U.S. GEOLOGICAL SURVEY— REDUCING THE RISK FROM VOLCANO HAZARDS MountMount RainierRainier---- Learning to Live with Volcanic Risk

olcano ount Rainier in V Washington state is Hazards Man active volcano reaching more than 2.7 miles Mount Rainier (14,410 feet) above sea level. Its majestic edifice looms LAVA FLOWS L ava is over expanding suburbs in molten rock that flows or oozes onto the earth's surface. Mount the valleys that lead to Rainier consists largely of numerous nearby Puget Sound. USGS lava flows interbedded with rock rubble. research over the last several decades indicates PYROCLASTIC FLOWS that Mount Rainier has been P the source of many volcanic yroclastic flows are mudflows (lahars) that buried hot avalanches of mudflows (lahars) that buried lava fragments and gas areas now densely formed by the collapse of thick lava populated. Now the USGS is FIGURE 1.1.—View across Puyallup River valley flows and working cooperatively with eruption toward Mount Rainier (Photograph by David columns. local communities to help Wieprecht, U.S. Geological Survey.) people live more safely with E the volcano. Lahars look and behave like flowing xplosive eruptions blast fragments concrete, and their impact forces destroy of rock high into the air. Large fragments fall to the most man-made structures. At Mount ground close to the Rainier, they have traveled 45-50 miles per volcano. Small fragments Lahars are the greatest hazard hour at depths of 100 feet or more in (ash) from Lahars are the greatest hazard TEPHRA confined valleys, slowing and thinning in large Mount Rainier (fig. 1) is an active eruptions can travel the wide, now-populated valleys. thousands of miles volcano that is currently at rest between downwind. eruptions. Its next eruption might At Mount Rainier, lahars are a greater produce volcanic ash, lava flows, or hazard than other volcanic products such pyroclastic flows. The latter can rapidly as lava and poisonous gases that have L ahars are fast-moving slurries of melt snow and ice, and the resulting been popularized by TV and film. Lava volcanic rock, mud, and water that meltwater torrent could produce lahars flows and pyroclastic flows are unlikely to look and behave like extend more than a few miles beyond the LAHARS flowing concrete. (a word of Indonesian origin that has They originate either come to mean volcanic mudflow) that National Park boundaries. Volcanic ash from massive volcanic landslides travel down valleys beyond the base of (tephra) will be distributed downwind, 80 or from surges of eruption- percent of the time toward the east away generated meltwater. the volcano to areas now densely popu- lated. Lahars caused by large landslides from large populations (fig. 2). can also occur during non-eruptive times– The USGS, in cooperation with the without the seismicity and other warnings University of Washington, monitors many that normally precede eruptions. Cascade Range volcanoes, including Mount Rainier, to detect precursors to U.S. Department of the Interior USGS Fact Sheet 034-02 U.S. Geological Survey 2002 eruptive activity. Mount Rainier last Probability of Tephra Accumulation a muddy slurry—a clay-rich (also erupted in the 19th century; one or called cohesive) lahar that is more small eruptions from one of funneled into one or more sur- the summit craters produced local WASHINGTON rounding valleys. ashfall. Sand-rich lahars, also known as noncohesive lahars, form during Mount Rainier is one of the eruptions of Mount Rainier when most hazardous volcanoes in hot pyroclastic flows melt snow the United States Seattle Wenatchee and ice. (Mount Rainier supports Tacoma more than one cubic mile of glacial Although Mount Rainier has ice—as much as all other Cascade erupted less often and less explosively in Range volcanoes combined.) Olympia Ellensburg recent millennia than its neighbor, Because sand-rich lahars occur Mount St. Helens, the proximity of Yakima during eruptive activity, they are large populations makes Mount Rainier Mount Rainier likely to be preceded by events that a far greater hazard to life and will warn of an impending erup- property. tion, and thus of increased lahar Vancouver potential. 1. The population at risk—More than 150,000 people reside on the Small lahars, traveling only a deposits of previous lahars. 1 in 10,000 1 in 1,000 1 in 500 few miles, are caused by local 2. The size and frequency of lahars— avalanches of rock debris, sudden During the past few millennia lahars FIGURE 2.——Map showing the annual probability that releases of glacial meltwater, or volcanic ash will be deposited to a thickness of 1/3 inch that have reached the Puget Sound intense rainfall. These lahars occur or more from an eruption of Mount Rainier. Volcanic many times each century. lowland have occurred, on average, ash, of this thickness or less, can cause disruption of at least every 500 to 1,000 years. ground and air transportation, and can cause damage New lahar deposits may get Smaller flows not extending as far as to electronics and machinery. redistributed downstream over a the lowland occur more frequently. period of many years as the disrupted If lahars of the future happen at logs, forming a buried forest. Some of drainage network is re-established. rates similar to those of the past, the deposits can be traced upstream to Thus, valley-floor areas that were not there is at least a one in seven the volcano’s flanks, and all contain inundated by the initial lahar deposits chance of a lahar reaching the Puget volcanic fragments unique to Mount may suffer enhanced flooding and Sound lowland during an average Rainier. Geologists map the deposits progressive burial by remobilized human life-span. and determine the tree ages to learn sediment (zone of post-lahar sedimenta- 3. We may not have advance warning– when the trees were engulfed and killed tion in figure 3). USGS research shows that some by the lahar. Old-timers recall encoun- lahars occur with little or no tering huge buried stumps and logs when Weakened rock and rising warning. Our only warning could be plowing fields and digging wells. The magma can cause flank youngest such forest was buried about a report that a flow is under way. collapse 500 years ago and uncovered during collapse Past lahars indicate future excavations for new homes in the Flank collapses can be triggered Puyallup River valley. when magma intrudes into a volcano and Geologists determine the size and destabilizes it, as happened at Mount timing of past lahars and use this St. Helens in 1980. Lahars form in several ways information