Redoubt Volcano, Southern Afaska: 

A Hazard Asselssment Based on · 

Eruptive Activj~lY through 1968 

Redoubt Volcano, Southern Alaska: 

A Hazard Assessmer1t Based on 

Eruptive Activity through 1968 

By ALISON B. TILL, M. ELIZABETH YOUNT, and j.R. RIEHLE 

U.S. GEOLOGICAL SURVEY BULLETIN 1996 
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

Any use of trade, product, or firm names 

in this publication is for descriptive purposes only 

and does not imply endorsement by the U.S. Government 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1993 

For sale by 

Book and Open-File Report Sales 

U.S. Geological Survey 

Federal Center, Box 25286 

Denver, CO 80225 

Library of Congress Cataloging in Publication Data 

Till, Alison B. 

Redoubt Volcano, southern Alaska :a hazard assessment based on eruptive 

activity through 1968 I by Alison B. Till, M. Elizabeth Yount, and J.R. Riehle. 

p. cm.-(U.S. Geological Survey bulletin; 1996) 

Includes bibliographical references. 

Supt. of Docs. no.: I 19.3:1996 

1. Volcanic hazard assessment-Alaska-Redoubt Volcano Region. 

2. Redoubt Volcano (Alaska) I. Yount, M.E. II. Riehle, J.R. Ill. Title. 

IV. Series. 

QE75.B9 no. 1996 

[QE523.R39] 

557.3 s-dc20 

[551.2'1 '0798] 91-22238 

CIP 
CONTENTS 

Abstract 1 

Introduction 1 

Geologic history 4 

Introduction 4 

Pleistocene history 4 

Holocene history 5 

Historical record 8 

Summary 9 

Volcanic hazards 9 

The hazard map 10 

Tephra distribution 12 

Summary 14 

References cited 15 

Appendix 17 

Typical events and products associated with volcanic eruptions 18 

Tephra fall 18 

PLATE 

Debris avalanches, debris flows and floods 18 

Lateral blasts 18 

Pyroclastic flows and surges 19 

Lava flows and lava domes 19 

1. Generalized geology and potential hazards from Redoubt Volcano, southern Alaska, 

based on eruptive activity through 1968 In pocket 

FIGURES 

1. Index map showing geographic features of Cook Inlet region, southern Alaska, 

with inset showing extent of snow and ice cover on Redoubt Volcano 2 

2-4. Photographs showing: 

2. Redoubt Volcano and sun-ouuding Chigmit Mountains, southern 

Alaska 3 

3. Summit crater of Redoubt Volcano, southern Alaska 3 

4. Drift River delta, southem Alaska 4 

5. Graph showing silica content of products erupted during the four periods of 

Redoubt Volcano's history 4 

6-9. Photographs showing: 

6. A 200-m-high cliff on we:st side of canyon of Drift glacier showing 

early cone-building deposits 5 

7. Late cone-building colurrmar-jointed andesite flows tens of meters 

thick exposed on south flank of Redoubt Volcano 5 

8. Southwest flank of Redoubt Volcano showing deposits of pyroclastic 

apron erupted during late cone-building period 5 

Contents Ill 
IV Contents 

9. Radially fractured block exposed on south flank of Redoubt 

Volcano 6 

10. Graph showing summary of volcanic activity at Redoubt Volcano during the 

past 10,000 yrs 6 

11. Map of upper Cook Inlet and occurrence of tephra deposits correlated to Redoubt Volcano 7 

12-16. Photographs showing: 

12. Debris flow deposits greater than 10m thick in Drift River canyon 

adjacent to toe of Drift glacier 7 

13. View up Drift River canyon showing surface of debris flow 

deposits 8 

14. Finely laminated, ripple-crossbedded fine sand just upstream from 

Drift glacier 8 

15. View up Drift glacier toward summit of Redoubt Volcano 11 

16. View down Drift River valley across lower Drift glacier 12 

17. Graph showing average wind direction and speed for four levels in atmosphere 

above Anchorage and King Salmon, Alaska 12 

18. Map showing approximate areas likely to receive noticeable tephra fall from 

explosive eruptions of Redoubt Volcano 14 

19. Photograph showing small pyroclastic flow descending north flank of Augustine Volcano on March 30, 1986 19 

GLOSSARY 

[see "Appendix" for more detailed definitions of certain eruption-related products] 

altered rock. Rock that has undergone changes in its chemical and mineralogical 

composition since its formation; in the case of volcanic rocks, the changes are commonly due to penetration of heat, gases, and fluids derived from the cooling lava or the 

magma body--these changes may be accompanied by a decrease in rock strength 

andesite. A type of volcanic rock with a silica content between 53 and 63 percent, 

considered "intermediate" in silica content 

ash cloud. The cloud of hot volcanic gases and very small tephra fragments that is 

derived from, travels above, and usually outdistances pyroclastic flows 

ash plume. A cloud of volcanic gases and ash that is the downwind extension of an 

eruption column; the cloud is shaped by the wind, therefore the shape may be complex 

owing to variation in wind direction at different altitudes 

basalt. A type of volcanic rock with less than 53 percent silica, considered "low" in 

silica content 

bedding. A characteristic of sedimentary rocks in which planar surfaces separate texturally or compositionally different layers 

columnar joints. A prismatic jointing pattern resembling columns, resulting from 

contraction of thick lava flows or pyroclastic deposits during cooling 

convergent arc. A linear band of volcanoes formed in a zone of contraction of the 

Earth's crust, generally where two tectonic plates collide 

crater. A bowl-shaped topographic depression at a volcanic vent 

dacite. A type of volcanic rock containing between 63 and 70 percent silica, considered "high" in silica content 

density current. A current that is denser than the surrounding air or water because of 

the material it contains 
glacial surge. A rapid flow of glacial ice that results in thickening of the glacier or 

advance of the glacial front 

Holocene. The period of time from the end of the Pleistocene, I 0,000 years ago, to 

the present 

hydromagmatic. Relating to the interaction of water and magma 

infrasonic wave. A sound wave with a frequency below the audible range 

lithology. The mineral composition and texture of a rock 

mass balance. The change in mass of a glacier over some defined time interval as 

determined at a point, averaged over an area, or for the entire glacier 

Pleistocene. The period of time from I. 7 million years ago to I 0,000 years ago 

pyroclastic. Pertaining to pieces or fragments of rock formed by explosion at a volcanic vent 

recurrence interval. The average time interval between events 

silicic. Rich in silica; used to describe the silica content of an igneous rock 

stratigraphic record. The succession of rocks preserved in a given area, representing 

the geologic history of the area through time 

stratigraphy. The arrangement of rocks in a sequence, recording successive geologic 

events in time 

tephra. A collective term for particles of all sizes and compositions that are explosively emitted by a volcanic eruption 

topography. The shape of the Earth's surface, the configuration of valleys and ridges 

vent. An opening in the Earth's surface through which volcanic materials are erupted 

Contents V 

Redoubt Volcano, Southern Alaska: A Hazard Assessment 

Based on Eruptive Activity through 1968 

By Alison B. Till, M. Elizabeth Yount, and J.R. Riehle 

Abstract 

Redoubt Volcano is one of four composite volcanoes on 

the west side of Cook Inlet, Alaska, which have been active 

during Holocene time and which pose a hazard to the 

State's main population center. The volcano, a typical convergent-arc volcano, is composed of about 40 cubic kilometers (km3) of interbedded lava flows, rubble, and pyroclastic 

material, which are chiefly of andesitic composition. Minor 

basaltic and dacitic rocks make up a small part of the volcano. Most of the cone building occurred during middle and 

late Pleistocene time. At least 30 large tephra-forming eruptions have occurred during the past 1 0,000 years. Eruptions 

in 1902, 1966, 1968, and 1989-90 produced ash and generated floods on the Drift River, which drains the north side of 

the volcano, by melting part of the volcano's extensive glacial cover. 

