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THE ERUPTION OF REDOUBT VOLCANO, ALASKA, 

DECEMBER 14, 1989-AUGUST 31, 1990 
Frontispiece. Aerial photograph of north flank of Redoubt Volcano. Drift glacier descends from the breached summit crater 

down the north flank. Arrow marks location of lava dome that formed in 1966. Photograph by Austin Post on August 27, 1987. 

(Photograph identification no. 87R3-095). 
The Eruption of Redoubt Volcano, Alaska, 

December 14, 1989-. August 31, 1990 

Steven R. Brantley, Editor 

With contributions by 

Thomas J. Casadevall 

Bernard A. Chouet 

John N. Davies 

Steven A. Estes 

Cynthia A. Gardner 

Richard P. Hoblitt 

John C. Lahr 

Richard G. LaHusen 

Jon J. Major 

Robert G. McGimsey 

Thomas P. Miller 

Thomas L. Murray 

Christina A. Neal 

Christopher J. Nye 

Robert A. Page 

Thomas C. Pierson 

John A. Power 

Christopher D. Stephens 

Richard B. Waitt 

U.S. GEOLOGICAL SURVEY CIRCULAR 1061 
U.S. DEPARTMENT OF THE INTERIOR 

MANUEL LUJAN, JR., 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 

First printing 1990 

Second printing 1991 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1990 

Free on application to the 

Books and Open-File Reports Section 

U.S. Geological Survey 

Federal Center, Box 25425 

Denver, CO 80225 

library of Congress Cataloging-in-Publication Data 

The Eruption of Redoubt Volcano, Alaska, December 14, 1989-August 31, 

1990 I edited by Steven R. Brantley : with contributions by Thomas 

j. Casadevall ... [et al.]. 

p. em.- (U.S. Geological Survey circular: 1061) 

Includes bibliographical references. 

Supt. of Docs. no.: I 19.4/2:1061 

1. Redoubt Volcano (Aiaska)-Eruption, 1989-1990. I. Brantley, 

Steven R. II. Casadevall, Thomas j. Ill. Geological Survey (U.S.) 

IV. Series. 

QE523.R39E78 1990 

551.2'1'097983-dc20 90-21779 

CIP 

Cover. Redoubt Volcano emits a vigorous plume consisting mostly of water vapor (steam), sulfur dioxide, and 

carbon dioxide. Plume is drifting to the west. View is to the north. Photograph by Steven R. Brantley on January 22, 

