U.S. Department of the Interior

U.S. Geological Survey

VOLCANO HAZARDS IN THE MOUNT HOOD

REGION, OREGON

By

W.E. Scott1

, T.C. Pierson1, S.P. Schilling1, J.E. Costa1,

C.A. Gardner1

, J.W. Vallance2, and J.J. Major1

1. U.S. Geological Survey, Cascades Volcano Observatory, 5400 MacArthur Boulevard, Vancouver, WA 98661

2. McGill University, Department of Civil Engineering and Applied Mechanics, 817 Sherbrooke St. West, Montreal,

QC, H3A 2K6, Canada

Open-File Report 97-89

This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards or with the North American

Stratigraphic Code. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

1997
CONTENTS

Summary 1

Introduction 1

Past hazardous events 1

Lava eruptions, pyroclastic flows, and related lahars 2

Debris avalanches and lahars 4

Tephra falls 6

Volcano-hazards-zonation map 7

Proximal hazard zones 8

Distal hazard zones 9

Sandy River drainage 9

White River drainage 10

Hood River drainage 10

Tephra-hazard zones 10

Regional lava-flow hazard zone 11

How large an event is possible at Mount Hood? 11

Hazard forecasts and warnings 11

Protecting our communities and our families from volcano hazards 12

References and suggested additional reading 14

End notes 14

ILLUSTRATIONS

Volcano hazards in the Mount Hood region, Oregon (large map) In pocket

Key geologic events in the Mount Hood region during the past 30,000 years 2

Hazardous geologic events at Mount Hood 3

Photograph of upper south flank of Mount Hood showing the lava dome of Crater Rock and

the head of the fan of pyroclastic-flow and lahar deposits below it. The steep scarp above

Crater Rock was formed by a landslide (debris avalanche) about 1,500 years ago. 5

Large boulders in Wemme transported by a lahar that originated from a debris avalanche

on the upper southwest flank of Mount Hood about 1,500 years ago. 6

This report is also available in digital form through the World Wide Web.

URL: http://vulcan.wr.usgs.gov/Volcanoes/Hood/Hazards/

Additional paper copies of this report can be purchased from the U.S. Geological Survey

Branch of Distribution, P.O. Box 25286, Denver, CO 80225

(303) 202-4210

Cover photo: Mount Hood and Portland, Oregon. View is to east from the Washington Park Rose Garden.

Tom Iraci, USDA - Forest Service
VOLCANO HAZARDS IN THE MOUNT HOOD REGION,

OREGON

By

W.E. Scott, T.C. Pierson, S.P. Schilling, J.E. Costa, C.A. Gardner,

J.W. Vallance, and J.J. Major

SUMMARY

Mount Hood is a potentially active volcano close to rapidly growing communities and recreation areas.

The most likely widespread and hazardous consequence of a future eruption will be for lahars (rapidly

moving mudflows) to sweep down the entire length of the Sandy (including the Zigzag) and White River

valleys. Lahars can be generated by hot volcanic flows that melt snow and ice or by landslides from the

steep upper flanks of the volcano. Structures close to river channels are at greatest risk of being destroyed.

The degree of hazard decreases as height above a channel increases, but large lahars can affect areas more

than 30 vertical meters (100 vertical feet) above river beds. The probability of eruption-generated lahars

affecting the Sandy and White River valleys is 1-in-15 to 1-in-30 during the next 30 years, whereas the

probability of extensive areas in the Hood River Valley being affected by lahars is about ten times less.

The accompanying volcano-hazard-zonation map outlines areas potentially at risk and shows that some

areas may be too close for a reasonable chance of escape or survival during an eruption. Future eruptions

of Mount Hood could seriously disrupt transportation (air, river, and highway), some municipal water

supplies, and hydroelectric power generation and transmission in northwest Oregon and southwest

Washington.

INTRODUCTION

Snow-clad Mount Hood dominates the

Cascade skyline from the Portland metropolitan

area to the wheat fields of Wasco and Sherman

Counties. The mountain contributes valuable

water, scenic, and recreational resources that help

sustain the agricultural and tourist segments of the

economies of surrounding cities and counties.

Mount Hood is also one of the major volcanoes of

the Cascade Range, having erupted repeatedly for

hundreds of thousands of years, most recently

during two episodes in the past 1,500 yr. The last

episode ended shortly before the arrival of Lewis

and Clark in 1805. When Mount Hood erupts

again, it will severely affect areas on its flanks and

far downstream in the major river valleys that head

on the volcano. Volcanic ash may fall on areas up

to several hundred kilometers downwind.

The purpose of this report is to describe the

kinds of hazardous geologic events that have

happened at Mount Hood in the past and to show, in

the accompanying volcano-hazard-zonation maps,

which areas will be at risk when such events occur

in the future.

PAST HAZARDOUS EVENTS

Mount Hood has erupted intermittently for

hundreds of thousands of years, but historical

observations are meager, so most of our

information about its past behavior comes from

geologic study of the deposits produced by

prehistoric events[1; numerals in brackets refer to

end notes listed at the end of the report]. We also

use observations of recent eruptions at other

similar volcanoes around the world to better

understand what future eruptions of Mount Hood

INTRODUCTION 1
might be like. A brief description of the kinds of

events that have occurred at Mount Hood and are

likely to happen in the future follows.