The fieldwork and data analysis on which this report is 

based were completed before Redoubt erupted in 1989. The 

report was in final preparation stages at the time of the eruption. Because of this, discussion of events related to the 

1989-90 eruption are included in other reports. 

Tephra from future eruptions of Redoubt will probably 

be carried by prevailing winds to the northeast and could 

have a substantial impact on human activities as far as several hundred kilometers from the volcano. Half the State's population would be affected by such events. In addition, flows 

or floods in any of several flanking valleys could reach the 

sparsely inhabited coast of Cook Inlet, where seasonal fishing and recreational use occurs and an oil tanker termi nal is 

located, near the mouth of Drift River. Floods and debris 

flows large enough to reach the Drift River delta have occurred during historical time; they are thought to have been 

generated during eruptions. Less likely but potentially more 

hazardous events than the debris flows and floods that occurred during historical eruptions are partial cone collapse, 

prolonged ash-rich emissions, and generation of extremely 

large-volume debris flows. 

Manuscript approved for publication May 15, 1992. 

A least-likely scenario is the occurrence of highly explosive eruptions of silicic magma that would produce, in addition to the products of the events cited above, fast-moving 

pyroclastic flows in valley bottoms and (or) a lateral blast, 

the damaging effects of which could extend as far as 30 or 

40 kilometers from the volcano. Such· activity is precedented 

at Redoubt, as the oldest deposits of Redoubt Volcano are 

early Pleistocene dacitic pyroclastic deposits. Eruptions of 

dacitic material are potentially much more explosive than 

andesitic eruptions owing to the higher silica content of dacite. 

INTRODUCTION 

Redoubt Volcano, one of four active volcanoes on the 

west side of upper Cook Inlet in southern Alaska, is I70 

kilometers (km) southwest of Anchorage and 80 km west 

of Kenai (fig. I); other active volcanoes in the area are 

Mount Spurr, Mount Iliamna, and Augustine Volcano. Redoubt is an eroded cone-shaped mountain, mostly covered 

with snow and ice, that rises to a summit height of 3, I 08 

meters (m), some I ,500 to I ,800 m above the surrounding 

Chigmit Mountains (fig. 2). The volume of the cone is approximately 40 cubic kilometers (km3

). The volcano is 

drained on the north by the Drift River, on the east by 

Redoubt Creek, and on the south by the Crescent River, 

all of which flow into Cook Inlet. A I Yi-km-wide ice-filled 

summit crater is breached by a north-flowing glacier (informally known as the Drift glacier; fig. 3). Redoubt is the 

second most historically active of the Cook Inlet volcanoes, having erupted four times since I900 ( I902, I966, 

1968, I989-90). The vent active during the I966, I968, 

and 1989-90 eruptions is on the north edge of the summit 

crater (fig. I, inset; fig. 3). 

Redoubt has erupted at least 30 times during the past 

I 0,000 years. The effects of these eruptions have been 

recognized as far as 240 km from the volcano. Half of 

Alaska's population lives downwind from the volcano in 

the Cook Inlet area; the distal effects of future tephra 

eruptions of Redoubt could reach this population readily. 

Introduction 
L, 

, .J 

LAKE CLARK 

NATIONAL 

PARK 

l/r---, _j-'--''--'----,, 

A Hayes Volcano 

--i 

! I A Mount Spurr 

i 

i_l 

i 

\ ----1 

r_J>_J 

·-.....r-1 

S ' River 

.? 

50 

I 

100 

I 

Palmer 

• 

150 

I 

200 KILOMETERS 

_j 

Figure 1. Index map showing geographic features of Cook Inlet region, southern Alaska. Inset shows extent of snow and ice cover 

Redoubt Volcano. 

2 Redoubt Volcano, Alaska: Hazard Assessment Based on Eruptive Activity through 1968 
Human activity near the volcano is limited. The volcano is 

located in Lake Clark National Park and is infrequently 

used for recreational purposes on a seasonal basis. During 

the summer fishing season, set-netters work the beaches 

near Redoubt and boats fish directly offshore. The Drift 

River terminal, an oil storage and tanker terminal serving 

offshore oil platforms in western Cook Inlet, approximately 35 km downstream from Redoubt near the mouth of the 

Drift River, is particularly vulnerable to eruption-related 

floods (figs. I and 4). Air traffic from Anchorage to communities in southwest Alaska and the Orient passes near 

the volcano and could be seriously affected during periods 

of eruptive activity. 

No detailed geologic mapping or hazard assessment of 

Redoubt was done prior to this investigation. Effects of the 

1966 and 1968 eruptions on the mass balance of Drift glacier have been studied by Sturm and others ( 1983, 1986). 

Figure 2. Redoubt Volcano and surround ing Ch igmit Mountains, southern Alaska; Cook Inlet in foreground . View from 

east. 

Riehle and Emmel (1980) and Riehle and others (1981) 

recognized 3,500-year-old volcanic debris flow deposits 

(lahars) in sea-cliff exposures southeast of the volcano. In 

a study of ash deposited in the upper Cook Inlet region 

within the past 10,000 years, Riehle (1985) identified at 

least 30 ash layers that were produced by eruptions of Redoubt. Forbes and others ( 1969) and Pulpan and Kienle 

(1979, 1980) published chemical analyses of volcanic rock 

collected during reconnaissance work. 

The fieldwork and data analysis on which this report is 

based were completed before Redoubt erupted in late 1989. 

The report was in final preparation stages at the time of the 

eruption. Because of this, little discussion of events related 

to the 1989-90 eruption is included. Detailed discussions of 

the 1989-90 eruption are included in other reports. 

Mapping of Redoubt's cone and the valleys that drain 

it provides the basis for reconstructing its geologic history 

and for assessing the volcanic hazards associated with future eruptions. In assessing hazards to life and property we 

considered the range of past eruptive behavior at Redoubt 

as well as that at other volcanoes of similar type and eruptive style. The past eruptive behavior· of Redoubt, as indicated by the type, age, and volume of deposits, is a specific 

analog for its future behavior, although a change in eruptive style is possible. Also, the types of past eruptions of 

similar volcanoes can be used as analogs, in a generalized 

sense, to aid in identifying types or magnitudes of unprecedented eruptions that might occur at Redoubt. 

The geologic history of Redoubt is briefly outlined 

here because it has not been described elsewhere. An assessment of potential hazards from future eruptions follows. A description of volcanic processes and resulting 

deposits is included in the "Appendix." 

Figure 3. Summit crater of Redoubt Volcano; true summit is· on left 

(east) side. Drift gl ac ier drains northward past 1966-68 vent (shown by 

circle). View from northwest. 

Introduction 3 
GEOLOGIC HISTORY recorded by hot pyroclastic flow deposits and shallow intrusive rocks, an early cone-building period recorded by 

thin lava flows and interlayers of volcanic rubble, a late 

cone-building period recorded by thick lava flows and voluminous pyroclastic deposits, and the latest period recorded by voluminous debris flow deposits and widespread 

tephra deposits. Each period encompasses many individual 

eruptions that are characterized by a similarity in style and 

general lava composition. Volcanic activity at Redoubt 

probably began more than 400,000 years ago, but most of 

the cone has been built in the past 200,000 years. Volcanic 

products of the early explosive period are largely dacitic 

(62-68 percent Si02). Basalt and basaltic andesite (49-59.5 

percent Si02) erupted during the early cone-building period around 190,000 years ago (J. VonEssen, written commun., 1988); later cone-building flows and pyroclastic 

deposits are andesitic in composition (54-62 percent 

Si02). Holocene tephra deposits of Redoubt are silicic andesite (59-61.5 percent Si02). After the early cone-building period, average magma composition has generally 

become more silicic with time (fig. 5). 

