2
MOUNT RAINIER, ACTIVE CASCADE VOLCANO

Mount Rainier (Figure 2.1) is one of about two dozen recently active volcanoes in the Cascade Range, a volcanic arc formed by subduction of the Juan de Fuca plate beneath the North American plate. Volcanism in this arc began at least 37 million years ago and has continued intermittently to the present. During that time, numerous volcanoes have formed, flourished, died, and eroded away, generally leaving behind only those deposits in protected, low-lying areas surrounding the easily eroded cones. These deposits have been buried by younger eruptions, altered by burial metamorphism, and exposed at the Earth's surface by erosion. The modern volcanoes and volcanic fields of the Cascades, which rest on this older volcanic landscape, have formed in the past 2 million years, and mostly in the past 1 million years or less (Crandell, 1963; Crandell and Miller, 1974).

The volcanic cone, or edifice, has been constructed from thousands of lava flows and breccias and a few ash deposits. Some of the lava flows are more than 60 m thick at the base of the edifice, in what is now Mount Rainier National Park (Figure 2.2). Some of the breccias were deposited by moving water, but others were probably emplaced during volcanic explosions or by fragmentation of moving lava flows.

The volcano is located about 35 km southeast of the Seattle-Tacoma metropolitan area (see Figure 2.1), which has a population of approximately 2.5 million people (Figure 2.3). This metropolitan area is the high-technology industrial center of the Pacific Northwest and one of the commercial-aircraft-manufacturing centers of the United States. The rivers draining the volcano empty into Puget Sound, which has two major shipping ports, and into the Columbia River, a major shipping lane and home



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2 MOUNT RAINIER, ACTIVE CASCADE VOLCANO Mount Rainier (Figure 2.1) is one of about two dozen recently active volcanoes in the Cascade Range, a volcanic arc formed by subduction of the Juan de Fuca plate beneath the North American plate. Volcanism in this arc began at least 37 million years ago and has continued intermittently to the present. During that time, numerous volcanoes have formed, flourished, died, and eroded away, generally leaving behind only those deposits in protected, low-lying areas surrounding the easily eroded cones. These deposits have been buried by younger eruptions, altered by burial metamorphism, and exposed at the Earth's surface by erosion. The modern volcanoes and volcanic fields of the Cascades, which rest on this older volcanic landscape, have formed in the past 2 million years, and mostly in the past 1 million years or less (Crandell, 1963; Crandell and Miller, 1974). The volcanic cone, or edifice, has been constructed from thousands of lava flows and breccias and a few ash deposits. Some of the lava flows are more than 60 m thick at the base of the edifice, in what is now Mount Rainier National Park (Figure 2.2). Some of the breccias were deposited by moving water, but others were probably emplaced during volcanic explosions or by fragmentation of moving lava flows. The volcano is located about 35 km southeast of the Seattle-Tacoma metropolitan area (see Figure 2.1), which has a population of approximately 2.5 million people (Figure 2.3). This metropolitan area is the high-technology industrial center of the Pacific Northwest and one of the commercial-aircraft-manufacturing centers of the United States. The rivers draining the volcano empty into Puget Sound, which has two major shipping ports, and into the Columbia River, a major shipping lane and home

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FIGURE 2.1 Map of Mount Rainier and surrounding regions in the State of Washington. The approximate outline of the volcano is indicated by the solid fill in Mount Rainier National Park. The locations of Puget Sound and the Seattle-Tacoma metropolitan area are indicated by dark and light stippling, respectively. Also shown are the generalized locations of the dammed reservoirs on the White, Nisqually, and Cowlitz rivers (after Swanson and others, 1992).

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FIGURE 2.2 Generalized geologic map of Mount Rainier National Park (modified from Walsh and others, 1987).

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FIGURE 2.3 Population density map of the general region shown in Figure 2.1. Incorporated areas and areas with population densities greater than or equal to 200 persons per square mile are shaded. In other areas, population densities are denoted with dots, each dot representing 100 people (courtesy of Carol Jenner, State of Washington Office of Financial Management).

