M

FIGURE 1.—View across Puyallup River valley

toward Mount Rainier (Photograph by David

Wieprecht, U.S. Geological Survey.)

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

U.S. Department of the Interior USGS Fact Sheet 034-02

U.S. Geological Survey 2002

Mount Rainier Mount Rainier Mount Rainier-- Learning to Live with Volcanic Risk Learning to Live with Volcanic Risk Learning to Live with Volcanic Risk

Lahars look and behave like flowing

concrete, and their impact forces destroy

most man-made structures. At Mount

Rainier, they have traveled 45-50 miles per

hour at depths of 100 feet or more in

confined valleys, slowing and thinning in

the wide, now-populated valleys.

At Mount Rainier, lahars are a greater

hazard than other volcanic products such

as lava and poisonous gases that have

been popularized by TV and film. Lava

flows and pyroclastic flows are unlikely to

extend more than a few miles beyond the

National Park boundaries. Volcanic ash

(tephra) will be distributed downwind, 80

percent of the time toward the east away

from large populations (fig. 2).

The USGS, in cooperation with the

University of Washington, monitors many

Cascade Range volcanoes, including

Mount Rainier, to detect precursors to

Lahars are the greatest hazard

Mount Rainier (fig. 1) is an active

volcano that is currently at rest between

eruptions. Its next eruption might

produce volcanic ash, lava flows, or

pyroclastic flows. The latter can rapidly

melt snow and ice, and the resulting

meltwater torrent could produce lahars

(a word of Indonesian origin that has

come to mean volcanic mudflow) that

travel down valleys beyond the base of

the volcano to areas now densely populated. Lahars caused by large landslides

can also occur during non-eruptive times–

without the seismicity and other warnings

that normally precede eruptions.

ount Rainier in

Washington state is

an active volcano

reaching more than 2.7 miles

(14,410 feet) above sea level. (14,410 feet) above sea level.

Its majestic edifice looms

over expanding suburbs in

the valleys that lead to

nearby Puget Sound. USGS

research over the last research over the last

several decades indicates

that Mount Rainier has been

the source of many volcanic

mudflows (lahars) that buried

areas now densely

populated. Now the USGS is

working cooperatively with

local communities to help

people live more safely with

the volcano. the volcano.

Mount Rainier

L

P

LAVA FLOWS

PYROCLASTIC FLOWS

L

LAHARS

yroclastic flows are 

hot avalanches of 

lava fragments and gas 

formed by the collapse 

of thick lava 

flows and 

eruption 

columns.

Volcano

 Hazards

TEPHRA

E

 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 eruptiongenerated meltwater.

 ava is 

molten rock that flows or oozes 

onto the earth's surface. Mount 

Rainier consists largely of numerous 

lava flows interbedded with rock 

rubble.

xplosive eruptions blast fragments 

of rock high into the air. 

Large fragments fall to the 

ground close to the 

volcano. Small fragments 

(ash) from 

large 

eruptions can travel 

thousands of miles 

downwind.
eruptive activity. Mount Rainier last

erupted in the 19th century; one or

more small eruptions from one of

the summit craters produced local

ashfall.

Mount Rainier is one of the

most hazardous volcanoes in

the United States

Although Mount Rainier has

erupted less often and less explosively in

recent millennia than its neighbor,

Mount St. Helens, the proximity of

large populations makes Mount Rainier

a far greater hazard to life and

property.

1. The population at risk—More than

150,000 people reside on the

deposits of previous lahars.

2. The size and frequency of lahars—

During the past few millennia lahars

that have reached the Puget Sound

lowland have occurred, on average,

at least every 500 to 1,000 years.

Smaller flows not extending as far as

the lowland occur more frequently.

If lahars of the future happen at

rates similar to those of the past,

there is at least a one in seven

chance of a lahar reaching the Puget

Sound lowland during an average

human life-span.

3. We may not have advance warning–

USGS research shows that some

lahars occur with little or no

warning. Our only warning could be

a report that a flow is under way.

Past lahars indicate future

 Geologists determine the size and

timing of past lahars and use this

information to indicate future hazard

potential. For example, in figure 3, the

areas inundated by the 2,300 year-old

National Lahar in the Nisqually Valley,

and the 500 year-old Electron Mudflow

in the Puyallup Valley are superimposed

on all valleys surrounding Mount Rainier.

They illustrate areas that could be

inundated if flows of those sizes

occurred in each valley.

A lahar flowing downvalley from

Mount Rainier leaves a thick valleybottom deposit of boulders and hardened mud that may envelop stumps and

logs, forming a buried forest. Some of

the deposits can be traced upstream to

the volcano’s flanks, and all contain

volcanic fragments unique to Mount

Rainier. Geologists map the deposits

and determine the tree ages to learn

when the trees were engulfed and killed

by the lahar. Old-timers recall encountering huge buried stumps and logs when

plowing fields and digging wells. The

youngest such forest was buried about

500 years ago and uncovered during

excavations for new homes in the

Puyallup River valley.

Lahars form in several ways

Some of the largest lahars have

originated by collapse of weakened rock

from the flanks of the volcano—a large

landslide known as a flank collapse.

During eruptions, molten rock is injected

into cracks of the volcano and solidifies

as slabs of rock called dikes. The

cooling magma releases gases and heat

into groundwater, making it hot and

acidic. The hot, acidic waters convert

hard volcanic rock into soft, clay-rich

rock by a process called hydrothermal

alteration. When masses of water-rich

rock collapse, they transform rapidly into

a muddy slurry—a clay-rich (also

called cohesive) lahar that is

funneled into one or more surrounding valleys.