Introduction 

The geologic history of Redoubt Volcano can be divided into four eruptive periods: an early explosive period 

Pleistocene History 

Figure 4. View across Drift River delta toward river mouth. 

Buildings in upper left are Drift River terminal, a storage and 

shipment point for oil produced in Cook Inlet. 

Deposits of the early explosive period crop out low 

on the north, south, and southeast flanks of the volcano. 

BASALT 

50 55 

ANDESITE DACITE 

~ Products of recent eruptions 

I 

Products of late 

cone building 

Products of early cone building 

Products of early 

explosive phase 

60 65 70 

SILICA CONTENT, IN PERCENT 

Figure 5. Silica content of products erupted during the four periods of Redoubt Volcano's history. Recent phase represents analyses of lithic fragments collected from thin 

air-fall deposits that mantle ridges on north and east sides of volcano and may have 

erupted as recently as 1966 or 1968. Redoubt Volcano is in southern Alaska. 

4 Redoubt Volcano, Alaska: Hazard Assessment Based on Eruptive Activity through 1968 
They consist of debris flow and glacial deposits containing 

dacitic pumice, dacitic pyroclastic deposits, and a dacitic 

rock mass that may be a shallow intrusion or dome. The 

magma extruded during these eruptions was the most silicic in Redoubt's history (fig. 5), implying that these may 

have been Redoubt's most explosive eruptions. 

The present cone began forming during the succeeding early cone-building period, during which numerous 

basalt and basaltic andesite lava flows and less tephra 

erupted. Exposures of these thin flows (each less than 6 m 

thick) have distinctive color bands: the flows are gray to 

black, and intervening layers of tephra and rubble are red. 

The banded sequences are exposed on all sides of the volcano and provide a marker horizon for determining relative ages of deposits (fig. 6). 

Andesitic lava flows and a voluminous apron of pyroclastic debris erupted during the late cone-building period. 

These thick (30-60 m) columnar-jointed lava flows form 

the upper edifice of the volcano and also occur low on the 

north flank (fig. 7). The pyroclastic apron consists of unconsolidated, poorly sorted deposits that interfinger with 

the lava flows and dominate the southern slope of Redoubt 

(fig. 8). The. deposits are sand-sized tephra to bouldersized blocks of andesitic lava. Many of the blocks are radially fractured (fig. 9); such fractures indicate that they 

were transported while hot, in avalanches shed from lava 

flows or pyroclastic flows. 

Holocene History 

The eruptive history of Redoubt Volcano during Holocene time (the past 10,000 years) is summarized in figure 

10. The prehistoric eruption record was inferred from the 

Figure 6. A 200-m-high cliff on west side of canyon of Drift 

glacier, southern Alaska, exposes early cone-building deposits of Redoubt Volcano; light bands are thin lava flows, dark 

layers are rubble and ash. 

examination and dating of Holocene tephra deposits identified at 31 localities in the Cook Inlet region (Riehle, 

1985). Locations at which Redoubt tephra was identified 

are shown in figure 11. At least 30 tephra layers from Holocene eruptions of Redoubt were identified within 22 km 

of the volcano. Tephra deposits correlative with Redoubt 

Volcano on the basis of chemical and mineralogic similarity to one or more proximal Redoubt deposits are identifiable at least as far as Tyonek (112 km northeast), and 

perhaps as far as Anchorage and Talkeetna (as far as 240 

km away) (figs. I, 11). The 1966 and 1968 eruptions of 

Redoubt did not produce enough tephra to be identified at 

the sites studied by Riehle; this absence indicates that the 

Holocene tephra record includes only deposits larger than 

those produced during the 1966 and 1968 eruptions. Eruptions of a size sufficient to impact human activity also 

Figure 7. Late cone-building columnar-jointed andesite flows 

tens of meters thick exposed on south flank of Redoubt Volcano, southern Alaska. 

Figure 8. View west along southwest flank of Redoubt Volcano, southern Alaska, showing deposits of pyroclastic apron 

eru pted during late cone-building period. Width of view approximately 0.8 km. 

Geologic History 5 
may not be large enough to produce deposits in the stratigraphic record. Therefore, during Holocene time there 

may have been many more than 30 eruptions of sufficient 

size to have affected human activity. 

Many more eruptions occurred during the Holocene 

than the 30 eruptions documented in the tephra stratigraphy. Such minor eruptions produced several very thin tephra deposits as far as 160 km east of the volcano on the 

Kenai Peninsula. These deposits are difficult to adequately 

sample for identification, but they are generally similar in 

composition to Redoubt tephra deposits. The deposits 

probably represent several eruptions of Redoubt Volcano 

during the past one to two thousand years (Douglas Reger, 

written commun., 1986; dating based on proximity of the 

deposits to archeological horizons). Although no data exist 

on the distribution of Holocene tephra to the west or south, 

wind data indicate that most tephra deposits are likely to 

be found in the northeast quadrant (fig. 11). 

Figure 9. Radially fractured block, 3 m in diameter, exposed 

on south flank of Redoubt Volcano, southern Alaska. Fracturing indicates that block was deposited hot and cooled in 

place; otherwise it would likely have disintegrated during 

transport. 

Eruptions of the size represented by tephra in the stratigraphy likely produced numerous pyroclastic flows and 

6000 

5000 

4000 

3000 

Twenty three tephra layers deposited at 

Bear Lake, 22 km east of Redoubt 1 

c_j 2000 

a:i Crescent River mudflows 2 

UJfooo 

~ 0 Seven tephra layers deposited at Bear Lake 1 0 

1000 

ci 1700 <i. ........,... -..SCALE CHANGE~ 

1700 

1800 

Present 

1778 "White smoke but no fire" observed by 

Captain James Cook 3 

1819}Wat er vapor(?); in 1850, reported "smoking since 1819" by Wrangell and others 4 

1850 alternat e translation: "Smoking in 1819" 5 

/

Explosions 6; ashfall from Lake Clark to 

the Skwentna valley 7 

1900 1902 L.Water vapor(?) emissions 8 

:11965 : Water vapor emitted from new 

fissures 9 1 0 

1950 1966: Explosive ash bursts, outburst 

floods 11, 12 

:--11967: "Series of clouds" 13 

'\ l}-968 : Explosive ash bursts 13,14 

2000 

1989-90 Explosive ash burst, flooding 15 

1 Riehle, 198 5 

2 Riehle & others, 1981 

3 Beagle hole, 196 7 

4 Petroff, 1884 

5 Dall, 1870 

6 The Alaskan (Sitka) March 29 , 1902 

7 Martin & Katz, 1912 

8 Anchorage Daily Times, May 23, 1933 

9 Anchorage, Daily News, Jan. 29-30, 1965 

10 Fairbanks News-Miner, Feb. 4, 1965 

11 Anchorage Daily News, Jan. 25-29, 1966 

12 Sturm & others, 1986 

13 Wilson & Forbes, 1969 

14 Anchorage Daily News, Jan. 7, 1968 

15 Alaska Volcano Observatory, 1990 

Figure 10. Volcanic activity at Redoubt Volcano, southern Alaska, duri ng the past 10,000 yrs. Oldest events shown at top. 

6 Redoubt Volcano, Alaska: Hazard Assessment Based on Eruptive Activity through 1968 
(or) debris flows, by analogy with other volcanoes. Deposits from these events may exist on Redoubt but are indis·-

tinguishable from older deposits owing to similarity or 

extensive reworking. Indirect estimation of the likely size 

of the deposits related to the large Holocene eruptions is 

hampered by the lack of thickness and distribution data for 

specific tephra beds in the regional stratigraphy. 

