U.S. GEOLOGICAL SURVEY—REDUCING THE RISK FROM VOLCANO HAZARDS Mount Rainier—Living Safely With a Volcano in Your Backyard
ajestic Mount Rainier M soars almost 3 miles (14,410 feet) above sea level and looms over the expanding suburbs of Seattle and Tacoma, Washington. Each year almost two million visitors come to Mount Rainier National Park to admire the volcano and its glaciers, alpine meadows, and forested ridges. However, the volcano’s beauty is deceptive— U.S. Geological Survey (USGS) research shows that Mount Rainier is one of our Nation’s most dangerous volcanoes. It has been the source of countless eruptions and volcanic mudflows (lahars) that have surged down valleys on its flanks and buried broad areas now densely populated. To help people live more safely with the volcano, USGS scientists are working closely with local communities, emergency managers, and the National Park Service.
Mount Rainier, an active volcano currently at rest between eruptions, is the highest peak in the Cascade Range. Its edifice, capped by snow and 25 glaciers, has been built up by untold eruptions over the past 500,000 years. It last erupted in 1894–95, when small summit explosions were reported by observers in Seattle and Tacoma. Mount Rainier’s next eruption The flat floor of the Puyallup River valley near Orting, Washington, is formed by deposits of the 500-year-old might be of similar or larger size and Electron lahar, which surged down from Mount Rainier (in background). Lahars, or volcanic mudflows, are rapidly flowing slurries of mud and boulders that destroy or bury most manmade structures in their paths. could produce volcanic ash, lava flows, Lahars from Mount Rainier can travel for tens of miles along river valleys and reach Puget Sound. (USGS and avalanches of intensely hot rock photograph by D.E. Wieprecht.) and volcanic gases, called “pyroclastic flows.” Some of these events swiftly than 10 miles from the volcano’s summit plumes of volcanic ash can greatly endan- melt snow and ice and could produce and remain within Mount Rainier National ger aircraft in flight and seriously disrupt torrents of meltwater that pick up loose Park, the largest lahars can travel for tens aviation operations. Although seldom life rock and become rapidly flowing slurries of miles and reach Puget Sound. Volcanic threatening, volcanic ash fallout on the of mud and boulders known as “lahars.” ash will be distributed downwind, most ground can be a nuisance to residents, In contrast to lava flows and pyroclastic often toward the east, away from Puget affect utility and transportation systems, flows that are unlikely to extend farther Sound’s large population centers. Airborne and entail substantial clean-up costs.
U.S. Department of the Interior Fact Sheet 2008–3062 U.S. Geological Survey 2008 Armero, Colom- rock slide away, they transform rapidly bia, was battered into a lahar. Although most large in 1985 by lahars landslides at Mount Rainier occurred generated by an during eruptive periods and were eruption of gla- probably triggered by magma intru- cier-clad Nevado del Ruiz volcano. sion or by explosive eruptions rocking More than 20,000 the volcano, the origin of at least one, people died in the the 500-year-old Electron lahar, may city. Note empty not be related to eruptions. This lahar street blocks left deposits as much as 20 feet thick, where structures were swept and buried an old-growth forest in the away. Tragically, vicinity of modern-day Orting. safety lay nearby in areas of higher ground. (USGS Are All Parts of the Volcano photograph by Susceptible to Landslides? R.J. Janda.) The west flank of Mount Rainier, including the head of the Puyallup River, has the greatest potential for unleashing large landslides that become Lahars Pose the Greatest Risk future large lahars happen at rates simi- far-traveled lahars, because it has the At Mount Rainier, the risk from lahars lar to those of the past, there is roughly largest amount of weakened clay-rich is greater than from lava flows, volcanic a 1-in-10 chance of a lahar reaching the rock at high altitude. Therefore, the ash fall, or other volcanic phenomena Puget Sound lowland during an average Puyallup River valley and, to a lesser because some pathways for future lahars human lifespan. extent, the Nisqually River valley, whose are densely populated and contain impor- • There may be little or no advance basin includes some of the weakened tant infrastructure such as highways, warning—Studies by U.S. Geological rock, are at most risk from such events. bridges, ports, and pipelines. Lahars look Survey (USGS) scientists show that Little Tahoma Peak on the east side of the and behave like flowing concrete, and at least one of Mount Rainier’s recent volcano and many other cliffs and steep they destroy or bury most manmade struc- large landslide-generated lahars may slopes can fail in landslides, such as one tures in their paths. Past lahars probably have occurred when the volcano was in December 1963 that