Lava Eruptions, Pyroclastic Flows,

and Related Lahars

Lava has erupted at Mount Hood chiefly in two

modes. Numerouslava flowsissued from vents on

the upper flanks and traveled up to 12 kilometers (7

miles) down valleys. Erosion of new valleys along

flow margins has left many of these lava flows as

ridges, such as Cathedral Ridge, that radiate out

from the center of the volcano. Observations of

lava flows at similar volcanoes suggest that Mount

Hood flows move down valleys as tongues of fluid

lava a few to tens of meters thick (10 to 200 feet)

encased in a cover of hardened lava rubble. Such

lava flows can destroy all structures in their paths,

but they advance so slowly that they seldom

endanger people. Lava domes formed stubby lava

2 VOLCANO HAZARDS IN THE MOUNT HOOD REGION, OREGON

1900-1996 Numerous small lahars and debris avalanches

Key geologic events in the Mount Hood region

during the past 30,000 years

Steam explosions, minor tephra falls

Lava dome near Crater Rock; pyroclastic flows and lahars in

Sandy, Zigzag, Salmon and White River valleys; tephra falls

Small lahars in Zigzag River valley

Small lahars in upper Sandy RIver valley

Lava dome near Crater Rock; pyroclastic flows and lahars in Sandy,

Zigzag, and Salmon River valleys; lahars in White River valley; tephra falls

Parkdale lava flow erupted in Middle Fork Hood River valley

Tephra fall from eruption of Mount Mazama (Crater Lake)

Multiple episodes of lava domes and lava flows, pyroclastic flows,

tephra falls, and lahars immediately following, during, and just

before last ice age; drainages on all flanks affected

Debris avalanche from upper south flank; lahars in Zigzag

and lower Sandy River valleys; small debris avalanches and

lahars in valleys on north and east flanks

1856-1865

about

200 years ago

about

500 years ago

about

1,000 years ago

about

1,500 years ago

about

7,600 years ago

about 15,000 to

30,000 years ago

Age of

events Types of events
masses on the upper flanks and summit of Mount

Hood as lava welled out of a vent and piled up, too

viscous to flow away. A recent example is the lava

dome that grew in the crater of Mount St. Helens

between 1980 and 1986. Past lava domes growing

on the steep upper flanks of Mount Hood were

typically unstable and collapsed repeatedly as they

grew higher and steeper.

Collapse of a growing lava dome or the front of

a thick lava flow generates landslides of hot rock

called pyroclastic flows. Pyroclastic flows are

fluid mixtures of hot rock fragments, ash, and gases

that sweep down the flanks of volcanoes at speeds

of 50 to more than 150 kilometers per hour (30 to

90 miles per hour) destroying vegetation and

structures in their paths. Most are confined to

valley bottoms, but pyroclastic surges, overriding

clouds of hot ash and gases, are more mobile and

can overwhelm even high ridge tops. At Mount

Hood, pyroclastic flows have traveled at least 12

PAST HAZARDOUS EVENTS 3

Hazardous Geologic Events at Mount Hood

Collapsing

Lava Dome

Pyroclastic

Flow and Surge

Lahar

Ballistic

Fragments

Debris

Avalanche

Magma

Reservoir

Magma

Conduit

Tephra

Fallout

Lava Flow

Steam Explosion

Lahar

Ash Cloud

Mount Hood generates a wide

variety of geologic events that can

endanger people and property.

Most of these events are associated

with eruptions, but some, like

debris avalanches, can occur even

during quiet times. Small events

may pose a hazard only within a

few kilometers of the summit,

while large events can directly or

indirectly endanger people and

property tens to hundreds of

kilometers away.
kilometers (7 miles) from lava domes; pyroclastic

surges probably traveled even farther. Pyroclastic

flows and surges also produce ash clouds that can

rise thousands of meters (tens of thousands of feet)

into the atmosphere and drift downwind for

hundreds of kilometers (hundreds of miles). The

consequences of this ash are discussed in a later

section called Tephra Fall.

Pyroclastic flows and surges can also melt

snow and ice and generate lahars (also called

volcanic mudflows or debris flows). Lahars are

rapidly flowing, water-saturated mixtures of mud

and rock fragments, as large as truck-size boulders,

that range in consistency from mixtures resembling

freshly mixed concrete to very muddy water.

Lahars can travel more than 100 kilometers (60

miles) down valleys. They move as fast as 80

kilometers per hour (50 miles per hour) in steep

channels close to a volcano, but slow down to about

15 to 30 kilometers per hour (10-20 miles per hour)

on gently sloping valley floors farther away. Past

lahars at Mount Hood completely buried valley

floors in the Sandy and Hood River drainages all

the way to the Columbia River and in the White

River drainage all the way to the Deschutes River.

Eruptive activity at Mount Hood during the

past 30,000 years has been dominated by growth

and collapse of lava domes. The last two episodes

of eruptive activity occurred 1,500 and 200 years

ago. Repeated collapse of lava domes extruded

near the site of Crater Rock, Mount Hood’s

youngest lava dome, generated pyroclastic flows

and lahars and built much of the broad smooth fan

on the south and southwest flank of the volcano.

The newly formed fans of debris on the lower

flanks of Mount Hood and deposits of lahars in

river valleys were highly erodible, which caused

additional impacts. Normal rainfall, snowmelt,

and streams remobilized the sediment and

continued to move it farther downstream for years

after eruptions. For example, after the last eruptive

period, the Sandy River became choked with

sediment and within about a decade buried the

preeruption valley floor over 20 meters (65 feet)

deep between Sandy and Troutdale. Ultimately,

much of the sediment from past eruptions entered

the Columbia River. A recurrence of such events

would greatly affect the Columbia River, its

shipping channel, and, potentially, hydroelectric

installations, such as Bonneville Dam.

Debris Avalanches and Lahars

Rapidly moving landslides, called debris

avalanches, occurred numerous times in the past

when the steep upper parts of Mount Hood

collapsed under the force of gravity. Warm acidic

ground water that circulates in cracks and porous

zones inside volcanoes alters strong rock to weak

slippery clay, thereby gradually weakening them

and making them more susceptible to debris

avalanches than other mountains. Volcanoes are

further weakened as erosion, especially by

glaciers, oversteepens slopes. The destabilizing

forces of magma (molten rock) pushing up into a

volcanic cone prior to an eruption can trigger

debris avalanches as occurred at Mount St. Helens

in 1980. Unexpected earthquakes (both smaller

local ones and larger distant ones) or steam

explosions can also trigger debris avalanches. A

debris avalanche can attain speeds in excess of 160

kilometers per hour (100 miles per hour); the larger

the avalanche, the faster and farther it can move.