Approximately 0.5 km3 of volcanic debris was deposited in the Crescent River valley about 3,500 years ago. 

These debris flow deposits contain abundant material derived from the south slope of Redoubt. The debris flows 

reached Cook Inlet, 32 km away (where deposits are 2-5 

m thick), and filled the valley from Squarehead Cove on 

Tuxedni Bay to Polly Creek (pl. 1, map A). Part of the 

deposit forms the natural dam that contains Crescent Lake 

(Riehle and others, 1981 ). Riehle and others ( 1981) interpreted these deposits to be the result of a cone-destructive 

eruption of Redoubt and suggested that the hummocky topography of the natural dam at Crescent Lake might have 

60 

50 

e Sample srtes wrth Re doubt 

tephr a laye rs 

0 Sa m p le srtes w rt h teph ra 

layers belreved to be from 

Redoubt 

o Sample srtes w rt h no rde nt rfr ed Redoubt t eph ra lay e rs 

.A. Volcano 

100 KI LOME T ER S 

Figure 11. Upper Cook Inlet, southern Alaska, and occurrence of tephra deposits correlated to Redoubt Volcano 

(modified from Riehle, 1985). Samples of tephra layers from 

Christianson Lake near Talkeetna and Circle Lake near Homer (see fig. 1 ), beyond the limits of this figure, may also be 

from Redoubt (Riehle, unpub. data, 1985; samples provided 

by Thomas Ager). 

been formed by a debris avalanche. It is unclear from their 

study what the temperature of emplacement of the debris 

flow deposits might have been, and no specific evidence 

relating the deposits to an eruption has been found. Theirs 

is not, however, an unreasonable hypothesis. 

After 3,500 years ago, no major debri:o flows were deposited in the Crescent River valley. Activity moved to the 

Drift River valley, possibly as the result of a shift in location of the volcano's vent. Debris flow deposits in the 

Drift River valley are younger and smaller in volume than 

those in the Crescent River valley, and they have been 

generated fairly frequently. Four to six debris flows were 

deposited on the Drift River after about 2,000 years ago 

and probably within the past 200 to 400 years. Drift River 

deposits are composed largely of sand in contrast to the 

bimodal, boulder-in-clay composition common in those of 

the Crescent River (fig. 12). Clasts in the Drift River debris flow deposits are dominated by a single lithology. Material carried in the Drift River debris flows was probably 

derived from ejection of hot pyroclastic material onto 

snow and ice or mobilization of unconsolidated deposits 

on the cone by large amounts of water. Drift River debris 

flows apparently had a higher ratio of water-to-rock debris 

than the Crescent River debris flows when deposited; the 

Drift River flows were mapped 32 km downstream from 

the volcano (pl. 1, map A). Some of these debris flows 

deposited boulders with mean diameters of 0.5 to 2 m as 

far as 19 km downstream from the volcano (13 km upstream from the Drift River terminal site) (fig. 13). Ice 

rafting may have transported the few anomalously large 

boulders observed on the lower reaches of Drift River. The 

debris flow deposits grade into flood deposits farther down 

the drainage. 

Figure 12. Debris flow deposits greater than 1 0 m thick in 

Drift River Canyon, southern Alaska, adjacent to toe of Drift 

glacier. 

Geologic History 7 
Lake deposits are present at one locality in the Drift 

River valley immediately upstream from the Drift glacier 

(pl. 1, map A). There, tan, planar-laminated, locally ripplecrossbedded fine sand is interlayered with gray, coarse 

sand- to pebble-sized tephra layers (fig. 14 ). The deposit is 

a minimum of 1.5 m thick, and it may have formed when 

the river was dammed either by volcanic debris or by the 

advance of the Drift glacier. In either case, subsequent catastrophic failure of the unstable dam (a common occurrence in glacier- and debris-dammed lakes) would have 

caused flooding on Drift River. However, there is no direct 

evidence that debris flow deposits downstream from the 

glacier formed in this manner. 

An area of altered rock is present on the east flank of 

Redoubt (pl. 1, map A). Although the age and origin of 

these rocks are not known, they are important in assessing 

volcanic hazards, because they may constitute a weakened 

part of the cone. Lava flows and pyroclastic deposits within this area are orange, rust, mustard, and locally green 

from weathering or alteration. The varicolored stains coat 

clasts and fractures and are locally dispersed through the 

matrix of pyroclastic deposits. Phenocrysts in lava flows 

are altered to clays; silicified material occurs locally. Unlike typical volcanic deposits, bedding of these altered 

flows dips uniformly toward the summit of the volcano. 

The bedding orientation is probably not related to an adjacent volcanic edifice east of Redoubt, inasmuch as no associated deposits have been found. The strata on the east 

flank of the mountain were hydrothermally altered and 

probably slid as a coherent block (like a large landslide) 

down that flank at some time during the past 10,000 years. 

Pumice fragments were found sitting on and intergrown with mats of vegetation on ridges east, south, 

southwest, and north of Redoubt. The presence of the 

pumice indicates that eruptions of Redoubt during the last 

1,000 to 2,000 years have produced more than fine ash. 

Figure 13. View up Drift River canyon, southern Alaska, 

showing surface of debris flow deposits originating at Redoubt Volcano; large boulders on left were deposited by 

1966 debris flows. Photograph taken approximately 9 km 

from Drift glacier terminus. 

Historical Record 

Records of volcanic activity in the Cook Inlet region 

before the late 19th century are found in notes and diaries 

of explorers. Volcanoes were often observed to be "smoking," but in most cases the smoke was probably water 

vapor rather than tephra. Captain James Cook in 1778 observed that Redoubt was "emitting a white smoke but no 

fire" (Beaglehole, 1967, p. 370). In 1794, George Vancouver saw Redoubt but apparently did not note any "smoke" 

(Lamb, 1984). In an 1850 compilation of volcanic phenomena in Alaska, Grewingk attributed to Baron von 

Wrangell the observation that Redoubt "had been smoking 

since 1819" (Petroff, 1884) or "in 1819 the Redoubt Volcano of Cook's Inlet smoked" (Dall, 1870, p. 499). In our 

opinion, these early reports are not accounts of actual 

eruptions. 

In 1902, explosive eruptions of Redoubt deposited 

tephra from Lake Clark to the Skwentna valley (Martin 

and Katz, 1912). Tephra fell on the settlement of Hope, on 

the Kenai Peninsula (fig. 1), January 22 and repeated explosions were heard there February 17 and 18 (The Alaskan, Sitka, March 29, 1902). An English tourist reported 

that "at Kenai and Kusiloff [Kasilof]***[tephra] lay on the 

ground several inches thick in places" (Cane, 1903). This 

is probably an exaggeration, because there is no sign of 

1902 tephra in terrestrial sites or in lake cores near Kasilof 

or Kenai (Thomas Ager, written commun., 1985). Tephra 

deposits from this eruption have been recognized elsewhere, however, and an analysis of tephra from this eruption was published by Pulpan and Kienle (1979). 

Following an earthquake in May 1933, Redoubt was 

observed "belching smoke," which was unusual since it 

"had never been known to emit anything more than a 

slight wisp of smoke" (Fairbanks Daily News-Miner, May 

25, 1933). The report does not mention tephra or that the 

Figure 14. Finely laminated, ripple-crossbedded fine sand 

just upstream from Drift glacier, southern Alaska, interpreted 

to be lake deposits (see pl. 1 for location). Pencil for scale. 

