The Alaska Volcano Observatory (AVO) was established in 1988 to carry out volcano monitoring,

eruption notification, and volcanic hazards assessments in Alaska. The cooperating agencies of

AVO are the U.S. Geological Survey (USGS), the University of Alaska Fairbanks Geophysical

Institute (UAFGI), and the Alaska Division of Geological and Geophysical Surveys (ADGGS).

AVO also plays a key role in notification and tracking of eruptions on the Kamchatka Peninsula of

Russia as part of a formal working relationship with the Kamchatkan Volcanic Eruptions Response

Team (KVERT).

Cover photo: Iliamna Volcano and Umbrella Glacier as viewed from the valley of

West Glacier Creek. View is toward the northeast.
Preliminary Volcano-Hazard 

Assessment for Iliamna Volcano, 

Alaska

by Christopher F. Waythomas1 and Thomas P. Miller1

Open-File Report 99-373 

1999

This report is preliminary and subject to revision as new data become available. Any use of trade, product or firm 

names is for descriptive purposes only and does not imply endorsement by the U.S. Geological Survey

U.S. DEPARTMENT OF THE INTERIOR

U.S. GEOLOGICAL SURVEY

1Alaska Volcano Observatory, Anchorage, Alaska
U.S. DEPARTMENT OF THE INTERIOR

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

For additional information: Copies of this report may be purchased from:

U.S. Geological Survey U.S. Geological Survey

Alaska Volcano Observatory Branch of Information Services

4200 University Drive Box 25286

Anchorage, AK 99508 Denver, CO 80225-0286

http://www.avo.alaska.edu
CONTENTS

Summary of hazards at Iliamna Volcano. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Purpose and scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Physical setting of Iliamna Volcano . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Prehistoric eruptive history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Historical eruptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Hazardous phenomena at Iliamna Volcano . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Volcanic hazards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Volcanic ash clouds. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Volcanic ash fallout and volcanic bombs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Lahars, lahar-runout flows, and floods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Debris avalanche . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Pyroclastic flow and surge. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Directed blasts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Volcanic gases. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Lava flow. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Event frequency and risk at Iliamna Volcano . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Hazard warning and mitigation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

References cited. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

ILLUSTRATIONS

PLATE

1. Preliminary volcano hazard zonation map for Iliamna Volcano, Alaska [In pocket]

FIGURES

1. Map showing location of Iliamna Volcano and other volcanoes in the Aleutian Arc 3 

2. Map showing location of Iliamna Volcano in Lake Clark National Park and 

Wilderness and place names mentioned in text . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

3. Photograph of Iliamna Volcano as viewed from the Johnson River Valley 

toward the southwest . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

4. Generalized geologic map of Iliamna Volcano . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

5. Diagram showing known history of eruptive activity and other volcanic events 

at Iliamna Volcano for the past 10,000 years . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

6. Photograph of Iliamna Volcano and distinct steam plume trailing westward . . . . . . 10 

7. Simplified sketch of a stratovolcano and associated hazardous phenomena . . . . . . . 12

8. Sketch maps of average wind direction and likely travel paths for volcanic ash

 clouds from Iliamna Volcano . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
9. Map showing the approximate extent of volcanic ash fallout for small to 

moderate eruptions of Iliamna Volcano . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

10. Hazard zonation map for lahars, lahar-runout flows, and floods . . . . . . . . . . . . . . . . 16

11. Photograph of noncohesive lahar deposit exposed along the coastline near the 

mouth of the Red River . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 

12. Photograph of the southeast flank of Iliamna Volcano showing debris-avalanche 

deposits from 1997, the fumarole zone near the summit, and older avalanche 

scar at the head of Red Glacier. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

13. Hazard zonation map for debris avalanche. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

14. Topographic map of summit area of Iliamna Volcano. . . . . . . . . . . . . . . . . . . . . . . . 20

15. Photograph of the north side of Red Glacier showing debris-avalanche deposits 

formed in 1997 and an older slightly more extensive deposit that is probably a 

few hundred years old or less . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

16. Hazard zonation map for pyroclastic flow and surge . . . . . . . . . . . . . . . . . . . . . . . . . 23

17. Hazard zonation map for directed blast . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

18. Map showing location of seismic-monitoring stations on Iliamna Volcano . . . . . . . 27

19. Chart showing level of concern color code for volcanic eruptions . . . . . . . . . . . . . . 28

CONVERSION FACTORS

VERTICAL DATUM

Sea level: In this report, “sea level” refers to the National Geodetic Vertical Datum of 1929--A geodetic 

datum derived from a general adjustsealvelment of the first-order level nets of the United States and Canada, formerly called Sea Level Datum of 1929. In the area of this report, datum is mean lower low water.

Multiply By To obtain

meter (m) 3.281 foot

kilometer (km) 0.6214 mile

cubic meter (m3) 35.3 cubic foot

cubic kilometer (km3

) 0.2399 cubic mile

degree Celsius (°C) °F = 1.8 x °C + 32 degree Fahrenheit (°F)
Summary of Hazards at Iliamna Volcano 1

Preliminary Volcano-Hazard Assessment for 

Iliamna Volcano, Alaska

By Christopher F. Waythomas and Thomas P. Miller 

SUMMARY OF HAZARDS AT ILIAMNA VOLCANO

 Iliamna Volcano is a 3,053-meter-high, ice- and snow-covered stratovolcano in the southwestern Cook Inlet region about 225 kilometers southwest of Anchorage and about 100 kilometers northwest of Homer. Historical eruptions of Iliamna Volcano have not been positively

documented; however, the volcano regularly emits steam and gas, and small, shallow earthquakes

are often detected beneath the summit area. The most recent eruptions of the volcano occurred

about 300 years ago, and possibly as recently as 90-140 years ago. Prehistoric eruptions have generated plumes of volcanic ash, pyroclastic flows, and lahars that extended to the volcano flanks

and beyond. Rock avalanches from the summit area have occurred numerous times in the past.

These avalanches flowed several kilometers down the flanks and at least two large avalanches

transformed to cohesive lahars. The number and distribution of known volcanic ash deposits from

Iliamna Volcano indicate that volcanic ash clouds from prehistoric eruptions were significantly

less voluminous and probably less common relative to ash clouds generated by eruptions of other

Cook Inlet volcanoes. Plumes of volcanic ash from Iliamna Volcano would be a major hazard to

jet aircraft using Anchorage International Airport and other local airports, and depending on wind

direction, could drift at least as far as the Kenai Peninsula and beyond. Ashfall from future eruptions could disrupt oil and gas operations and shipping activities in Cook Inlet. Because Iliamna

Volcano has not erupted for several hundred years, a future eruption could involve significant

amounts of ice and snow that could lead to the formation of large lahars and downstream flooding. The greatest hazards in order of importance are described below and shown on plate 1.

• Volcanic ash clouds

Clouds of fine volcanic ash will drift away from the volcano with the wind. These ash clouds are a hazard to all aircraft downwind. Airborne volcanic ash can drift thousands of kilometers from its source

volcano. Ash from future eruptions could interfere with air travel especially during a large sustained

eruption.

• Volcanic ash fallout

Ash fallout from prehistoric eruptions of Iliamna Volcano reached parts of south-central Alaska where

accumulations of several millimeters or more of fine ash are known. Fine ash is a nuisance and may

cause respiratory problems in some humans and animals. Heavy ashfall can disrupt many human

activities and may interfere with power generation, affect visibility, and could damage electrical components and equipment. Resuspension of ash by wind could extend the unpleasant effects of ash fallout.

THE ALASKA VOLCANO HAZARD ASSESSMENT SERIES 

 This report is part of a series of volcano hazard assessments being prepared by the Alaska Volcano Observatory. The reports are intended to describe the nature of volcanic hazards at Alaska 

volcanoes and show the extent of hazardous areas with maps, photographs, and other appropriate 

illustrations. The reports are preliminary and subject to revision as new data become available.
2 Preliminary Volcano-Hazard Assessment for Iliamna Volcano, Alaska

• Lahars and floods

Hot volcanic debris interacts with snow and ice to form fast-moving slurries of water, mud, rocks, and

sand. These flows, called lahars, are expected to form during most future eruptions of Iliamna. Lahars

tend to follow streams and drainageways, and will probably flow to the coastline. Lahars could be hazardous to people and facilities in major valleys during an eruption.

•Debris avalanches

A debris avalanche is a rapidly moving mass of solid or incoherent blocks, boulders, and gravel initiated by a large-scale failure of the volcano flank. Multiple prehistoric and several small historical

debris avalanches have occurred at Iliamna Volcano but did not extend very far beyond the base of the

volcano. Large debris avalanches could form that may involve a significant amount of the volcanic

edifice. A major flank collapse could transform to a lahar that would inundate streams and drainageways on the distal slopes of the volcano.

• Pyroclastic flow and surge

Hot material expelled from the volcano may travel rapidly down the volcano flanks as flows of volcanic debris called pyroclastic flows and surges. These flows will primarily travel along major valleys

and are not expected to reach the coast. They pose little hazard except to people or facilities in major

valleys during an eruption.

Other hazardous phenomena that may occur but are uncommon during typical eruptions of

Iliamna Volcano include:

• Directed blasts

A directed blast is a lateral explosion of the volcano caused by rapid release of internal pressure commonly caused by a slope failure or landslide. Directed blasts are rare volcanic events, and evidence for

a directed blast has not been identified at Iliamna Volcano.

• Volcanic gases

Some volcanoes emit gases in concentrations that are harmful to humans. However, the frequently

windy conditions, lack of closed depressions that could collect gases, and steep terrain at Iliamna prevent the buildup of volcanic gases. Thus, the hazard from volcanic gases is minimal. 

