Past Eruptive Behavior

Mount Hood is more than 500,000 years

old. The volcano has grown in fits and starts,

with decades to centuries of frequent eruptions

separated by quiet periods lasting from

centuries to more than 10,000 years. In the

recent past, Mount Hood has had two

significant eruptive periods, one about 1,500

years ago and the other about 200 years ago.

Unlike its neighbor to the north, Mount St.

Helens, Mount Hood does not have a history of

violent explosive eruptions. Instead,

(see inside pages for definitions of bold terms),

rarely traveling more than 6 to 8 miles from

their source, have built up the flanks of the

volcano one sector at a time. Sometimes,

instead of flowing slowly downhill, lava piles

up over its vent forming a many

hundreds of feet high. On the steep upper

slopes of Mount Hood, growing lava domes

have repeatedly collapsed to form extremely

hot, fast-moving . Few of

these pyroclastic flows have traveled more than

8 miles. But because they are extremely hot,

such flows can melt significant quantities of

snow and ice to produce that flow down

river valleys, often far beyond the flanks of the

volcano. Over the past 30,000 years, growth

and collapse of lava domes and generation of

lahars have dominated Mount Hood's eruptive

activity.

lava flows

lava dome

pyroclastic flows

lahars

Throughout Mount Hood's history, rapid

landslides, called , of

various sizes have occurred. The largest ones

removed the summit and sizable parts of the

volcano's flanks and formed lahars that flowed

to the Columbia River. Large debris avalanches

occur infrequently and are usually triggered by

eruptive activity. But small ones not associated

with eruptive activity occur more frequently.

Small avalanches can occur when rocks,

altered and weakened by acidic volcanic fluids

or by weathering, such as freezing and

thawing, fail spontaneously.

debris avalanches

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

Mount Hood's last major

eruption occurred in the

1790's not long before Lewis and

Clark's expedition to the Pacific

Northwest. In the mid-1800's, local

residents reported minor explosive

activity, but since that time the

volcano has been quiet. Someday,

however, Mount Hood will erupt

again. What will those eruptions be

like and how will they affect us?

Scientists from the U.S. Geological

Survey (USGS) are studying the

volcano's past eruptive behavior to

better anticipate and prepare for

future eruptive activity.

Mount Hood from Portland, Oregon. When Mount Hood next erupts, Portland could be affected by light ashfalls

similar to those it experienced during the 1980 eruptions of Mount St. Helens. The city will not be directly

affected by lava flows, pyroclastic flows, or lahars, but regional transportation and water supplies could be

disrupted. (Photo by David Wieprecht, USGS)

U.S. Department of the Interior

U.S. Geological Survey

USGS Fact Sheet 060-00

August 2000

Mount Hood story and Hazards of Oregon’s Most

Recently Active Volcano

—Hi

Eruptions at Mount Hood

During the Past 30,000 Years

Mid-1800’s

Small steam and ash

explosions

Multiple episodes of lava

dome growth, pyroclastic

flows, lava flows, lahars,

and tephra fall; valleys on

all flanks affected

30,000 to 15,000 years ago

About 1,500 years ago

Debris avalanche from

upper south flank; lava

dome near Crater Rock,

pyroclastic flows, lahars in

south and west valleys;

substantial tephra falls

near volcano

About 200 years ago

Lava dome at Crater Rock;

pyroclastic flows, lahars in

south and west valleys, and

minor tephra falls

Subduction of the Juan de Fuca Plate under the

North American Plate controls the distribution of

earthquakes and volcanoes in the Pacific Northwest.

Mount Hood is just one of several recently active,

major volcanic centers in the Cascade Range.

CANADA

Juan de Fuca Plate North

American

Plate

USA

Casca i d a

b S

du u ction Zone

N

Volcano Earthquakes
Are You at Risk from the Next Eruption of Mount Hood?

The Next Eruption of Mount

Hood

Planning for the Future

Scientists expect the next eruption to

consist of small explosions and the growth

and collapse of lava domes, generating

pyroclastic flows, ash clouds, and lahars.

Lahars pose the greatest hazard because

more people live downstream in laharprone river valleys than live on the

volcano's flanks. Thus, it is important to

know where one lives, works, and plays in

relation to Mount Hood's hazard zones.