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to approximately a million people in southwestern Washington and northwestern Oregon. Mount Rainier is an active volcano. It last erupted approximately 150 years ago (Mullineaux, 1974), and numerous large floods and debris flows have been generated on its slopes during this century. More than 100,000 people live on the extensive mudflow deposits that have filled the rivers and valleys draining the volcano during the past 10,000 years (Table 2.1). A major volcanic eruption or debris flow that is not prepared for could kill hundreds or thousands of residents and cripple the economy of the Pacific Northwest. Despite the potential for such danger, Mount Rainier has received little study. Most of the geologic work on Mount Rainier was done more than two decades ago. Fundamental topics such as the development, history, and stability of the volcano are poorly understood. The recent eruptions of Mount St. Helens in the southern Washington Cascades, as well as the eruptions of other arc volcanoes such as Mount Pinatubo (Philippines), El Chichón (Mexico), Mount Unzen (Japan), and Nevado del Ruiz (Colombia), have focused the awareness of the science community, government, and the general public on volcanic hazards in the densely populated Puget Lowland area (see Figure 2.3). Public awareness of natural hazards has been heightened by the recent recognition of an active fault crossing Seattle (Atwater and Moore, 1992; Bucknam and others, 1992; Jacoby and others, 1992; Karlin and Abella, 1992; Schuster and others, 1992), which is considered capable of generating an earthquake of magnitude 7 or greater. As explained later, such an earthquake could trigger a catastrophic collapse of a portion of Mount Rainier's volcanic edifice. Volcanic Hazards at Mount Rainier The term volcanic hazard is used here to refer to a volcanic or related events that pose a threat to persons or property in surrounding regions. Table 2.2 lists 15 volcanic hazards relevant to Mount Rainier. Also shown in the table are provisional estimates of risk, defined as the

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TABLE 2.1 Major Holocene Volcanic Events at Mount Rainer Eventa Dates Explosive Eruptionb Lahar Number Length (km) Reference Glacier outburst floods and lahars 1992-1986   15 4.1 Scott and others (1992) Driedger and Walder (1991) Lahar, Nisqually River 1970-1930   7 8 Crandell (1971) Lahar, Tahoma Creek 1967   1 13 Crandell (1971) Lahar, Kautz Creek 1947   2 16 Crandell (1971) Phreatic eruption? 1894 (2-3?)c     Majors and McCollum (1981) Lahar, Nisqually River 1870-1860   1 12 Crandell (1971) Tephra X 1854-1820 (1)     Mullineauz (1974) Lahar, West Fork White River 1695-1480   1 37 Crandell (1971); Yamaguchi (1983) Lahar, Nisqually River ca. 400 B.P.   1 13.5 Crandell (1971) Lahar, North Puyallup River > 400 B.P.   2 ? Crandell (1971) Lahar, Tahoma Creek ca. 400 B.P.   1 14.5 Crandell (1971) Lahar, Tahoma Creek 470-2800 B.P.   2 10.5 Crandell (1971); Yamaguchi (1983) Lahar, Ohanapecosh River 470-3600 B.P.   1 17.5 Crandell (1971); Yamaguchi (1983) Lahar, South Mowich River 470-3600 B.P.   1 12 Crandell (1971) Lahar, Kautz Creek 470-3600 B.P.   3 10 Crandell (1971); Yamaguchi (1983) Lahar, Muddy Fork Cowlitz River 470-3600 B.P.   1 13.5 Crandell (1971) Yamaguchi (1983) Lahar, South Mowich River < 1480   1 11 Crandell (1971); Yamaguchi (1983) Lahar, South Puyallup River < 1480   4 ? Crandell (1971); Yamaguchi (1983) Lahar, Kautz Creek < 1480   2 11.5 Crandell (1971); Yamaguchi (1983)