8 Redoubt Volcano, Alaska: Hazard Assessment Based on Eruptive Activity through 1968 
smoke was dark in color. Hence, it seems likely that the 

activity was water-vapor emissions rather than an eruption. 

In January and February of 1965, pilots reported that 

five fissures had opened on the southeast wall of Redoubt 

and were emitting water vapor and hot gases (Anchorage 

Daily News, January 29 and 30, 1965; Fairbanks Daily 

News-Miner, February 4, 1965). One year later on January 

24, 1966, Redoubt erupted. "Giant black puffs" of tephra 

rising 6,000 m above the summit and tephra falling over a 

24-km radius were reported (Anchorage Daily News, January 25, 1966). A 22-man seismic crew camping and working 

along the lower Drift River was evacuated following flooding of the river as a result of the eruption. According to their 

account (Anchorage Daily News, January 26, 1966), the icebound river broke up suddenly and water rose 1 to 1.2 m in 

15 minutes, carrying chunks of ice "the size of a D-7 cat". 

The flood crested and conditions returned to normal within 

about 30 minutes. During this flood, boulders with mean 

diameters of 1 to 2 m were deposited within approximately 

3 to 5 km of the volcano. Other smaller floods occurred 

through February and March (Sturm and others, 1986). 

Infrasonic waves attributed to explosive eruptions of 

Redoubt were recorded 550 km away at College, Alaska, 

during two periods: six explosions from January 24 to February 20, 1966, and five explosions from December 7, 

1967, to April 28, 1968 (Wilson and Forbes, 1969). Wilson 

and Forbes also reported "another series of clouds" erupted 

from January 1 to 15, 1967, that reached altitudes of 6,000 

m, but no accompanying infrasonic waves were recorded. 

Sturm and others (1986), in a study of long-term effects 

of the eruptions on the Drift glacier that drains the summit 

crater, estimated that 60,000,000 m3 (.06 km3) of ice were 

"blasted, melted, scoured, or washed away by the cumulative events of 1966-68." When parts of the glacier that were 

separated by the eruption reconnected, a dramatic thickening of the lower glacier occurred and the surface speed 

increased by a factor of ten. This surge apparently culminated in 1986 without causing a damming of the Drift River. 

In mid-December 1989, anomalous seismic activity 

developed under Redoubt and on December 14 the volcano started a series of ash-rich eruptions (Alaska Volcano 

Observatory Staff, 1990). During late December and January, dome building alternated with explosive activity. A 

large flood inundated the Drift River drainage and reached 

as far as Cook Inlet (Alaska Volcano Observatory Staff, 

1990). Ash clouds generated during explosive events 

caused damage to aircraft and interrupted air traffic. Oil 

production in Cook Inlet was temporarily curtailed because of the flood and hazards associated with the ongoing 

eruption (Alaska Volcano Observatory Staff, 1990). 

Summary 

Initial activity of Redoubt Volcano in early Pleistocene time included explosive eruptions of dacitic material. 

The composition of the subsequent cone-building deposits, 

which included both air-fall debris and lava flows, evolved 

from basalt and low-silica andesite in the early period to 

andesite in the late period. Holocene eruptions have produced andesitic and dacitic air-fall debris and lava domes, 

but no lava flows; some destruction of the cone has occurred. Large debris flows and floods have inundated major valleys that drain the volcano, a result of closely 

coupled glacial and volcanic processes. At least 30 eruptions of Redoubt have produced enough tephra to be recorded in the stratigraphy of upper Cook Inlet in the past 

10,000 years. Approximately 3,500 years ago, debris flows 

and possibly a debris avalanche deposited abundant material in the Crescent River valley. These deposits may have 

resulted from an eruption. Since then, minor eruptions 

have produced debris flows originating in drainages on the 

north side of the volcano, domes at the vent, and lapillisized tephra fragments. In the Drift River valley, coupling 

of volcanic and glacial processes may have resulted in formation and subsequent collapse of ice or debris dams, creating floods downstream up to 20 years after an eruption. 

VOLCANIC HAZARDS 

The fundamental assumption made in this assessment 

is that Redoubt's past eruptive style indicates its future 

eruptive style, an assumption proven reasonable for other 

volcanoes, such as Mount St. Helens in Washington (Crandell and Mullineaux, 1975, 1978). However, another factor 

to be considered is that eruptions of a type or magnitude 

unprecedented in Redoubt's past but like those that have 

occurred at similar volcanoes may also occur in the future. 

Life- and property-threatening effects of volcanic 

eruptions decrease in severity with distance from a volcano. The safe distance from the volcano depends on the 

magnitude and explosivity of the eruption and the nature 

of the eruptive products. 

Explosivity depends chiefly on viscosity and volatile 

content of the magma or erupted material. The composition 

of the magma erupted influences its explosivity. Low-silica 

basaltic lava is fluid (has low viscosity), which allows easy 

escape of gases, and is therefore not explosive. Gases 

trapped by pasty, viscous magma escape violently upon 

eruption. The higher the silica content, and (or) the lower 

the temperature of the magma, the higher the viscosity and 

the greater the potential explosivity of the eruption. 

Explosive activity can also be caused by factors independent of magma composition, viscosity, and volatile 

content. Rapid heating of large volumes of ground water 

and surface water can lead to highly explosive hydromagmatic eruptions. Therefore, magma rising into a groundwater system or water circulating into a magma system 

may cause magma to erupt explosively, independent of its 

composition and gas content. 

Volcanic Hazards 9 
To judge the potential effects of an eruption, the types 

of events likely to occur are also an important factor. For 

instance, debris avalanches, debris flows, floods, and large 

pyroclastic flows generally travel farther and faster from 

their source than lava flows. Lateral blasts may affect 

large areas and may be unimpeded by topographic barriers. The safe distance, therefore, is strongly dependent on 

the type of events associated with an eruption. Moreover, 

eruptive effects are interrelated; hot tephra fall or pyroclastic flows mixed with snow and ice can generate debris 

flows and floods. 

Finally, the size of the eruption is critical in determining its potential hazard. Products from small eruptions 

may be flowage deposits limited to drainages immediately 

around the volcano and a small tephra deposit; large eruptions may affect both valleys and highlands in a sizable 

area around the volcano. Large tephra plumes, or a large 

volume of tephra erupted over a prolonged period, can affect large areas at great distances from the volcano. 

The 1966-68 eruptions of Redoubt produced material 

containing 59 to 61.5 percent silica (fig. 5). Products of 

future eruptions will likely be of material similar to or 

slightly more silica rich than the recent products, that is, 

silica-rich andesite or possibly dacite. If the magma produced has a high silica content, it will be viscous and contain more volatiles. Future eruptions, therefore, are likely 

to be explosive, regardless of their size and duration. 

The size of eruptions is difficult to predict. The 1966-

68 eruptions produced minor tephra deposits and floods 

that are probably typical of the least intense and the most 

frequent type of eruption at Redoubt. The 1966-68 events 

are useful as a model for high-frequency, low-volume debris flows and floods on the Drift River, bqt they are not 

representative of larger eruptions such as the 30 significant 

tephra-forming eruptions observed in the Holocene stratigraphy of the Cook Inlet basin. 

Importantly, the fact that floods reached the Drift 

River delta as a result of historical eruptions indicates that 

eruptions need not be as large as the 30 recorded in Holocene stratigraphy to have an impact on the population in 

south-central Alaska. Smaller eruptions could certainly 

have a noticeable effect on human activities in upper Cook 

Inlet if tephra dispersal were directed toward population 

centers. The eruptions of Redoubt in 1966-68 and Augustine Volcano in 1976 and 1986, for example, are not considered significant in volume of tephra produced, and yet 

the Augustine eruptions did impact human activities 

(Yount and others, 1987). 