Small-volume debris avalanches typically move

only a few kilometers (1 to 3 miles), but largevolume debris avalanches are capable of reaching

tens of kilometers (tens of miles) from the volcano.

Debris avalanches destroy everything in their paths

and can leave deposits 10 to more than 100 meters

(30 to more than 300 feet) thick on valley floors.

Depending upon their water content, debris

avalanches can transform into lahars, which, like

lahars formed by pyroclastic flows, can move

down valleys for even greater distances.

About 1,500 years ago, a moderate-size debris

avalanche originating on the upper southwest flank

of Mount Hood (see photograph) produced a lahar

that flowed down the Zigzag and Sandy River

valleys. It swept over the entire valley floor in the

Zigzag-Wemme-Wildwood area, and inundated a

broad area near Troutdale, where the Sandy flows

into the Columbia River—a total distance of about

90 kilometers (55 miles). More than 100,000 years

ago, a much larger debris avalanche and related

lahar flowed down the Hood River, crossed the

Columbia River, and flowed several kilometers up

the White Salmon River on the Washington side.

Its deposit must have dammed the Columbia River

at least temporarily.

During noneruptive periods, relatively small

lahars present a hazard along channels and on

4 VOLCANO HAZARDS IN THE MOUNT HOOD REGION, OREGON
PAST HAZARDOUS EVENTS 5

Photograph of upper south flank of Mount Hood showing the lava dome of Crater Rock and the

head of the fan of pyroclastic-flow and lahar deposits below it. The steep scarp above Crater

Rock was formed by a landslide (debris avalanche) about 1,500 years ago.
floodplains on the flanks of Mount Hood.

Although of modest size compared to lahars

generated by eruptions or large debris avalanches,

they occur much more frequently. Twenty-one

lahars, including single flows as large as several

hundred thousand cubic meters (cubic yards),

whose effects were chiefly limited to areas within

15 kilometers (9 miles) of Mount Hood’s summit,

are reported in the historical record. Most occurred

during autumn and early winter rains. Glacial

outburst floods caused at least two and probably as

many as seven others. A highly damaging lahar

occurred in December 1980 when intense warm

rain (with rapid snowmelt) triggered a flow in

Polallie Creek that killed a camper at the creek

mouth and temporarily dammed the East Fork

Hood River. The ensuing dambreak flood

destroyed about 10 kilometers (6 miles) of Oregon

Highway 35 and other downstream facilities and

caused about $13 million in damage.

Tephra Falls

Mount Hood has typically not produced thick,

extensive deposits of tephra (fragmented

solidified lava that rises into the air, is carried by

winds, and falls back to the ground) as has nearby

Mount St. Helens. Rather, relatively modest

amounts of tephra were produced during past lavaflow and lava-dome eruptions. Most tephra fallout

was caused by clouds of sand- and silt-size

particles that rose from moving pyroclastic flows

produced by lava-dome collapse. Tephra was also

generated by explosions driven by volcanic gases.

Both types of tephra clouds probably reached

altitudes of 1,000 to 15,000 meters (3,000 to

50,000 feet) above the volcano and were then

carried away by the prevailing wind, which blows

toward sectors northeast, east, or southeast of

Mount Hood about 70 percent of the time. Winds

that would carry tephra toward the Portland

6 VOLCANO HAZARDS IN THE MOUNT HOOD REGION, OREGON

Large boulders in Wemme transported by a lahar that originated from a debris

avalanche on the upper southwest flank of Mount Hood about 1,500 years ago.
metropolitan area are rather uncommon, occurring

only a few percent of the time. On the flanks of the

volcano, each event deposited, at most, a few

centimeters (inches) of tephra. Thickness of tephra

fallout decreased rapidly downwind to probably

just a few millimeters (one-tenth inch) or less at

100 to 200 kilometers (60-120 miles) from the

volcano. During future explosions at Mount Hood,

large, dense ballistic fragments(more than 5 cm (2

inches) in diameter) that can damage structures and

kill or injure people may be thrown up to 5

kilometers (3 miles) from vents.

Tephra fallout produced by future eruptions of

Mount Hood poses little threat to life or structures

in nearby communities. But tephra clouds can

create tens of minutes or more of darkness as they

pass over a downwind area, even on sunny days,

and reduce visibility on highways. Tephra

ingested by vehicle engines can clog filters and

increase wear. Deposits of tephra can short-circuit

electric transformers and power lines, especially if

the tephra is wet and thereby highly conductive,

sticky, and heavy. This effect could seriously

disrupt hydroelectric power generation and

transmission along the Columbia River and

powerline corridors north and east of the volcano.

Tephra clouds often spawn lightning, which can

interfere with electrical and communication

systems and start fires. A serious potential danger

of tephra stems from the grave effects of even

small, dilute tephra clouds on jet aircraft that fly

into them. Major air routes pass by Mount Hood,

and tephra clouds produced repeatedly during an

eruptive episode would interfere greatly with air

traffic.

Lessons learned in eastern Washington during

the 1980 eruption of Mount St. Helens can help

prepare governments, businesses, and citizens for

future tephra falls. Communities experienced

significant disruptions in transportation, business

activity, and services during fallout of from 0.5 to 8

centimeters (1/4 to 3 inches) of tephra and for

several days thereafter. The greater the amount of

tephra that fell, the longer the recovery time. As

perceived by residents, tephra falls of less than 0.5

centimeter (1/4 inch) were a major inconvenience,

whereas falls of more than 1.5 centimeters (2/3

inch) constituted a disaster. Nonetheless, all

communities resumed normal activities within

about two weeks. On the basis of the type and

magnitude of tephra production we would expect

from Mount Hood in the future, only nearby

communities, such as Government Camp,

Rhododendron, and Parkdale, would likely receive

a tephra thickness approaching 1.5 centimeters

(2/3 inch) in any one event. However, some other

nearby volcanoesin the Cascade Range do produce

large explosive tephra eruptions that could affect

the Mount Hood region.

