Some physical processes, such as celestial mechanics, are so regular that their histories can be ex trapolated with near certainty into the future. Predictions of the onset times, durations, and locations of solar eclipses can thus be done on a deterministic basis. Other physical phenomena, such as volcanic eruptions, are much more variable in nature. Extrapolating their histories and current processes into the future can only be done on a probabilistic basis. In the science of probabilistic forecasting, there are no sure predictions, only statistical gains over predic tions based only on historical averages. Pasteur said it well, "Chance favors the prepared mind."
Experience and insight are important in the interpretations of highly variable phenomena such as human nature, and they should not be discounted in the interpretation of highly complex and variable physical phenomena. However, what is needed in the science of probabilistic forecasting are objective criteria for interpreting the historical and monitoring data, and more basic research into the processes of volcanism so that the precursor events of eruptions can be interpreted in terms of their physical and chemical meanings. This should lead to better predictions of the nature, time, location, and magnitude of eruptive events.
The 1980 eruptions of Mount St. Helens make available a case study showing where eruption forecasting stands today and the directions it may take in the near future. In general, the forecasting of the Mount St. Helens activity was accurate and may have saved thousands of lives. In terms of the exact timing, nature, and scale of the huge May 18 eruption and avalanche, it was not accurate enough. Experience was painfully gained at the cost of 62 lives.
Mount St. Helens has been more active and more explosive during the last 4,500 years than any other volcano in the conterminous United States. In the future, Mount St. Helens probably will erupt violently and intermittently * * * and will affect human life and health, property, agriculture, and general economic welfare over a broad area. The volcano's behavior pattern suggests that the current quiet interval will not last as long as a thousand years; instead, an eruption is more likely to occur within the next hundred years, and perhaps even before the end of this century.
The hazards report by Crandell and Mullineaux (1978) was accurate in predicting the areas affected by the May 18 eruption with regard to pyroclastic flows, air-fall ash, mudflows, and floods. The areas affected by the avalanche and directed blast are beyond the limits of any recognized prehistoric eruption of the same kind at the volcano and, therefore, were un precedented in scale in the 4,500 yr history upon which the hazards report was based.
Careful geologic mapping during several field seasons in the Mount St. Helens area gave Crandell and Mullineaux an additional benefit in dealing with hazard warning problems. They had become well ac quainted with the operations of the U.S. Forest Serv ice, lumber industry, and utilities in the area, and this knowledge led to much better credibility for their warnings during the phase of small eruptions prior to the May 18 catastrophe.
deformation, temperature, electric and magnetic fields, gas emissions, and the chemistry and mineral ogy of erupted lavas or pyroclastic materials. All of these techniques have been used during the recent activity at Mount St. Helens. Some techniques have worked better than others at forecasting, but all have yielded important data from the standpoint of obtaining a basic understanding of explosive volcanism.
Major visible deformation was first seen on March 27 and was monitored directly after April 23. The very large rates of deforrnation—1.5-2.5 m/day in an area of 1.5x2.0 km on the high north flank of Mount St. Helens—were of major concern. The close connection in time and space between the earthquake foci and the bulging area led most of the scientists studying the volcano to conclude that a shallow intru sion was taking place beneath an area just north of the summit. The possibility of a major avalanche and a lateral explosion were considered. Reports on the lateral explosions of Lassen Peak (Day and Allen, 1925), Bandai-san (Kuno, 1962), Bezymianny volcano (Gorshkov, 1959), Shiveluch volcano (Gorshkov and Dubik, 1970), and Mount Lamington (Taylor, 1958) were being reread by Mount St. Helens workers in late April and early May.
The possibility that a lava dome might extrude at Mount St. Helens without much accompanying damage, as happened at Syowa-Sinzan in Japan in 1944-45 (Kuno, 1962), was also considered, as well as the possibility that the apparent intrusion at Mount St. Helens might cease before it caused any major avalanche or eruption. One of the most perceptive possibilities suggested between March 27 and May 18 was by Barry Voight of Pennsylvania State Univer sity. He compared Mount St. Helens to the areas of large historic avalanches, at Gros Ventre, Wyo., in 1925, and Madison Canyon, Mont., in 1959, and decided that a potential failure of the north flank of Mount St. Helens could generate an avalanche of 1-3 km3. Voight (written commun., May 1, 1980) noted that:
A catastrophic event of the kind observed at Bandai-san—in which an explosively motivated fragmental flow devastated an area of more than 70 square km must be regarded as a legitimate possibility, particularly in view of the enhanced hydraulic pressure conditions implied by frequent summit steam explosions and the relatively high level of released seismic energy.
