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

Glacier Peak — History and Hazards of a Cascade Volcano

Glacier Peak lies in Washington State’s North Cascade Mountains,

in the heart of a wilderness area bearing its name. Its past

eruptions have melted snow and ice to inundate downstream

valleys with rocks, mud, and debris. Large eruptions from Glacier

Peak have deposited ash throughout much of the western United

States and southwestern Canada. Photo by D.R. Mullineaux, USGS.

The stunning snow-capped volcanoes

of Washington State have long been

recognized by Native Americans in their

language and legends, and they immediately caught the eyes of U.S. and European explorers in the late 18th and early

19th centuries. By the 1790’s, Mounts

Baker, Rainier, and St. Helens were noted

and named in the first written descriptions

of the Columbia River and Puget Sound

regions. In 1805 Lewis and Clark noted

Mount Adams. By the mid-19th century

each of these four volcanoes had their

place on a published map.

Glacier Peak wasn’t known by settlers

to be a volcano until the 1850’s, when

Native Americans mentioned to naturalist

George Gibbs that “another smaller peak

to the north of Mount Rainier once

smoked.” Not until 1898 did Glacier Peak

appear on a published map under its

current name.

Glacier Peak lies only 70 miles

northeast of Seattle—closer to that city

than any volcano except Mount Rainier.

But unlike Mount Rainier, it rises only a

few thousand feet above neighboring

peaks, and from coastal communities it

appears merely as a high point along a

snowy saw-toothed skyline. Yet Glacier

Peak has been one of the most active and

explosive of Washington’s volcanoes.

Known eruptive episodes at Glacier Peak during the past 15,000 years. Each episode (depicted by a single

icon) represents many individual eruptions. The ages of these episodes, in calendar years before present are

corrected from dates based on a radiocarbon time scale. The uncorrected radiocarbon ages for these

episodes, which appear in some publications, are 11,200, 5,100, 2,800, 1,800, 1,100, and 300 years before

present.

U.S. Department of the Interior USGS Fact Sheet 058-00

U.S. Geological Survey 2000

US SG

Since the continental ice sheets receded

from the region, Glacier Peak has erupted

repeatedly during at least six episodes.

Two of these eruptions were among the

largest in Washington during the past

15,000 years. These pages describe some

of the effects of past eruptions and

possible consequences of future activity.

15 10

THOUSANDS OF YEARS AGO

5 0

Multiple small tephra

eruptions, dome collapses,

lahars large enough

to reach the sea

Dome collapses,

lahars large

enough to reach

the sea

Dome

collapses,

lahars

Dome

collapses,

lahars

Small

steam

eruptions

Glacier Peak's eruption history

Multiple large tephra eruptions,

dome collapses, lahars large

enough to reach the sea

Glacier Peak is the most

remote of the five active

volcanoes in Washington

State. It is not prominently

visible from any major population center, and so its

attractions, as well as its

hazards, tend to be overlooked. Yet since the end of

the last ice age, Glacier Peak

has produced some of the

largest and most explosive

eruptions in the state. During

this time period, Glacier Peak

has erupted multiple times

during at least six separate

episodes, most recently

about 300 years ago. What

were these eruptions like?

Could similar ones affect us

today? Scientists from the

U.S. Geological Survey

(USGS) are working to

answer these questions and

help prepare for future

activity.

Glacier

Peak 

Rainier

Adams St. Helens 

Baker

Pacific Ocean

WASHINGTON
Chelan Spokane

Ephrata

60

1.0

1.3

0.3

>0.2 <0.1

0.7

Missoula

Glacier

Peak

During Past Eruptions . . .

Tephra Covered the Landscape

Glacier Peak and Mount St. Helens

are the only volcanoes in Washington

State that have generated large, explosive

eruptions in the past 15,000 years. Their

violent behavior results from the type of

molten rock (magma) they produce.

Dacite, the typical magma type of Mount

St. Helens and Glacier Peak, is too

viscous to flow easily out of the eruptive

vent; it must be pressed out under high

pressure. As it approaches the surface,

expanding gas bubbles within the magma

burst and break it into countless

fragments. These fragments are

collectively known as tephra; the smallest

are called ash.

About 13,100 years ago, Glacier Peak

generated a sequence of nine tephra

eruptions within a period of less than a

few hundred years. The largest ejected

more than five times as much tephra as

the May 18, 1980, eruption of Mount

St. Helens and was one of the largest in

the Cascade Range since the end of the

last ice age.

Some of the tephra from these

eruptions fell back onto the volcano and

avalanched down its flanks. Much of the

rest rose high into the atmosphere and

drifted hundreds to thousands of miles

downwind. Deposits from these eruptions

are more than a foot thick near Chelan,

Washington, and an inch thick in western

Montana.

Since these events, Glacier Peak has

produced several tephra eruptions, all of

much smaller volume.

Lava Domes Collapsed onto the

Volcano’s Flanks

During most of Glacier Peak’s

eruptive episodes, lava domes have

extruded onto the volcano’s summit or

steep flanks. Parts of these domes

collapsed repeatedly to produce pyroclastic flows and ash clouds. The remnants of

prehistoric lava domes make up Glacier

Peak’s main summit as well as its “false

summit” known as Disappointment Peak.