Some hazardous events can occur without an eruption. Crandell ( 1971) described a debris flow deposit that 

extends 35 km from Mount Rainier and formed at a time 

for which there is no evidence of an eruption. The debris 

flow deposit is clay rich and originated in an area of altered lava flows on the flank of the mountain. A debris 

flow or debris avalanche could originate on the east flank 

of Redoubt in a similar fashion, without an associated 

eruption. However, Crandell (1971) noted that such flows 

or avalanches are most commonly triggered by earthquake 

or volcanic activity. 

Large eruptions of Redoubt might result in catastrophic failure of the volcano's flank. A lateral blast, debris avalanche, and (or) large-volume debris flow might 

occur. Blockage of major drainages-c' around the volcano 

and movement of large debris flows as far as tidewater are 

possible and precedented at Redoubt. Pyroclastic flows, 

domes, and lava flows on the cone could trigger landslides 

and debris flows. Large volumes of tephra, from single or 

repeated eruption events, could be directed toward the upper Cook Inlet area by the prevailing winds and would 

affect population centers. 

Although the duration of an eruption of a circum-Pacific volcano can range from several hours to many years, 

most eruptions last no longer than several months, with 

their most violent phases lasting several days. For example, the 1986 eruption of Augustine Volcano consisted of 

three explosive phases, each 5 to 10 days long, spanning a 

5 month period. Within each of the three phases, the intensity of explosive activity fluctuated considerably (Yount 

and others, 1987). Accounts by Wilson and Forbes ( 1969) 

suggested similar fluctuations occurred during the 1966 

and 1968 eruptions of Redoubt. 

The Hazard Map 

The area around the volcano was divided into four 

hazard zones on the basis of hazards that arise from different types of eruptive activity as well as different sizes of 

eruption (plate 1, map B). Certain zone boundaries can be 

located precisely owing to the overriding influence of topography on distribution of debris flows, pyroclastic flows, 

and lava flows. Other zone boundaries are approximate, 

and their placement is more arbitrary, as discussed below. 

For any future eruptions, the violence and duration of the 

eruption and the position of the vent will influence the location and extent of the hazards created; therefore, the 

hazard zones shown on plate 1 should be considered 

guides, rather than rigid predictions. 

Hazard zones were differentiated by types of deposits 

(pl. 1, map B). Zones A and B are "flowage-hazard zones" 

delineating hazards created by eruption products flowing 

down valleys. The extent of zones A and B is limited by 

the topography, the nature of the eruptive products, and 

the size of the eruption. Zones A and B delineate hazard 

zones due to lava flows, pyroclastic flows, debris avalanches, debris flows, and floods. Zones A 1 and A2 are 

most likely to be impacted by debris flows and floods generated by the present vent or any vent in the summit crater. Zone A 1 is likely to be affected by products of any 

eruption, including eruptions similar to or smaller in mag1 0 Redoubt Volcano, Alaska: Hazard Assessment Based on Eruptive Activity through 1968 
nitude than the 1966 and 1968 events. During a single 

small eruption, the zone might be repeatedly inundated. 

Zone A2 is likely to be affected by short periods of flooding during a small eruption from the summit crater or inundated by one or several large-volume debris flows 

during a larger eruption. Both zones A 1 and A2 could be 

affected by outburst flooding as a result of damming of 

Drift River by pyroclastic flows, glacial advances, or avalanches. Zone B delineates areas that, in addition to A 1 

and A2, are likely to be affected by debris avalanches, debris flows, floods, pyroclastic flows and related ash-cloud 

surges and (or) lava flows during an eruption larger than 

those in 1966 and 1968. Parts of zone B could also be 

affected by eruptions of any size from a vent outside the 

summit crater. 

Zone C and zone D (on the inset of map B, pl. 1) 

represent areas that may be affected by hot tephra clouds 

and lateral blasts; these areas are hard to define precisely 

owing to the extreme mobility of clouds and blasts. The 

size and direction of travel of those clouds and blasts will 

determine the extent of the area affected. The boundary of 

zone C was arbitrarily placed at the ridge crest closest to 

Redoubt to represent the likely distribution of hot gas and 

tephra clouds in a moderate to large eruption. It is common for clouds or blasts to overtop ridges; the divide at 

the edge of zone C varies considerably in height along its 

length. Large blasts and clouds will likely travel outside 

the boundaries of zone C, especially where the divide is 

low. The 35-km radius indicated in the inset as zone D 

shows the largest area likely to be affected by a large lateral blast (Crandell and Hoblitt, 1986). At Mount St. 

Helens, the lateral blast of the May 18, 1980, eruption leveled trees as far as 29 km away (U.S. Geological Survey, 

1981). 

Once the location of the vent is known for a given 

eruption, the areas in zones A and B most likely to be 

affected can be better delineated. The amount of ice and 

loose rock debris directly around the vents controls the 

supply of material to debris flows and floods. The particular drainage affected by debris flows or floods depends on 

the location of the vent. The most likely vent position during the next eruption is near or at the site of the vents 

active in 1966, 1968, and 1989-90, on the north side at 

the head of the Drift glacier. 

The north and the south sides of the cone contain different amounts of ice and unconsolidated material. The 

large volume of ice and small amount of unconsolidated 

material on the north side indicates that floods are more 

likely than debris flows in the Drift River drainage as a 

result of a small eruption. The south flank of the volcano is 

underlain by a large fan of unconsolidated pyroclastic deposits that could easily be mobilized by meltwater to produce large debris flows during a small eruption. Depending 

on the location of the vent, an eruption could produce large 

debris flows and floods on any side of the volcano. 

Sturm and others (1983, 1986) attributed the destruction of upper Drift glacier during the 1966 eruption to 

both melting and explosive disruption. They hypothesized 

that the large burst of water that formed the first 1966 

flood was derived from subglacially ponded meltwater, 

produced as the vent heated before the eruption and released by the eruption. The accumulation area for the Drift 

glacier is the summit crater; the outlet from the crater is 

the site of the most recent vent (figs. 3, 15). Heating of 

rocks in the vent area may melt a large quantity of ice 

before any other manifestations of an eruption are evident. 

In addition, the mass of the glacier extending immediately 

down-drainage from the vent and glaciers on the north 

fl ank are likely to be melted by hot eruptive debris. These 

two sources of water may combine to produce a large volume of water that would be released into the Drift River 

valley during the course of an eruption (fig. 16). 

Figure 15. View up Drift glacier toward summit of Redoubt 

Volcano, southern Alaska; glacier surface in foreground covered with deposits from 1966-68 debris flows; arrow marks 

site of 1966-68 vent. 

Volcanic Hazards 11 
Sturm and others (1986) showed that eruptions may 

result in surges of the Drift glacier that could dam the Drift 

River. Their data show that surges occur as the glacier attempts to reestablish equilibrium after its upper reaches 

have been altered or destroyed by an eruption. Associated 

advance of the glacier could temporarily block the Drift 

River and create a lake on the upstream side. Failure of the 

ice dam would result in flooding of the river and possibly 

endanger the oil terminal at the coast. Flood hazard created 

by glacial damming of Drift River could culminate as long 

as 10 to 20 years after the eruption. However, surging of 

the Drift glacier without damming of the Drift River was 

documented after the 1966-68 eruptions (Sturm and others, 1986), indicating that there is not a necessary relationship between ice damming and eruptive activity. 