• Lava flow

Streams of molten rock (lava) may extend a few kilometers from the active vent. Lava flows move

slowly, only a few tens of meters per hour, and pose little hazard to humans. Some lava flows may

develop steep, blocky fronts and avalanching of blocks could be hazardous to someone close to the

flow front.

SUGGESTIONS FOR READING THIS REPORT

Readers who want a brief overview of the hazards at Iliamna Volcano are encouraged to read the summary and consult plate 1 and the illustrations. Individual sections of this report provide a slightly more 

comprehensive overview of the various hazards at Iliamna Volcano. A glossary of geologic terms is 

included and additional information about Iliamna Volcano can be obtained by consulting the references cited at the end of this report.
Introduction 3

INTRODUCTION

Iliamna Volcano is an active snow- and

ice-covered stratovolcano located in the southwestern Cook Inlet area (fig. 1). Apparently,

the volcano has not erupted in historic time

(about the past 150 years) and we know of no

verifiable reports of historical eruptive activity.

However, Iliamna exhibits all the signs of an

active volcano, including the emission of

steam and gas from a prominent zone of fumaroles near the summit, and small, shallow

earthquakes that intermittently occur beneath

the volcano. Iliamna Volcano is located within

a few hundred kilometers of the major population, commerce, and industrial centers of

south-central Alaska, and future eruptions,

should they be explosive, could pose a hazard

to the citizens and economy of the region. The

geological record of eruptive activity at

Iliamna Volcano indicates that eruptions are

generally effusive in nature and occur infrequently. Large, rare eruptions could generate

substantial volumes of volcanic ash that may

rise to more than 12,000 meters above sea

level. Clouds of volcanic ash, if they are produced, would be hazardous to jet aircraft in the

Cook Inlet region and for thousands of kilometers downwind from the volcano.

Most of the area around Iliamna Volcano is

within Lake Clark National Park and Wilderness (fig. 2) and is uninhabited. Life and property are not at risk in the immediate vicinity of

the volcano.

Purpose and Scope

This report summarizes the principal volcanic hazards associated with eruptions of

Iliamna Volcano. Hazardous volcanic phenomena that have occurred on the volcano as well

as distal effects of eruptions are described. The

present status of monitoring efforts to detect

volcanic unrest and the procedure for eruption

notification and dissemination of information

also are presented. A series of maps and illustrations that show potentially hazardous areas

are included. A glossary of geologic terms is at

the end of the report. Terms defined in the

glossary are italicized at their first appearance

in the text.

▲

0 200 400 km

0 200 400 mi

40 41

39

38 37 36 35 34 33

32 31 28

30 29

27

24

26 25

23 21

20

18

19 17

16

15

14

12

13

11

10 8

9

7 6

3

2

1

5

22

ILIAMNA

ALASKA

Kodiak

Kiska Island

Island

Atka

Island Unalaska

Island

Anchorage

150 ̊ W

138 ̊ W

158 ̊ W

170 ̊ W

56 ̊ 

60 ̊ 

178 ̊ E

ALEUTIAN

VOLCANIC

ARC

United States

Canada

FOX ISLANDS

52 ̊ 

64 ̊ 

Ak.

Pen.

 1. Wrangell

 2. Spurr

 3. Redoubt

 Iliamna

 5. Augustine

 6. Katmai

 7. Novarupta

 8. Trident

9. Mageik

10. Martin

11. Peulik

12. Ukinrek

13. Chiginagak

14. Aniakchak

15. Veniaminof

16. Pavlof

17. Dutton

18. Isanotski

19. Shishaldin

20. Fisher

21. Westdahl

 22. Akutan

23. Makushin

24. Bogoslof

25. Okmok

26. Vsevidof

27. Kagamil

28. Carlisle

29. Cleveland

30. Yunaska

31. Amukta

32. Seguam

33. Korovin

34. Kasatochi

35. Great Sitkin

36. Kanaga

37. Tanaga

38. Gareloi

39. Cerberus

40. Little Sitkin

41. Kiska

BERING SEA

GULF OF ALASKA

PACIFIC OCEAN

Cook

Inlet

Figure 1. Location of Iliamna Volcano and other volcanoes in the Aleutian Arc. All of these volcanoes have 

exhibited some kind of eruptive activity in the past 200 years.
4 Preliminary Volcano-Hazard Assessment for Iliamna Volcano, Alaska

LAKE CLARK

NATIONAL PARK

AND WILDERNESS

BOUNDARY

(approximate)

100

100

100

100

100

500

500

500

500

500

500

500

500

500

500

500

1000

1000

1000

1000

1500

1500

2000

2500

153 ̊ 153 ̊30'

60 ̊

15'

60 ̊ H/L = 0.1

H/L = 0.3

(residential area)

Snug Harbor

Cannery (abandoned)

0 5

0 5 10 KILOMETERS

MILES

CONTOUR INTERVAL 500 FEET

Red 

Glacier

COOK INLET

CHINITNA BAY

TUXEDNI BAY

Crescent River

Tuxedni Glacier

Tuxedni 

River

Red River

Johnson

River

Creek

West

Glacier

ILIAMNA

VOLCANO

CHISIK

ISLAND

Red Creek

Silver Salmon

Glacier

Lateral

Umbrella

Glacier

airstrip

Figure 2. Location of Iliamna Volcano in Lake Clark National Park and Wilderness and place names 

mentioned in text. Red dashed lines indicate two estimates of the proximal hazard zone based on H/L = 

0.3 and H/L = 0.1 where H is the height of the volcano summit and L is the runout length of pyroclastic 

flows, lahars, and debris avalanches of comparable volume. Typical H/L values for other volcanoes 

range from 0.1 to 0.3.
Physical Setting of Iliamna Volcano 5

Acknowledgments

David Coray and the staff of the Silver

Salmon Lodge are gratefully acknowledged

for sharing their knowledge of the local area

and for logistical support during field activities. We thank J.E. Begét, R.B. Moore, and

T.E.C. Keith for review comments.

PHYSICAL SETTING OF ILIAMNA

VOLCANO

Iliamna Volcano (fig. 3) is a cone-shaped

stratovolcano with an extensive cover of glacier ice and snow. The volcano is 3,053 meters

high and several 5-to-10-kilometer-long valley

glaciers extend from the upper flanks of the

volcano (fig. 2). The volume of glacier ice on

Iliamna Volcano above 2,000 meters is about 1

cubic kilometer. The four largest glaciers have

a combined ice volume of about 15 cubic kilometers, which is about three times the total

volume of ice and perennial snow on Mount

Rainier, Washington (D.C. Trabant, USGS,

written commun., 1999). 

Iliamna Volcano consists primarily of a

stratified assemblage of andesite lava flows

and minor lahar, pyroclastic flow, and debrisavalanche deposits. A prominent zone of sulfurous fumaroles (fig. 3) is located on the south

side of the volcano near the summit; steam

from the fumaroles is common and is often

visible on calm cloudless days.

Iliamna Volcano is located in Lake Clark

National Park and Wilderness and is only

occasionally visited by people. Because the

volcano is located in a national park and many

of its drainages are important sport fisheries,

recreational use of bottomland areas near or on

the volcano is common in summer. Residential

developments along the Cook Inlet coastline

could be at risk from ashfall, should a large but

rare explosive eruption occur.

PRE-HISTORIC ERUPTIVE HISTORY

The modern volcanic edifice of Iliamna

Volcano consists of a stratified assemblage of

andesitic lava flows, lahar, pyroclastic-flow,

and debris-avalanche deposits. These deposits

are depicted on a preliminary geologic map of

the volcano (fig. 4) that was completed during

geologic studies in 1995-1997 (Miller and others, 1996). The oldest lava flows are of early

Pleistocene age (ca. 1 Ma) and were erupted

from a vent or vent complex associated with

the present edifice. Much of the volcano is

concealed by an extensive cover of ice and

snow, and consequently, little is known about

the eruptive history prior to about 7,000 years

before present (B.P.). At various times during

the Quaternary period (about the past 1.8 million years), Iliamna Volcano and the surrounding mountains were shrouded by glacier ice,

South Twin

fumarole zone Iliamna Volcano

Figure 3. Iliamna Volcano as viewed from the Johnson River Valley toward the southwest. Also shown is 

the fumarole zone near the summit and the South Twin volcanic center.
6 Preliminary Volcano-Hazard Assessment for Iliamna Volcano, Alaska

Jg

Jg

Jg

Qda V

152o30' 153o

60o

59o

45'

UMBRELLA GLACIER

MIDDLE GLACIER

EAST

GLACIER

RED GLACIER

LATERAL GLACIER

JOHNSON

GLACIER

TUXEDNI

GLACIER

Johnson

River

East Glacier Creek

West Glacier Creek

Contour Interval 1000 Feet

BRUIN

BAY

FAULT

Ql

Ql

0 1 2 3 4 5 MILES

0123 4 8 KILOMETERS 5 6 7

Mt. Nick

Iliamna

Volcano

Hickerson Lake

Qmh

Qmh

Qmh

Qmh

Qmh

Qmh

Qcaf

Qda

Qra

Qda

Qjgd

Qjgd

Qjgdt

Jg

Jg Jg

Jg

Jg

Jg

Jg

Jg

Jg

Qtv Qtv

Qtv

Qtv

Qtv Qcaf

V

Qtv

Js

Js

Js Js

Js

Js

Js

Js

Js

Js

Js

Qcaf

Qcaf

Qcaf

Qcaf

Qcaf

Qcaf

Qiv

Qiv

Qiv

Qiv

Qiv

Qiv

Qtv

Qiv

Jm

Jm

Jm

60o

15'

1000

2000

3000

2000

2000

1000

2000

1000

2000

2000

1000

1000

2000

2000 1000

2000

2000

3000

4000

4000

4000

5000

6000

7000

8000

9000

5000

6000

4000

3000

2000

1000

2000 2000

2000

1000

2000

3000

4000

1000

2000

2000

3000

3000

4000

4000

1000

3000

4000

2000

3000

4000

4000

4000

3000

5000

5000

4000 3000

3000

1000

5000

6000

Figure 4. Generalized geologic map of Iliamna Volcano (modified from Miller and others, 1996). Extent 

of debris cover on modern glaciers not shown.
Pre-Historic Eruptive History 7

Qcaf

Qda

Qra

Qmh

Ql

SURFICIAL DEPOSITS

VOLCANIC ROCKS

Qjgd

Qjgdt

Qiv

Qtv

Jg

NON-VOLCANIC ROCKS

Js

Jm

Q

U

A

T

E

R

N

A

R

Y

J

U

R

A

S

S

I

C

MAP UNITS

V

EXPLANATION OF SYMBOLS

Colluvium, alluvium, lacustrine, and floodplain deposits of Holcene age.