Hazardous Areas

Tephra Hazards

Hazard zones shown on the map were

determined on the basis of distance from

the volcano, vent location, and type of

hazardous evens. Proximal hazard zones

( ) are areas subject to rapidly moving

debris avalanches, pyroclastic flows, and

lahars that can reach the hazard boundary

in less than 30 minutes, as well as to slowmoving lava flows. Areas within proximal

hazard zones should be evacuated before an

eruption begins, because there is little time

to get people out of harm's way once an

eruption starts. Most pyroclastic flows, lava

flows, and debris avalanches will stop

within the proximal hazard zone, but lahars

can travel much farther.

Distal hazard zones ( ) are areas

adjacent to rivers that are pathways for

lahars. Estimated travel time for lahars to

reach these zones is more than 30 minutes,

which may allow individuals time to move

to higher ground and greater safety if given

warning. Shown are inundation areas for

lahars of a size similar to lahars that swept

through the Sandy River 1,500 year ago.

Smaller or larger lahars will affect smaller

or larger areas, respectively. Lahars could

affect transportation corridors by damaging

or destroying bridges and roads. Water

from the Bull Run Watershed, vital to

Portland, is transported in pipes that cross

distal hazard zones along the Sandy River.

Proximal and distal hazard zones are

subdivided into zones and on the basis

of the vent location during the next

eruption. During the past two eruptive

episodes, the vent was located near Crater

Rock. Scientists anticipate that the vent for

the next eruption will most likely be in the

same area. Thus, areas within hazard zones

and have a higher probability of

being affected during the next eruption than

do areas within hazard zones and ,

which reflect a vent located on the

volcano's west, north, or east flank.

During and after an eruption, large

amounts of sediment could be carried by

rivers and discharged into the Columbia

River. This sediment could narrow the

Columbia's channel, forcing it to the north

and potentially causing bank erosion along

the river's north bank.

Mount Hood does not have a history of

large explosive eruptions. Therefore, it is

unlikely that communities downwind

(typically east of the volcano) will receive

tephra accumulations thick enough to

collapse roofs. However, even minor

amounts of tephra can damage machinery

and electronic equipment or make driving

hazardous. Ash clouds from small

explosions or from pyroclastic flows can

reach tens of thousands of feet in altitude,

endangering jet aircraft by causing engine

failure. Minor amounts of tephra can make

breathing difficult for those with

respiratory problems, the elderly, and

infants. For most residents, however, tephra

will be a nuisance and 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.

Scientists do not know when Mount

Hood will erupt again or whether it will

erupt in our lifetimes, but, as Mount St.

Helens taught us, it is best to be prepared.

Scientists continuously monitor Mount

Hood for signs of unrest and are in

communication with responsible local,

State, and Federal agencies. Individuals too

can prepare by doing the following:

about the volcano hazards that

could affect your community and

determine whether you live, work, or

go to school in a volcano hazard zone.

what you and your family will do

if a hazardous volcanic event occurs.

in helping your community

be prepared.

A few moments spent now could help

keep the next eruption of Mount Hood from

becoming a disaster for you, your family,

and your community.

P

D

A B

PA DA

PB DB

Learn

Plan

Participate

●

●

●

House partly buried by lahars from the 1980 eruption of Mount St. Helens. Note the high mud line on the

house and large amounts of woody debris carried by the lahars. (Photo by Lyn Topinka, USGS)

Simplified volcano-hazards-zonation map for Mount Hood, Oregon. For more

information on hazard zones see text or for a more detailed map and description see

Scott and others, 1997, Volcano Hazards in the Mount Hood Region, Oregon, U.S.

Geological Survey Open-File Report 97-89.

PA PB

Major valleys

Proximal Hazard Zones (P)

(lava flow, pyroclastic flow,

debris avalanche, lahar)

DA DB

Bank Erosion

Distal Hazard Zones (D)

(lahar)

Mount Hood’s last

major eruption

occurred 200 years

ago and consisted

of growth and

collapse of a lava

dome that sent

numerous

pyroclastic flows

down the south

and west flanks. Crater Rock (above) is the

remnant of that dome. The left photo shows a

pyroclastic flow during the August 7, 1980,

eruption of Mount St. Helens. A billowing ash

cloud rises from the pyroclastic flow. Future

pyroclastic flows from Mount Hood could

devastate areas on the flanks of the volcano, and

fallout of ash from the ash cloud could disrupt

transportation and prove a nuisance to residents

downwind. (Photos by Peter Lipman and

, USGS)

David

Wieprecht

Explosive eruptions blast lava

fragments ( ) and gas into the

air. Tephra can also be carried

aloft in billowing ash clouds

above pyroclastic flows. Large

fragments fall to the ground

close to the volcano, but

smaller fragments ( ) can

travel hundreds to thousands

of miles downwind.

tephra

ash

Debris avalanches are rapid landslides of rock,

soil and overlying vegetation, snow, or ice. They

can bury or smash objects in their path. All or

some portion of debris avalanches can transform

into lahars. are fast-moving slurries of

rock, mud, and water that move down river

valleys. They can bury, move, or smash objects

in their path. Lahars form when pyroclastic

flows melt snow or ice, or by the

transformation of debris

avalanches, or by mobilization

of loose debris on the

flanks of

volcanoes.