 Sand-rich lahars, also known

as noncohesive lahars, form during

eruptions of Mount Rainier when

hot pyroclastic flows melt snow

and ice. (Mount Rainier supports

more than one cubic mile of glacial

ice—as much as all other Cascade

Range volcanoes combined.)

Because sand-rich lahars occur

during eruptive activity, they are

likely to be preceded by events that

will warn of an impending eruption, and thus of increased lahar

potential.

Small lahars, traveling only a

few miles, are caused by local

avalanches of rock debris, sudden

releases of glacial meltwater, or

intense rainfall. These lahars occur

many times each century.

 New lahar deposits may get

redistributed downstream over a

period of many years as the disrupted

drainage network is re-established.

Thus, valley-floor areas that were not

inundated by the initial lahar deposits

may suffer enhanced flooding and

progressive burial by remobilized

sediment (zone of post-lahar sedimentation in figure 3).

Weakened rock and rising

magma can cause flank

collapse

Flank collapses can be triggered

when magma intrudes into a volcano and

destabilizes it, as happened at Mount

St. Helens in 1980.

A neighboring volcano—Mount

Baker—produced flank collapses in the

1840’s that were apparently triggered by

steam explosions related to volcanic

activity. Steam explosions at Mount

Rainier could trigger flank collapses and

lahars with little or no advance warning.

Although many flank collapses occur

during eruptive periods, it is possible for

them to be triggered by earthquakes or

they may be the result of progressive

weakening of the rock, saturation by

groundwater and the continuing pull of

gravity.

FIGURE 2.—Map showing the annual probability that —

volcanic ash will be deposited to a thickness of 1/3 inch

or more from an eruption of Mount Rainier. Volcanic

ash, of this thickness or less, can cause disruption of

ground and air transportation, and can cause damage

to electronics and machinery.

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 OpenFile 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.
detection of the approaching lahar,

effective notification of people at risk,

public understanding of the hazard, and

prompt response by citizens.

Volcanoes often show signs that

they are getting ready to erupt days to

weeks or months in advance. Scientists

of the U.S. Geological Survey and

University of Washington evaluate signs

of unrest and look for increased seismic

activity, increased emission of volcanic

gases and swelling of the volcano.

When unrest is detected, scientists will

increase monitoring efforts and notify

emergency management officials.

What can you do?

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.

Inquire: Ask public officials to advise

you about how to respond during any

emergency.

Plan: Develop an emergency plan with

your family so that you are prepared

for natural hazards and emergencies.

by C.L.Driedger and K.M. Scott

Layout design and modification of

graphics by Christine Janda.

Original graphics by Lisa Faust

Flanks of volcano can unleash

flank collapses and rockfalls

The west flank at the head of the

Puyallup River valley has the greatest

potential for collapse because it has the

largest area of weakened clay-rich rock

remaining at high altitude. The east and

northeast sides of the volcano can let

loose large rockslides, such as that of

December 1963, because of fractures in

the ridges.

One collapse can yield lahars in

multiple valleys

Future lahars will follow river

valleys that drain Mount Rainier. Four

of the five major river systems flow

westward into suburban areas of Pierce

County. These flow pathways are

mapped by the USGS (fig. 3), just as

flood-inundation maps show the areas at

risk of flooding. Lahars occurring

during an eruption may affect valley

areas miles from the volcano, but a

precursory warning should allow ample

time for evacuation.

A catastrophic flow will likely

spread into multiple valleys. The largest

known flow entered all five drainages,

and most of the known large flows have

entered two or more.

Monitoring and emergency

planning are ongoing

The USGS, in cooperation with the

University of Washington, monitors the

state of the volcano and assesses

hazards from volcanic activity. The

lahar pathways mapped by the USGS

guide the hazard-area regulations of the

comprehensive land-use plan for local

counties. The plan’s urban growth

boundary and its proposed land uses in

unincorporated areas are designed to

minimize population growth, where

possible, within hazard zones.

Local, county, state, and federal

agencies including the USGS have joined

to develop a Mount Rainier volcanic

hazards response plan that addresses

such issues as emergency-response

operations and strategies for expanded

public awareness and mitigation of

volcanic hazard.

Lahar-warning system reduces

risk

Because of the higher level of risk

from lahars in the Carbon and Puyallup

River valleys, the USGS and Pierce

County Department of Emergency

Management have installed a lahardetection and warning system. The

system consists of arrays of five

acoustic flow monitors (AFM’s) that

detect the ground vibrations of a lahar.

Computerized evaluation of data

confirms the presence of a flowing lahar

and issues an automatic alert to emergency management agencies. Emergency managers then can initiate

response measures such as evacuations.

This system for automatic detection and

notification of a lahar reduces, but does

not eliminate, risk in the lahar pathways.

Lahar travel times are short Lahar travel times are short

The estimated time between

detection of a lahar and its arrival in

Orting is about 40 minutes. Dispersed

populations closer to Mount Rainier

would be affected sooner. Time is short,

and successful evacuation will depend on

Mount Rainier

is 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. 

For up-to-date information contact:

U.S. Geological Survey

Cascades Volcano Observatory

1300 SE Cardinal Court, Suite 100

Vancouver, WA 98683

Telephone: 360-993-8900

World Wide Web:

http://vulcan.wr.usgs.gov

or

USGS Volcano Hazards Program

http://volcanoes.usgs.gov

This fact sheet supersedes Mount Rainier–Living

with Perilous Beauty (Fact Sheet 065-97).

See also Volcano Hazards from Mount

Rainier, Washington (U.S. Geological Survey

Open-File Report 98–428).

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