1990. 
CONTENTS 

Summary 1 

Highlights of the eruption and monitoring 1 

Introduction 2 

Redoubt Volcano 3 

Eruptive history 6 

Regional geologic setting 7 

Chronology of the 1989-90 eruption 7 

Pre-eruption, November 20--December 13, 1989 7 

December 14-December 21, 1989 8 

December 22, 1989-January 2, 1990 12 

January 2, 1990 13 

January 8-February 14, 1990 14 

February 15, 1990 15 

February 21-April 20, 1990 17 

April 21-August 31, 1990 18 

Effects of the 1989-90 eruption 19 

Introduction 19 

Tephra 19 

Debris flows 24 

Hazard information provided by Alaska Volcano Observatory 24 

Introduction 24 

Volcano update 25 

Color code 25 

Forecast plume trajectory 25 

Monitoring the 1989-90 eruption 26 

Introduction 26 

Seismicity 27 

Field observation 28 

Sulfur-dioxide emission 28 

Slow-scan video monitor 29 

Lightning-detection system 29 

Debris-flow detection system 30 

Future eruptions in Alaska 31 

Conclusion 31 

Acknowledgments 32 

References 33 

FIGURES 

Frontispiece. Aerial photograph of north flank of Redoubt Volcano 

1. Map showing historically active volcanoes in Alaska 3 

2. Map showing geographic features of Cook Inlet region 4 

3. Diagram showing institutional components of the Alaska Volcano 

Observatory 5 

4. Map showing topography of Redoubt Volcano and location of drainages 

and the Drift River Terminal 5 

Contents V 
VI Contents 

5o Aerial photograph of Redoubt Volcano, Drift River valley, Rust Slough, 

Cannery Creek, and Drift River Terminal 6 

60 Diagram showing the Pacific plate descending beneath the North American 

plate along the Aleutian trench 7 

7 0 Map and cross section showing location of earthquakes beneath Cook Inlet 

region between January 1980 and November 1989 8 

80 Photograph of seismic center at the University of Alaska Geophysical 

Institute in Fairbanks 9 

9 0 Typical seismograms illustrating the two main types of seismic events 

recorded at Redoubt Volcano 9 

100 Photograph of tephra plume rising from Redoubt's crater on December 16, 

1989 10 

11. Nighttime Advanced Very High Resolution Radiometer image from a 

National Oceanic and Atmospheric Administration satellite of Redoubt 

Volcano in eruption, December 16, 1989 10 

12o Photographs showing how a scanning electron microscope reveals the shape 

and type of ash particles erupted from Redoubt Volcano 12 

13o Map, cross section, and diagram of seismicity at Redoubt Volcano, 

December 13-31, 1989 13 

14o Sketch of the eruption's first lava dome 13 

150 Map showing approximate boundaries of area inundated by debris flow in 

Drift River valley on January 2, 1990 14 

16-21. Photographs showing: 

160 Storage tanks of Drift River Terminal between Rust Slough and main 

channel of Drift River IS 

17. Huge, eerie, anvil-shaped ash cloud moving toward the Kenai 

Peninsula from Redoubt Volcano on January 8, 1990 16 

18o Daytime darkness in downtown Soldotna on the Kenai Peninsula 

during an ash fall from Redoubt Volcano 16 

19 0 Block of the first lava dome, deposited in Drift River valley 5 km 

downstream from Redoubt Volcano 17 

200 North flank of Redoubt Volcano and irregular surface of Drift 

glacier 17 

21. Redoubt Volcano during a moderate explosive episode on April21, 

1990 18 

22. Graph showing duration of Redoubt's explosive episodes determined by the 

duration of seismic signals recorded by seismic station SPU 18 

23-260 Photographs showing: 

23. Ice canyon eroded into Drift glacier by pyroclastic flows generated by 

dome collapses 19 

240 Pyroclastic-flow deposits atop Drift glacier 20 

25o Third-largest dome of the eruption, which formed after the explosive 

episode on April 21, 1990 20 

26. Workers repairing an electric transformer that short-circuited because 

of ash fall in Soldotna on December 16, 1989 21 

27 0 Graph showing particle-size distribution of ash that fell downwind of 

Redoubt Volcano 22 

28-30. Photographs showing: 

280 Flights canceled during the busy Christmas holiday season owing to 

numerous explosive episodes 22 

29 0 Anchorage International Airport 23 

300 Crew examining one of the Boeing 747 jet engines that suffered 

extensive damage when the jet encountered an ash cloud on December 

15, 1989 23 

31o Text of Redoubt Volcano Update, March 22, 1990 25 
32. Map showing plume trajectory forecast for 3:00 a.m. on February 15, 

1990 25 

33. Sketch showing scientific methods used to monitor volcanoes 26 

34. Map showing location of seismic stations and debris-flow sensors around 

Redoubt Volcano 27 

35-38. Graphs showing: 

35. Seismicity at Redoubt Volcano between December 13 and 22, 1989, 

expressed in average amplitude values (RSAM) 28 

36. Cumulative energy release recorded by seismic station RDN, 

December 26, 1989-January 3, 1990 28 

37. Record from Seismic Spectral Amplitude Measurement system 

(SSAM) showing signature of a swarm of low-frequency events that 

preceded the moderate explosion on March 23, 1990 29 

38. Daily sulfur-dioxide emission rates of Redoubt Volcano 29 

39. Map showing location of lightning strikes during the February 15, 1990, 

explosive episode 30 

40. Records from seismic station DFR and three debris-flow sensors used to 

detect and track a debris flow triggered by an explosive episode on April 6, 

1990 30 

41. Compass-rose diagrams showing average wind direction at four altitudes 

above Anchorage and King Salmon, Alaska 31 

TABLES 

1. Chronology of the 1989-90 eruption of Redoubt Volcano 11 

2. Chemical composition, in percent, of rock samples from several recent 

eruptions in Alaska 21 

3. Color-code scheme developed by Alaska Volcano Observatory to characterize 

eruptive status of Redoubt 26 

4. Status of monitoring and geologic studies at Alaskan volcanoes 32 

GLOSSARY 

Accretionary lapilli. More or less spherical masses of loosely aggregated ash, 

mostly between 1 mm and 1 em in diameter. 

Andesite. A volcanic rock consisting of 53-63 percent silica; has a moderate 

viscosity when in a molten state. 

Ash. Fragments of lava or rock less than 2 mm in diameter that are blasted into 

the atmosphere by volcanic explosions. 

Basalt. A volcanic rock consisting of less than 53 percent silica; has a low 

viscosity when in a molten state. 

Caldera. A large volcanic depression, usually circular in map view. 

Composite volcano. A steep-sided volcano composed of layers of volcanic rock, 

usually of high-viscosity lava, and unconsolidated material such as debris-flow 

and pyroclastic deposits. 