Lahars

Volcano Hazards

Ash

Cloud

Lava Flows and Domes

Lava

Lava flows

Lava domes

is molten rock that flows onto the Earth's

surface. move downslope away from

a vent and bury or burn everything in their paths.

form when lava piles up over a vent.

Debris Avalanches and Lahars

Pyroclastic Flows

Pyroclastic flows are highspeed avalanches of hot rock,

gas, and ash that are formed

by the collapse of lava domes

or eruption columns. They

can move up to 100 miles

per hour and have

temperatures to 1500 F.

They are lethal, burning,

burying, or asphyxiating

all in their paths.

o

Pyroclastic

Flow

Tephra

Pyroclastic Flow

Ash Cloud

Crater Rock

CLARK COUNTY

SKAMANIA COUNTY

WASCO COUNTY

HOOD RIVER COUNTY

The Dalles

Hood River

Welches

Bull Run

Watershed

WASHINGTON

OREGON

0:30

0:30

0:30

0:30

0:30

1:00

1:30

2:00

3:00

1:00

1:30

2:00

Sandy

CLACKAMAS COUNTY

MULTNOMAH COUNTY

Camas

Washougal

Troutdale

White

Salmon

River

Wemme

Wildwood

Parkdale

KLICKITAT COUNTY

1:30

Minimum time

for lahar to

reach an area,

in hours:minutes

Watershed

The Dalles

MOUNT

HOOD

OREGON

Crater Rock

Hazards Zonation Map

Columbia River

Sandy River

Salmon River

White River

0:30

3:30

MOUNT HOOD

1:00

2:30

Tygh Valley

River

Deschutes

Government

Camp

2:30

84

14

197

26

35 Bonneville

Dam

The Dalles

Dam

0 5 MILES

N

Polallie

Creek

A River Transformed

In 1806, Meriwether Lewis and

William Clark named a river on the

south side of the Columbia River gorge

the “Quicksand River.” Their description

of a wide, shallow with a bed “formed

entirely of quicksand,” bears little resemblance

to the narrow, moderately deep river we call today

the Sandy River. What happened? The answer lay

50 miles away at Mount Hood. An eruption in the

1790's caused a tremendous amount of volcanic

rock and sand to enter the Sandy River drainage.

That sediment was still being flushed downstream

when Lewis and Clark saw and named the river.

Since 1806, the river has removed the excess

sediment from its channel. The Toutle River in

southwest Washington was similarly affected by the

1980 eruptions of Mount St. Helens. A sedimentretention dam was built in the late 1980's to trap

sediment and ease flooding in the lower Toutle

River below the dam.

river
Hazards Can Occur Even Without

Eruptive Activity

Past Catastropic Events

Mount Hood Today

Monitoring for the Next Eruption

Lahars are often associated with eruptive

activity, but they can also be generated by

rapid erosion of loose rock during heavy rains

or by sudden outbursts of glacial water. On

Christmas Day 1980, an intense rainstorm

rapidly melted snow and triggered a small

landslide in fragmental debris in upper Polallie

Creek. The resulting lahar moved downvalley

at 25 to 35 miles per hour. At the mouth of

Polallie Creek, the lahar spread out, killing a

camper and temporarily damming the East

Fork Hood River. Flooding after failure of this

temporary dam destroyed 5 miles of highway,

three bridges, and a state park—at a cost of at

least $13 million. Small lahars such as this

occur every few years at Mount Hood, but few

have been as destructive.

Two past eruptions at Mount Hood provide

perspective on the impact of future large

events. Both were associated with eruptive

activity that triggered debris avalanches and

were accompanied by lava extrusion,

pyroclastic flows, and lahars. One represents a

truly catastrophic event.