Debris-avalanche deposits from Iliamna Volcano of late Holocene age.

Rock-avalanche deposits.

Glacial deposits of Holocene age. Primarily ice-stagnation drift,

outwash, and end moraine.

Lahar deposits of late Holocene age.

Johnson Glacier dome tephra deposits.

Johnson Glacier dome complex.

Lava flows from the present Iliamna volcanic center.

Lava flows from the North and South Twin volcanic center.

Granitic rocks of the Aleutian-Alaska Range batholith.

Sedimentary rocks, undifferentiated.

Metamorphic rocks, undifferentiated.

High-angle reverse fault; dotted where concealed.

Collapse scar

Major vent, location approximate

Former vent

Fumarole

Flow direction of lava flows

Figure 4. Continued.
8 Preliminary Volcano-Hazard Assessment for Iliamna Volcano, Alaska

and much of Cook Inlet and most of southern

Alaska lay beneath a thick and areally extensive ice sheet. Early eruptions of Iliamna Volcano may have been subglacial which would

limit the preservation of proximal volcanic

deposits and volcanic ash, therefore making it

difficult to determine the early eruptive history. Because Quaternary glaciation was episodic, proximal volcanic deposits generated

during times of relatively restricted ice cover

may have been removed by erosion during

times of glacier expansion. 

Prehistoric eruptions of Iliamna Volcano

have generally been from a central vent associated with the present edifice, although an older

vent existed in the vicinity of South Twin peak

(fig. 3). Flank vents present on the south side

of Iliamna Volcano and the southeast side of

South Twin experienced dome-building eruptions of unknown size during the Pleistocene

epoch, but because they are inaccessible, they

have not been studied in detail. An assemblage

of nested lava domes on the northeast side of

Iliamna near Mt. Nick (fig. 3) was the site of

eruptive activity 4,000 to 5,000 years ago.

The history of eruptive activity since about

7,000 yr B.P. (fig. 5) is determined by studying

the stratigraphic relations of volcanic deposits

exposed in riverbanks and gullies on the flanks

of the volcano and by analyzing volcanic ash

deposits preserved in areas beyond the volcano. In general, volcanic activity is episodic

and long periods of inactivity are punctuated

by periods of rapid deposition of volcanic sediment during eruptions. Vegetation growth and

soil development may occur during non-eruptive periods which then are buried by volcanic

deposits during subsequent eruptions. Over

time, a stacked, vertical sequence of buried

soils and vegetation, volcanic deposits, and

volcanic ash develops. Because the stratigraphic sequence evolves in this manner over

many thousands of years, it is possible to

determine the eruptive history of the volcano

by dating buried soils and plant remains associated with volcanic deposits. This methodology has been applied at Iliamna Volcano and is

the basis for deciphering the eruptive history

of the volcano.

At least two fine-grained volcanic-ash

deposits from Iliamna Volcano have been identified on the Kenai Peninsula. These ash layers

are within a vertical sequence of peat and volcanic-ash deposits from other Cook Inlet volcanoes and have been radiocarbon dated and

geochemically correlated with a pumiceous

lapilli tephra found on the proximal flanks of

Iliamna Volcano. A discontinuous mantle of

pumiceous lapilli tephra is common on bedrock pinnacles that extend above modern glaciers northeast of Iliamna Volcano. This tephra

deposit is found as much as 50 kilometers

northeast of the present summit of Iliamna

Volcano. The pumiceous lapilli tephra and fine

ash layer on the Kenai Peninsula are evidence

for a large plinian eruption of Iliamna Volcano

from a vent on the northeast upper flank of the

volcano about 4,000 yr B.P.

The second of the two fine-grained ash layers found on the Kenai Peninsula is geochemically similar to the proximal lapilli tephra and

dates to about 7,000 yr B.P. This ash bed is

evidence for an older plinian(?) eruption of

Iliamna Volcano but so far no associated volcanic deposits have been identified on the

proximal flanks of the volcano.

During the past 4,000 years, many of the

major valleys on the volcano were inundated

by volcanic mudflows called lahars (fig. 5). In

most areas, the lahars were generated by the

dynamic interaction of pyroclastic flows with

ice and snow and are the direct products of

eruptive activity. However, a few lahars appear

to have been generated from large-scale collapses of a discrete part of the volcanic cone.

These lahars are slightly clay rich and contain

an abundance of hydrothermally altered rock

debris. Such lahars are called cohesive lahars

and apparently, they may develop without any

relation to an eruption.
Pre-Historic Eruptive History 9

0

500

1000

2000

3000

4000

5000

6000

7000

EXPLOSIVE ERUPTION

EXPLOSIVE(?) ERUPTION(~300 yr B.P.)

EXPLOSIVE(?) ERUPTION(~140 yr B.P.) { Johnson River Valley

Johnson River Valley

Red River Valley

EXPLOSIVE(?) ERUPTION (>2300 yr B.P.)

FLANK COLLAPSE & ERUPTION(?) (~2300 yr B.P.) W. Glacier Creek

W. Glacier Creek

Johnson River Valley.

Tephra fallout on 

Kenai Peninsula

EXPLOSIVE ERUPTION Tephra fallout on 

Kenai Peninsula

Ash emission & tephra fallout

Pyroclastic flow

Noncohesive lahar

Cohesive lahar

Lava dome

Debris avalanche

AGE, IN

YEARS B.P. TYPE OF VOLCANIC ACTIVITY AREAS AFFECTED

EXPLANATION

FLANK COLLAPSE & ERUPTION(?) (~90 yr B.P.)

Figure 5. Known history of eruptive activity and other volcanic events at Iliamna Volcano for the past 

10,000 years.
10 Preliminary Volcano-Hazard Assessment for Iliamna Volcano, Alaska

HISTORICAL ERUPTIONS

Iliamna Volcano has had no documented

historical eruptions. References to historical

eruptions in the 1860’s and 1870’s are poorly

documented and reports of “smoke” issuing

from the volcano (fig. 6) do not clearly indicate eruptive activity or emission of volcanic

ash (Miller and others, 1998). Two lahars inundated the lower Johnson River Valley (fig. 2)

in the past 150 years and one of these was

probably related to an eruption. Another lahar

flowed down Red River to the coast about 300

years ago. This lahar was initiated by a pyroclastic eruption of Iliamna Volcano and is the

best known evidence for recent eruptive activity.

Elevated levels of seismic activity beneath

Iliamna Volcano were recorded by the Alaska

Volcano Observatory (AVO) in 1996 (McGimsey and Neal, 1997). Earthquakes as large as

magnitude 3.2 and as many as 82 earthquakes

per day were recorded between May and September 1996. During this period, an increased

flux of carbon dioxide and sulfur dioxide gas

was detected over the volcano. 

Figure 6. Iliamna Volcano and distinct steam plume trailing westward. Distant observations and photographs of the volcano like this one, taken near Seldovia, resulted in several false reports of eruptive activity. 

A vigorous steam plume (composed primarily of water vapor) is often visible on clear calm days and does 

not indicate that an eruption is imminent or in progress. Date and name of photographer unknown but the 

photograph was probably taken sometime before 1950.
Hazardous Phenomena at Iliamna Volcano 11

HAZARDOUS PHENOMENA AT 

ILIAMNA VOLCANO

A volcanic hazard (fig. 7) is any volcanic

phenomenon that is potentially threatening to

life or property. In general, hazards associated

with volcanic eruptions are grouped as proximal or distal depending on the areas most

likely to be affected by specific volcanic phenomena. The classification of hazardous phenomena at Iliamna Volcano as proximal or

distal is an approximate classification because

the extent of a particular hazard is in part

related to the scale of the eruption. Thus, a

large eruption may cause some phenomena to

affect areas well beyond the volcano, whereas

during a smaller eruption, the same phenomena may only affect areas in the immediate

vicinity of the volcano. 

Proximal hazards are those phenomena

that occur in the immediate vicinity of the volcano, typically within a few tens of kilometers

of the active vent. The proximal hazard zone is

delineated by the ratio of the volcano summit

height (H) to the runout length (L) of onground hazardous phenomena such as pyroclastic flows, debris avalanches, and lahars.

Typical H/L values range from 0.1 to 0.3. Life

and property within the proximal hazard zone

may be at risk during eruptions depending on

the eruptive style and duration of activity.

