confined valleys, slowing and thinning inthe wide, now-populated valleys.

At Mount Rainier, lahars are agreater hazard than other volcanicproducts such as lava and poisonousgases that have been popularized by TVand film. Lava flows and pyroclasticflows are unlikely to extend more than afew miles beyond the National Parkboundaries. Volcanic ash (tephra) willbe distributed downwind, 80 percent ofthe time toward the east away from largepopulations (fig. 2).

The USGS, in cooperation with theUniversity of Washington, monitorsmany Cascade Range volcanoes, includ-ing Mount Rainier, to detect precursorsto eruptive activity. Mount Rainier waslast active in the 19th century; one ormore small eruptions produced localashfall from one of the summit craters.

FL A N K C OLL A PSES, L A H A RS, A N D V OLC A NIC FLO W RISK A T M O U N T R A INIER

W hat are the hazards?Mount Rainier (fig. 1) is an active

volcano that is currently at rest betweeneruptions. Its next eruption mayproduce volcanic ash, lava flows, orpyroclastic flows. The latter can rapidlymelt snow and ice, and the resultingmeltwater torrent may produce lahars(the widely used Indonesian word forvolcanic mudflows) that travel downvalleys beyond the base of the volcano toareas now densely populated. Laharsmay also occur during non-eruptivetimes–without the profound seismicityand other warnings that normallyprecede eruptions.

Lahars look and behave like flowingconcrete, and their impact forces destroymost man-made structures. At MountRainier, they have traveled 45-50miles/hr at depths of 100 feet or more in

FIGURE 1.—View across Puyallup Rivervalley toward Mount Rainier

U.S. G E OLO GIC A L S U RV E Y— RE D U CIN G T H E RISK FR O M VOLC A N O H A Z A R D S

U .S. Department of the Interior USGS Fact Sheet 065–97U .S. Geological Survey 1998

ount Rainier is an active volcano

reaching more than 2.7 miles(14,410 feet) above sea level.Its majestic edifice loomsover expanding suburbs inthe valleys that lead tonearby Puget Sound. USGSresearch over the lastseveral decades indicatesthat Mount Rainier has beenthe source of many volcanicmudflows (lahars) thatburied areas now denselypopulated. Now the USGS isworking cooperatively withlocal communities to helppeople live more safely withthe volcano.

MMount Rainier

ava is molten rock that flowsor oozes onto the earth's surface.Mount Rainier consists largely ofnumerous lava flows interbeddedwith rock rubble.

L

L

P

LAVA FLOWS

PYROCLASTIC FLOWS

LAHARS

yroclastic flows are hot avalanches of

lava fragments and gas formed by the

collapse of thick lava flows and

eruptioncolumns.

ahars are fast-moving slurries of volcanic rock, mud, and water that

look and behave like flowing concrete.

They originate either from massive volcanic landslides or

from surges of eruption-generated meltwater.

Volcano Hazards

xplosive eruptions blast fragments of rock high

into the air. Large fragments fall to the

ground close tothe volcano.

Small fragments(ash) from large

eruptions can travel thousands of miles

downwind.

TEPHRA

xplosive eruptionsE

GUS S

Mount Rainier--

Living with Perilous Beauty

W hy is Mount Ra inier themost dangerous volcanoin the United States?

Although Mount Rainier haserupted less often and less explo-sively in recent millennia than itsneighbor, Mount St. Helens, theproximity of large populationsmakes Mount Rainier a far greaterhazard to life and property.

1. The population at risk--More than100,000 people reside on thedeposits of previous lahars. Therisk that an individual structurewill be impacted by a lahar fromMount Rainier in much of thisarea is comparable to its risk ofdamage by fire.

2. The size and frequency of lahars--During the past few millennialahars that have reached well intothe Puget Sound lowland haverecurred, on average, at least every500 to 1,000 years. Smaller flowsoccur more frequently. A flow witha 500-yr return period has about aone in seven chance of occurring inan average human life-span.

3. We may not have advance warning–USGS research shows that somelahars may occur with little or nowarning. Our only warning couldbe a report that a flow is underway.

How are lahar depositsident if ied?

A lahar flowing down valley fromMount Rainier leaves a thick valley-bottom deposit of boulders and hard-ened mud that may envelop stumps andlogs of a buried forest. Some of thedeposits can be traced upstream to thevolcano’s flanks, and all contain volcanicfragments unique to Mount Rainier.Geologists map the deposits and deter-mine the tree ages to learn when the treeswere engulfed and killed by the lahar.Old-timers recall encountering hugeburied stumps and logs when plowingfields and digging wells. The youngestsuch forest was buried about 500 yearsago and uncovered during excavations fornew homes in the Puyallup River valley.

