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Home , Carbon River, Cascade Range, Mount Rainier, Puyallup River

FLANKUS COLLAPSESG S , , AND VOLCANIC FLOW RISK AT

U.S. GEOLOGICAL SURVEY— REDUCING THE RISK FROM HAZARDS Mount Rainier-- Living with Perilous Beauty

olcano ount Rainier is an V M active volcano Hazards reaching more than 2.7 miles (14,410 feet) above sea level. Mount Rainier Its majestic edifice looms over expanding suburbs in FLOWS the valleys that lead to nearby . USGS ava is molten rock that flows Lor oozes onto the earth's surface. research over the last Mount Rainier consists largely of several decades indicates numerous lava flows interbedded with rock rubble. that Mount Rainier has been the source of many volcanic PYROCLASTIC FLOWS (lahars) that buried areas now densely populated. Now the USGS is Pyroclastic flows are hot of working cooperatively with lava fragments and local communities to help gas formed by the people live more safely with collapse of thick FIGURE 1.—View across Puyallup lava flows and the volcano. valley toward Mount Rainier eruption columns.

TEPHRA What are the hazards? confined valleys, slowing and thinning in xplosxivpelo esirvuep teiorunps tions the wide, now-populated valleys. E blast fragments of rock high Mount Rainier (fig. 1) is an active into the air. Large volcano that is currently at rest between At Mount Rainier, lahars are a fragments fall to the eruptions. Its next eruption may greater hazard than other volcanic ground close to the volcano. produce volcanic ash, lava flows, or products such as lava and poisonous pyroclastic flows. The latter can rapidly gases that have been popularized by TV Small fragments (ash) from large melt and ice, and the resulting and film. Lava flows and pyroclastic eruptions can travel meltwater torrent may produce lahars flows are unlikely to extend more than a thousands of miles downwind. (the widely used Indonesian word for few miles beyond the National Park volcanic mudflows) that travel down boundaries. Volcanic ash () will valleys beyond the base of the volcano to be distributed downwind, 80 percent of the time toward the east away from large L ahars are fast-moving slurries of areas now densely populated. Lahars , mud, and water that may also occur during non-eruptive populations (fig. 2). look and behave like LAHARS flowing concrete. times–without the profound seismicity The USGS, in cooperation with the and other warnings that normally They originate either from University of , monitors massive volcanic landslides or precede eruptions. many volcanoes, includ- from surges of eruption- ing Mount Rainier, to detect precursors generated meltwater. Lahars look and behave like flowing concrete, and their impact forces destroy to eruptive activity. Mount Rainier was most man-made structures. At Mount last active in the 19th century; one or Rainier, they have traveled 45-50 more small eruptions produced local miles/hr at depths of 100 feet or more in ashfall from one of the summit craters.