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Eventa Dates Explosive Eruptionb Lahar Number Length (km) Reference Electron Mudflow, Puyallup River 550-530 B.P.   1 48 Crandell (1971); Scott and others (1992) Lahar, Nisqually River 800-360 B.P.   7 12 Crandell (1971) Lahar, Puyallup River 1000-1050 B.P.   1 30 Crandell (1971); Scott and others (1992)   2200-2500 B.P.   2 10 R.P. Hoblitt (unpublished data) Tephra Cd 2200-2500 B.P. 1 (?) 1 24 Mullineaux (1974)   2200-2500 B.P.   1 12 Crandell (1971) Lahar, White River 2300-2700 B.P.   3 13 Crandell (1971) Lahar, West Fork White River 2300-3600 B.P.   1 37 Crandell (1971) Lahar, White River 2300-3600 B.P.   1 33 Crandell (1971) Round Pass Mudflow, Tahmoa Creek and Puyallup River 2170-2790 B.P.   1 31 Crandell (1971); Scott and others (1992) Lahar, Nisqually River < 3600 B.P.   1 47 Crandell (1971) Lahar, Kautz Creek > 3600 B.P.   1 11.5 Crandell (1971) Lahar, South Mowich River > 3600 B.P.   1 18.5 Crandell (1971) Lahar, South Puyallup River > 3600 B.P.   1 ? Crandell (1971) Lahar, West Fork White River 3600-5700 B.P.   2 18 Crandell (1971) Lahar, Nisqually River 3600-6800 B.P.   3 30.5 Crandell (1971); Bacon (1983) Lahar, Carbon River 3600-6800 B.P.   1 18.5 Crandell (1971); Bacon (1983) Lahar, Ohanapecosh River 3600-6800 B.P.   1 17.5 Crandell (1971); Bacon (1983) Tephras B and H 3900-5000 B.P. (2)     Mullineaux (1974) Tephra F 4500-5000 B.P. (1)     Mullineaux (1974)

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Eventa Dates Explosive Eruptionb Lahar Number Length (km) Reference Osceola Mudflow, West Fork White River 4500-5000 B.P.   1 113 Crandall (1971); Scott and others (1992) Paradise Lahar, Nisqually River and Paradise River 4500-5000 B.P.   1 29 Crandall (1971); Scott and others (1992) Tephras S and N 5000-5800 B.P. (2)     Mullineaux (1974) Lahars, White River 5700-6800 B.P.   5 13 Crandall (1971) Tephra D 5800-6400 B.P. (1)     Mullineaux (1974) Tephra L 6400 B.P. (1)     Mullineaux (1974) Tephra A 6400-6700 B.P. (1)     Mullineaux (1974) Lahar, Ohanapecosh River > 6800 B.P.   1 9.5 Crandall (1971) Tephra R > 8750 B.P. (1)     Mullineaux (1974) SOURCE: Modified from Hoblitt and others, 1987. a Many smaller events are given by Scott and others (1992, Table e and text). b Number of events. Parentheses indicate volume less than 0.1 km3. c Questionable phreatic explosions. d At least one pyroclastic flow of 12 km length, and one or more lava flows of at least 3.5 km length, roughly coincide with eruption of layer C.