VOLCANO-HAZARDS-ZONATION

MAP

The accompanying volcano-hazards zonation

map shows areas that could be affected by future

hazardous geologic events at Mount Hood [2;

numerals in brackets refer to end notes listed at the

end of the report]. Hazardous areas are divided

into proximal and distal hazard zones depending

on distance from the volcano. Some zones are split

further on the basis of their relative degree of

hazard. Zone boundaries are based on (1) the

frequency and magnitude of past events at the

volcano, as recorded by their deposits, (2)

computer models that predict the extent, depth, and

travel time of future lahars, and (3) our experience

and judgment derived from observations and

understanding of events at other volcanoes. Other

hazard zones shown on inset maps include zones

for tephra fall, for lava flows from other vents

scattered through the Mount Hood area, and for

very large-magnitude, but highly unlikely, events

at Mount Hood.

Most future events will affect only part of a

hazard zone. The location and size of an affected

area will depend on the location of the erupting

vent or landslide, the volume of material involved,

the snow and ice conditions around and down slope

from the vent, and the character of an eruption,

especially its explosivity. For lava domes, the

duration of an eruptive episode is also important.

Long-lived dome extrusion with numerous

collapses can fill entire valleys on the flanks of the

volcano with debris and form broad fans that

eventually can affect large parts of a hazard zone.

Although we show boundaries of hazard zones

by lines, the degree of hazard does not change

abruptly at these boundaries. Rather, the hazard

decreases gradually as distance from the volcano

VOLCANO-HAZARDS-ZONATION MAP 7
increases and decreases more rapidly as elevation

above the valley floor increases. Areas

immediately beyond outer hazard zones should not

be regarded as hazard-free, because the boundaries

can only be approximately located, especially in

areas of low relief. Too many uncertainties exist

about the source, size, and mobility of future events

to locate the boundaries of zero-hazard zones with

confidence.

Proximal Hazard Zones

Proximal hazard zones include areas from the

summit out to 24 km (15 miles) along major valleys

and out to about 12 kilometers (7 miles) in between

major valleys. These zones are subject to several

types of rapidly moving, devastating flows.

Pyroclastic flows and surges will travel out to a

maximum distance of about 12 kilometers in less

than 10 minutes, whereas lahars and debris

avalanches can travel out to the 24-km hazard

boundary in as little as 30 minutes[3]. Areas up to

5 kilometers (3 miles) from a vent could also be

subject to showers of large (more than 5

centimeters or 2 inches) ballistic fragments within

a few minutes of an explosion. Owing to such high

speeds, escape or survival is unlikely in proximal

hazard zones. Therefore, evacuation of proximal

hazard zones prior to onset of an event is

realistically the only way to protect lives. Lava

flows issuing from vents on the upper flanks of

Mount Hood would be largely restricted to

proximal hazard zones, but they would move much

more slowly than these other types of flows.

During the past 1,500 years, lava-dome growth

has been localized in the area around Crater Rock,

the youngest lava dome on Mount Hood, which lies

in a steeply sloping, breached crater south of the

summit ridge. We think that this same area is the

most likely vent location during the next eruption

as well. Therefore, we define a proximal hazard

zone A (PA), which encompasses those areas that

could be affected by events accompanying dome

growth at or near Crater Rock. A less likely event is

the opening of a vent elsewhere on the upper east,

north, or west flank. Should this occur, the

corresponding hazard zone would be all or part of

proximal hazard zone B (PB). Depending on vent

location, especially if at the summit, all or part of

zone PA also could also be at risk. On the lower

south and west flanks, hazard zone PB extends

beyond the limit of zone PA because a lava dome

growing at the summit would be at a higher altitude

than Crater Rock and would have the potential to

generate farther-reaching pyroclastic flows. On

the basis of past eruption frequency, we estimate

the probability of an eruption impacting part of

zone PA in the next 30 years (the 30-year

probability) to be about 1 in 15 to 1 in 30 [4]. In

contrast, the 30-year probability of part of zone PB

being affected is on the order of 1 in 300 [4]. We

caution that these probabilities are based solely on

the long-term behavior of the volcano. Any signs

of increased restlessness at Mount Hood will

increase these probabilities dramatically.

Several major valleys within the proximal

hazard zones are highlighted on the map by a

hachured line pattern because they are more likely

than others to be affected by future pyroclastic

flows and lahars related to collapse of growing lava

domes, especially during initial stages of dome

building. These valleys, along with Polallie Creek

valley, are also areas subject to frequent small

lahars, floods, and debris avalanches triggered by

storms or other noneruptive causes. If a lava dome

grows near Crater Rock, the White and Zigzag

River valleys and the valley of Zigzag Glacier and

its meltwater stream, an unnamed tributary of the

upper Sandy, are the most likely pyroclastic-flow

and lahar paths. If an eruptive episode continues

for a long enough time period that debris fills the

heads of these drainages, pyroclastic flows and

lahars will be able to sweep over a broader area,

which could include the Little Zigzag River, Still

Creek (including the area around Government

Camp), and Salmon River valleys. Likewise in

zone PB on the north or east flank, the main valleys

below a growing lava dome would initially be the

most likely flow paths. For example, dome growth

on the upper northeast flank would initially affect

the valleys of Newton Creek and Eliot Branch. The

large area in the proximal hazard zone between

these valleys that is drained by Polallie and several

other creeks does not presently head directly on the

upper flanks and probably would not be affected

initially. Before these drainages could be

inundated by pyroclastic flows, the valley heads of

Newton Creek and(or) Eliot Branch would have to

be partly filled with debris.

8 VOLCANO HAZARDS IN THE MOUNT HOOD REGION, OREGON
While the subdivision of the proximal area into

zones PA and PB based on vent location applies

well to pyroclastic flows and lahars produced by

lava dome collapse, several other types of events

are not so neatly restricted by this hazard zonation.

First, the earthquakes and deformation associated

with future intrusion of magma into Mount Hood

can trigger landslides of fractured and weakened

rock from the steep upper slopes. Therefore, even

though dome building is localized at one site,

landslides elsewhere on the upper flanks can

generate debris avalanches and related lahars in

valleys not otherwise affected by dome growth.