A large eruption of Redoubt could result in a lateral 

blast, caused by catastrophic failure of a side of the cone as 

magma moved up into it. Lateral blasts have occurred several times at Mount St. Helens and Lassen Peak (Crandell 

and Hoblitt, 1986). In general, blasts at Redoubt occur infrequently and are smaller than the lateral blast from Mount 

St. Helens in 1980 (Crandell and Hoblitt, 1986). A lateral 

blast can occur on any side of a volcano, but weakened 

rock provides a preferred pathway for magma entering the 

cone. The presence of a large volume of altered rock on the 

east side of Redoubt suggests that side of the cone may be 

structurally weak (pl. 1, map A). If this is the case, the east 

flank of the mountain would provide a pathway for eruption 

of gas, lava, or a lateral blast in the future. 

Tephra Distribution 

Tephra dispersal is likely to have the greatest impact on 

human activity in the Cook Inlet area. None of the hazard 

Figure 16. View down Drift River valley, southern Alaska, 

across lower Drift glacier (foreground). Glacier confines Drift 

River against north wall of valley. Volcano lies right of field 

of view. Drift River oil terminal is 35 km downstream, out of 

sight to right. 

zones shown on plate 1, map B indicate the distribution of 

tephra that could be expected from future eruptions of Redoubt because the distribution can be expected to extend 

beyond the area of the plate. Factors that affect the distribution of tephra are the altitude reached by the tephra-laden 

eruption column, the speed and direction of winds at altitudes reached by the column, the size and density of the 

contained particles, the amount of precipitation during the 

eruption, and the length and continuity of the eruption. 

In February 1966, an explosion at Redoubt sent tephra to 13,700 m (Wilson and others, 1966). Small eruptions 

such as those that occurred in 1966 and 1968 from Redoubt's most recently active vent at an elevation of 2,500 

m can be expected to propel tephra as far as 12,000 m 

above the vent. Larger eruptions can be expected to produce larger and higher eruption columns. The dramatic 

May 18, 1980 eruption of Mount St. Helens propelled 

tephra more than 25,000 m into the atmosphere (SarnaWojcicki and others, 1981). Scheduled and private air traffic from Anchorage to other points in Alaska and the 

Orient passes near Redoubt; an eruption column reaching 

12,000 m would intersect both high- and low-level flight 

routes used daily by commercial and private aircraft. 

Summaries of upper air wind data for Anchorage and 

King Salmon were compiled to characterize the prevailing 

wind conditions in the region (fig. 17). Data for Anchorage should more accurately reflect winds at Redoubt inasmuch as the volcano is on the east side of the Alaska 

Range and closer to Anchorage. Winds 3,000 m and higher above Anchorage more commonly blow toward the 

north and east rather than the south and west. Thus, it is 

highly likely that an eruption of Redoubt would result in 

tephra fall on the upper Cook Inlet region and the south 

flank of the Alaska Range. If winds were more typical of 

King Salmon, tephra might be distributed in a northeast or 

southeast direction, depending on the season. 

Any future eruptions of Redoubt Volcano can be expected to send tephra at least tens of thousands of meters into the 

atmosphere; most often, prevailing winds would blow tephra 

toward the north and east. Heavier concentrations of tephra 

will be deposited directly downwind from the volcano following more violent explosions. If an eruption is prolonged, 

the direction of tephra transport would vary as wind direction changes. It is likely that small amounts of tephra would 

be distributed over a wide area south of the Alaska Range 

and east of the Chigmit Mountains (fig. 18). If heavy precipitation occurs during an eruption, tephra would be washed 

...... 

Figure 17. Average wind direction and speed for four levels 

in atmosphere above Anchorage and King Salmon, Alaska. 

Data from National Climatic Data Center, Asheville, North 

Carolina. Anchorage data cover period 1948-72; King Salmon data cover period 1953-60. Compass roses depict percentage of time winds blew toward a given direction. 

12 Redoubt Volcano, Alaska: Hazard Assessment Based on Eruptive Activity through 1968 
ANCHORAGE KING SALMON 

Winter and Spring Summer and Autumn Winter and Spring Summer and Autumn 

12,000 m w 

3,000 m E 

s s 

D J F M A M A S 0 13 

ANCHORAGE KING SALMON 

TIME, IN MONTHS 
from the air more rapidly than it would otherwise settle to 

the ground, and therefore would be less widely distributed. 

Summary 

The 1966 and 1968 eruptions provide a good model 

for the smallest type of eruption that occurs at Redoubt. 

Any future eruption of Redoubt has a high probability of 

being at least as large as those small events and is likely 

to produce floods, debris flows, pyroclastic flows, and possibly lava flows and domes (see pl. 1, map B, zones A and 

B). Floods and debris flows on the Drift or Crescent Rivers (depending on vent position) and tephra fall are the 

events likely to occur during a small eruption that would 

GULF 

0 100 200 Kl LOMETE RS 

Figure 18. Approximate areas likely to receive noticeable tephra fall from explosive eruptions of Redoubt Volcano, southern 

Alaska. Area denoted by parabola (within circle) most likely to be affected because of prevailing winds; area outside of parabola 

would receive ash fall if wind direction were unusual. Thickness of tephra deposited will decrease with distance from Redoubt. 

14 Redoubt Volcano, Alaska: Hazard Assessment Based on Eruptive Activity through 1968 
threaten human activity. Duration of the eruption, size of 

the eruption column, and wind speed and direction would · 

determine whether tephra fall reaches population centers 

on Cook Inlet. If an eruption results in severing of the 

Drift glacier, a surge of the glacier could be triggered 10 

to 20 years after the eruption (Sturm and others, 1986). 

A large-volume eruption of Redoubt is less probable 

than a small event. A large eruption may produce tephra 

fall, pyroclastic flows, debris flows, debris avalanches, 

floods, lava flows, and possibly a lava dome or lateral blast 

(pl. 1, map B). For such events, the position of the vent is 

important in determining the nature and distribution of the 

products. The east flank of the mountain in particular may 

be more prone to destruction by lateral blast. 

A large eruption from a vent located on the south side 

could trigger voluminous debris flows in the Crescent 

River drainage, as has occurred in the past (Riehle and 

others, 1981 ). Eruptions from a vent located on the north 

side would most likely produce water-rich debris flows or 

floods on the Drift River. The most harmful effect of any 

eruption would be tephra distributed in the Cook Inlet region. 

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of two sound channels in the polar atmosphere from infrasonic observations of an Alaskan volcano: Nature, v. 211, 

no. 5045,p. 163-165. 

Yount, M.E., Miller, T.P., and Gamble, B.M., 1987, The 1986 

eruptions of Augustine Volcano: hazards and effects, in 

Hami1ton, T.D., and Galloway, J.P., eds., Geologic studies in 

Alaska by the U.S. Geological Survey during 1986: U.S. 

Geological Survey Circular 998, p. 4-13. 

16 Redoubt Volcano, Alaska: Hazard Assessment Based on Eruptive Activity through 1968 
APPENDIX 
Typical Events and Products Associated With 

Volcanic Eruptions 

Events associated with the eruption of composite volcanoes such as Redoubt range from those that throw material many kilometers into the atmosphere to those that 

slowly squeeze material out of the vent; airborne particles 

may travel great distances from the volcano, whereas lava 

oozing out of the vent may move a very short distance. 

Tephra Fall 

Material thrown into the air during explosive eruptions can vary in size from dust to large blocks and can 

consist of pulverized rock from the existing cone, from 

other rocks underneath the volcano, or from molten lava 

supplying the volcano. Tephra refers to particles of all 

sizes explosively emitted during an eruption. Explosive 

fragments smaller than 2 millimeters (mm) are called ash. 

Vapor emissions accompanied by minor amounts of ash 

are commonly among the first manifestations of an impending eruption. Explosive eruptions of composite volcanoes are commonly characterized by vigorous tephra-andvapor plumes and periodic tephra-rich explosions. 