Pyroclastic-flow deposits cover the valley

floors east and west of the volcano.

Ridges east of the summit are mantled by

deposits from ash clouds.

Lahars Inundated River Valleys

Past eruptions have severely affected

river valleys that head on Glacier Peak.

Pyroclastic flows mixed with melted snow

and glacial ice to form rapidly flowing

slurries of rock and mud known as lahars.

About 13,100 years ago, dozens of

eruption-generated lahars churned down

Glacier Peak from the east, showing the main

summit and Disappointment Peak, which are

remnants of prehistoric lava domes. Photo by

Austin Post, USGS.

(TOP) Total thickness of tephra (inches) erupted from Glacier Peak during a

series of large eruptions about 13,100 years ago. Light blue indicates

approximate area covered by ash during these eruptions.

(LEFT) Tephra deposit from a Glacier Peak eruption 13,100 years ago, at a

site about 10 miles south of Glacier Peak. Overlying this deposit is an ash

bed from an eruption at Crater Lake, Oreg., 7,800 years ago. Photo by

R.B. Waitt, USGS.

Eruption column from Mount St. Helens on

May 18, 1980. Rock fragments (tephra) give the

column the gray color. Tephra from the eruption

fell as far away as Colorado. About 13,100 years

ago, an explosive eruption from Glacier Peak

generated a sequence of tephra eruptions, the

largest of which ejected more than five times as

much tephra as the May 18,1980 eruption of

Mount St. Helens. Photo by Austin Post, USGS.

Ash cloud

River

valley

Pyroclastic-flow

deposit

Pyroclastic

flow

Growing

lava dome

Dome collapsing

When gas is not abundant 

enough to drive explosive 

eruptions, lava oozes out of 

a vent and accumulates as 

a thick, viscous dome.

On steep slopes, sections of 

the dome may break away 

and disintegrate into hot 

avalanches of debris known 

as pyroclastic flows.

Pryoclastic flows cascade downhill, crushing 

or incinerating everything in their path, and 

come to rest in nearby river valleys. Behind the 

front of the pyroclastic flow, ash clouds separate from the coarse debris, rise vertically in 

bouyant plumes, and drift downwind for many 

miles.

Pyroclastic flows fill river valleys with tens to 

hundreds of feet of loose debris that chokes 

rivers and is readily transported by water.
A collapsing lava dome generates a pyroclastic flow and ash cloud at Colima Volcano, Mexico, November 22,

1998. Photo by Abel Cortes, ©1998, University of Colima.

the White Chuck, Suiattle, and Sauk

Rivers, inundating valley floors. Lahars

then flowed down both the North Fork

Stillaguamish (then an outlet of the upper

Sauk River) and Skagit Rivers to the sea.

In the Stillaguamish River valley at

Arlington, more than 60 miles downstream from Glacier Peak, lahars deposited more than seven feet of sediment.

Shortly after the eruptions ended, the

upper Sauk’s course via the Stillaguamish

was abandoned and the Sauk River began

to drain only into the Skagit River, as it

does today.

About 5,900 years ago and 1,800

years ago, dome-building eruptions

generated lahars that extended once again

to the sea, this time only along the Skagit

River. In small eruptions since 1,800 years

ago, lahars have extended the entire

length of the White Chuck River and part

way down the Suiattle.

Lahars can also be generated by landslides (also called flank collapses) on

volcanoes, as has happened repeatedly at

Glacier Peak’s neighbor to the north,

Mount Baker. At Mount Baker, lahars

from numerous landslides, some without

accompanying eruptive activity, have

affected valley floors near the volcano. A

few much larger landslides during

eruptive periods generated lahars that

flowed hundreds of feet deep through

upper valleys and reached the sea. At

Glacier Peak landslide-generated lahars

have occurred less frequently than at

Mount Baker.

During Future Eruptions . . .

Glacier Peak’s eruptive episodes are

typically separated by several hundred to

a few thousand years. Thus in any given

year, the probability of a new episode

beginning is roughly one in a thousand. It

is unlikely that we will see an eruption

within our lifetimes. If one does take

place, its impact would vary dramatically

in different geographic areas depending

on the size of the eruption, wind direction,

and type of hazards produced.

In undeveloped areas near the

volcano, the landscape would be severely

altered by lava domes, pyroclastic flows,

ash clouds, lahars, and associated

phenomena.

In river valleys downstream from the

volcano, lahars could block transportation

routes, destroy highways and bridges,

bury houses in mud, cover farmland with

debris, choke river channels, and increase

the severity of floods for years or decades

after the eruptions stop. These effects will

be most frequent in the White Chuck and

upper Suiattle River valleys. They will be

less frequent, but potentially more

damaging due to greater population and

infrastructure, in the Sauk and Skagit

River valleys. Still less likely would be

lahars in the Stillaguamish River valley,

which would occur only if the Sauk River

became choked with enough debris to be

diverted west into the Stillaguamish River

valley. When a lahar takes place, residents of communities along these rivers

should move to high ground as quickly as

possible.

