1 U.S.Geological Survey, David A. Johnston Cascades Volcano Observatory, 5400 MacArthur blvd., Vancouver, WA 98661
This report is preliminary and has not been reviewed for conformity with U.S.Geological Survey editorial standards or with the North American Stratigraphic Code. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S.Government.
Mount Rainier at 4393 meters (14,410 feet) the highest peak in the Cascade Range is a dormant volcano whose load of glacier ice exceeds that of any other mountain in the conterminous United States. This tremendous mass of rock and ice, in combination with great topographic relief, poses a variety of geologic hazards, both during inevitable future eruptions and during the intervening periods of repose.
The volcano's past behavior is the best guide to possible future hazards. The written history of Mount Rainier encompasses the period since about A.D. 1820, during which time one or two small eruptions, many small debris flows, and several small debris avalanches have occurred. This time interval is far too brief to serve as a basis for estimating the future behavior of a volcano that is several hundreds of thousands of years old. Fortunately, prehistoric deposits record the types, magnitudes, and frequencies of past events, and show which areas were affected by them. At Mount Rainier, as at other Cascade volcanoes, deposits produced since the latest ice age (approximately during the past 10,000 years) are well preserved. Studies of these deposits reveal that we should anticipate potential hazards from some phenomena that only occur during eruptions and from others that may occur without eruptive activity. Tephra falls, pyroclastic flows and pyroclastic surges, ballistic projectiles, and lava flows occur only during eruptions. Debris avalanches, debris flows, and floods commonly accompany eruptions, but can also occur during dormant periods.
This report (1) explains the various types of hazardous geologic phenomena that could occur at Mount Rainier, (2) shows areas that are most likely to be affected by the different phenomena, (3) estimates the likelihood that the areas will be affected, and (4) recommends actions that can be taken to protect lives and property. It builds upon and revises a similar document prepared by D.R. Crandell in 1973. Our revision was motivated by the availability of new information about Mount Rainier's geologic history, by advances in the field of volcanology, and by the need to assess hazards in a more quantitative manner than in Crandell's pioneering report.
Most of the many geologic phenomena that we describe here would only affect the immediate vicinity of Mount Rainier. However, tephra falls and debris flows could affect great numbers of people far from the volcano. Tephra is commonly dispersed by winds over broad areas, and although its effects can be quite disruptive, it is usually not lethal. In contrast, debris flows are restricted to valleys that originate at the volcano, but their effects can be very severe. In terms of their potential effects, debris flows from Mount Rainier constitute the greatest volcano hazard in the Cascade Range.
Explosive eruptions typically produce vertical plumes of hot gases mixed with volcanic rock particles. If the mixture is less dense than air, it rises over the volcano's vent until it reaches an altitude at which it ceases to be buoyant. As the plume rises, its ability to support particles progressively diminishes. Eventually, the particles in the plume (tephra, or volcanic ash) will be carried downwind and will fall to produce a deposit that covers a broad area. Tephra deposit thicknesses and particle sizes usually decrease with increasing distance from the volcano. Near the vent, large eruptions can produce tephra thicknesses of many meters (yards), containing fragments as large as tens of centimeters (10-20 inches) across. At hundreds of kilometers (hundreds of miles) from the vent, tephra deposits typically consist of a trace to a few cm (few inches) of dust to silt-sized particles.
Large tephra fragments are capable of causing death or injury by impact, and may be hot enough to start fires where they land. These hazards usually do not extend beyond about 10 kilometers (6 miles) from the vent. Most tephra-related injuries, fatalities, and social disruption occur at a greater distances from the vent, where tephra fragments are less than a few centimeters (1 inch) across. Clouds of fine tephra can block sunlight, greatly restrict visibility, and thereby slow or stop vehicle travel. Such clouds are commonly accompanied by frequent lightning. The combination of near or total darkness, lightning, and falling tephra can be terrifying. When inhaled, tephra can create or aggravate respiratory problems. Accumulation of more than about 10 centimeters (4 inches) of tephra on the roof of a building may cause it to collapse. Even thin tephra accumulations ruin crops. Wet tephra can cause power lines to short out. Fine tephra is abrasive and can damage mechanical devices and increase maintenance problems. Finally, tephra clouds are extremely hazardous to aircraft, because engines may stop and pilots may not be able to see.
The hazard from tephra fall is, in general, less severe than that of some other volcanic phenomena and therefore may not be given adequate attention during planning for volcanic crises. However, the 1980 eruptions of Mount St. Helens show that even thin accumulations of tephra can profoundly disrupt social and economic activity over broad areas. For example, the Washington communities of Yakima, Ritzville, and Spokane experienced significant disruptions in transportation, business activity, and community services when 6 to 80 millimeters (1/4 to 3 inches) of tephra fell. The greater the amount of tephra that fell, the longer a community took to recover. Residents found that tephra falls of less than 6 millimeters (1/4 inch) were a major inconvenience, and that falls of more than 17 millimeters (2/3 inch) were a disaster. Nonetheless, all three communities returned to nearly normal activities within two weeks.
Mount Rainier is a moderate tephra producer relative to other Cascade volcanoes. Eleven eruptions have deposited layers of frothy tephra (pumice) near Mount Rainier in the past 10,000 years (fig. 1), most recently in the first half of the nineteenth century. Pumice layers are produced by eruptions of gas-rich magma (molten rock). At least 25 layers of non-pumice-bearing (lithic) material lie between the pumice layers. Some if not all of this material was probably produced by eruptions of gas-poor magma, or by eruptions driven by steam rather than by magma.