Such events, largely restricted to the hachured

areas in zone PB, occurred on the east, north, and

west flanks during the past 1,500 years, while

dome growth and collapse affected valleys on the

south and southwest flanks. Furthermore, owing to

the pronounced filling of valleys on the south side

by debris during the past 1,500 years, the majority

of high cliffs and spurs subject to landsliding lie on

other flanks. Thus, regardless of which zone a

dome is growing in, potential hazards from debris

avalanches and lahars exist in other parts of the

proximal zones. Second, explosive eruptions

driven by volcanic gases can also affect both

proximal zones. Explosions can generate highly

mobile pyroclastic flows as material falls back to

the ground and can hurl large ballistic fragments

outward up to 5 kilometers (3 miles). Such events

are less constrained by topographic features than

are pyroclastic flows from dome collapse, so

explosions at a vent in one proximal zone could

impact parts of the other proximal zone, especially

with ballistics.

Distal Hazard Zones

Distal hazard zones lie beyond proximal zones

and include the major river valleys draining Mount

Hood that can be swept by lahars and debris

avalanches. Depending on distance from the

volcano, these areas will be affected 30 minutes to

several hours after the onset of an event, and escape

is possible given sufficient warning.

Sandy River drainage

Numerous lahars inundated the Sandy River

valley and its tributaries that head on Mount Hood

during the two eruptive episodes of the past 1,500

years. Thus, plentiful deposits are available for

study so that we can reconstruct extent and depth of

past flows. Areas most likely to be affected by

future events are included in distal hazard zone A

(DA), which encompasses areas known to have

been inundated by these prehistoric flows and

adjacent areas up to 12 meters (40 feet) higher.

Deposits do not always record the highest level of

inundation, and we use our judgment based on

experience with historical lahars to locate the

boundaries higher on valley walls where the valley

is relatively narrow and flows would deepen, or

where flows sweeping around bends would run up

higher on the outside of a bend. In zone DA, the

probability of areas being inundated by a lahar or

debris avalanche depends on such factors as height

above the river channel, distance downstream from

Mount Hood, and valley shape. The 30-year

probability of areas in zone DA being inundated by

lahars is the same as that for renewed dome growth

near Crater Rock, about 1 in 15 to 1 in 30 [4].

Lahars that flow south from the Crater Rock

area can also enter the valleys of other Sandy River

tributaries, such as Still Creek and Salmon River.

During an eruptive period, we consider these

tributaries less likely to be inundated than the upper

Sandy and Zigzag Rivers, at least initially, because

the topography of the upper debris fan will funnel

future lahars toward the upper Sandy and Zigzag

valleys. Furthermore, events of the past 1,500

years suggest that only small lahars will enter the

lower canyons of Still Creek and Salmon River,

and these should transform into muddy floods

before traveling far down either channel.

Lahars and sediment-rich floods that flowed to

the Columbia River during and immediately after

past eruptive periods built the delta at the mouth of

the Sandy River near Troutdale. The delta area is in

hazard zone DA because its low relief offers little

protection from lahars or from the shifting of

channels that can occur during periods of high

sedimentation. Sediment deposition at the edge of

the delta has narrowed the Columbia River and

pushed it against the Washington shore. Future

lahars and eruption-induced sedimentation are

likely to build the delta farther out into the

Columbia River and narrow the existing channel,

which could lead to progressive bank erosion and

inundation of land in the Camas-Washougal area.

VOLCANO-HAZARDS-ZONATION MAP 9
The hazard zone on the north side of the river

outlines areas thought to be vulnerable to these

processes. More specific forecasts are impossible

other than to note that areas close to the river and

land composed of erodible deposits are the most

vulnerable.

White River drainage

Lahars spawned by lava-dome collapses swept

through the White River valley about 200 years ago

and inundated large parts of Tygh Valley. Hazard

zone DA encompasses these deposits as well as

adjacent areas that lie up to 12 meters (40 feet)

higher depending on valley width (see discussion

in Sandy River drainage). Lahars of this

magnitude would inundate the broad flood plain of

White River in Tygh Valley, but probably not reach

the town itself. Lahars that reach the Deschutes

River probably would be diluted to muddy floods

that would transport large amounts of sediment

into the Columbia River upstream from The Dalles

Dam. The 30-year probability of an area in zone

DA along White River being inundated by a debris

avalanche or lahar is about 1 in 15 to 1 in 30 [5].

Hood River drainage

The Hood River and its tributaries drain about

one-half of Mount Hood, but have apparently not

been affected by lahars that extended outside of the

proximal hazard zone, or substantially out of

channels, during the past 15,000 years. Recent

eruptions that produced pyroclastic flows and

lahars in the Sandy and White River valleys only

indirectly affected upper parts of the Hood River

basin by producing modest debris avalanches and

related lahars. However, dome growth on the east

or north flank, a condition that we consider of

lower probability than renewed activity near Crater

Rock, would substantially impact Hood River and

its tributaries.

Owing to the lack of evidence of young events

in the Hood River Valley, we include areas along

Hood River in distal hazard zone DB, and estimate

a 30-year probability of inundation by lahars or

debris avalanches on the order of 1 in 300 [4]. We

consider two types of events. First, several masses

of partly altered and highly fractured rock on the

steep upper east and north flanks could generate a

debris avalanche and related lahar with a volume of

about 50 million cubic meters (65 million cubic

yards), which is roughly the volume of the largest

debris avalanche and lahar generated in the Sandy

River valley during the past 1,500 years. Second,

dome growth on the upper east or north flank could

generate lahars similar to those produced by dome

growth and collapse near Crater Rock during the

past 1,500 years. We use a computer model

calibrated from analysis of these Sandy River

events to simulate flows in the three forks of Hood

River and to define hazard zone DB [6].