On the more prosaic side, a bulging slope associated with rock creep (increased tilt) may lead to an increase of rockfall hazards from exposed rock areas such as Goat Rocks, increased snow avalanche hazard, and increased risk of glacier falls.
In general, for most natural catastrophes, the larger the event, the more rarely it occurs. Therefore, most of the scientists studying Mount St. Helens in early May considered that a significant avalanche and eruption were distinctly possible, but that extremely large events were not very probable. Even so, the possibility of danger was clearly evident from prehistoric eruptions of Mount St. Helens, historic eruptions at other volcanoes, and the ongoing high rate of seismicity and deformation. These factors were considered by authorities responsible for land management around the volcano in their decision to continue restricting access to the volcano, even in the face of mounting public pressure to open the Spirit Lake recreation area.
The October 16 eruptions, which began at 2158 PDT, were preceded by more than 30 hr of increased local seismicity, 12 days of small northward displace ments of the crater rampart, and a decrease of CO^2 emission and CO^2/SO^2 ratios between October 9 and 15.
On December 13-14, a small sector of the lava dome was apparently blown out. Because of bad weather, this eruption was not witnessed. Local seismicity began to increase on December 25 and reached a maximum by December 26-27, and the
Table 111.—Geological, geochemical, and geophysical changes that occurred before significant eruptions of Mount St. Helens [hr, hours; leaders (---) phenomenon did not occur or was not detected] ______________________________________________________________________________ Onset of changes before eruptions Eruptions Thermal Gas Small SeIsmicity Tremor Deformation anomaly emission eruptions _____________________________________________________________________________ Mar. 27, 7 days --- --- --- --- --- 1230 PST. May 18, 60 days 49 days (1) 53 days 53 days --- 53 days (1) 0832 PDT. May 25, --- 7 days --- - --- --- 10 hr 0228 PDT. June 12, --- 9 hr --- --- --- 2hr 2110 PDT. July 22, 8 hr --- 4 days --- 9 days --- 1714 PDT. Aug 7, --- 41/2 hr --- --- 5 days --- 1624 PDT. Oct. 16, 30 hr --- 12 days --- 7 days 6 days 2158 PDT. Dec. 27-28 2 days --- 14 days --- --- 14 days ____________________________________________________________________________ (1) Intermittent .___
crater rampart moved tens of centimeters northward. On December 28, renewed growth of the lava dome was observed. This growth continued through January 2-4, 1981, doubling the volume of the dome. The summary of eruptions in table 111 shows that more than one precursory phenomenon was evident before the beginning of each eruption. Most of the precursors were clearly recognized and were reported before the eruptions. The pattern of tremor before the May 25 eruption and the pattern of gas emissions before the July 22 eruption were recognized as ap parent precursors after the eruption had occurred. Even though there was no short-term warning of the May 18 eruption, five different, long-term phe nomena indicated the possibility of a future eruption. Precursory events not followed by eruptions (false alarms) did occur, and these probably are inevitable in any probabilistic forecasting system. Table 112 lists eight occurrences of precursory-type events that were not followed by significant eruptions. Only before August 15 was there more than one potential precursory event.
Although there is ample evidence that several phenomena, which can be observed and measured, give warnings of impending eruptions for hours to months in advance, no single pattern has clearly repeated itself before all or even most of the eruptions. Basic scientific research, as well as human experience and judgment, are still needed in large measure.
Because of the generally long repose intervals between eruptive periods of explosive volcanoes, several volcanic centers or areas should be studied and monitored from one observatory. Most impor tant of all, the geologic, geophysical, and gochemical studies on several volcanoes should be done by a group of scientists at one headquarters. Volcanology is such a broad field of research that many of its problems can only be solved by scientists from different disciplines—geology, chemistry, physics, and mathematics—working closely together.
Table 112.—Precursory phenomena that were not followed by significant eruptions at Mount St. Helens [Leaders (---), phenomenon did not occur or was not detected] _____________________________________________________ Type and operation Date Thermal Gas Small Seismicity Tremor Deformation anomoly emission eruptions June 3 --- Several --- --- --- --- hours June 26 --- --do--- --- --- --- --- July 28 --- --- --- --- --- 20 min. Aug. 15 --- --- --- --- 2 days Several minutes - Aug 25 --- --- --- --- 4 days --- Sept 24 --- --- --- --- --- 4 hr Oct. 1 --- --- --- --- --- Duration unknown Nov18-19 --- --- --- --- --- Do___
Taylor, G. A. M., 1958, The 1951 eruption of Mount Lamington, Papua: Australia Bureau of Mineral Resources Bulletin 38, 117 p.