traveled several traveled 45 to 50 miles per hour and were quiet and not providing the warning miles, but such events are too small to as much as 100 feet or more thick where signs typical of a restless and erupting generate lahars. In contrast to landslides, confined in valleys near the volcano. They volcano. In such a rare case, the only lahars generated by eruptions could thinned and spread out in the wide valleys warning could be a report that a lahar is descend any of the valleys originating on downstream, slowing to 15 to 25 miles already underway. Mount Rainier. per hour. Deposits of past lahars are found in all of the valleys that start on Mount Two Types of Lahars Long-Term Effects of Lahars Rainier’s flanks. Mount Rainier can generate two types Lahars fill stream channels and bury of lahars that can threaten surrounding valley floors with deposits of boulders, How Hazardous is Mount Rainier? valleys: sand, and mud a few feet to tens of Mount Rainier has erupted less often feet thick. These deposits readily erode • Meltwater-generated lahars—Mount as rivers and streams reestablish their and less explosively in recent millennia Rainier supports more than one cubic than its well-known neighbor, Mount St. channels, shedding abundant sediment mile of glacial ice—as much as all downstream over years to decades. Helens. However, the proximity of large other Cascade Range volcanoes com- population centers in valleys susceptible Because of this, downstream valley floors bined. During past eruptive episodes, initially unaffected by a lahar may later to lahars from Mount Rainier makes it swift melting of snow and ice by pyro- suffer increased flooding and progressive a far greater threat to life and property clastic flows and other events caused burial by remobilized sediment. Recent than Mount St. Helens for the following numerous lahars. Such lahars would studies have revealed extensive layers reasons: be preceded by events that warn of an of sandy sediment from Mount Rainier • Population and development at impending eruption. that extend to the Port of Seattle along risk—About 80,000 people and their • Landslide-generated lahars—Land- the Green and Duwamish River valleys. homes are at risk in Mount Rainier’s slides can be triggered when molten This sediment was rapidly eroded from the deposits of lahars caused by eruptions lahar-hazard zones. Key infrastructure rock (magma) intrudes into a volcano about 1,000 years ago, even though such as major highways and utilities and destabilizes it, as happened at the lahars themselves did not extend cross through these zones, which also Mount St. Helens in 1980, or they may contain economically important busi- much past present-day Auburn, which be triggered by large earthquakes. They lies about 20 miles south of downtown nesses, hydroelectric dams, and major may also be the result of the eventual seaports. Seattle. failure of rocks that were weakened • Size and frequency of lahars— by the action of acidic fluids. Magma During the past several thousand years releases gases and heat creating hot, Debris Flows Threaten Areas in large lahars have reached the Puget acidic ground water that, over time, Mount Rainier National Park Sound lowland on average at least can convert hard volcanic rock into Almost annually, water released from once every 500 to 1,000 years. Smaller weak, clay-rich rock by a process glaciers or runoff from intense rainfall flows not extending as far as the called hydrothermal alteration. When incorporates rocks and sediment to lowland occurred more frequently. If masses of water-saturated clay-rich form “debris flows” that affect valleys
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on the east side of Mount Rainier traveled north and west along the White White the along west and north traveled Rainier Mount of side east the on
deposit, called the Osceola Mudflow, was formed when a massive landslide landslide massive a when formed was Mudflow, Osceola the called deposit,
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MOUNT RAINIER MOUNT HAZARD ZONES FOR DEBRIS FLOWS, LAHARS, LAVA FLOWS, AND PYROCLASTIC FLOWS FROM FROM FLOWS PYROCLASTIC AND FLOWS, LAVA LAHARS, FLOWS, DEBRIS FOR ZONES HAZARD on the flanks of Mount Rainier. Such debris flows behave like lahars, but are HOW TO PREPARE FOR A LAHAR OR OTHER typically of such small size that they VOLCANO HAZARD seldom travel beyond the base of the volcano and only affect vulnerable areas Learn: Determine within the boundaries of Mount Rainier whether you live, work, National Park. Summer and autumn are or go to school in a lahar the seasons during which debris flows are hazard zone. Learn about most common—times when glaciers are all volcanic processes producing large amounts of meltwater that could affect your and intense rains can fall on little- community. vegetated, snow-free areas with abundant Plan: Develop an loose debris. Because debris flows pose emergency plan with risks to park visitors and infrastructure, your