to indicate future hazard A neighboring volcano—Mount potential. For example, in figure 3, the Some of the largest lahars have Baker—produced flank collapses in the areas inundated by the 2,300 year-old originated by collapse of weakened rock 1840’s that were apparently triggered by National Lahar in the Nisqually Valley, from the flanks of the volcano—a large steam explosions related to volcanic and the 500 year-old Electron Mudflow landslide known as a flank collapse. activity. Steam explosions at Mount During eruptions, molten rock is injected in the Puyallup Valley are superimposed Rainier could trigger flank collapses and on all valleys surrounding Mount Rainier. into cracks of the volcano and solidifies as slabs of rock called dikes. The lahars with little or no advance warning. They illustrate areas that could be Although many flank collapses occur inundated if flows of those sizes cooling magma releases gases and heat into groundwater, making it hot and during eruptive periods, it is possible for occurred in each valley. acidic. The hot, acidic waters convert them to be triggered by earthquakes or A lahar flowing downvalley from hard volcanic rock into soft, clay-rich they may be the result of progressive Mount Rainier leaves a thick valley- rock by a process called hydrothermal weakening of the rock, saturation by bottom deposit of boulders and hard- alteration. When masses of water-rich groundwater and the continuing pull of ened mud that may envelop stumps and rock collapse, they transform rapidly into gravity. 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). The map shows areas that could be inundated if events similar in size to those of the past occurred today. Lahar hazard is not equal in all valleys. Puyallup Valley is the valley most susceptible to lahars caused by flank collapse. Risk to individual drainages will be refined as scientists learn more about the volcano. Flanks of volcano can unleash Lahar-warning system reduces detection of the approaching lahar, flank collapses and rockfalls risk effective notification of people at risk, public understanding of the hazard, and The west flank at the head of the Because of the higher level of risk prompt response by citizens. Puyallup River valley has the greatest from lahars in the Carbon and Puyallup potential for collapse because it has the River valleys, the USGS and Pierce Volcanoes often show signs that largest area of weakened clay-rich rock County Department of Emergency they are getting ready to erupt days to remaining at high altitude. The east and Management have installed a lahar- weeks or months in advance. Scientists northeast sides of the volcano can let detection and warning system. The of the U.S. Geological Survey and loose large rockslides, such as that of system consists of arrays of five University of Washington evaluate signs December 1963, because of fractures in acoustic flow monitors (AFM’s) that of unrest and look for increased seismic the ridges. detect the ground vibrations of a lahar. activity, increased emission of volcanic Computerized evaluation of data gases and swelling of the volcano. When unrest is detected, scientists will One collapse can yield lahars in confirms the presence of a flowing lahar and issues an automatic alert to emer- increase monitoring efforts and notify multiple valleys gency management agencies. Emer- emergency management officials. Future lahars will follow river gency managers then can initiate valleys that drain Mount Rainier. Four response measures such as evacuations. What can you do? of the five major river systems flow This system for automatic detection and Learn: Determine whether you live, westward into suburban areas of Pierce notification of a lahar reduces, but does work, or go to school in a lahar County. These flow pathways are not eliminate, risk in the lahar pathways. hazard zone. Learn about all volcanic mapped by the USGS (fig. 3), just as processes that could affect your flood-inundation maps show the areas at Lahar travel times are short community. risk of flooding. Lahars occurring during an eruption may affect valley The estimated time between Inquire: Ask public officials to advise areas miles from the volcano, but a detection of a lahar and its arrival in you about how to respond during any precursory warning should allow ample Orting is about 40 minutes. Dispersed emergency. time for evacuation. populations closer to Mount Rainier Plan: Develop an emergency plan with would be affected sooner. Time is short, A catastrophic flow will likely your family so that you are prepared and successful evacuation will depend on spread into multiple valleys. The largest for natural hazards and emergencies. known flow entered all five drainages, and most of the known large flows have entered two or more. by C.L.Driedger and K.M. Scott Mount Rainier Monitoring and emergency is an active volcano... Layout design and modification of planning are ongoing graphics by Christine Janda. The USGS, in cooperation with the Original graphics by Lisa Faust University of Washington, monitors the state of the volcano and assesses For up-to-date information contact: hazards from volcanic activity. The lahar pathways mapped by the USGS U.S. Geological Survey Cascades Volcano Observatory guide the hazard-area regulations of the 1300 SE Cardinal Court, Suite 100 comprehensive land-use plan for local R educing population growth Vancouver, WA 98683 counties. The plan’s urban growth in the paths of lahars, Telephone: 360-993-8900 boundary and its proposed land uses in implementing a warning system, unincorporated areas are designed to and planning and practicing World Wide Web: minimize population growth, where evacuations can lower the http://vulcan.wr.usgs.gov possible, within hazard zones. potential loss of life and or Local, county, state, and federal property during future eruptions USGS Volcano Hazards Program agencies including the USGS have joined and lahars. These actions can http://volcanoes.usgs.gov to develop a Mount Rainier volcanic reduce the risk from lahars and This fact sheet supersedes Mount Rainier–Living hazards response plan that addresses provide a measure of safety for with Perilous Beauty (Fact Sheet 065-97). such issues as emergency-response those who enjoy living, working, See also Volcano Hazards from Mount operations and strategies for expanded and playing in valleys Rainier, Washington (U.S. Geological Survey public awareness and mitigation of surrounding Mount Rainier. Open-File Report 98–428). volcanic hazard.

Printed on recycled paper