U.S. Department of the Interior USGS Fact Sheet 065–97 U.S. Geological Survey 1998 Why is Mount Rainier the Probability of Tephra Accumulation sudden releases of glacial meltwater, or most dangerous volcano storms. These lahars (Case 3 in figure 3) happen commonly–many times each WASHINGTON in the ? century. Although Mount Rainier has New deposits may get redistrib- erupted less often and less explo- uted downstream over a period of many sively in recent millennia than its years as the disrupted drainage network neighbor, Mount St. Helens, the Wenatchee becomes re-established. Thus valley-floor areas that were not impacted by the proximity of large populations Tacoma makes Mount Rainier a far greater initial lahar deposits may suffer enhanced flooding and progressive burial by hazard to life and property. Olympia Ellensburg 1. The population at risk--More than remobilized (zone of post-lahar 100,000 people reside on the Mount Yakima sedimentation in figure 3). deposits of previous lahars. The Rainier risk that an individual structure What triggers a flank collapse? will be impacted by a lahar from Mount Rainier in much of this Flank collapses can be triggered when area is comparable to its risk of 1 in 10,000 1 in 1,000 1 in 500 magma intrudes into a volcano and damage by fire. destabilizes it, as happened at Mount St. 2. The size and frequency of lahars-- FIGURE 2.—Map showing the annual probability Helens in 1980. However, scientists During the past few millennia that volcanic ash will be deposited to a studying the timing of flank collapses thickness of 1/3 inch or more from an lahars that have reached well into find that some flank collapses were eruption of Mount Rainier. Volcanic ash, of apparently not accompanied by erup- the Puget Sound lowland have this thickness or less, can cause disruption recurred, on average, at least every of ground and air transportation, and can tions. They fear that a collapse could 500 to 1,000 years. Smaller flows cause damage to electronics and happen suddenly during non-eruptive occur more frequently. A flow with machinery. periods and without the swarms of small a 500-yr return period has about a earthquakes and other detectable phe- one in seven chance of occurring in enormous landslides--flank collapses-- nomena that typically accompany rising an average human life-span. from the slopes of the volcano. Hot, acid magma. 3. We may not have advance warning– water percolates through the volcano and A large earthquake unrelated to rising USGS research shows that some slowly changes hard volcanic rock into magma could trigger a flank collapse on lahars may occur with little or no soft rock that is clay-rich in a process Mount Rainier. At least one flank warning. Our only warning could called hydrothermal alteration. The collapse at Mount Rainier could have be a report that a flow is under progressively weakens, like a been caused by a great prehistoric house infested with termites. Eventually, way. earthquake that occurred at about the a section of the volcano collapses, same time. perhaps unexpectedly, and the muddy, How are lahar deposits A neighboring volcano--Mount water-rich mass transforms rapidly into a Baker--produced flank collapses in the identified? muddy slurry--a clay-rich, or cohesive, 1840’s that were apparently triggered by lahar (Case 1 in figure 3)--that is A lahar flowing down valley from steam explosions. Steam explosions at funneled at high speed into one or more Mount Rainier leaves a thick valley- Mount Rainier could possibly trigger surrounding valleys. bottom deposit of boulders and hard- flank collapses and lahars with no ened mud that may envelop stumps and Sand-rich, or noncohesive, lahars advance warning. logs of a buried . Some of the form during eruptions of Mount Rainier Some flank collapses and lahars may deposits can be traced upstream to the when hot pyroclastic flows melt snow happen with no obvious trigger. They volcano’s flanks, and all contain volcanic and ice. (Mount Rainier supports more may simply result from progressive fragments unique to Mount Rainier. than one cubic mile of glacial ice--as hydrothermal alteration of the rock, Geologists map the deposits and deter- much as all other Cascade Range saturation by groundwater, and the mine the tree ages to learn when the trees volcanoes combined.) These lahars may continuing pull of gravity. were engulfed and killed by the lahar. be less of a risk than cohesive lahars because they commonly have been Old-timers recall encountering huge What areas are at risk? buried stumps and logs when plowing smaller, although more frequent in the fields and digging wells. The youngest volcano’s history (Case 2 in figure 3). The courses of lahars will be the river such forest was buried about 500 years Because noncohesive lahars occur with valleys that drain Mount Rainier. Four of ago and uncovered during excavations for volcanic activity, they are likely to be the five major river systems flow west- new homes in the valley. preceded by events that will warn of an ward into suburban areas of Pierce impending eruption; thus, they will be County. These flow pathways are expected. How and why do lahars form? mapped by the USGS (fig. 3), just as Small lahars, only a few miles long are flood-inundation maps show the areas at The most dangerous lahars start as caused by local avalanches of rock debris, risk of flooding. Lahars occurring during

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EXPLANATION Small lahars with recurrence interval <100 years (Case 3) Moderate lahars with recurrence interval 100-500 years (Case 2) Large lahars with recurrence interval 500-1000 years (Case 1) Packwood r Area most likely to be affected by ve lava flows and pyroclastic flows Ri Packwood Cowlitz Lake Post-lahar sedimentation Randle