About 100,000 years ago, a large portion of

the volcano's north flank and summit

collapsed. The resulting debris avalanche

transformed into a lahar that swept down the

Hood River valley. At the river's mouth, where

the town of Hood River now stands, the lahar

was 400 feet deep. The lahar crossed the

Columbia River and surged up the White

Salmon River valley on the Washington side.

Since that time lava has filled in the scar left

by the debris avalanche.

On the south side of the volcano, the scar

from a 1,500-year-old debris avalanche is still

visible, forming the amphitheater around

Crater Rock. A lahar formed by this event

traveled the length of the Sandy River valley,

depositing boulders as large as 8 feet in

diameter, 30 feet above present river level

where the towns of Wemme and Wildwood

now stand. The lahar spread out over the delta

at the mouth of the Sandy River and pushed

the Columbia River to the north. This event,

although large by Mount Hood's standards,

was only about one-tenth the size of the

100,000-year-old event.

Mount Hood's next eruption is much more

likely to be smaller than or similar in size to

the 1,500-year-old event. An eruption similar

in size to the 100,000-year-old event, although

possible, is much less likely.

Today Mount Hood shows no signs of

imminent volcanic activity, but hot steam

vents, or fumaroles, near Crater Rock attest to

heat below. On clear, cold, windless days, a

steam plume is often seen rising from the

fumaroles. Visitors to Mount Hood frequently

smell the "rotten egg" odor of the fumarole

gas, whose composition indicates that magma

lies a few miles below the summit.

Renewed activity at most volcanoes begins

with increasing numbers of earthquakes

beneath the volcano as magma moves towards

the surface. Since 1977 the University of

Washington's Geophysics Program and the

USGS have continuously monitored

earthquakes at Mount Hood. Typically, one to

three small earthquake swarms (tens to more

than one hundred earthquakes lasting 2 to 5

days) occur every year. What scientists are

looking for as a sign of renewed activity is for

a swarm to persist, for the number of

earthquakes to increase dramatically, or for the

depths of earthquakes to become shallower.

Such signs of reawakening might also be

accompanied by changes in composition or

temperature of fumarole gases, or by

deformation of the volcano's flanks.

Scientists can generally detect when a

volcano becomes "restless," thereby providing

some warning to officials and the public. But

they cannot say precisely when an eruption

will begin, how big it will be, or how long it

will last. Thus, we will all have to confront

many uncertainties when Mount Hood

reawakens. Recent eruptions around the world

reveal that lava-dome eruptions, like those

typical of Mount Hood, can begin after weeks

to months of restlessness, last for time periods

of months to years, and generate tens to

hundreds of pyroclastic flows and lahars of

varying size. Unfortunately, the end of an

eruption doesn't always mean the end of

eruption-related problems. New deposits of

rock debris on the volcano's slopes and in river

valleys can be reworked to form lahars for

many years after an eruption ends.

For more information contact:

Volcano Hazards in the Mount Hood Region,

Oregon

What are Volcano Hazards?

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/

See also

(USGS Open-File Report 97-89) and

(USGS Fact Sheet 002-97)

COOPERATING ORGANIZATIONS

U.S. Department of Agriculture, Forest Service

University of Washington, Geophysics Program

Geologist examining

100,000-year-old lahar

deposit exposed along

Underwood Hill Road,

near the mouth of the

White Salmon River in

Washington. The lahar,

derived from a large

debris avalanche off the

north side of Mount

Hood, flowed down the

Hood River, crossed the

Columbia River, and

traveled several miles

up the White Salmon

River before stopping.

Here, the deposit is

about 40 feet thick and

about 300 feet above

present river level.

(Photo by William Scott,

USGS)

The two faces of Mount Hood. In the photo of the south flank taken from Trillium Lake, the dashed line marks

the steep scarp above Crater Rock formed by a landslide 1,500 years ago. Dome growth and collapse from

Crater Rock during the last two eruptive episodes (1,500 and 200 years ago) sent numerous pyroclastic flows

down the south flank, resulting in the smooth south-facing slope. On the north flank, lack of recent volcanic

activity has preserved the deeply carved glacial landscape on this side of the mountain. In its long history,

Mount Hood has experienced numerous ice ages, each lasting for thousands of years, when glaciers were

more extensive than today. During the last one, which ended about 15,000 years ago, glaciers extended 4 to 5

miles beyond their present limits. (Photos by William Scott, USGS)

Crater Rock

South Flank North Flank

Cynthia A. Gardner, William E. Scott,

Jon J. Major, and Thomas C. Pierson

Graphics and Design by

Bobbie Myers, Lisa Faust, and Christine Janda