Anyone in this zone would have little or no

time to escape from the area in the advent of an

eruption. Because most of the area around

Iliamna Volcano is uninhabited, only the occasional visitor is at risk from the various proximal hazards. Residential areas along the Cook

Inlet coastline are within the proximal hazard

zone for H/L = 0.3 (fig. 2) but because of the

local topography these areas are not likely to

be subjected to any volcanic hazards except

ash fallout. 

Distal hazards pose less risk to people

because there is usually adequate time for

warning and evacuation. This group of hazards

affects people and structures that are more than

about 10-30 kilometers from the active vent.

Volcanic ash, either in explosive eruption columns or ash clouds that drift far away from the

volcano, can be both a proximal and a distal

hazard, especially to aircraft. 

The geologic map of Iliamna Volcano (fig.

4) depicts deposits formed by various volcanic

phenomena. Most of these phenomena are

confined to the flanks of the volcano and the

major drainages that extend from the summit.

Only volcanic ash clouds, ash fallout, pyroclastic flow and surge, and unusually large volume lahars could affect areas well beyond

Iliamna Volcano (fig. 2). 

VOLCANIC HAZARDS

Volcanic Ash Clouds

Because volcanic ash deposits from

Iliamna Volcano are uncommon in the geologic record, it appears that large-volume,

explosive, ash-forming eruptions of Iliamna

are probably unlikely in the future but significant disruptive small eruptions could occur.

Several times in the past 10,000 years, explosive eruptions of Iliamna propelled significant

quantities of fine ash particles or tephra into

the atmosphere forming an eruption cloud (fig.

7) that drifts away from the volcano with the

wind (fig. 8). The fine ash particles may

remain in the atmosphere for days to weeks

depending on the size of the eruption. Volcanic

ash clouds are a hazard to all aircraft downwind from the volcano (Casadevall, 1994).

Airborne ash from Iliamna Volcano has

traveled at least as far as the Kenai Peninsula

during Holocene eruptions. During future

eruptions, ash could be dispersed over the

Cook Inlet area and beyond. Variable winds

over Cook Inlet could temporarily detain a

drifting ash cloud causing dusty ash-laden air

to linger in the region. High winds after a significant ashfall can resuspend ash and prolong

the foul air condition. 
12 Preliminary Volcano-Hazard Assessment for Iliamna Volcano, Alaska

magma

pyroclastic flow

eruption column

lahar (debris flow)

lava flow

prevailing wind

magma reservoir

fumaroles

eruption cloud

ash (tephra) fall

acid rain

bombs

conduit

crack

ground water

landslide

(debris avalanche)

fumaroles

Volcanoes generate a wide variety of 

phenomena that can alter the Earth's 

surface and atmosphere and endanger 

people and property. While most of 

the natural hazards illustrated and 

described in this diagram are 

associated with eruptions, some, like 

landslides, can occur even when a 

volcano is quiet. Small events may 

pose a hazard only within a few 

kilometers of a volcano, while large 

events can directly or indirectly 

endanger people and property tens to 

hundreds of kilometers away.

Figure 7. Simplified sketch of a stratovolcano and associated hazardous phenomena (modified from 

Myers and others, 1997)
Volcanic Hazards 13

Ash clouds from other eruptions of Cook

Inlet volcanoes have drifted to the southeast

over western Canada and over parts of the

United States and eventually out across the

Atlantic Ocean, indicating that long-distance

transport of volcanic ash is possible during

future eruptions of Iliamna Volcano.

Volcanic Ash Fallout and Volcanic 

Bombs

As clouds of volcanic ash drift from the

volcano, a steady rain or fallout of ash usually

occurs. Volcanic ash is one of the most troublesome and hazardous products of explosive volcanism. Because it may be transported long

distances, it has the potential to affect areas

many hundreds of kilometers from the volcano. Few people have been killed by falling

ash, but the weight of a thick ashfall could

cause structures to collapse and inhaling ash

particles is a health hazard and can be life

threatening to some people with respiratory

problems. Sometimes a “mud rain” results if

airborne volcanic ash mixes with falling rain

or snow.

Blocks or bombs of volcanic rock debris

may be ejected as ballistic projectiles that fall

or strike areas near the vent. In extreme cases,

blocks may be ejected distances of 10 to more

than 30 kilometers from the vent. Typically,

the zone of bomb fallout is within a few kilometers of the vent. People or low-flying aircraft would be at risk only within a few

kilometers of the vent.

N

N

N

N

Anchorage

King Kodiak

Salmon ILIAMNA

VOLCANO

Cold Bay

13,000 

11,000

9,000

5,000

3,000

1,000

7,000

ALTITUDE, IN METERS

Figure 8. Average wind direction and likely travel paths for volcanic ash clouds from Iliamna Volcano. Cold 

Bay data from 1945-57; King Salmon data from 1953-60; Kodiak data from 1949-62; Anchorage data from 

1948-72. Original data obtained from the National Climatic Data Center, Asheville, NC. Windrose lengths 

are proportional to wind frequency determined by annual percent.
14 Preliminary Volcano-Hazard Assessment for Iliamna Volcano, Alaska

Ashfall from eruptions of Iliamna Volcano

may be a public health concern for parts of

south-central Alaska, especially the Kenai

Peninsula. The approximate extent of ash fallout from future eruptions is shown on figure 9.

Because wind direction and speed will control

the movement of the ash plume, the areas most

likely to receive ashfall are those in the zone of

prevailing winds. The strongest and most consistent winds are from the west, southwest, and

northwest (fig. 8). The thickness of ash fallout

will decrease in a downwind direction but it is

Iliamna

Volcano

Mount Spurr

Crater Peak

Redoubt

Volcano

Iliamna

Volcano

Mount Douglas

Seward

Kasilof

Hope

Anchorage

60o

Kenai Peninsula

152o 150o

Homer

Augustine

Volcano

Drift River

Oil Terminal

60 Tyonek o

30'

Nikiski

Sterling Cooper

Landing

Soldotna

Ninilchik

Kenai

EXPLANATION

59o

30'

Anchor 

Point

COMPOSITE AREA OF POSSIBLE 

ASH FALLOUT IN UPPER COOK 

INLET FOR A FUTURE ERUPTION 

OF ILIAMNA VOLCANO

(For any single eruptive event the

ash plume may affect only a

narrow sector of the 

stippled area 

depending on wind 

direction)

decreasing probability

Cook Inlet

Silver

Salmon

Seldovia

Airport

Oil platforms

Oil tanker

loading dock

Power plant

Oil refinery

Known deposits of

tephra and fine ash

from Iliamna Volcano

Approximate extent

of volcanic ash fallout

expected during small

to moderate eruptions

of Iliamna Volcano.

Stippled pattern indicates

areas likely to receive

ash fallout because of

prevailing wind direction. 

154o

100 KILOMETERS

60 MILES

50

30

0

0

Figure 9. Approximate extent of volcanic ash fallout for small to moderate eruptions of Iliamna Volcano.
Volcanic Hazards 15

impossible to predict how much ash will be

emitted during an individual eruption, though

any amount of ashfall could be disruptive.

Industrial facilities in the upper Cook Inlet

region such as oil drilling rigs, oil refineries,

manufacturing plants, and power plants could

be affected by ashfall.

Lahars, Lahar-Runout Flows, and 

Floods

Most of the volcanoes in Alaska support

glaciers or are snow-covered most of the year.

During typical eruptions, hot pyroclastic

debris expelled from the volcano interacts

dynamically with the snowpack or glacier

cover causing extensive melting and water

production. As meltwater mixes with available

unconsolidated sediment, various types of

flowage phenomena may occur on the volcano

flanks and in stream channels and drainages

downstream from the volcano. Most of these

phenomena are categorized as debris flows

(fig. 7) or more specifically as noncohesive

lahars. Lahars consist of a poorly sorted mixture of boulders, sand, and silt. Noncohesive

lahars typically undergo a downstream transformation to finer grained, watery flows,

called hyperconcentrated flows or laharrunout flows. If enough sediment is lost from a

lahar during flowage, the lahar may transform

into a normal streamflow or flood and consist

mostly of water.

Lahars also may form directly from watersaturated, clay-rich volcanic rock avalanches

(Hoblitt and others, 1995; Vallance and Scott,

1997) that may or may not be triggered by an

eruption. Such lahars are called cohesive

lahars because the matrix sediment of typical

deposits contains more than about 3 percent

clay. Cohesive lahars are thought to develop

from large volcanic avalanches that remove a

significant amount of the volcanic edifice.

Hydrothermal alteration of volcanic bedrock

within the volcano decreases the strength of

the rock mass and eventually a sector of the

volcano becomes gravitationally unstable, collapses, and forms a large rock avalanche.

Because the weathered volcanic rock is water

saturated and contains clay minerals, the avalanche debris may evolve into a cohesive lahar

that can flow for several to tens of kilometers

downstream from the volcano. Cohesive lahars

can be more hazardous than their noncohesive

counterparts because they tend to be more

dense, and may develop from rock avalanches

that are unrelated to volcanic eruptions and

thus may occur without the usual precursory

volcanic activity that heralds an eruption.

Lahar deposits of Holocene age have been

identified in several of the drainages on

Iliamna Volcano (fig. 10) and cohesive lahar

deposits are present in the Johnson River and

Red Creek Valleys and at one location along

West Glacier Creek. The cohesive lahar deposits in the Red Creek- Johnson River area are

about 90 years old and indicate that a major(?)

flank collapse that transformed to a lahar has

occurred in the recent past. Noncohesive lahar

deposits also are present in the Red

Creek/Johnson River area and these deposits

formed during an eruption of Iliamna Volcano

about 140 years ago. 