Tephra-Hazard Zones

Mount Hood has produced relatively modest

tephra falls, but it is not the only source of tephra

fall in the region. For example, Mount St. Helens,

which lies 100 kilometers (60 miles) northwest of

Mount Hood has been the most prolific producer of

tephra in the Cascades during the past few

thousand years. The accompanying maps indicate

the 30-year probability of tephra fall affecting the

Mount Hood region from all Cascade sources. The

maps are based on the combined likelihood of

tephra-producing eruptions occurring at Cascade

volcanoes, the relationship between thickness of a

tephra-fall deposit and distance from its source

vent, and regional wind patterns [7]. Probability

zones extend farther east of the range because

winds blow from westerly directions most of the

time. One map shows 30-year probabilities for a

fall of one centimeter (about 0.4 inch) or more and

the other for a fall of 10 centimeters (about 4

inches) or more. The map pattern illustrates clearly

the dominating influence of Mount St. Helens.

Most of the region around Mount Hood has a

30-year probability of about 1 in 30 of receiving

one centimeter or more of tephra; for 10

centimeters or more the 30-year probability is

about 1 in 150. In detail, the probability of such

thicknesses accumulating on the flanks of Mount

Hood and a short distance beyond are somewhat

higher owing to the additional probability of

multiple tephra falls from ash clouds of pyroclastic

flows being produced at Hood during an episode of

lava-dome growth. This area of higher probability

is too small to show at the scale of the figures, but

includes the Upper Hood River Valley around

Parkdale, the upper parts of Bull Run and The

Dalles Watersheds, and the upper Sandy and

10 VOLCANO HAZARDS IN THE MOUNT HOOD REGION, OREGON
Zigzag River valleys, including Rhododendron

and Government Camp.

Regional Lava-Flow Hazard Zone

In addition to Mount Hood, other volcanoes in

the region from the Portland metropolitan area to

the east flank of the Cascade Range have erupted

during the past two million years. The most recent

such event, about 7,500 years ago, produced a

small tephra fall and a 6-kilometer-long (3.7-milelong) lava flow in the valley of the Middle Fork of

Hood River, west of Parkdale. Some older events

built large shield volcanoes composed chiefly of

lava flows, such as Mount Defiance, Lost Lake

Butte, and Clear Lake Butte. Future eruptions will

probably occur at new vents within this same broad

region, rather than at existing vents. On the basis of

the rate at which past events have buried the

landscape with lava flows, the 30-year probability

of a given site within the hazard zone (excluding

the proximal hazard zones of Mount Hood) being

buried by a lava flow is estimated to be on the order

of 1 in 100,000, a very low probability indeed [8].

How Large an Event Is Possible at

Mount Hood?

The largest magnitude event that is possible at

Mount Hood is one of very low 30-year probability

—less than 1 in 10,000—but one that would have

very serious consequences. Although preparing

for such a rare event probably is not warranted,

understanding the worst-case scenario is

nonetheless prudent. As demonstrated by Mount

St. Helens’ catastrophic eruption on May 18, 1980,

two types of large-scale hazardous events can

occur at volcanoes like Mount Hood—a largevolume debris avalanche and lahar and a large

directed blast (a type of highly mobile pyroclastic

flow). Hazard zones for a large-volume debris

avalanche and lahar are shown on the hazard map,

and the hazard zone for a directed blast is shown on

an inset map.

Debris avalanches and lahars of much greater

size than those considered in constructing hazard

zones DA and DB have occurred in the past and are

possible again at Mount Hood. The hazard map

portrays the potential extent of a debris avalanche

and lahar of about 500 million cubic meters in the

Sandy and Hood River valleys [9]. (The limited

sector of the volcano drained by the White River

precludes a landslide of this volume being confined

to the White River.) An event of this size or larger

has happened at least once in both the Sandy and

Hood River valleys during the past 500,000 years.

Such an avalanche today would entail removal of a

significant portion of the upper part of Mount

Hood. Sizable deformation driven by intrusion of

magma into the volcano (and thus a warning

period) would probably be a necessary condition to

generate such an event.

A directed blast devastated more than 500

square kilometers (200 square miles) north of

Mount St. Helens on May 18, 1980. A directed

blast of similar size and mobility at Mount Hood

could engulf a broad sector of the hazard zone

outlined in the inset figure on the hazard-zonation

map [10]. The irregular shape of the hazard zone

reflects the topography of the surrounding area.

The zone extends farther from the volcano where

elevations drop more steeply, as down the Hood

River Valley toward the Columbia River.

HAZARD FORECASTS AND

WARNINGS

Scientists recognize several signs of

impending volcanic eruptions. The upward

movement of magma, or molten rock, into a

volcano prior to an eruption causes changes that

can usually be detected by geophysical instruments

and visual observation. Swarms of small

earthquakes are generated as rocks break to make

room for rising magma or as heating of fluids

causes underground pressures to increase. Heat

from the magma can increase the temperature of

ground water and boost temperatures and steaming

from fumaroles; it can also generate small steam

explosions. The composition of gases emitted by

fumaroles can change as magma nears the surface.

Injection of magma into the volcano can cause

swelling or other types of surface deformation.

A regional seismic network operated jointly by

the U.S. Geological Survey and the Geophysics

Program at the University of Washington detects

and locates earthquakes around Mount Hood. As

many as 50 earthquakes have been recorded in one

HAZARD FORECASTS AND WARNINGS 11
year, but the events are seldom strong enough to be

felt by people on the volcano. Most occur in

swarms of several events that are located below the

summit area or the south flank at depths of less than

10 kilometers (6 miles). An increase in this level of

earthquake activity would be noticed quickly. At

monitored volcanoes similar to Mount Hood, a

notable increase in seismicity has occurred days to

months before the onset of eruptions.

Scientists have conducted other surveys aimed

at detecting precursory activity at Mount Hood. A

network of precisely surveyed points has been

remeasured several times. Gases from fumaroles

are analyzed periodically. An increase in

seismicity would prompt deployment of additional

seismometers to better locate earthquakes, the

resurvey of the deformation network to detect

slight ground movements, and the analysis of

volcanic gases. Changes in some or all of these

baselines might indicate intrusion of new magma

into the volcano.