Tephra is the most widely distributed product from 

eruption of volcanoes similar to Redoubt. The area covered by tephra deposits depends on the height to which the 

tephra is injected into the atmosphere, the direction and 

speed of the wind, the size of the particles erupted, and 

the violence and duration of the eruption. This area can be 

very large, as much as tens of thousands of square kilometers. Small- to moderate-sized eruptions commonly produce explosive eruption columns that may rise at speeds 

from 8 to 30 m per second as high as of 29 km (Blong, 

1984). Winds at different levels in the atmosphere commonly blow in different directions, sometimes in direct 

opposition to each other, and thus increase the area of 

tephra dispersal. Tephra from small eruptions can be 

blown tens to hundreds of kilometers from the volcano. 

Harmful effects of tephra are due to its abrasive and 

corrosive characteristics or to its low melting temperature. 

Wilson (1959) documented the effects of tephra fall with 

emphasis on Alaskan volcanoes. Eyes and nasal passages 

of humans and animals may be irritated by tephra. Grazing 

animals may suffer tooth abrasion or gastric irritation by 

ingesting tephra-covered foliage. As was discovered during the 1980 eruptions of Mount St. Helens, tephra can 

have detrimental effects on transportation of goods and 

people, generation of electricity, communication, operation 

of computers, and of any other activity dependant on machinery. The effects on jet turbines can be particularly serious, because their operating temperatures are similar to 

the melting temperatures of tephra. Tephra drawn into a jet 

engine can melt and coat the engine. Blong (1984) cited 

two incidents of jets inadvertently flying through tephra 

plumes over Indonesia and having engine failure; in both 

cases the pilots were able to restart their engines. Damage 

to aircraft was also reported during the 1976 and 1986 

eruptions of Augustine Volcano and the 1989 eruption of 

Redoubt (Kienle and Shaw, 1979; Yount and others, 1987; 

Alaska Volcano Observatory Staff, 1990; Kienle and others, 1990). 

Other effects of tephra are due to its weight and acidity (Wilcox, 1959). Acidity and turbidity of water supplies 

may temporarily increase. Buildings may not be able to 

support the weight of a thick accumulation of tephra. Sewage systems may become clogged. Plants may be smothered. Cloth and (or) other susceptible materials may be 

damaged by exposure to mildly acidic conditions caused 

by the mixing of tephra and moisture. 

Debris Avalanches, Debris Flows and Floods 

A debris avalanche results from a massive failure of 

the flank of a volcano. Debris avalanche deposits are characterized by hummocky topography consisting of small 

hills and closed depressions. Longitudinal and transverse 

ridges may occur. The deposits are dry compared to debris 

flows and floods and commonly occur in areas of the cone 

that have been weakened by faulting, hydrothermal alteration, or both. 

Debris flows are moving masses of rock, soil, and 

water. As the proportion of rock to water decreases, debris 

flows grade into floods. Debris flows and floods are common during eruptions of ice-covered volcanoes such as 

Redoubt, because the hot eruptive material melts the snow 

and ice. Debris flows and floods are highly mobile and 

capable of traveling many kilometers down valleys draining the volcano. The Osceola debris flow moved 100 km 

from Washington's Mt. Rainier to the city of Puyallup, 

within 9.5 km of Tacoma (Crandell and Waldron, 1956). 

The volume and rate of flow govern the size of the material that can be transported downstream. In larger flows, 

boulders several meters in diameter can be transported. 

lateral Blasts 

An explosive eruption may occur as a vertical column 

or as a lateral blast. During a lateral blast, the thrust of the 

eruption is directed horizontally from the vent rather than 

vertically. A lateral blast results from catastrophic failure 

of a flank area weakened by cracking, faulting, or alteration. When gaseous (highly explosive) magma is injected 

into the cone, it takes the weakest pathway to the surface; 

if the flank is weaker than the summit area, a lateral blast 

will occur. When one occurs, material from the failed portion of the cone as well as the invading magma body 

move in part as a density current, in the from of a debris 

avalanche or pyroclastic flow, and in part as an inflated 

gas-and-tephra cloud. The volume of the material moving 

18 Redoubt Volcano, Alaska: Hazard Assessment Based on Eruptive Activity through 1968 
as an avalanche or flow and the local topography will 

partially control or influence the area of impact. The gasand-tephra cloud rapidly travels great distances from the 

volcano, generally unimpeded by topography, and will be 

the biggest influence on the area of impact. The May 18, 

1980, eruption of Mount St. Helens produced a lateral 

blast that leveled trees in a broad area as far as 29 km 

from the vent (U.S. Geological Survey, 1981; Crandell and 

Hoblitt, 1986). 

Pyroclastic Flows and Surges 

Pyroclastic flows are fast-moving, dry, turbulent mixtures of eruptive particles of all sizes that are buoyed up 

by hot, rapidly expanding gases. They are of higher temperature and travel faster than debris flows, and they are 

commonly generated during explosive eruptions as the debris in the eruptive column falls back to earth and flows 

down the volcano's flank. As pyroclastic flows travel, 

rapid expansion of internal gases lifts finer tephra or ash 

particles upward into a billowing cloud called an ashcloud surge or glowing cloud (fig. 19). 

Pyroclastic flows may move at speeds of greater than 

225 km per hour on steep slopes (Moore and Melson, 

1969), and internal temperatures of flows commonly exceed 590°C. In 1902, 30,000 people in the town of St. 

Pierre, Martinique, died from the blast of a hot ash cloud 

associated with pyroclastic flows from an eruption of Mt. 

Pelee (Fisher and others, 1980). The blast leveled most 

structures in its path and upset, sank, or burned ships anchored in the harbor (Macdonald, 1972). 

Although pyroclastic flows and surges can surmount 

topographic barriers and travel tens of kilometers if their 

speed, gas inflation, and volume are great enough, smaller 

flows are usually confined or strongly influenced by topography. The area affected by ash-cloud surges is determined by the size of the associated pyroclastic flow, the 

height of the cloud above the flow, and the wind direction 

and strength (Crandell, 1980). The main hazard zones 

from pyroclastic flows are the valleys leading from the 

volcano; the area threatened by associated ash-cloud surges extends up valley walls as well and is influenced by 

wind direction. 

Because of the heat contained in pyroclastic flows 

and surges, they may generate debris flows and floods 

when they occur on snow- and-ice-covered volcanoes. 

lava Flows and lava Domes 

Lava flows and domes result when magma oozes 

from the vent. If the magma is viscous, it will mound up 

around the vent, to create a dome. If magma is more fluid, 

it will flow away from the vent under the influence of 

gravity. Viscosity and slope angle control the speed at 

which the lava moves; the rate at which the lava is extruded may influence the size of the flow. 

The path of a lava flow, more so than other volcanic 

phenomena, is determined by topography. However, lava 

flows are not always erupted from the summit of the volcano and may erupt from vents on the side of the cone. 

Composite volcanoes similar to Redoubt tend to produce 

high-viscosity lava flows that do not travel much beyond 

the base of the cone, unless they are of unusually large 

volume. Lava flows move more slowly than pyroclastic 

flows and surges, debris avalanches, debris flows, or 

floods, and they pose less of a threat to life; nevertheless, 

they totally devastate property in their path. 

Eruptions that produce volcanic domes are commonly 

accompanied by explosions and small pyroclastic flows. 

Volcanic domes are usually extruded after early, gas-rich 

explosive activity and may grow intermittently over several months or years. 

Figure 19. Small pyroclastic flow descending north flank of 

Augustine Volcano, southern Alaska, on March 30, 1986. 

Billowing ash cloud rising from flow is an ash-cloud surge; 

field of view approximately 600 m. 

Appendix 19