The distribution and age of lahar deposits

in the Red Creek Valley and in the lower

Johnson River Valley indicate that this area has

been inundated by lahars at least twice in about

the past 150 years. The lahar flows were at

least several meters deep and probably

extended across the entire width of the lower

Red Creek Valley and the central part of the

Johnson River Valley. The noncohesive flows

may have transformed to sediment-laden water

floods by the time they reached the coast north

of Silver Salmon (fig. 10), a residential area. It

would take approximately 45-90 minutes for

moderate-to-large-volume lahars (>10 million

cubic meters) to travel this far.
16 Preliminary Volcano-Hazard Assessment for Iliamna Volcano, Alaska

LAKE CLARK

NATIONAL PARK

AND WILDERNESS

BOUNDARY

(approximate)

100

100

100

100

100

500

500

500

500

500

500

500

500

500

500

500

1000

1000

1000

1000

1500

1500

2000

2500

153 ̊ 153 ̊30'

60 ̊

15'

60 ̊

0 5

0 5 10 KILOMETERS

MILES

CONTOUR INTERVAL 500 FEET

Red 

Glacier

COOK INLET

CHINITNA BAY

TUXEDNI BAY

Crescent River

Tuxedni Glacier

Tuxedni 

River

Red River

Johnson

River

Creek

West

Glacier

ILIAMNA

VOLCANO

CHISIK

ISLAND

Red Creek

Silver Salmon

Glacier

Lateral

Umbrella

Glacier

Areas likely to be inundated by lahars, lahar-runout flows, and floods during 

most eruptions.

Areas susceptible to inundation by lahars, lahar-runout flows, and floods during

most eruptions but less likely to be affected than zone L1.

Areas that could be inundated by lahars, lahar-runout flows, and floods during 

large sustained eruptions but less likely to be affected than zones L1 or L2. 

Known lahar deposits of Holocene age.

Boulders of volcanic rock transported by lahars.

L1

L2

L3

Greater

Lesser

Degree

of

Hazard

EXPLANATION

L3

L3

L3

L2

L3

L1

L1

L2

L3

Figure 10. Hazard zonation map for lahars, lahar-runout flows, and floods.
Volcanic Hazards 17

Noncohesive lahar deposits also are preserved along the Cook Inlet coastline near the

mouth of the Red River (fig. 11). These deposits contain cobble- and boulder-size clasts of

juvenile andesite that were probably still hot

when the lahar flowed to the sea. Spruce trees

growing on the lahar deposit are about 300

years old and indicate that the lahar and an

associated eruption of Iliamna Volcano

occurred at about this time (Begét, 1996). Several large unweathered boulders and blocks of

andesite from Iliamna Volcano are present on

the alluvial fan of the Red River (fig. 10).

These blocks are part of the same lahar deposit

exposed along the coast and further document

an extensive lahar that must have swept down

Red Glacier and inundated the Red River Valley and the Red River alluvial fan. Moderateto-large-volume lahars could reach the coast at

the mouth of the Red River in as little as 30

minutes.

Hazard zones for lahars, lahar-runout

flows, and floods are shown on figure 10 and

on plate 1. Three hazard zones (L1, L2, and

L3) that depict differing degrees of hazard are

indicated. Hazard zone L1 indicates areas that

are likely to be inundated by lahars, laharrunout flows, and floods during most eruptions

of any magnitude. Hazard zone L1 also

includes areas that have been affected by

lahars, lahar-runout flows, and floods during

the past 300 years. Hazard zone L2 includes

areas that are susceptible to inundation by

lahars, lahar-runout flows, and floods but is

less likely to be affected than hazard zone L1

during most eruptions from the present vent. If

the location of the vent changes such that

activity is more likely in a drainage presently

in hazard zone L2, the degree of hazard would

change to L1. Hazard zone L3 includes areas

that could be affected by lahars, lahar-runout

flows, and floods only during large, sustained

eruptions. These areas contain no known lahar

deposits. 

Because lahars, lahar-runout flows, and

floods can move rapidly, can be several meters

deep, and can transport boulder- and blocksize particles, they would be hazardous to life

and property in the flow path. The distribution

and age of lahar deposits indicate that future

developments in the Johnson and Red River

Valleys, as well as the areas where these rivers

enter Cook Inlet, could be at risk from lahars,

lahar-runout flows, and floods. At present

however, lahars pose no hazard in these areas

because they are uninhabited and have no permanent structures or facilities. 

Figure 11. Noncohesive lahar deposit exposed along the coastline near the mouth of the Red River. 

The maximum age of spruce trees growing on the lahar deposit is about 300 years indicating the 

lahar and associated eruption occurred at about this time.
18 Preliminary Volcano-Hazard Assessment for Iliamna Volcano, Alaska

Debris Avalanche

Volcanic rock or debris avalanches (fig. 7)

typically form by structural collapse of the

upper part of the volcano. The ensuing avalanche moves rapidly down the volcano flank

and forms a bouldery gravel deposit many

kilometers from the source that may exhibit a

characteristic hummocky surface and broad

areal extent. Most debris-avalanche deposits

are traceable up the slopes of the volcano to an

arcuate-shaped scar at or near the volcano

summit that marks the zone of collapse and

origin of the avalanche (fig. 12). 

Although some debris avalanches occur

during an eruption, large-scale collapse of a

volcanic cone may occur during a distinctly

non-eruptive period, sometimes as a result of

long-term chemical alteration of volcanic rock

by hot, acidic ground water. As the interior

structure of the volcano becomes weakened,

the flank may collapse and produce a debris

avalanche. Debris avalanches that form this

way produce deposits that contain a significant

amount of matrix clay (usually more than 3

percent) and the avalanche itself may transform into a cohesive lahar if it contains or

acquires sufficient water. Numerous debrisavalanche deposits are present on the flanks of

Iliamna Volcano but it is uncertain whether

these deposits formed during eruptions or were

initiated by other mechanisms, such as large

earthquakes or simple gravity-driven collapse.

Debris-avalanche deposits have been identified in the Red River Valley and on Red Glacier, on Umbrella Glacier and in West Glacier

Creek Valley, on Lateral Glacier, and on Tuxedni Glacier (fig. 13). All of these deposits are

fresh appearing and relatively young (less than

Avalanche

scar

1997 debrisavalanche

deposit

Fumarole

zone

Figure 12. View of the southeast flank of Iliamna Volcano showing debris-avalanche deposits from 

1997 (solid line), the fumarole zone near the summit (yellow dashed line), and older avalanche scar at 

the head of Red Glacier (red dashed line).
Volcanic Hazards 19

LAKE CLARK

NATIONAL PARK

AND WILDERNESS

BOUNDARY

(approximate)

100

100

100

100

100

500

500

500

500

500

500

500

500

500

500

500

1000

1000

1000

1000

1500

1500

2000

2500

153 ̊ 153 ̊30'

60 ̊

15'

60 ̊

EXPLANATION

Debris-avalanche deposits associated 

with Iliamna Volcano. 

Debris-avalanche and rockfall deposits 

from Iliamna Volcano reworked by glaciers.

Rock-avalanche deposits not associated 

with Iliamna Volcano.

Maximum likely extent of a debris avalanche from 

Iliamna Volcano for H/L = 0.2

Maximum likely extent of a debris avalanche from 

Iliamna Volcano for H/L = 0.4

Probable travel path of debris avalanches from 

Iliamna Volcano.

Generalized extent of rockfall debris on glacier surfaces. 

0 5

0 5 10 KILOMETERS

MILES

CONTOUR INTERVAL 500 FEET

Red 

Glacier

COOK INLET

CHINITNA BAY

TUXEDNI BAY

Crescent River

Tuxedni Glacier

Tuxedni 

River

Red River

Johnson

River

Creek

West

Glacier

ILIAMNA

VOLCANO

CHISIK

ISLAND

Red Creek

Silver Salmon

Glacier

Lateral

Umbrella

Glacier (residential area)

Snug Harbor

Cannery (abandoned) airstrip

Figure 13. Hazard zonation map for debris avalanche.
20 Preliminary Volcano-Hazard Assessment for Iliamna Volcano, Alaska

about 1,000 years old) and some may be of

historic age (<200 years). The debris-avalanche deposits consist of angular gravel that is

composed of poorly sorted mixtures of sand,

cobbles, and boulders. All deposits contain

clasts of grey andesitic lava and yellow-orange

hydrothermally altered rock debris.

Most of the exposed summit of Iliamna

Volcano consists of hydrothermally altered and

weakened volcanic bedrock. Periodically,

small volumes of rock break loose from the

upper part of the volcano and form minor

debris avalanches (fig. 12), especially in spring

when the snowpack is melting. These avalanches incorporate snow and ice and in some

cases may begin as snowslides or ice falls that

scour exposed altered bedrock. In 1994, 1996,

and 1997, several such avalanches originated

from the edifice of Iliamna Volcano (fig. 14)

and flowed down upper Red and Umbrella

Glaciers for a distance of about 3-8 kilometers.

The characteristic rocky debris cover on Red,

Lateral, and Umbrella Glaciers is mostly

debris avalanche material and indicates that

these glaciers are frequently swept by rock

avalanches (fig. 15).

Hazard zonation for debris avalanche at

Iliamna Volcano is based in part on the runout

distance of recent avalanches. The maximum

likely runout distance of a future debris avalanche was estimated using the ratio of the fall

height (H) to runout distance (L) of known

avalanche deposits. The H/L ratio for the 1994,

1996, and 1997 debris avalanche deposits and

Area of hydrothermally

altered bedrock likely to be

the source of a future

debris avalanche. Volume of

altered, weakened rock

is about 0.4 cubic kilometers.

