Geological Survey Bulletin 1238 Volcanic Hazards at Mount Rainier, Washington

Debris flows and eruptions have occurred sporadically since the last major glaciation. If this pattern continues with no significant change, both phenomena must be expected in the future. The best available guide to the possible variety, frequency, and location of future events is table 1, which shows all known postglacial debris flows and eruptions. This tabulation goes beyond the short time span of direct observation of Mount Rainier volcano, which embraces little more than 100 years. Discretion must be used, however; for such a tabulation not only "mixes" deposits formed in different ways but also has the effect of telescoping time; from it one might conclude that disaster is imminent. For better perspective, it is helpful to examine the relative frequency and location of each kind of event (table 2). It can be seen from tables l and 2 that some volcanic events (lava flows) have occurred perhaps not more than once in 10,000 years; others (small-scale steam explosions), as often as once each century and perhaps as often as once each decade. Records of the National Park Service indicate that from 1932 to 1961, five debris flows and floods not caused by volcanic activity occurred in the Kautz Creek and Nisqually River valleys alone; thus, for the entire park, these phenomena probably have occurred at a rate of one in 3-10 years. It is clear that valley floors are the localities most often affected and that some valleys are affected more often than others.

We conclude that debris flows are by far the greatest hazard because of their frequency and possibly very large size and because they travel along valley floors where highways, dwellings, and other works of man are concentrated. Furthermore, we believe that any future major eruption of Mount Rainier, and even some minor ones, will be accompanied by passage of debris flows down one or more valleys that head on the volcano. If a flow as large as the Osceola Mudflow were to occur today, it doubtlessly would result in wholesale destruction and death, perhaps on a scale comparable to that accompanying some large mudflows caused by volcanic eruptions in Japan and Indonesia within the last century. Flows of this kind could result from steam explosions or possibly from processes unrelated to volcanism and could thus occur without warning. Fortunately, however, debris flows of extremely large volume are infrequent. Debris flows of relatively small volume, which may not be accompanied by volcanic activity and which are a rather common occurrence in many of the valleys, also present a definite hazard to the roads and recreational facilities that have been developed on valley floors adjacent to the volcano.

A second conclusion reached from inspection of table 1 is that debris flows are more common in some valleys than in others. The reasons for this are not fully understood, but two important factors are the presence of masses of altered and weakened rock on the volcano at some valley heads and the availability of large volumes of loose glacial drift on some steep valley sides. If, as we believe, the largest debris flows from Mount Rainier in the past did originate in avalanches of altered rock from high parts of the volcano, the largest avalanches and debris flows in the future probably will originate in similar areas. The most extensive known mass of altered rock near the summit of the volcano is in the east wall of Sunset Amphitheater. At least four large avalanches and debris flows probably have originated in Sunset Amphitheater during the last thousand years or so, and similar phenomena will very likely occur there again.

Table 1 implies that debris flows have become more frequent with the passage of time, but this may be only because younger deposits are relatively well preserved and readily seen, whereas some older ones may have been either removed by erosion or buried.

Although future volcanic eruptions are expectable, prediction of such events is not possible in terms of a specific time, place, or scale. Not only do we lack knowledge of the events that preceded eruptions of the past, but these events might not necessarily be identical during each successive eruption. Our inability to predict specifics of future activity leaves two principal methods by which destructive effects might be minimized: careful site selection for future construction away from areas likely to be affected, and recognition of signs of an impending eruption. Special importance should be attached to the recognition of phenomena which, if correctly evaluated, might warn of an impending eruption. Eruptions preceded by the rise of molten rock into the volcano might thus be anticipated (table 2), even though some kinds of steam explosions could occur with little or no warning.

TABLE 2.—Types of eruptions that have occurred at Mount Rainier in postglacial time, anticipated effects and frequency of similar eruptions in the future, and possible warning signs of an impending eruption
[The flanks of the volcano include an area within 4 or 5 miles from the summit]