Periods of unrest at volcanoes are usually times

of great uncertainty. Although outstanding

advances have been made in volcano monitoring

and eruption forecasting over the past few decades,

scientists are often able to make only very general

statements about the probability, type, and scale of

an impending eruption. Precursory activity can go

through accelerating and decelerating phases, and

sometimes die out without leading to eruption.

Government officials and the public must realize

the limitations in forecasting eruptions and be

prepared for such uncertainty.

Once lava-dome growth begins, scientists will

be able to detect most collapses that could generate

pyroclastic flows, lahars, and tephra falls by using

a combination of visual observations, seismic

monitoring, flow detectors, and weather radar.

Warnings that an event is in progress could then be

broadcast by emergency-response agencies. At

times when darkness or clouds hamper visual

observations, great uncertainty will probably exist

about the character and size of an event. Specific,

advance warning of individual dome-collapse

events probably would not be possible; however,

seismic or other evidence would signal the onset of

a period of dome growth, during which dome

collapses would be likely.

Some other hazardous events at Mount Hood,

including debris avalanches and related lahars,

may occur with little or no advance warning.

However, the onset of earthquakes and ground

deformation related to magma intrusion would

increase the probability of debris avalanches,

especially those of large size that have the greatest

chance of inundating developed areas.

PROTECTING OUR COMMUNITIES

AND OUR FAMILIES FROM

VOLCANO HAZARDS

Communities, businesses, and citizens need to

plan ahead to mitigate the effects of future

eruptions, debris avalanches, and lahars. Longterm mitigation includes using information about

volcano hazards when making decisions about land

use and siting of critical facilities. Development

should avoid areas judged to have an unacceptably

high risk or be planned to reduce the level of risk.

For example, a real-estate development along a

valley could set aside low-lying areas at greatest

risk from lahars for open space or recreation, and

use valley walls or high terraces for houses and

businesses.

When volcanoes erupt or threaten to erupt,

emergency responses are needed. Such responses

will be most effective if citizens and public

officials have an understanding of volcano hazards

and have planned the actions needed to protect

communities. Mount Hood has a settlement

(Government Camp), major highways (US 26 and

OR 35), and popular tourist and recreation areas

(Timberline Lodge and Mount Hood Meadows Ski

Area) on its flanks. Furthermore, several thousand

people live within 35 kilometers (22 miles) of

Mount Hood along the channels and flood plains of

rivers that drain the volcano. Such areas are at

greatest risk from lahars and debris avalanches and

could be inundated within one hour of an event’s

onset.

Because an eruption can occur within days to

months of the first precursory activity and because

some hazardous events can occur without warning,

suitable emergency plans should be made before

hand. Public officials need to consider issues such

as public education, communications, and

evacuations. Emergency plans already developed

for floods may apply, with modifications, to

hazards from lahars.

12 VOLCANO HAZARDS IN THE MOUNT HOOD REGION, OREGON
Businesses and individuals should also make

plans to respond to volcano emergencies. Planning

is prudent because once an emergency begins,

public resources can often be overwhelmed, and

citizens may need to provide for themselves and

make informed decisions. The Red Cross

recommends numerous items that should be kept in

homes, cars, and businesses for many types of

emergencies that are much more probable than a

volcanic eruption. A map showing the shortest

route to high ground will also be helpful.

The most important additional item is

knowledge about volcano hazards and, especially,

a plan of action based on the relative safety of areas

around home, school, and work. Lahars pose the

biggest threat to people living in valleys that drain

Mount Hood. The best strategy for avoiding a lahar

is to move to the highest possible ground. A safe

height above river channels depends on many

factors including size of the lahar, distance from

the volcano, and shape of the valley. For areas

beyond the proximal hazard zone, few lahars will

rise more than 30 meters (100 feet) above river

level. Be aware that an approaching lahar will

cause a loud roaring noise like a gradually

approaching jet plane. Once audible, a lahar may

be only a few minutes away.

REFERENCES AND SUGGESTED

ADDITIONAL READING

Brantley, S.R., and Scott, W.E., 1993, The danger of

collapsing lava domes: Lessons for Mount Hood,

Oregon: Earthquakes and Volcanoes, v. 24, no. 6, p. 244-

269.

Cameron, K.A., and Pringle, P.T., 1987, A detailed

chronology of the most recent major eruptive period at

Mount Hood, Oregon: Geological Society of America

Bulletin, v. 99, p. 845-851.

Casadevall, T.J., 1994, Volcanic Ash and Aviation Safety:

Proceedings of the First International Symposium on

Volcanic Ash and Aviation Safety: U.S. Geological

Survey Bulletin 2047, 450 p.

Crandell, D.R., 1980, Recent eruptive history of Mount

Hood, Oregon, and potential hazards from future

eruptions: U.S. Geological Survey Bulletin 1492, 81 p.

REFERENCES AND SUGGESTED ADDITIONAL READING 13

END NOTES

[1] The geologic data upon which this report is based comes from reports and maps published during the past 35 years, as well as

from our ongoing investigations of Mount Hood. Sources include: D.E. Trimble, 1963 , Geology of Portland, Oregon, and

adjacent areas: U.S. Geological Survey Bulletin 1119; W.S. Wise, 1969, Geology and petrology of the Mt. Hood area: A study of

High Cascade volcanism: Geological Society of America Bulletin, v. 80, p. 969-1,006; D.R. Crandell, 1980, Recent eruptive

history of Mount Hood, Oregon, and potential hazards from future eruptions: U.S. Geological Survey Bulletin 1492; K.A.

Cameron and P.T. Pringle, 1986, Postglacial lahars of the Sandy River basin, Mount Hood, Oregon: Northwest Science, v. 60, p.

225-237; K.A. Cameron and P.T. Pringle, 1987, A detailed chronology of the most recent major eruptive period at Mount Hood,

Oregon: Geological Society of America Bulletin, v. 99, p. 845-851; and unpublished work by former and present U.S. Geological

Survey staff (J.E. Costa, C.A. Gardner, J.J. Major, T.C. Pierson, W.E. Scott, D.R. Sherrod, R.I Tilling, and J.W. Vallance).