Scars of former

debris-avalanches 

CONTOUR INTERVAL 100 FEET

0 1 Mile

0 1 Kilometer

153o

60o

153o05'

60o

02'

Figure 14. Topographic map of summit area of Iliamna Volcano. Zone of 

hydrothermally altered, weakened bedrock and likely source area for 

future debris avalanches indicated by dashed red line. Probable flow 

paths for avalanches indicated by arrows.
Volcanic Hazards 21

the debris avalanche deposits on the northern

margins of Red and Umbrella Glaciers is about

0.4. Future similar debris avalanches are

expected to extend several kilometers from the

volcano summit but not far beyond the upper

reaches of the major glaciers on the volcano

(fig. 13). The runout distance of a debris avalanche is proportional to its volume, and if

larger avalanches occur at Iliamna Volcano

they will travel a greater distance down the

major valleys. For debris avalanches having

volumes of 0.5-1.0 cubic kilometers, an H/L

ratio of 0.2 gives a maximum runout distance

of about 15 kilometers (fig. 13). For all drainages except Red Creek, the avalanches will be

generally unconfined and able to spread and

thin laterally which will limit their mobility.

For drainages like Red Creek, the avalanche

will be directed down a discrete flow path

causing it to behave like a channelized flow. In

these settings, the avalanche may transform to

a cohesive lahar that could travel a considerable distance beyond the hazard zone boundary for debris avalanche.

About 0.4 cubic kilometer of hydrothermally altered volcanic bedrock makes up the

summit of Iliamna Volcano. If this amount of

rock were to break loose and form a debris

avalanche, it would most likely inundate the

Red or Umbrella Glacier Valleys and could

extend approximately 10-15 kilometers down

these glaciated valleys. If an avalanche this

size transformed to a cohesive lahar that

entered the Johnson River Valley via Red

Creek, the lahar would probably inundate the

lower Johnson River Valley across the entire

width of the present valley and would probably

extend to the coastline. 

Pyroclastic Flow and Surge

A pyroclastic flow is a hot, dry mixture of

volcanic rock debris and gas that flows rapidly

downslope (fig. 7). A pyroclastic surge is similar to a pyroclastic flow but has a higher gas

content. Because it is mostly gas, a pyroclastic

surge moves more rapidly than a pyroclastic

flow and may not be confined by topography,

Red 

Glacier

1997 

avalanche

debris

late Holocene

avalanche debris

Figure 15. View down the north side of Red Glacier showing debrisavalanche deposits formed in 1997 and an older slightly more extensive 

deposit that is probably a few hundred years old or less.
22 Preliminary Volcano-Hazard Assessment for Iliamna Volcano, Alaska

and therefore may climb up and over ridges.

Pyroclastic flows are relatively dense and will

generally follow topographically low areas

such as stream valleys. Any of the major drainages that head on the volcano could be

engulfed by pyroclastic flows even during

modest eruptions. Because they are hot and

fast-moving, both pyroclastic flows and surges

could be lethal to anyone on the proximal

flanks of Iliamna Volcano during an eruption.

Pyroclastic flows and surges at Iliamna

Volcano will most likely form by collapse of a

cooling lava dome. Should a large, explosive,

and rare plinian-style eruption occur, the eruption column would collapse and fall back

toward the volcano forming a fast-moving

pyroclastic flow. It is likely that pyroclastic

flows and surges generated by lava-dome collapse were associated with former eruptions of

Iliamna Volcano, and these probably led to the

development of noncohesive lahars. To date

however, pyroclastic-flow or surge deposits

have not been identified on the flanks of

Iliamna Volcano.

Pyroclastic flows and surges from most

eruptions would be expected to reach at least

several kilometers beyond the vent and could

travel in almost any direction. The runout distance of pyroclastic flows is estimated with

H/L values of 0.2 and 0.3 (fig. 16). These values give runout distances of 10-15 kilometers

and are most relevant to pyroclastic flows generated by a collapsing eruption column. Pyroclastic flows initiated by lava-dome collapse

usually affect only a discrete sector of the volcano. During large eruptions pyroclastic flows

could reach the sea in the Red River Valley and

possibly in West Glacier Creek Valley (fig.

16). Pyroclastic flows and surges from eruptions of Iliamna Volcano that reach Cook Inlet

would be unlikely except for a rare, extreme

eruption.

It is difficult to accurately predict the

extent of a pyroclastic surge. However,

because of their genetic relation to pyroclastic

flows, they have a slightly greater lateral

extent. Thus, the extent of the hazard boundary

is uncertain (fig. 16). Because surges are hot

(300 to 800 °C) and gaseous, death or injury

from asphyxiation and burning is likely.

Because the surge cloud may travel very fast

(at least tens of meters per second), pre-eruption evacuation of the area near the volcano is

the only way to eliminate risk from pyroclastic

surges.

Directed Blasts

A directed blast is a large-scale lateral volcanic explosion caused by a major landslide or

slope failure that uncaps the internal vent system of the volcano. Such an event is rare in the

history of a volcano. Although geologic evidence indicates that landslides and slope failures have occurred at Iliamna Volcano, thus far

evidence for a directed blast has not yet been

discovered. The hazard zone boundary showing the area most likely to be affected by a

directed blast (fig. 17) is based on data from

the 1980 eruption of Mount St. Helens. The

directed blast associated with the 1980 Mount

St. Helens eruption is one of the largest known

historical events and thus is a “worst case”

example. If a directed blast were to occur from

the summit of Iliamna Volcano, it could affect

a broad area, possibly a 180° sector from the

vent. A directed blast will usually happen in

the first few minutes of an eruption and thus

there is no time for warning or evacuation.

Living things in the path of a directed blast

will be killed or destroyed by impact, burning,

abrasion, burial, and heat.
Volcanic Hazards 23

LAKE CLARK

NATIONAL PARK

AND WILDERNESS

BOUNDARY

(approximate)

100

100

100

100

100

500

500

500

500

500

500

500

500

500

500

500

1000

1000

1000

1000

1500

1500

2000

2500

153 ̊ 153 ̊30'

60 ̊

15'

60 ̊

0 5

0 5 10 KILOMETERS

MILES

CONTOUR INTERVAL 500 FEET

Red 

Glacier

COOK INLET

CHINITNA BAY

TUXEDNI BAY

Crescent River

Tuxedni Glacier

Tuxedni 

River

Red River

Johnson

River

Creek

West

Glacier

ILIAMNA

VOLCANO

CHISIK

ISLAND

Red Creek

Silver Salmon

Glacier

Lateral

Umbrella

Glacier (residential area)

Snug Harbor

Cannery (abandoned) airstrip

Extent of pyroclastic flows for H/L = 0.3. Pyroclastic flows associated with eruptioncolumn collapse could extend to about this boundary during small to moderate eruptions.

Extent of pyroclastic flows for H/L = 0.2. Pyroclastic flows associated with eruptioncolumn collapse could extend to about this boundary during moderate to large eruptions.

Most likely flow path for pyroclastic flows and surges. During moderate to large eruptions,

pyroclastic flows and surges could be directed along topographically low areas such as

valleys and drainages and could extend beyond the indicated hazard-zone boundaries in

these areas.

EXPLANATION

Figure 16. Hazard zonation map for pyroclastic flow and surge.
24 Preliminary Volcano-Hazard Assessment for Iliamna Volcano, Alaska

LAKE CLARK

NATIONAL PARK

AND WILDERNESS

BOUNDARY

(approximate)

100

100

100

100

100

500

500

500

500

500

500

500

500

500

500

500

1000

1000

1000

1000

1500

1500

2000

2500

153 ̊ 153 ̊30'

60 ̊

15'

60 ̊

0 5

0 5 10 KILOMETERS

MILES

CONTOUR INTERVAL 500 FEET

Red 

Glacier

COOK INLET

CHINITNA BAY

TUXEDNI BAY

Crescent River

Tuxedni Glacier

Tuxedni 

River

Red River

Johnson

River

Creek

West

Glacier

ILIAMNA

VOLCANO

CHISIK

ISLAND

Red Creek

Silver Salmon

Glacier

Lateral

Umbrella

Glacier (residential area)

Snug Harbor

Cannery (abandoned) airstrip

Extent of area that could be affected by a directed blast similar to the blast generated 

during the 1980 eruption of Mt. St. Helens, Washington. This hazard-zone boundary is 

a worst-case condition for Iliamna Volcano. If a directed blast were to occur, it could 

engulf a large portion of the indicated hazard zone. 

EXPLANATION

Figure 17. Hazard zonation map for directed blast. Directed blasts are uncommon and deposits 

formed by a directed blast have not been found at Iliamna Volcano. A directed blast is most likely to 

be directed down one or more of the major drainages and less likely to the west.
Event Frequency and Risk at Iliamna Volcano 25

Volcanic Gases

Gases are emitted by most active volcanoes, because magma contains dissolved gases

and boils off shallow ground water that is typically present within volcanoes. The most common volcanic gases are water vapor, carbon

dioxide, carbon monoxide, sulfur dioxide, and

hydrogen sulfide. Volcanic sulfur and halide

gases that encounter water form large amounts

of sulfuric acid (H2SO4) and minor amounts of

hydrochloric (HCl) and hydrofluoric acid (HF)

as aerosols or droplets. Both carbon monoxide

and carbon dioxide are colorless and odorless

and thus impossible to detect without a measuring device. Carbon dioxide is heavier than

air and may displace the available oxygen in

confined spaces or low-lying areas causing

suffocation. In high concentrations, carbon

dioxide, hydrogen sulfide, and sulfur dioxide

may be harmful or toxic to humans and may

damage vegetation downwind from the volcano. Acid precipitation may develop from the

mixing of snow or rain with acidic volcanic

aerosols, which may cause various types of

skin and respiratory irritations and cause corrosive damage to paint, fabric, and structures.