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probability of loss of life, property, and productive capacity in the area affected by the hazard. Risk depends in part on proximity to the hazard, which is defined here in terms of distance from the volcanic edifice. The proximal zone refers to areas on and adjacent to the volcanic edifice. For Mount Rainier, the proximal zone generally lies within the boundaries of Mount Rainier National Park (see Figure 2.1). The distal zone refers to areas beyond the edifice that could be affected significantly by volcanic activity. For Mount Rainier, the distal zone includes areas up to about 100 km outside of the National Park. Risk also depends on the size (magnitude) of the event and its frequency of occurrence. In general, high-magnitude events pose greater risks to people and property than low-magnitude events. Relatively little is known about magnitudes and frequencies of volcanic events at Mount Rainier, so the estimates of risk shown in Table 2.2 are necessarily qualitative. Studies of the geologic history of Mount Rainier and other Cascades volcanoes (see Chapter 3 in this report) suggest that major volcanic hazards are likely to include the following: Volcanic eruptions. The eruption of lava flows and tephra (particulate materials such as ash). Edifice failure. The gravitational collapse of a portion of the volcano. Glacier outburst floods (jökulhlaups). The sudden release of meltwater from glaciers and snowpack or from glacier-dammed lakes on the edifice. Lahars, or debris flows, and debris avalanches. Gravitational movement of commonly water-saturated volcanic debris down the steep slopes of the volcano and into nearby valleys. The most likely volcanic hazards at Mount Rainier are from debris avalanches, lahars, and floods like those of the past that have repeatedly swept down the valleys heading on the edifice (Crandell and Mullineaux, 1967; Crandell, 1973; Scott and others, 1992). Frequency and magnitude estimates for such events can be made by reconstructing the spatial and

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TABLE 2.2 Potential Volcanic Hazards at Mount Rainier Hazard Probable Risk Need to knowa   Prox- imal Distal   Lava flows M? L How far, how fast, role in producing melting of snow and ice Phreatic and phreatomagmatic eruptions H? L Generation, potential size, favored eruption site(s) Ballistic projectiles H L Size of ballistics Tephra H H Frequency of small falls Pyroclastic flows and surges H? H Frequency of occurrence; role is producing melting of snow and ice Laharsb H H Origin, how far, how fast Jõkulhlaupsb M L Role of heat flow in production Sector collapseb H H Causes, sizes Landslidesb M L   Rock and debris ava- lanchesb H L Causes, sizes, association with steam blasts Volcanic earthquakesb L? L   Ground deformationb L? L   Tsunami L L?  

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Hazard Probable Risk Need to knowa   Proximal Distal   Airshocks L L   Gases and aerosolsb M? L   NOTES: Proximal and distal refer to areas within and outside Mount Rainer National Park, respectively. L = low; M = moderate; H = high a With more refined information on magnitude, frequency and areas affected for all hazards. b Hazards that can occur when volcano is not in eruption. temporal distributions of lahars and debris avalanches preserved in the stratigraphic record on and around the edifice. For example, the frequency with which lahars have affected areas more than 20 km from the volcano in the past (Table 2.1) suggests that the annual probability of such an event is about 0.001. An event of this magnitude would be expected to occur an average of once every 1,000 years. Similarly, lahars that extend to distances of 50 km or more from the volcano have an estimated annual probability of about 0.0001. An event of this magnitude would be expected to occur an average of once every 10,000 years. These larger lahars could affect the Puget Lowland, inundating tens to hundreds of square kilometers in relatively densely populated areas. These probabilities should be considered as minimum estimates, because they are based on incomplete mapping of lahar distributions. As additional lahars are identified through field investigations, these probabilities could be revised upward. That is, these events could be seen as occurring with greater frequency than present estimates would suggest. The most voluminous debris avalanches and lahars at Mount Rainier originated from parts of the volcano that contained large volumes of hydrothermally altered materials (Crandell, 1971; Scott and others, 1992). Frank (1985) concluded that the upper west flank and the summit

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FIGURE 2.5 Map of Mount Rainier and surrounding regions showing the generalized locations of three large lahars indicated by dark stippling: the Osceola Mudflow (White River), Paradise Lahar (Nisqually River), and Electron Mudflow (Puyallup River). After Swanson and others (1992) with data from Crandell (1971).