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FIGURE 3.—Hazard zones for lahars, lava flows, and pyroclastic flows from Mount Rainier (Hoblitt and others, 1998; US Geological Survey Open-File Report 98–428). an eruption may affect valley areas miles collapse lahar could reduce, but not How much warning will from the volcano, but a precursory eliminate, the risk in the lahar pathways. we have? warning should allow ample time to Two critical parts of such a detection and prepare for evacuation. warning system include: (1) seismic Travel time of a lahar increases with A catastrophic flow will likely spread detection and automatic notification of a distance. Thus the travel time of a large into multiple drainages. The largest major collapse event on Mount Rainier; lahar from Mount Rainier to the lowland known flow entered all five drainage and (2) detection, location, and auto- valleys is estimated at 1 to 2 hours, sectors, and most of the known large matic notification of a lahar in progress. whereas the travel time to the nearest area flows have entered two or more. Drain- of large population may be as little as 45 1. Seismic detection of collapse—work age sectors converge toward the summit minutes. However, as the AFM arrays is under way to structure the and are separated by sharp, unstable must be located beyond the volcano seismic network run cooperatively divides. Lahars have been as much as flanks, as little as 30 minutes, after by the 800-1000 feet deep on the flanks of warning from the pilot lahar-detection Mount Rainier, overtopping divides even and the USGS to discriminate the system, may be available for evacuation of when originating in one drainage. seismic signal of a major collapse urban areas closest to Mount Rainier in event on Mount Rainier and issue the Puyallup valley. Time is short, and Are all parts of the volcano an automatic notice. successful evacuation will depend on susceptible to landslides? 2. Lahar detection—the acoustic flow detection of the approaching lahar, monitor (AFM) developed by effective notification of people at risk, All flanks of the volcano are suscep- USGS scientists can detect the public understanding of the hazard, and tible to landslides and rockfalls. How- ground vibrations of a lahar. The practiced response by citizens. ever, the east and west flanks are the least USGS and Pierce County Depart- stable and have been the source of the ment of Emergency Management, What can you do? largest landslide-generated lahars because together, are installing a pilot lahar- of an east-west-trending dike and fracture detection system in the Puyallup Learn: Determine whether you live, zone that promotes circulation of and valleys. Com- work, or go to school in a lahar hydrothermal fluid in those sectors. In puterized evaluation of data from hazard zone. Learn about all addition, large lahars commonly enter arrays of five AFM’s in each valley volcanic processes that could affect more than one drainage, placing multiple are expected to confirm either your community. valleys at risk. presence or absence of a flowing Plan: Develop an emergency plan lahar and issue an automatic alert. How are we responding to the with your family so that you are prepared for natural hazards and risks? emergencies. The USGS, in cooperation with the Inquire: Ask public officials to advise University of Washington, monitors the Mount Rainier you about how to respond during state of the volcano and assesses eruptive is an active volcano... any emergency. and hydrologic hazards stemming from volcanic activity. by The lahar pathways mapped by the USGS guide the hazard-area regulations K.M. Scott, E.W. Wolfe, of the comprehensive land-use plan for and C.L. Driedger Pierce County. The plan’s urban growth boundary and its proposed land uses in Graphic design by Christine Janda unincorporated areas are designed to minimize population growth, where possible, within hazard zones. R educing population growth For up-to-date information contact: in the paths of lahars, Local, county, state, and federal U.S. Geological Survey implementing a warning system, agencies including the USGS have joined Volcano Observatory to develop a Mount Rainier volcanic and planning and practicing 5400 MacArthur Boulevard hazards plan that, when complete, will evacuations can lower the Vancouver, WA 98661-7095 address such issues as emergency- potential loss of life and response operations and strategies for property during future eruptions Telephone: 360-696-7693 expanded public awareness and mitiga- and lahars. These actions can World Wide Web: tion of volcanic hazard. reduce the risk from lahars and http://vulcan.wr.usgs.gov provide a measure of safety for What about a lahar-warning those who enjoy living, working, See also Volcano Hazards from system? and playing in valleys Mount Rainier, Washington (U.S. Geological Survey surrounding Mount Rainier. A system for automatic detection and Open-File Report 98–428 notification of an unanticipated flank-