, Hoblitt and others (1987) published frequency, order-of-magnitude, and probability information for a wide range of volcanic events, using as a model those recorded in the geologic history of individual Cascade Range volcanoes.
Details about improvements in emergency response and land-use planning as they relate to the learning experience of Mount St. Helens are beyond the scope of this article. However, as noted above, these two subjects are significantly dependent on technical information about volcanic hazards.
In the Pacific Northwest, debris flows and debris avalanches constitute some of the greatest volcanic hazards. However, detailed information on probabilities of flow types of varying magnitudes and frequencies are not yet published for specific drainages (although several such studies are in progress). This information is critical for planning purposes because urban/suburban growth typically occurs along river valleys; those valleys near volcanoes could be vulnerable to future flows.
Eisbacher and Clague (1984) discuss a range of active and passive hazards-management measures for flows of various sizes. Their techniques range from forestry practices, control works, protective works, and planning and zoning. They note that decisions about these debris-flow and avalanche hazards must be founded on the recurrence and magnitude information and a broad-based socio-economic consensus relating to the hazards and acceptance of risk. As growth continues in the Pacific Northwest, hazards management and emergency response decisions will no doubt be influenced by similar factors.
Despite the destruction it caused, Mount St. Helens provides a laboratory for many kinds of research that are now beginning to effectively prepare us for living with the hazards this volcano and others still pose.
Acknowledgments
I thank Dallas Childers, Randy Dinehart, Richard Hoblitt, Richard Janda, Bobbie Myers, Linda Noson, and John Pitlick for suggestions which have improved the contents of this summary.
Field guides and further reading:
Phillips, W.M., 1987, Geologic Guide to the Monitor Ridge climbing route, Mount St. Helens, Washington: Washington Geologic Newsletter, v. 15, no. 4, p. 3-15. (Also, reprinted in Oregon Geology, v. 50, no. 3, 1988).
Swanson, D.A.; Cameron, K.A.; Evarts, R.C.; Pringle, P.T.; Vance, J.A., 1989, Excursion 1A – Cenozoic volcanism in the Cascade Range, southern Washington and northernmost Oregon. In: Chapin, C.E.; Zidek, Jiri, editors, Field excursions to volcanic terranes in the western United States, Vol. II -–Cascades and intermountain west: New Mexico Bureau of Mines and Mineral Resources Memoir 47, p. 1-50.
Waitt, R.B.; Hoblitt, R.P.; Criswell, C.W.; Scott, K.M.; Gilcken, Harry; Brantley, S.R., 1989, Excursion 2A – Recent volcaniclastic deposits and processes at Mount St. Helens Volcano, Washington. In: Chapin, C.E., Zidek, Jiri, editors, Field excursions to volcanic terranes in the western United States, Vol. II – Cascades and intermountain west: New Mexico Bureau of Mines and Mineral Resources Memoir 47, p. 51-89.