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from an old-growth forest that was buried by the mudflow (Patrick Pringle, Washington State Department of Natural Resources, oral communication, 1993; see Figure 2.6(B)). In the past 45 years, several dozen debris flows and outburst floods have occurred at Mount Rainier, the majority in the Tahoma Creek-Nisqually River drainage (Table 2.1; Scott and others, 1992). These flows did not directly threaten communities, but they did affect areas frequented by visitors to the National Park and required the expenditure of Park Service funds for cleanup and reconstruction. One popular road remains closed. The largest debris flow extended about 16 km from its origin on the volcanic edifice (Crandell, 1971). Debris flows of this magnitude are essentially unpredictable at current levels of understanding, but they serve as a reminder of the dynamic landscape surrounding Mount Rainier. Events such as edifice failures, glacier outburst floods, and debris flows can occur in the absence of volcanic eruptions. Mount Rainier is the high- est volcano in the Cascade Range (4,392 m above sea level, with approximately 3,000 m of relief) and contains about 140 km3 (Sherrod and Smith, 1990) of structurally weak, locally altered rock capped by about 4.4 km3 of snow and ice (Driedger and Kennard, 1986), all of which stand near the angle of repose. The volcano is inherently unstable (Figure 2.7). Ground shaking during an earthquake could cause the gravitational failure of a large sector of the volcanic edifice, producing catastrophic avalanches and debris flows, and possibly triggering an eruption. Indeed, several large debris flows from Mount Rainier have been generated apparently without eruptive products. A pertinent example is the Round Pass Mudflow, which is approximately 2,600 years old (Scott and others, 1992; Patrick Pringle, Washington State Department of Natural Resources, oral communication, 1994). Catastrophic edifice failure is generally recognized to be a severe hazard at stratovolcanoes such as Mount Rainier (Siebert, 1992; López and Williams, 1993). On a worldwide basis, such collapses have occurred an average of four times a century for the past 500 years (Siebert, 1992). A

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FIGURE 2.6 (A) Cross-section of the Osceola Mudflow near Buckley, Washington, approximately 85 km downstream from the volcano. The mudflow, which caps the 30-m high terrace, is 3 to 5 m thick. The boundary between the mudflow and underlying glacial deposit is indicated by an arrow in the photo.

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FIGURE 2.6 (B) Stump of a 2-m diameter Douglas fir uncovered during excavation near Orting, located near the confluence of the Carbon and Puyallup rivers (see Figure 2.5). The stump is from an old-growth forest that was buried by the mudflow. (Photos courtesy of Patrick Pringle, Washington State Department of Natural Resources.)

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FIGURE 2.7 Interlayered lava flows and mudflows in a 30-m high cliff near the summit of the volcano. These layers probably formed when thin lava flows moved downslope, melting ice and snow to create mudflows. The rubble layers dip away from the summit at angles up to 30° and are inherently unstable (from Fiske and others, 1963, Figure 51).

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particularly pertinent example of such a collapse occurred at the Bandaisan Volcano in Japan in 1888; this failure occurred without any precollapse or postcollapse eruptive activity (Sekiya and Kikuchi, 1889). Glacier outburst floods and lahars can occur during warm summer days, heavy rainfalls, or as a result of transient heating events that melt the snow and ice cover on the volcano. Several lahars were generated on Mount Rainier in 1947 as a result of heavy rainfall (Crandell, 1971). The volcanic edifice contains a well-developed hydrothermal circulation system that transfers heat from depth to the surface (Frank, 1985, in press). This system is supplied by precipitation at the surface of the volcano, and pathways for fluid flow are provided by the numerous faults and fractures in the edifice. Changes in this ''plumbing system" due to the formation of new faults and fractures could bring heated fluids into contact with snow and ice on the volcanic edifice, causing rapid melting and runoff. Such heating could occur without warning. Transient thermal events have been observed on other volcanoes in the Cascades, for example, at Mount Baker, Washington, in 1975 (Malone and Frank, 1975; Frank and others, 1977). Eight of the Cascade Range volcanoes or volcanic fields have erupted in the past 500 years. Six of these, including Mount Rainier, have erupted in the past 200 years (Table 2.3). Four of the eruptions in the past 200 years were relatively large and could have caused considerable property damage and loss of life if they occurred today. Pyroclastic flows and lahars that entered the Columbia River were produced from Mount Hood in the 1790s and about 1800 (Cameron and Pringle, 1987). Mount St. Helens erupted explosively in late 1799 or early 1800 (Yamaguchi, 1983), producing a widespread tephra deposit (Mullineaux, 1986). In 1915, Lassen Peak in California erupted pyroclastic flows with accompanying lahars that created a "devastated" zone north and northeast of the volcano. Mount St. Helens erupted violently in 1980, killing 57 people through the combined effects of a debris avalanche formed by a giant landslide, a lateral blast expelled as the slide depressurized the volcanic system, and lahars generated by the eruption (Lipman and Mullineaux, 1981).