In areas downwind from the volcano,

even small tephra eruptions could disrupt

air and ground transportation and dust

towns with ash. Tephra could clog

drainage ducts and ventilation filters,

short-circuit power transformers, damage

machinery and electronic equipment,

reduce visibility, exacerbate respiratory

ailments, and stall transportation. Large

tephra eruptions (comparable to Glacier

Peak’s largest) would have more widespread effects and could deposit enough

tephra to collapse roofs in nearby downwind communities. Owing to prevailing

wind patterns, tephra fall during future

eruptions is most likely east of Glacier

Peak. But tephra could affect communities

in all directions from the volcano depending on wind patterns during an eruption.

Five tephra eruptions from Mount

A few hundred yards upstream from its

confluence with the Sauk River, the White

Chuck River erodes into lahar deposits

produced by prehistoric eruptions of

Glacier Peak. Photo by R.B. Waitt, USGS.

More than 200 homes and over

120 miles of roads were destroyed

by the 1980 lahars at Mount

St. Helens. Pictured is a damaged

home along the South Fork Toutle

River. Photo by Lyn Topinka,

USGS.
Concrete

Marblemount

Rockport

Darrington

Arlington

N. Fork Stillaguamish R.

Sa uk R. Skagit Riv er

Sauk

R.

Skagit R.

Lake

Shannon

Samish Bay

Skagit

Bay

Granite Falls

N

White Chuck R.

Glacier

Peak

Suiattle

River

EXPLANATION

More frequent

Less frequent

Least frequent

Pyroclastic flows, flank

collapses, and associated phenomena

LAHARS

0 5 10 MILES

10,541'

CHELAN COUNTY

Stanwood

Sedro

Woolley

Conway

SKAGIT COUNTY

SNOHOMISH COUNTY

Burlington

Edison

Mount Vernon

Marysville

Everett

LaConner 

Areas at risk from lahars, lava domes, pyroclastic flows, and associated phenomena from Glacier Peak. Map

modified from R.B. Waitt and others, U.S. Geological Survey Open-File Report 95-499.

St. Helens in 1980, for example, deposited ash north, east, west, and south of

the volcano. Damage from tephra can be

mitigated by such actions as shutting

down and covering equipment, frequently

replacing air filters in machinery, wearing

dust masks, and avoiding unnecessary

travel.

Preparing for the Next Eruption

Future eruptions from Glacier Peak

will almost certainly be preceded by an

increase in earthquake activity, and

possibly by measurable swelling of the

volcano and emission of volcanic gases.

In cooperation with the USGS, the

University of Washington’s Geophysics

Program continuously monitors earthquakes that could portend Glacier Peak’s

next eruption. The USGS also works with

Federal, State, Provincial, and local

agencies to prepare for disruption that

might accompany renewed activity. A

coalition of these agencies, known as the

Mount Baker–Glacier Peak Facilitating

Committee, has drafted a plan outlining

how agencies will work together in the

event of unrest at Mount Baker or Glacier

Peak. If Glacier Peak were to reawaken,

the USGS would rapidly deploy additional monitoring instruments and,

together with these agencies, establish a

local volcano observatory and command

center that would keep nearby communities informed of developments.

What You Can Do

• Learn about the volcano hazards that

could affect your community, and

determine whether you live, work,

play, or go to school in a volcanohazard zone.

• Plan what you and your family will

do if a hazardous volcanic event

occurs.

• Participate in helping your community be prepared.

A few moments spent now could help

prevent the next eruption from becoming a

disaster for you, your family, and your

community.

Larry Mastin and Richard Waitt

Graphics and design by

Christine Janda, Bobbie Myers, and Lisa Faust

Printed on recycled paper

Annual probability of tephra fall exceeding 0.5 inch

thick from an eruption of Glacier Peak.

Communities east of the volcano are more

susceptible to tephra fall because the wind is

normally from the west. Glacier Peak has produced

large tephra eruptions, but not frequently.

For more information contact:

U.S. Geological Survey

Cascades Volcano Observatory

5400 MacArthur Blvd., Vancouver, WA 98661

Tel: (360) 993-8900, FAX: (360) 993-8980

http://vulcan.wr.usgs.gov/

or

USGS Volcano Hazards Program

http://volcanoes.usgs.gov/

or

your local emergency management agency:

Skagit County (360) 428-3250

Snohomish County (425) 423-7635

See also Volcanic-Hazard Zonation for Glacier

Peak Volcano, Washington (USGS Open-File

Report 95-499) and What are Volcano

Hazards? (USGS Fact Sheet 002-97)

COOPERATING ORGANIZATIONS

U.S. Department of Agriculture, Forest Service

University of Washington, Geophysics Program

More than 1 in 10,000 1 in 20,000 to 50,000

1 in 10,000 to 20,000 Less than 1 in 50,000

W A S H I N G T O N

GLACIER PEAK