U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2008–3062

2008

U.S. GEOLOGICAL SURVEY—REDUCING THE RISK FROM VOLCANO HAZARDS

Mount Rainier—Living Safely With a Volcano in Your Backyard

Majestic Mount Rainier 

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.

The flat floor of the Puyallup River valley near Orting, Washington, is formed by deposits of the 500-year-old 

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. 

Lahars from Mount Rainier can travel for tens of miles along river valleys and reach Puget Sound. (USGS 

photograph by D.E. Wieprecht.)

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 

might be of similar or larger size and 

could produce volcanic ash, lava flows, 

and avalanches of intensely hot rock 

and volcanic gases, called “pyroclastic 

flows.” Some of these events swiftly 

melt snow and ice and could produce 

torrents of meltwater that pick up loose 

rock and become rapidly flowing slurries 

of mud and boulders known as “lahars.” 

In contrast to lava flows and pyroclastic 

flows that are unlikely to extend farther 

than 10 miles from the volcano’s summit 

and remain within Mount Rainier National 

Park, the largest lahars can travel for tens 

of miles and reach Puget Sound. Volcanic 

ash will be distributed downwind, most 

often toward the east, away from Puget 

Sound’s large population centers. Airborne 

plumes of volcanic ash can greatly endanger aircraft in flight and seriously disrupt 

aviation operations. Although seldom life 

threatening, volcanic ash fallout on the 

ground can be a nuisance to residents, 

affect utility and transportation systems, 

and entail substantial clean-up costs. 
Armero, Colombia, was battered 

in 1985 by lahars 

generated by an 

eruption of glacier-clad Nevado 

del Ruiz volcano. 

More than 20,000 

people died in the 

city. Note empty 

street blocks 

where structures 

were swept 

away. Tragically, 

safety lay nearby 

in areas of higher 

ground. (USGS 

photograph by 

R.J. Janda.)

Lahars Pose the Greatest Risk

 At Mount Rainier, the risk from lahars 

is greater than from lava flows, volcanic 

ash fall, or other volcanic phenomena 

because some pathways for future lahars 

are densely populated and contain important infrastructure such as highways, 

bridges, ports, and pipelines. Lahars look 

and behave like flowing concrete, and 

they destroy or bury most manmade structures in their paths. Past lahars probably 

traveled 45 to 50 miles per hour and were 

as much as 100 feet or more thick where 

confined in valleys near the volcano. They 

thinned and spread out in the wide valleys 

downstream, slowing to 15 to 25 miles 

per hour. Deposits of past lahars are found 

in all of the valleys that start on Mount 

Rainier’s flanks.

How Hazardous is Mount Rainier?

 Mount Rainier has erupted less often 

and less explosively in recent millennia 

than its well-known neighbor, Mount St. 

Helens. However, the proximity of large 

population centers in valleys susceptible 

to lahars from Mount Rainier makes it 

a far greater threat to life and property 

than Mount St. Helens for the following 

reasons:

•	 Population and development at 

risk—About 80,000 people and their 

homes are at risk in Mount Rainier’s 

lahar-hazard zones. Key infrastructure 

such as major highways and utilities 

cross through these zones, which also 

contain economically important businesses, hydroelectric dams, and major 

seaports.

future large lahars happen at rates similar to those of the past, there is roughly 

a 1-in-10 chance of a lahar reaching the 

Puget Sound lowland during an average 

human lifespan.

•	 There may be little or no advance 

warning—Studies by U.S. Geological 

Survey (USGS) scientists show that 

at least one of Mount Rainier’s recent 

large landslide-generated lahars may 

have occurred when the volcano was 

quiet and not providing the warning 

signs typical of a restless and erupting 

volcano. In such a rare case, the only 

warning could be a report that a lahar is 

already underway.

Two Types of Lahars

 Mount Rainier can generate two types 

of lahars that can threaten surrounding 

valleys:

•	 Meltwater-generated lahars—Mount 

Rainier supports more than one cubic 

mile of glacial ice—as much as all 

other Cascade Range volcanoes combined. During past eruptive episodes, 

swift melting of snow and ice by pyroclastic flows and other events caused 

numerous lahars. Such lahars would 

be preceded by events that warn of an 

impending eruption.

•	 Landslide-generated lahars—Landslides can be triggered when molten 

rock (magma) intrudes into a volcano 

and destabilizes it, as happened at 

Mount St. Helens in 1980, or they may 

be triggered by large earthquakes. They 

may also be the result of the eventual 

failure of rocks that were weakened 

by the action of acidic fluids. Magma 

releases gases and heat creating hot, 

acidic ground water that, over time, 

can convert hard volcanic rock into 

weak, clay-rich rock by a process 

called hydrothermal alteration. When 

masses of water-saturated clay-rich 

rock slide away, they transform rapidly 

into a lahar. Although most large 

landslides at Mount Rainier occurred 

during eruptive periods and were 

probably triggered by magma intrusion or by explosive eruptions rocking 

the volcano, the origin of at least one, 

the 500-year-old Electron lahar, may 

not be related to eruptions. This lahar 

left deposits as much as 20 feet thick, 

and buried an old-growth forest in the 

vicinity of modern-day Orting.