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TABLE 2.3 Principal Volcanoes and Volcanic Fields in the Cascade Range and Dates of Their Most Recent Volcanic Activity, Listed from North to South Volcano Location Date of most recent volcanisma Silverthrone British Columbia Possibly younger than 1000 A.D. Bridge River cones British Columbia Possibly younger than 500 A.D. Meagher Mountain field British Columbia About 300 B.C. Mount Cayley British Columbia About 20,000 years ago Mount Caribaldi British Columbia Early Holocene Mount Baker Washington 1880 (1884?) A.D. Glacier Park Washington 18th century(?) Mont Rainier Washington 1820-1854 (1894?) A.D. Goat Rocks Washington Late Pleistocene(?) Mount Adams field Washington Eleven Holocene eruptions, possibly none younger than 1500 B.C. Mount St. Helens Washington 1980s A.D. Indian Heaven field Washington About 6000 B.C. Mount Hood Oregon 1865 A.D. Mount Jefferson area Oregon About 4500 B.C. Belknap Center Oregon About 360 A.D.

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Volcano Location Date of most recent volcanisma North Sister field (Collier Cone) Oregon About 950 A.D. South Sister (south flank) Oregon About 100 A.D. Mount Bachelor Oregon Early Holocene Newberry Volcano Oregon About 620 A.D. Mount Mazama (Crater Lake) Oregon About 5000 B.C. Mount McLoughlin Oregon 20,000 - 30,000 years ago Mount Shasta California 1786 A.D. Medicine Lake Highland California About 1000 A.D. Lassen Peak California 1914-1917 A.D. Cinder Cone California About 1650 A.D. NOTES: See Figure 3.1 in this report for geographic reference. a Sources: Simkin and others, 1981, 1984; Hildreth and Fierstein, 1983; Hoblitt and others, 1987; Harris, 1988; Souther and Yorath, 1991; Wood and Kienle, 1990; M. A. Clynne, U.S. Geological Survey, oral communications, 1992. These examples illustrate that Cascade Range volcanoes are capable of major eruptions, especially after long periods of quiescence. About 1,000 years of quiet at Lassen Peak preceded the 1914-1917 eruption, some 200 to 400 years of inactivity predated the late-nineteenth century eruptions at Mount Hood, and some 600 years of quiescence foreshadowed the activity at Mount St. Helens that began in 1480 and continued intermittently until 1857. In fact, circumstantial evidence suggests that long periods of inactivity at some intermittently active volcanoes end with particu-