Figure 1 shows that pumice-producing eruptions have been irregularly spaced through time, so it is impossible to predict when the next one will occur. On the basis of the evidence summarized in Figure 1, the average time interval between eruptions is about 900 years. This is a maximum estimate of the average time between eruptions because it considers neither eruptions that did not produce pumice nor small eruptions that did not produce recognizable deposits.
Volcanoes usually provide warning signals days to months before they erupt. As magma pushes its way upward, it shoulders aside the old rocks and produces earthquakes, and causes the sides of the volcano to deform slightly. Neither the earthquakes nor the deformation may be apparent to people, but they are detectable by sensitive instruments. Heat and gases from the rising magma may cause changes in the temperature, discharge rate, and composition of hot springs and fumarolic vapors.
Earthquakes near Mount Rainier are continuously monitored by a network of seismometers maintained under the auspices of the U.S. Geological Survey Volcano Hazards Program and the University of Washington Geophysics Program. In a typical year, this network detects a few hundred earthquakes that occur at or near Mount Rainier. At the first sign of unusual earthquake activity, scientists from the Geological Survey and other institutions will deploy additional instruments on and around Mount Rainier to monitor earthquakes, deformation, and other symptoms of volcanic unrest. The monitoring information will be used to assess the state of unrest and to issue appropriate advisories and warnings to emergency-response officials and the public. Symptoms of volcanic unrest at Mount Rainier would greatly increase the probability of debris avalanches, especially those of large size that might affect populated areas in the Puget Sound lowland.
Periods of volcanic unrest are usually times of great uncertainty. Although outstanding advances have been made in volcano monitoring and eruption forecasting over the past few decades, scientists are often able to make only very general statements about the probability, type, and scale of an impending eruption. Precursory activity can wax and wane, and sometimes dies out without leading to an eruption. Government officials and the public should realize the limitations in forecasting eruptions and be prepared for such uncertainty.
Communities, businesses, and citizens can undertake several actions to mitigate the effects of future eruptions, debris avalanches, and debris flows. Decisions about land use and siting of critical facilities can incorporate information about volcano hazards. Areas judged to have an unacceptably high risk can be left undeveloped. Alternatively, development can be planned to reduce the level of risk, or even include engineering measures to mitigate risk. For example, areas along the channels and flood plains of debris flow-prone rivers could be set aside for open space or recreation, and valley walls or high terraces could be used for houses, schools, and businesses.
An eruption or the threat of an eruption requires short-term emergency responses. Such responses will be most effective if citizens and public officials understand volcano hazards and have planned the actions needed to protect communities. Because the time can be short (days to months) between onset of precursory activity and an eruption, and because some hazardous events can occur without warning, appropriate emergency plans should be made and practiced beforehand. Public officials need to consider issues such as public education, communications, and evacuations. Emergency plans already developed for floods may be applicable, with modifications, to hazards from debris flows in valleys that head on Mount Rainier.
Businesses and individuals should also make plans to deal with volcano emergencies. Planning is prudent because once an emergency begins, public resources can often be overwhelmed, and citizens may need to provide for themselves and make informed decisions. The Red Cross recommends numerous items that should be kept in homes, cars, and businesses for many types of emergencies that are much more probable than a volcanic eruption. Other items that will help include a map showing the best route to high ground.
The most important additional item is knowledge about volcano hazards and, especially, a plan of action based on the relative safety of areas around home, school, and work. Be aware of the location of the volcano and valleys that may be affected by debris flows. If your house is within a hazard zone for debris avalanches and debris flows, and if you learn that a hazardous event may be in progress, move to higher ground nearby. If this is not possible, move downvalley and then move to higher ground at the first opportunity. A safe height above river channels depends on the size of the debris flow, distance from the volcano, and shape of the valley. For all but the largest debris flows, areas 50 meters (160 feet) or more above river level will be safe.
The accompanying maps show areas that could be affected in the future by (1) debris avalanches and debris flows, (2) pyroclastic flows, surges, lava flows, and ballistic projectiles, (3) tephra falls, and (4) lateral blasts. Although we show boundaries of hazard zones by lines, the degree of hazard does not change abruptly at these boundaries. Rather, the hazard decreases gradually away from the volcano and, for flows, with height above the valley floor. Areas immediately beyond outer hazard zones should not be regarded as hazard-free, because the boundaries can only be approximately located, especially in areas of low relief. Too many uncertainties exist about the source, size and mobility of future events to locate hazard-free zones with absolute confidence.
Even small thicknesses of tephra can profoundly disrupt social and economic activity over broad areas. The thickness of tephra necessary to cause buildings to collapse depends on construction practices, but experience shows that failures tend to increase as the thickness approaches 10 centimeters (4 inches). Consequently, tephra hazard is portrayed here with contour maps of the estimated annual probability of tephra accumulations of one centimeter (0.4 inch) or more and ten centimeters (4 inches) or more. Inset Maps B1 and B2 consider all major Cascade volcanoes, while Inset Maps C1 and C2 consider only eruptions from Mount Rainier. These estimates take into account the probability that the volcano will erupt, the probability that the specified tephra thickness will occur at a specified distance, and the probability that the wind will be blowing in a specified direction. Inset Map C2 shows that tephra loads of 10 centimeters (4 inches) or more from eruptions of Mount Rainier are most likely to occur east of the volcano, within a few tens of kilometers (miles) of the summit. Most buildings within this area are designed to support substantial snow loads and thus may be relatively resistant to damage by tephra loading.
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