Are All Parts of the Volcano 

Susceptible to Landslides?

 The west flank of Mount Rainier,

including the head of the Puyallup 

River, has the greatest potential for 

unleashing large landslides that become 

far-traveled lahars, because it has the 

largest amount of weakened clay-rich 

rock at high altitude. Therefore, the 

Puyallup River valley and, to a lesser 

extent, the Nisqually River valley, whose 

basin includes some of the weakened 

rock, are at most risk from such events. 

Little Tahoma Peak on the east side of the 

volcano and many other cliffs and steep 

slopes can fail in landslides, such as one 

in December 1963 that traveled several 

miles, but such events are too small to 

generate lahars. In contrast to landslides, 

lahars generated by eruptions could 

descend any of the valleys originating on 

Mount Rainier.

Long-Term Effects of Lahars

 Lahars fill stream channels and bury 

valley floors with deposits of boulders, 

sand, and mud a few feet to tens of 

feet thick. These deposits readily erode 

as rivers and streams reestablish their 

channels, shedding abundant sediment 

downstream over years to decades. 

Because of this, downstream valley floors 

initially unaffected by a lahar may later 

suffer increased flooding and progressive 

burial by remobilized sediment. Recent 

studies have revealed extensive layers 

of sandy sediment from Mount Rainier 

that extend to the Port of Seattle along 

the Green and Duwamish River valleys. 

This sediment was rapidly eroded from 

the deposits of lahars caused by eruptions 

about 1,000 years ago, even though 

the lahars themselves did not extend 

much past present-day Auburn, which 

lies about 20 miles south of downtown 

Seattle. 

 

Debris Flows Threaten Areas in 

Mount Rainier National Park

 Almost annually, water released from 

glaciers or runoff from intense rainfall 

incorporates rocks and sediment to 

form “debris flows” that affect valleys 

•	 Size and frequency of lahars—

During the past several thousand years 

large lahars have reached the Puget 

Sound lowland on average at least 

once every 500 to 1,000 years. Smaller 

flows not extending as far as the 

lowland occurred more frequently. If 
 

HAZARD ZONES FOR DEBRIS FLOWS, LAHARS, LAVA FLOWS, AND PYROCLASTIC FLOWS FROM 

MOUNT RAINIER

This map shows areas 

that could be affected 

by debris flows, 

lahars, lava flows, and 

pyroclastic flows from 

Mount Rainier if events 

similar in size to past 

events occurred today. 

Because small lahars 

are more common than 

large ones, most lahars 

would be less extensive 

than the hazard zone 

shown on the map and 

a few would be more 

extensive. The lahar 

hazard is not equal in 

all valleys. For example, 

the greatest hazard 

from lahars generated 

by landslides is on the 

west side of the volcano 

because it contains 

the largest amount 

of hydrothermally 

weakened rock. 

Scientists continue 

to reevaluate the 

hazard zones as they 

learn more about the 

volcano. Dams and 

reservoirs on several 

rivers could lessen the 

extent of future lahars 

by trapping all or much 

of the flow, but they 

could also increase a 

lahar’s extent if a lahar 

displaced reservoir 

water and caused 

dams to fail. A zone of 

flooding and post-lahar 

sedimentation is shown 

only in the Green and 

Duwamish River valleys 

(see Long-Term Effects 

of Lahars), because 

in other valleys it is 

included in the lahar 

hazard zone. Darkgray shading indicates 

urbanized areas. (Map 

simplified from U.S. 

Geological Survey OpenFile Report 98-428.)

Geologists study the deposits of past lahars to assess potential future 

hazards. Here a U.S. Geological Survey geologist samples a log buried in a 

bouldery lahar deposit east of Enumclaw, Washington. This 5,600 year-old 

deposit, called the Osceola Mudflow, was formed when a massive landslide 

on the east side of Mount Rainier traveled north and west along the White 

River valley. (USGS photograph by A. Durant.)
Printed on recycled paper

Carolyn L. Driedger and William E. Scott

Edited by James W. Hendley II

Graphic design by Jeanne DiLeo and Lisa Faust

For more information contact:

U.S. Geological Survey

David A. Johnston Cascades Volcano Observatory

1300 SE Cardinal Court, Building 10, Suite 100

Vancouver, WA 98683

(360) 993-8900

or

http://vulcan.wr.usgs.gov

http://www.pnsn.org/RAINIER/welcome.html

This Fact Sheet supersedes Mount Rainier—Living with 

Perilous Beauty ((Fact Sheet 065-97) and Mount Rainier—

Learning to Live with Volcanic Risk (Fact Sheet 034-02).

See also Volcano Hazards from Mount Rainier, Washington 

(U.S. Geological Survey Open-File Report 98-428).