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larly violent eruptions; apparently, energy is being stored rather than released in small eruptions (Simkin and others, 1981). Mount Rainier is capable of eruptions of moderate to very large magnitude, as measured by the Volcanic Explosivity Index (Newhall and Self, 1982) of 4 to 5 tentatively assigned to the explosive eruption that occurred between 30,000 and 100,000 years ago (Hoblitt and others, 1987). Its record of inactivity in the twentieth century and minor activity in the past few hundred years is not unusual for an active volcano. Indeed, based on past history, there is good reason to believe that the volcano will erupt again. Even relatively small eruptions could generate large floods and debris flows from melting of snow and ice on the summit. As noted previously, these debris flows and floods could cause significant property damage and loss of life along the river valleys draining the volcano, which tend to be heavily populated (Figure 2.3). Based on the known Holocene history of the volcano, the most likely future eruptive event at Mount Rainier is the extrusion of a lava flow at the summit, possibly accompanied by tephra eruptions. Geologic mapping (Fiske and others, 1963) has documented that numerous lava flows have been erupted recently in Mount Rainier's history, and the youngest of these, stubby flows up to 60 m thick, are preserved on the floor of present-day valleys and extend only a few kilometers away from the base of the volcano. Past history suggests that future lava flows will likely be restricted to valley floors within Mount Rainier National Park or will extend only a short distance outside the park. Although limited in areal extent, lava flows from Mount Rainier would destroy roads, buildings, and other fixed installations in and near valley bottoms and would be disruptive to many activities in the park. The sluggish motion of these flows would likely permit people to safely evacuate areas that were at risk, which means that little loss of life from this hazard would be expected. The public perception of risks associated with lava flows, if they occurred, could be far greater than the actual risks. The glowing surface of a flow could be exposed for many days, and the nighttime reflection of this glow from the underside of weather clouds might be visible to the

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hundreds of thousands of residents of the Puget Lowland. Similarly, columns of steam produced by the interaction of lava flows with snow and ice on the edifice might be visible to people at great distances from the volcano. These phenomena could convey a sense of impending crisis that would not be warranted because, as noted above, lava flows are likely to be restricted to National Park land. Explosive eruptions from Mount Rainier could send clouds of tephra high into the atmosphere, where it would be carried laterally by prevailing winds before settling to the ground. Past eruptions of the volcano have deposited up to 2.5 centimeters (cm) of ash 40 km downwind of the edifice (Crandell, 1973). Tephra would be a hazard to crops and other vegetation, machinery, and poorly built structures (the weight of the tephra could cause these structures to collapse). People living and working in such structures and those with respiratory problems would be at risk. The prevailing winds in western Washington are from southwest to northeast, so tephra from Mount Rainier would normally be carried away from the Seattle-Tacoma metropolitan area (Figure 2.1). Less frequently, winds blow from east to west, and during these times tephra could be scattered over much of the Puget Lowland. This would disrupt commerce, travel (especially at Seattle-Tacoma International Airport), and the daily lives of hundreds of thousands of people. Assessing frequencies and magnitudes of tephra eruptions is difficult. None of the thousands of flows on the edifice has been isotopically dated, nor is the age known for any tephra older than about 6,700 years. Ten tephras younger than about 6,700 years have been recognized and dated either directly or by bracketing between dated lahars. In postglacial times, according to Hoblitt and others (1987), the annual probability of a tephra eruption of small volume, between 0.01 and 0.1 km3, is about 1 in 1,000. The effects of such an eruption would be minor beyond a distance of approximately 50 km from the edifice. Hoblitt and others (1987) also estimate that the annual probability of an explosive eruption producing more than 0.1 km3 of tephra, which would have serious effects beyond approximately 50 km, is about 1 in 10,000. A task of future studies of Mount Rainier is to refine these estimates and, in particular, to estimate the

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average recurrence intervals for the lower magnitude but more frequent eruptions of lava flows. Recommendations Mount Rainier poses a significant hazard to life and property in heavily populated areas surrounding the volcano, particularly in the Seattle-Tacoma metropolitan area. The most likely hazards include edifice failures, glacier outburst floods, and lahars, with or without volcanic eruptions. Coordinated research that involves both geoscientists and social scientists should be undertaken to determine potential magnitudes and frequencies of potential hazards, their human and economic impacts, and strategies for using such information effectively to mitigate risk as part of this Decade Volcano Demonstration Project. A plan to achieve these objectives is outlined in the remainder of this report.