[2] The hazard map that accompanies this report differs in several respects from the map produced by D. R. Crandell in his 1980

hazard map of Mount Hood (see REFERENCES AND SUGGESTED ADDITIONAL READING). In part this reflects

differences in the amount of detail that can be shown at the different scales of the maps. For example the larger-scale 1980 map

subdivides what on this map is distal hazard zone DA along Sandy River into two zones. Other differences reflect new

information and interpretations and the inclusion of low probability-high consequence events.

[3] Approximate travel times of lahars from their initiation high on Mount Hood comes from an analysis of historical lahars at

volcanoes in the Cascades and elsewhere. The 30-minute time used to separate proximal and distal zones and travel-time marks

shown along distal valleys is a minimum estimate for flows of the size used to define zones DA and DB. The travel time for an

event of larger size could be shorter.
14 VOLCANO HAZARDS IN THE MOUNT HOOD REGION, OREGON

[4] We estimate the probability of future events by determining their average frequency of occurrence over a given time period

from the geologic and historical record of past events. We assume that the average frequency of future events should be about the

same as the average frequency of past events, which we determine by dividing the time period of record by the number of events.

The probability or likelihood of an event in the future derives from this average frequency. For instance, much of hazard zone PA

was affected by at least two episodes of eruptive activity centered near Crater Rock during the past 1,500 years, an average

frequency of one episode every 750 years, or roughly one episode every 500 to 1,000 years. For relatively rare events such as

volcanic eruptions, we commonly report probabilities for a time interval of 30 years rather than one year, because the longer time

perspective pertains to such human milestones as the span of a generation, the term of a home mortgage, or a substantial portion of

the useful life of public structures such as schools and hospitals. The probability is about 1 in 15 to 1 in 30 that an episode of lavadome growth will commence near Crater Rock in the next 30 years. Zone PB has not been extensively affected during the past

10,000 years, but has on numerous occasions between 10,000 and 100,000 years ago. Therefore we estimate the 30-year

probability of lava-dome growth shifting to the east, north, or west flank and extensively affecting zone PB to be on the order of 1

in 300. For comparison, the 30-year probability of a given house being damaged by fire in the United States is about 1 in 90. We

caution that these probabilities are based solely on the long-term behavior of the volcano and that they will rise dramatically with

any sign of increased restlessness at Mount Hood.

[5] Essentially all of hazard zone DA along White River has been inundated during both eruptive episodes of the past 1,500 years

and therefore is assigned the same 30-year probability as that for renewed dome growth near Crater Rock, 1 in 15 to 1 in 30.

[6] We use one-dimensional hydraulic flood-routing models, calibrated for lahar flows, to predict lahar depth and extent in

downstream valleys. Two different models were used for this analysis: a USGS model called HYDRAUX and a National

Weather Service model called DAMBRK. Both models are fully dynamic, one-dimensional, unsteady-state, finite-difference

mathematical models developed to simulate floods, but were calibrated with data from actual lahars at Mount Hood and other

Cascade volcanoes in order to get realistic values for flow resistance. We assume a debris avalanche of 50 million cubic meters,

which would entail the removal of a mass measuring about 800 meters long 400 meters wide and 150 meters thick (2600 by 1300

by 500 feet) from the upper flank. In running the model, we assume that this initial volume of 50 million cubic meters moves past

the first upstream valley cross section as a triangular-shaped hydrograph with a peak discharge of 70,000 cubic meters per second

and a duration of 0.4 hour.

[7] Maps were generated by computer program developed by R.P. Hoblitt (U.S. Geological Survey, Cascades Volcano

Observatory, written communication, 1996).

[8] Distribution of volcanic vents is modified from Sherrod and Smith (1989), U.S. Geological Survey Open-File Report 89-14,

and Sherrod and Scott (1995), U.S. Geological Survey Open-File Report 95-219. Hazard-zone boundary was made by assuming

that future vents could be located anywhere within the broad region defined by vents of the past two million years and that lava

flows can extend up to 15 kilometers (9 miles) from vents, where permitted by topography.

[9] Estimate of extent of 500 million cubic meter debris avalanche and lahar was obtained by using relationship of lahar volume to

cross-sectional area of flow that was developed by R.M. Iverson and described in W.E. Scott and others, 1995, Volcano hazards in

the Mount Adams region, Washington: U.S. Geological Survey Open-File Report 95-492. The zone on this map represents the

projection of a cross-sectional area of about 33,000 square meters (355,000 square feet) down the Sandy and Hood River valleys.

A volume of 500 million cubic meters was selected to represent a large-volume debris avalanche based on the following

comparison to the 1980 debris avalanche at Mount St. Helens. The 1980 avalanche removed about 2,300 million cubic meters

from an area on the north flank that had a surface slope of 0.5. The avalanche removed about 25% of the cone’s volume above the

altitude at which the failure plane intersected the lower north flank. In contrast to St. Helens, Mount Hood has much more concave

flanks so that slopes steeper than 0.5 are chiefly confined to the upper part of the volcano except in the heads of a few canyons on

the west flank. Thus, there is less volume available in the upper edifice from which to generate a large debris avalanche. The 25%

value from Mount St. Helens applied to Hood yields potential avalanche volumes of several hundred million cubic meters from

south and east flanks, 500 million cubic meters from the north flank, and up to 1000 million cubic meters or more from the steep

valleys on the west flank. Structural (presence of a pre-Mount Hood edifice forming much of the steep valley heads) and

topographic (narrowness of west flank canyons would probably result in a large failure not being restricted to a single drainage)

considerations probably reduce the potential volume on the west flank to less than 1000 million cubic meters. Thus we regard 500

million cubic meters as a reasonable maximum volume for a debris avalanche but caution that, under extreme conditions,

avalanches up to twice as large might originate on the west flank.

[10] A ratio of height of drop (measured from the summit of Mount Hood) to length of runout of 0.09, similar to that of the 1980

directed blast at Mount St. Helens, was used to estimate the potential extent of a directed blast at Mount Hood.