How and why do lahars form?The most dangerous lahars start as

enormous landslides--flank collapses--from the slopes of the volcano. Hot, acidwater percolates through the volcano andslowly changes hard volcanic rock intosoft rock that is clay-rich in a processcalled hydrothermal alteration. Themountain progressively weakens, like ahouse infested with termites. Eventually,a section of the volcano collapses,perhaps unexpectedly, and the muddy,water-rich mass transforms rapidly into amuddy slurry--a clay-rich, or cohesive,lahar (Case 1 in figure 3)--that isfunneled at high speed into one or moresurrounding valleys.

Sand-rich, or noncohesive, laharsform during eruptions of Mount Rainierwhen hot pyroclastic flows melt snowand ice. (Mount Rainier supports morethan one cubic mile of glacial ice--asmuch as all other Cascade Rangevolcanoes combined.) These lahars maybe less of a risk than cohesive laharsbecause they commonly have beensmaller, although more frequent in thevolcano’s history (Case 2 in figure 3).Because noncohesive lahars occur withvolcanic activity, they are likely to bepreceded by events that will warn of animpending eruption; thus, they will beexpected.

Small lahars, only a few miles long arecaused by local avalanches of rock debris,

sudden releases of glacial meltwater, orstorms. These lahars (Case 3 in figure 3)happen commonly–many times eachcentury.

New lahar deposits may get redistrib-uted downstream over a period of manyyears as the disrupted drainage networkbecomes re-established. Thus valley-floorareas that were not impacted by theinitial lahar deposits may suffer enhancedflooding and progressive burial byremobilized sediment (zone of post-laharsedimentation in figure 3).

W hat triggers a f lank collapse?

Flank collapses can be triggered whenmagma intrudes into a volcano anddestabilizes it, as happened at Mount St.Helens in 1980. However, scientistsstudying the timing of flank collapsesfind that some flank collapses wereapparently not accompanied by erup-tions. They fear that a collapse couldhappen suddenly during non-eruptiveperiods and without the swarms of smallearthquakes and other detectable phe-nomena that typically accompany risingmagma.

A large earthquake unrelated to risingmagma could trigger a flank collapse onMount Rainier. At least one flankcollapse at Mount Rainier could havebeen caused by a great prehistoricearthquake that occurred at about thesame time.

A neighboring volcano--MountBaker--produced flank collapses in the1840’s that were apparently triggered bysteam explosions. Steam explosions atMount Rainier could possibly triggerflank collapses and lahars with noadvance warning.

Some flank collapses and lahars mayhappen with no obvious trigger. Theymay simply result from progressivehydrothermal alteration of the rock,saturation by groundwater, and thecontinuing pull of gravity.

W hat areas are at risk?The courses of lahars will be the river

valleys that drain Mount Rainier. Four ofthe five major river systems flow west-ward into suburban areas of PierceCounty. These flow pathways aremapped by the USGS (fig. 3), just asflood-inundation maps show the areas atrisk of flooding. Lahars occurring during

FIGURE 2.—Map showing the annual probabilitythat volcanic ash will be deposited to athickness of 1/3 inch or more from aneruption of Mount Rainier. Volcanic ash, ofthis thickness or less, can cause disruptionof ground and air transportation, and cancause damage to electronics andmachinery.

WASHINGTON

Seattle

Tacoma

Olympia

Wenatchee

Ellensburg

Yakima

Vancouver

Mount

Rainier

1 in 10,000 1 in 1,000 1 in 500

Probability of Tephra Accumulation

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).

5 10 KILOMETERS0

5 10 MILES0

Orting

Packwood

KingCounty

PierceCounty

PackwoodLake

Riffe Lake

CowlitzRiver

NisquallyRiver

Greenwater

Puyallup

Buckley

Wilkeson

Enumclaw

Lake Kapowsin

Graham

Ashford

Eatonville

ElbeThurston County

Thurston County

Lewis County

AlderDam

MudMountain

Dam

Puy allup River

Carbon River

White Rive rSumner

MOUNTRAINIER

National Park Boundary

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)Area most likely to be affected by lava flows and pyroclastic flows

Post-lahar sedimentation

E X P L A N A T I O N

FederalWay

Pierce County

Auburn

Randle

Mount Rainier

Carbonado

Tacoma

Lake Tapps

Puget Sound

collapse lahar could reduce, but noteliminate, the risk in the lahar pathways.Two critical parts of such a detection andwarning system include: (1) seismicdetection and automatic notification of amajor collapse event on Mount Rainier;and (2) detection, location, and auto-matic notification of a lahar in progress.