Wind tends to disperse volcanic gas and it is

typically not found near the ground in concentrations hazardous to humans or animals more

than about 10 kilometers from the volcano.

During large eruptions, significant volumes of

gas can travel high in the atmosphere downwind from the volcano for days and thousands

of kilometers.

The hazard from volcanic gases at Iliamna

Volcano is extremely unlikely to be greater

than that posed by other volcanic phenomena.

Fumaroles located near the summit of the volcano produce a nearly constant plume of steam

and carbon dioxide and sulfur dioxide gas and

on calm days this plume is readily apparent. At

times, the steam plume is especially vigorous

and has been mistaken for an eruption cloud.

Volcanic gas may pose a health concern to

someone near the active fumarole area; however, frequent windy conditions at Iliamna

Volcano and the absence of an efficient trapping mechanism inhibit localized buildup of

volcanic gas. Therefore, the hazard from volcanic gases is minor.

Lava Flow

Narrow streams of molten rock or lava

may form during a future eruption of Iliamna

Volcano and commonly, lava flows (fig. 7)

develop after explosive activity at the volcano

declines. Most of Iliamna Volcano is composed of bedded lava flows (Qiv, Qtv, fig. 4).

Typical Iliamna flows are andesitic in composition and, when molten, are relatively viscous.

Future eruptions will probably generate lava

flows similar to those preserved on the volcano. The lava flows are expected to move

slowly downslope, probably not more than a

few tens of meters per hour. Lava flows of this

type pose little hazard to people who could

easily walk from them; however, lava flows at

Iliamna may develop steep fronts and could

shed blocks and debris downslope. Lava flows

that reach snow and ice could generate localized flooding.

EVENT FREQUENCY AND RISK AT 

ILIAMNA VOLCANO

An eruption of Iliamna Volcano can be

expected in the future, but the timing of the

next eruption is uncertain. The primary proximal hazard during a future eruption will be

lahars, lahar-runout flows, and floods that

could inundate significant portions of major

drainages on the volcano. Thick accumulations

of sediment in affected valleys and drainages

will occur and sediment-laden runoff could

persist for months to years after the eruption.
26 Preliminary Volcano-Hazard Assessment for Iliamna Volcano, Alaska

Because the immediate area around

Iliamna Volcano is uninhabited and no permanent structures or facilities are present, nothing

within about 20 kilometers of the volcano is at

risk from future eruptions. During any future

eruption, ash could fall on residential areas

along the Cook Inlet coastline, such as Silver

Salmon (fig. 2), and several millimeters of ash

could accumulate. The major drainages on the

volcano, especially the Johnson River, Red

River, and West Glacier Creek, are vulnerable

to inundation by lahars, lahar-runout flows,

and floods. In the event of a large, explosive

eruption, pyroclastic flows could also engulf

these valleys. 

Should a sustained explosive eruption

occur, clouds of volcanic ash would be generated that could drift thousands of kilometers

downwind. All aircraft, some facilities, and

living things—including humans—downwind

from the volcano are at risk from effects of

volcanic ash clouds and ash fallout. Ash

clouds from Iliamna Volcano could rise to altitudes of 12,000 meters or more and move into

the flight paths of jet aircraft using Anchorage

International and other airports in south-central and central Alaska. Aircraft using air

routes over the North Pacific Ocean and other

areas downwind of Iliamna, especially the

Gulf of Alaska and Pacific Northwest region,

could encounter clouds of volcanic ash. The

frequency at which dangerous clouds of volcanic ash are produced and the amount of ashfall

cannot be estimated with certainty. Signs of

volcanic unrest will precede an eruption and

permit reasonable estimates of the likelihood

of volcanic ash emission once an eruptive

phase is detected. However, it is not possible to

determine the characteristics of an ash cloud

before an eruption occurs, except that it is

likely to be similar to those generated by eruptions of other Cook Inlet volcanoes. 

HAZARD WARNING AND MITIGATION

Typically, eruptions at Cook Inlet volcanoes are preceded by a period of precursory

earthquake activity giving some degree of seismic warning prior to an eruption. Many eruptions are preceded by at least several weeks of

increased gas emission from the summit area.

When volcanic unrest is detected, other monitoring techniques, such as satellite observations, measurement of volcanic gas flux,

remote observation with real-time video or

time-lapse cameras, and geodetic surveying,

are used to develop a comprehensive assessment of the likelihood of an eruption and its

potential effects. 

 The AVO monitors Iliamna Volcano with a

real-time seismic network (fig. 18) equipped

with an alarm system that is triggered by elevated levels of seismic (earthquake) activity

indicating volcanic unrest. A network of six

radio-telemetered seismometers sends realtime radio signals to AVO offices in Anchorage and Fairbanks. In addition, AVO maintains

a field-based data collection program that

includes geodetic, temperature, and gas measurements.

One of the primary roles of the Alaska Volcano Observatory (AVO) is to communicate

timely warnings of volcanic unrest and potential eruptions (Eichelberger and others, 1995,

p. 4). The AVO distributes by fax and electronic mail a weekly update of volcanic activity that summarizes the status of the more than

40 historically active volcanoes along the

Aleutian Arc. During periods of unrest or volcanic crises, updates are issued more frequently to advise the public of significant

changes in activity. Recipients of these updates

include the Federal Aviation Administration,

air carriers, the National Weather Service, the

Alaska Department of Emergency Services,

local military bases, the Governor’s office,

various State offices, television and radio stations, news wire services, and others. Updates
Hazard Warning and Mitigation 27

LAKE CLARK

NATIONAL PARK

AND WILDERNESS

BOUNDARY

(approximate)

100

100

100

100

100

500

500

500

500

500

500

500

500

500

500

500

1000

1000

1000

1000

1500

1500

2000

2500

153 ̊ 153 ̊30'

60 ̊

15'

60 ̊

Seismic-monitoring station

airstrip

0 5

0 5 10 KILOMETERS

MILES

CONTOUR INTERVAL 500 FEET

Red 

Glacier

COOK INLET

CHINITNA BAY

TUXEDNI BAY

Crescent River

Tuxedni Glacier

Tuxedni 

River

Red River

Johnson

River

Creek

West

Glacier

ILIAMNA

VOLCANO

CHISIK

ISLAND

Red Creek

Silver Salmon

Glacier

Lateral

Umbrella

Glacier (residential area)

Snug Harbor

Cannery (abandoned)

EXPLANATION

Figure 18. Location of seismic-monitoring stations on Iliamna Volcano.
28 Preliminary Volcano-Hazard Assessment for Iliamna Volcano, Alaska

also are distributed by electronic mail to various volcano information networks and are

posted on the AVO world-wide web site

(www.avo.alaska.edu).

During the 1989-90 eruptions of Redoubt

Volcano, the AVO developed a “level of concern color code” (Brantley, 1990; fig. 19). This

code provides efficient and simple information

about the status of volcanic activity or unrest

and conveys the AVO’s interpretation of that

activity or unrest in terms of the potential for

an eruption and its likely effects. In the advent

of a volcanic crisis, various Federal, State, and

local officials are contacted by telephone,

advised of the situation, and the level of concern color code is established while an update

is being prepared. This approach has been used

successfully during recent periods of volcanic

unrest, such as the 1989-90 eruptions of

Redoubt Volcano, the 1992 eruptions of Mount

Spurr Volcano (Miller and Chouet, 1994;

Keith, 1995), and the 1996-97 eruption of Pavlof Volcano.

Minimizing the risks posed by eruptions of

Iliamna Volcano is possible through adequate

warning of potential hazards, and by avoiding

development or utilization of areas likely to be

affected by future eruptions (plate 1). Areas

within about 10-20 kilometers of Iliamna Volcano are at risk from all hazardous volcanic

phenomena. If for some reason, development

is unavoidable in hazardous areas, engineering

measures may be employed to minimize or

prevent undesirable consequences. 

Knowledge of potential hazards is required

to assess the risk associated with a specific

location on or near the volcano and to assess

whether or not movement to another location

would be safer. Recreational users of Lake

LEVEL OF CONCERN COLOR CODE

Volcano is in its normal "dormant" state.

 

Volcano is restless.

Seismic activity is elevated. Potential for eruptive activity is 

increased. A plume of gas and steam may rise several thousand 

feet above the volcano which may contain minor amounts of ash.

 

Small ash eruption expected or confirmed. Plume(s) not likely

to rise above 25,000 feet above sea level. Seismic disturbance 

recorded on local seismic stations, but not recorded at more distant

locations.

Large ash eruptions expected or confirmed. Plume(s) likely

to rise above 25,000 feet above sea level. Strong seismic signal 

recorded on all local and commonly on more distant stations.

GREEN:

YELLOW:

ORANGE:

RED:

LEVEL OF CONCERN COLOR CODE

Figure 19. Level of concern color code for volcanic eruptions.
References Cited 29

Clark National Park and Wilderness in the

vicinity of Iliamna Volcano should recognize

that low-lying terrain along streams and gullies

that extend toward the summit is subject to

pyroclastic flow and surge, lahars, lahar-runout

flows, floods, and avalanches. Given the

present configuration of the summit area, a

debris avalanche is most likely on the southeastern flank at the head of Red Glacier, or on

the southwestern flank at the head of Umbrella

Glacier. During an eruption, access closer than

about 10 kilometers from the volcano could be

impossible and the risks to human life great.

Small planes and helicopters seeking a view of

an eruption could be at risk from intermittent

and unpredictable discharge of ballistic projectiles (volcanic bombs) or sudden changes in

the travel direction of the eruption plume.