This Fact Sheet and any updates to it are available online 

at http://pubs.usgs.gov/fs/2008/3062/.

HOW TO PREPARE FOR A LAHAR OR OTHER 

VOLCANO HAZARD

Learn: Determine 

whether you live, work, 

or go to school in a lahar 

hazard zone. Learn about 

all volcanic processes 

that could affect your 

community.

Plan: Develop an 

emergency plan with 

your family so that you 

are prepared for natural 

hazards and emergencies.

Inquire: Ask public 

officials to advise you 

about how to respond 

during any emergency. Volcano evacuation signs direct traffic to safety on higher ground 

in Pierce County, Washington (USGS photograph by C.L. Driedger).

on the flanks of Mount Rainier. Such 

debris flows behave like lahars, but are 

typically of such small size that they 

seldom travel beyond the base of the 

volcano and only affect vulnerable areas 

within the boundaries of Mount Rainier 

National Park. Summer and autumn are 

the seasons during which debris flows are 

most common—times when glaciers are 

producing large amounts of meltwater 

and intense rains can fall on littlevegetated, snow-free areas with abundant 

loose debris. Because debris flows pose 

risks to park visitors and infrastructure, 

especially trails, roads, and bridges, 

Mount Rainier National Park educates 

staff and visitors about hazards from 

debris flows and how to avoid them by 

moving off valley floors. 

Past Lahars Provide Clues About 

Future Hazards

 Lahars leave behind thick layers of 

boulders, mud, and logs on valley floors. 

Geologists use this and other evidence 

to assess future hazard potential and to 

map zones in river valleys heading on 

Mount Rainier that could be inundated 

by future lahars. Not all valleys would 

necessarily be affected during a given 

eruption or large landslide, nor would 

all lahars in a valley be large enough to 

extend to hazard-zone boundaries. Lahar 

hazard zones mapped by the USGS are 

being used to guide the development of 

hazard-area regulations in comprehensive 

land-use plans by counties and cities that 

lie at the foot of Mount Rainier.

Lahar Warning System Reduces Risk

 Because there is higher level of risk 

from lahars generated by landslides 

on the west flank of Mount Rainier, 

the USGS, Pierce County Department 

of Emergency Management, and 

Washington State Emergency 

Management Division have established 

a lahar warning system. A detection 

component consists of arrays of monitors 

that record the ground vibrations of 

a lahar. Computerized evaluation of 

data assesses the presence of a flowing 

lahar and issues an automatic alert 

to emergency-management agencies. 

Emergency managers can then initiate 

appropriate response measures. City, 

county, and State agencies design 

and maintain notification procedures, 

evacuation routes, and public-education 

programs.

 If a large lahar were generated in the 

upper Puyallup River valley without the 

precursors that typically herald volcanic 

unrest and eruption, it could arrive at 

the City of Orting as little as 40 minutes 

after the initial warning is sounded. Time 

could be short, and successful mitigation 

will depend on effective notification of 

people at risk, public understanding of the 

hazard, and prompt response by citizens. 

This system for automatic detection and 

notification of a lahar reduces—but does 

not eliminate—risk in the lahar pathways. 

Monitoring and Emergency Planning 

Are Ongoing

 The USGS, in cooperation with the 

Pacific Northwest Seismic Network at the 

University of Washington, continuously 

monitors Mount Rainier and assesses 

potential hazards stemming from volcanic 

activity. Volcanoes often show signs 

of unrest, such as increased seismicity 

(earthquakes) and emission of volcanic 

gases and swelling of the volcano, days to 

months in advance of an eruption. When 

unrest is detected, scientists will notify 

emergency-management officials and 

increase monitoring efforts. 

 The Mount Rainier Volcanic Hazards 

Response Plan, which was created by 

cooperating local, county, State, and 

Federal agencies, is on the Web at 

http://www.co.pierce.wa.us/pc/Abtus/

ourorg/dem/EMDiv/Mt%20Rainier%20

VHRP.htm. The plan describes the 

responsibilities of agencies and how they 

will communicate with each other and the 

public during a volcanic crisis. 

What to Do if Threatened by a Lahar 

or Debris Flow 

 Know the signs of debris flows and 

lahars. Experience from around the 

world shows that moving to high ground 

off the valley floor is the only way to 

ensure safety during a lahar. When 

hiking in valleys on the slopes of Mount 

Rainier during late summer or during 

intense rainfall, be alert for the signs of 

an approaching debris flow—ground 

shaking and roaring sound—and move 

up the valley wall to higher ground. The 

same is true for lahars, but, because they 

affect much larger areas, people need 

to move out of threatened areas before 

lahars get close. Lahars are almost 

always preceded by volcanic unrest, so 

in most instances there will be time to 

warn people when there is an increased 

risk. Obtain a NOAA weather radio to 

receive alerts about possible lahars, as 

well as other natural hazards (for further 

information on the Web go to http://www.

weather.gov/nwr.).