1. Seismic detection of collapse—workis under way to structure theseismic network run cooperativelyby the University of Washingtonand the USGS to discriminate theseismic signal of a major collapseevent on Mount Rainier and issuean automatic notice.

2. Lahar detection—the acoustic flowmonitor (AFM) developed byUSGS scientists can detect theground vibrations of a lahar. TheUSGS and Pierce County Depart-ment of Emergency Management,together, are installing a pilot lahar-detection system in the Puyallupand Carbon River valleys. Com-puterized evaluation of data fromarrays of five AFM’s in each valleyare expected to confirm eitherpresence or absence of a flowinglahar and issue an automatic alert.

an eruption may affect valley areas milesfrom the volcano, but a precursorywarning should allow ample time toprepare for evacuation.

A catastrophic flow will likely spreadinto multiple drainages. The largestknown flow entered all five drainagesectors, and most of the known largeflows have entered two or more. Drain-age sectors converge toward the summitand are separated by sharp, unstabledivides. Lahars have been as much as800-1000 feet deep on the flanks ofMount Rainier, overtopping divides evenwhen originating in one drainage.

Are a ll parts of the volcanosuscept ible to landslides?

All flanks of the volcano are suscep-tible to landslides and rockfalls. How-ever, the east and west flanks are the leaststable and have been the source of thelargest landslide-generated lahars becauseof an east-west-trending dike and fracturezone that promotes circulation ofhydrothermal fluid in those sectors. Inaddition, large lahars commonly entermore than one drainage, placing multiplevalleys at risk.

How are we responding to therisks?

The USGS, in cooperation with theUniversity of Washington, monitors thestate of the volcano and assesses eruptiveand hydrologic hazards stemming fromvolcanic activity.

The lahar pathways mapped by theUSGS guide the hazard-area regulationsof the comprehensive land-use plan forPierce County. The plan’s urban growthboundary and its proposed land uses inunincorporated areas are designed tominimize population growth, wherepossible, within hazard zones.

Local, county, state, and federalagencies including the USGS have joinedto develop a Mount Rainier volcanichazards plan that, when complete, willaddress such issues as emergency-response operations and strategies forexpanded public awareness and mitiga-tion of volcanic hazard.

W hat about a lahar-warningsystem?

A system for automatic detection andnotification of an unanticipated flank-

How much warning w illwe have?

Travel time of a lahar increases withdistance. Thus the travel time of a largelahar from Mount Rainier to the lowlandvalleys is estimated at 1 to 2 hours,whereas the travel time to the nearest areaof large population may be as little as 45minutes. However, as the AFM arraysmust be located beyond the volcanoflanks, as little as 30 minutes, afterwarning from the pilot lahar-detectionsystem, may be available for evacuation ofurban areas closest to Mount Rainier inthe Puyallup valley. Time is short, andsuccessful evacuation will depend ondetection of the approaching lahar,effective notification of people at risk,public understanding of the hazard, andpracticed response by citizens.

W hat can you do?Learn: Determine whether you live,

work, or go to school in a laharhazard zone. Learn about allvolcanic processes that could affectyour community.

Plan: Develop an emergency planwith your family so that you areprepared for natural hazards andemergencies.

Inquire: Ask public officials to adviseyou about how to respond duringany emergency.

byK.M. Scott, E.W. Wolfe,

and C.L. Driedger

Graphic design byChristine Janda

For up-to-date information contact:U.S. Geological Survey

Cascades Volcano Observatory5400 MacArthur BoulevardVancouver, WA 98661-7095

Telephone: 360-696-7693

World Wide Web:http://vulcan.wr.usgs.gov

See also Volcano Hazards fromMount Rainier, Washington(U.S. Geological Survey

Open-File Report 98–428

Mount Rainieris an active volcano...

R educing population growth in the paths of lahars, implementing a warning system, and planning and practicing evacuations can lower the potential loss of life and property during future eruptions and lahars. These actions can reduce the risk from lahars and provide a measure of safety for those who enjoy living, working, and playing in valleys surrounding Mount Rainier.