family so that you especially trails, roads, and bridges, are prepared for natural Mount Rainier National Park educates hazards and emergencies. staff and visitors about hazards from debris flows and how to avoid them by Inquire: Ask public moving off valley floors. officials to advise you Volcano evacuation signs direct traffic to safety on higher ground about how to respond in Pierce County, Washington (USGS photograph by C.L. Driedger). during any emergency. Past Lahars Provide Clues About Future Hazards Lahars leave behind thick layers of the City of Orting as little as 40 minutes off the valley floor is the only way to boulders, mud, and logs on valley floors. after the initial warning is sounded. Time ensure safety during a lahar. When Geologists use thi s and other evidence could be short, and successful mitigation hiking in valleys on the slopes of Mount to assess future hazard potential and to will depend on effective notification of Rainier during late summer or during map zones in river valleys heading on people at risk, public understanding of the intense rainfall, be alert for the signs of Mount Rainier that could be inundated hazard, and prompt response by citizens. an approaching debris flow—ground by future lahars. Not all valleys would This system for automatic detection and shaking and roaring sound—and move necessarily be affected during a given notification of a lahar reduces—but does up the valley wall to higher ground. The ion or large landslide, nor would erupt same is true for lahars, but, because they all lahars in a valley be large enough to not eliminate—risk in the lahar pathways. affect much larger areas, people need extend to hazard-zone boundaries. Lahar to move out of threatened areas before hazard zones mapped by the USGS are Monitoring and Emergency Planning lahars get close. Lahars are almost being used to guide the development of Are Ongoing always preceded by volcanic unrest, so hazard-area regulations in comprehensive The USGS, in cooperation with the in most instances there will be time to land-use plans by counties and cities that Pacific Northwest Seismic Network at the lie at the foot of Mount Rainier. University of Washington, continuously warn people when there is an increased monitors Mount Rainier and assesses risk. Obtain a NOAA weather radio to Lahar Warning System Reduces Risk potential hazards stemming from volcanic receive alerts about possible lahars, as Because there is higher level of risk activity. Volcanoes often show signs well as other natural hazards (for further from lahars generated by landslides of unrest, such as increased seismicity information on the Web go to http://www. on the west flank of Mount Rainier, (earthquakes) and emission of volcanic weather.gov/nwr.). the USGS, Pierce County Department gases and swelling of the volcano, days to of Emergency Management, and months in advance of an eruption. When Washington State Emergency unrest is detected, scientists will notify Carolyn L. Driedger and William E. Scott Management Division have established emergency-management officials and increase monitoring efforts. Edited by James W. Hendley II a lahar warning system. A detection Graphic design by Jeanne DiLeo and Lisa Faust component consists of arrays of monitors The Mount Rainier Volcanic Hazards that record the ground vibrations of Response Plan, which was created by For more information contact: a lahar. Computerized evaluation of cooperating local, county, State, and U.S. Geological Survey data assesses the presence of a flowing Federal agencies, is on the Web at David A. Johnston Cascades Volcano Observatory 1300 SE Cardinal Court, Building 10, Suite 100 lahar and issues an automatic alert http://www.co.pierce.wa.us/pc/Abtus/ Vancouver, WA 98683 to emergency-management agencies. ourorg/dem/EMDiv/Mt%20Rainier%20 (360) 993-8900 Emergency managers can then initiate VHRP.htm. The plan describes the or http://vulcan.wr.usgs.gov appropriate response measures. City, responsibilities of agencies and how they http://www.pnsn.org/RAINIER/welcome.html county, and State agencies design will communicate with each other and the and maintain notification procedures, public during a volcanic crisis. This Fact Sheet supersedes Mount Rainier—Living with Perilous Beauty ((Fact Sheet 065-97) and Mount Rainier— evacuation routes, and public-education Learning to Live with Volcanic Risk (Fact Sheet 034-02). programs. What to Do if Threatened by a Lahar If a large lahar were generated in the or Debris Flow See also Volcano Hazards from Mount Rainier, Washington upper Puyallup River valley without the Know the signs of debris flows and (U.S. Geological Survey Open-File Report 98-428). precursors that typically herald volcanic lahars. Experience from around the This Fact Sheet and any updates to it are available online unrest and eruption, it could arrive at world shows that moving to high ground at http://pubs.usgs.gov/fs/2008/3062/.
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