People and facilities located farther away

from the volcano may have additional time to

prepare for the adverse effects of an eruption;

however, an emergency plan developed and

ready prior to the onset of an eruption is useful. The planning for volcanic emergencies is

similar to that for other emergencies, such as

flooding or extreme weather. The sources of

emergency information are often the same and

the usual interruption of essential services may

result. Thus, planning for interruptions in electrical service, transportation (especially air

travel), and outdoor activities is appropriate

for volcanic emergencies.

REFERENCES CITED

Begét, J.E., 1996, Dendrochronologic, archeologic,

and radiocarbon data on recent eruptions of

Iliamna and Augustine Volcanoes, Alaska: Eos,

Transactions of the American Geophysical

Union, v. 77, no. 46, p. F814.

Blong, R.J., 1984, Volcanic hazards: Sydney, Academic Press, 424 p.

Brantley, S.R., ed., 1990, The eruption of Redoubt

Volcano, Alaska, December 14, 1989—August

31, 1990: U.S. Geological Survey Circular 1061,

33 p.

Casadevall, T.J., ed., 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.

Eichelberger, J.C., Keith, T.E.C., Miller, T.P., and

Nye, C.J., 1995, The 1992 eruptions of Crater

Peak vent, Mount Spurr Volcano, Alaska—Chronology and summary, in Keith, T.E.C., ed., The

1992 eruptions of Crater Peak vent, Mount Spurr

Volcano, Alaska: U.S. Geological Survey Bulletin

2139, p. 1-18.

Hoblitt, R.P., Walder, J.S., Driedger, C.L., Scott,

K.M., Pringle, P.T., and Vallance, J.W., 1995, Volcano hazards from Mount Rainier, Washington:

U.S. Geological Survey Open-File Report 98-

273, 12 p.

Keith, T.E.C., ed., 1995, The 1992 eruptions of Crater

Peak Vent, Mount Spurr Volcano, Alaska: U.S.

Geological Survey Bulletin 2139, 220 p.

McGimsey, R.G., and Neal, C.A., 1997, Volcanic

activity in Alaska and Kamchatka—Summary of

events and response of the Alaska Volcano Observatory 1996: U.S. Geological Survey Open-File

Report 97-433, 34 p.

Miller, T.P., Begét, J.E., Stephens, C.D., and Moore,

R.B., 1996, Geology and hazards of Iliamna Volcano, Alaska: Eos, Transactions, American Geophysical Union, v. 77, p. F815.

Miller, T.P., and Chouet, B.A, 1994, The 1989-1990

eruptions of Redoubt Volcano—An introduction:

Journal of Volcanology and Geothermal

Research, v. 62, p. 1-10. 

Miller, T.P., McGimsey, R.G., Richter, D.H., Riehle,

J.R., Nye, C.J., Yount, M.E., and Dumoulin, J.A.,

1998, Catalog of the historically active volcanoes

of Alaska: U.S. Geological Survey Open-File

Report 98-582, 104 p.

Myers, B., Brantley, S.R., Stauffer, P., and Hendley,

J.W., II, 1997, What are volcano hazards?: U.S.

Geological Survey Fact Sheet 002-97.

Vallance, J.M., and Scott, K.M., 1997, The Osceola

mudflow from Mount Rainier—Sedimentology

and hazard implications of a huge clay-rich debris

flow: Geological Society of America Bulletin, v.

109, p. 143-163. 
30 Preliminary Volcano-Hazard Assessment for Iliamna Volcano, Alaska

GLOSSARY 

Andesite. A fine-grained volcanic rock made up of

feldspars and ferromagnesian minerals; typically

has a Si02 content of 54 to about 62 percent.

Ash. Fine fragments (less than 2 millimeters across)

of lava or rock formed in an explosive volcanic

eruption.

Cohesive. As applied to lahars, any lahar that contains

more than about 3-5 percent clay in the deposit

matrix.

Debris avalanche. Rapidly moving, dry flows of dissaggregated rock debris, sand, and silt. Volcanic

debris avalanches often form by some type of

structural collapse of the volcano, usually the

steep front of the cooled lava dome, or other parts

of the upper edifice. A large portion of the volcano may become unstable, break away from the

volcanic massif, and become an avalanche. A

debris avalanche may be triggered by an eruption

or earthquake. Debris avalanches move at velocities ranging from a few tens of meters per second

to more than 100 meters per second and behave

like complex granular flows or slide flows. Often

they are quite voluminous (greater than 10 cubic

kilometers) and may run out considerable distances (up to 85 kilometers) from their source.

The resulting debris avalanche deposit usually

exhibits hummocky surface morphology.

Directed blast. Large-scale volcanic explosions

caused by a major landslide or slope failure that

results in a rapid drop in the pressure of the

intruding magma near the surface of the volcanic

edifice. The 1980 eruption of Mt. St. Helens was

triggered by a massive slope failure and the subsequent laterally directed blast affected a 180°

sector north of the volcano and extended for several tens of kilometers outward. A directed blast

typically travels away from the volcano at a low

angle and may not be deflected by ridges or other

topographic barriers. Rock debris propelled by a

directed blast moves much faster than typical

landslides and rockfalls. For example, at Mt. St.

Helens, the initial velocity of the directed blast

cloud was about 600 kilometers per hour decreasing to about 100 kilometers per hour at a distance

25 kilometers from the volcano.

Edifice. The upper part of the volcanic cone, including the vent and summit areas.

Eruption cloud. Cloud of gas, ash, and other fragments that forms during an explosive volcanic

eruption and travels long distances with the prevailing winds.

Eruption column. The vertical portion of the eruption cloud that rises above a volcanic vent.

Fallout. A general term for debris that falls to the

Earth from an eruption cloud.

Fumarole. Small, vent-like crack or opening of

escaping gas and steam.

Lahar. An Indonesian term for a debris flow containing angular clasts of volcanic material. For the

purposes of this report, a lahar is any type of sediment/water mixture originating on or from the

volcano. Most lahars move rapidly down the

slopes of a volcano as channelized flows and

deliver large amounts of sediment to the rivers

and streams that drain the volcano. The flow

velocity of some lahars may be as high as 20 to 40

meters per second (Blong, 1984) and sediment

concentrations of >750,000 parts per million are

not uncommon. Large volume lahars can travel

great distances if they have an appreciable clay

content (> 3 to 5 percent), remain confined to a

stream channel, and do not significantly gain sediment while losing water. Thus, they may affect

areas many tens to hundreds of kilometers downstream from a volcano. 

Lahar-runout flow. The downstream or distal component of a lahar. Lahar-runout flows are finer

grained and more watery than a typical lahar.

Most noncohesive lahars transform to laharrunout flows as they travel downstream.

Lapilli. Ejected rock or pumice fragments between 2

and 64 millimeters in diameter.

Lava. Molten rock that reaches the Earth’s surface.

Lava dome. A steep-sided mass of viscous and often

blocky lava extruded from a vent; typically has a

rounded top and roughly circular outline.

Magma. Molten rock beneath the Earth’s surface.

Noncohesive. As applied to lahars, any lahar that contains less than about 3-5 percent clay in the

deposit matrix.

Pleistocene epoch. The period of Earth history

between 1.8 million and 10 thousand years before

present.
GLOSSARY 31

Plinian. Volcanic eruptions associated with highly

explosive ejection of tephra and large-volume

emissions of ash. Ash plumes from plinian eruptions usually reach 10,000 meters altitude or more

above the vent.

Pumice. Highly vesicular volcanic ejecta; due to its

extremely low density, it often floats on water.

Pyroclastic. General term applied to volcanic products or processes that involve explosive ejection

and fragmentation of erupting material.

Pyroclastic flow. A dense, hot, chaotic avalanche of

rock fragments, gas, and ash that travels rapidly

away from an explosive eruption column, often

down the flanks of the volcano (synonymous with

“ash flow”). Pyroclastic flows move at speeds

ranging from 10 to several hundred meters per

second and are typically at temperatures between

300 and 800 °C (Blong, 1984). Pyroclastic flows

form either by collapse of the eruption column, or

by failure of the front of a cooling lava dome.

Once these flows are initiated, they may travel

distances of several kilometers or more and easily

override topographic obstacles in the flow path. A

person could not outrun an advancing pyroclastic

flow.

Pyroclastic surge. A low-density, turbulent flow of

fine-grained volcanic rock debris and hot gas.

Pyroclastic surges differ from pyroclastic flows in

that they are less dense and tend to travel as a low,

ground-hugging, but highly mobile cloud that can

surmount topographic barriers. Surges often

affect areas beyond the limits of pyroclastic

flows.

Sulfurous. Sulfur bearing or rich in sulfur compounds. 

Stratovolcano. (also called a stratocone or composite

cone) A steep-sided volcano, usually conical in

shape, built of lava flows and fragmental deposits

from explosive eruptions.

Tephra. Any type of rock fragment that is forcibly

ejected from the volcano during an eruption.

Tephra may be fine-grained dust or “ash” (0.0625

to 2 millimeter diameter—silt to sand sized),

coarser “lapilli” (2 to 64 millimeter diameter—

sand to pebble sized), or consist of large blocks or

bombs (>64 millimeter—cobble to boulder

sized). When tephra is airborne, the coarsest fraction will be deposited close to the volcano, but the

fine fraction may be transported long distances

and can stay suspended in the atmosphere for

many months. Tephra particles are typically

sharp, angular, and abrasive, and are composed of

volcanic glass, mineral, and rock fragments.

Vent. An opening in the Earth's surface through which

magma erupts or volcanic gasses are emitted.