Dacite. A volcanic rock composed of 63-68 percent silica; has a high viscosity 

when in a molten state. 

Debris flow. A flowing mixture of water and rock debris. Volcanic debris flows 

are sometimes referred to as volcanic mudflows or lahars (an Indonesian word). 

Contents VII 
VIII Contents 

Dome. A steep-sided mound that forms when highly viscous lava piles up near a 

volcanic vent. Domes are formed by andesite, dacite, and rhyolite lavas. 

Earthquake. A sudden motion or trembling in the Earth caused by the abrupt 

release of accumulated strain in rocks. 

Lava flow. Molten rock that erupts from a vent or fissure and flows downhill. 

Magma. Molten rock, which contains dissolved gas and crystals, formed deep 

within the Earth. When magma reaches the Earth's surface, it is called lava. 

Phreatic eruption. A steam explosion that occurs when water comes in contact 

with hot rocks or ash near a volcanic vent. 

Piedmont. A flat or low-angle slope, area, glacier, or other feature at the base of 

a mountain. 

Pumice. A light-colored volcanic rock containing abundant trapped gas bubbles 

formed by the explosive eruption of magma. Because of its numerous vesicles, or 

bubbles, pumice commonly floats on water. 

Pyroclastic flow. A hot, fast-moving, and high-density mixture of ash, pumice, 

rock fragments, and gas that flows downhill during explosive eruptions. 

Seismicity. The phenomenon of earthquakes or earth vibrations. 

Silica. A molecule formed of silicon and oxygen (Si02

) that is the basic building 

block of volcanic rocks and the most important factor controlling the viscosity of 

magma. The higher a magma's silica content, the greater its viscosity. 

Silicosis. Fibrosis of the lungs caused by long-term inhalation of silica dust and 

resulting in a chronic shortness of breath. 

Tephra. A general term for airborne rock fragments ejected during explosive 

eruptions. Tephra consisting of fragments less than 2 mm in diameter is called 

ash. 

Tremor. In relation to volcanoes, a type of seismicity characterized by 

continuous vibration of the ground. The vibration is related to the transport of 

fluids and gas within or beneath a volcano. 

Volcanic landslide. The downslope movement of soil, rock debris, and 

sometimes glacial ice, with or without water, from the flank of a volcano. 

Volcano. A vent on the surface of the Earth through which magma erupts, and 

also the landform that is produced by the erupted material. 
The Eruption of Redoubt Volcano, Alaska, 

December 14, 1989-August 31, 1990 

Steven R. Brantley, Editor 

SUMMARY 

The 1989-90 eruption of Redoubt Volcano, 1 77 km 

southwest of Anchorage, Alaska, began on December 14, 

1989, less than 24 hours after a swarm of earthquakes struck 

beneath the volcano. A huge cloud of ash heralded the 

volcano's fourth and most damaging eruption of this century. 

Volcanic ash generated by numerous explosive episodes from 

December 1989 through April 1990 caused significant damage to aircraft, severely disrupted air traffic above southern 

Alaska, and resulted in local power outages and school closures. The explosions produced hot, fast-moving clouds of 

ash, rock debris, and gas (pyroclastic flows} that swept across 

Redoubt's heavily glaciated north flank. These events triggered massive debris flows in Drift River valley that threatened an oil tanker terminal near the river's mouth. Partial 

flooding of the terminal compound on two occasions forced 

authorities to modify its operating procedures, which temporarily curtailed oil production from 10 platforms in Cook Inlet. 

The damage and loss of revenue from ash and debris flows 

are estimated to total more than $100 million, which makes 

this the second most costly volcanic eruption in the history 

of the United States, exceeded only by the 1980 eruption of 

Mount St. Helens in Washington. 

The eruption not only focused public attention on the 

hazards posed by Redoubt Volcano and other volcanoes in 

the Cook Inlet region, but also demonstrated the value of 

monitoring volcanoes and the capability of providing forecasts of impending eruptions. The recently established Alaska 

Volcano Observatory (AVO} monitors the active volcanoes 

west of Cook Inlet in order to detect signs of volcanic unrest 

that may lead to an eruption and conducts geologic studies 

to assess the hazards the volcanoes pose to life and property. 

A seismic network centered on Redoubt became fully operational in October 1989 as part of the AVO monitoring program, and a detailed assessment of hazards from Redoubt 

(Till and others, 1990} was nearly finished when the volcano 

began showing signs of renewed activity in mid-December. 

These measures-plus the experience gained by scientists 

monitoring eruptions in the 1980's at Mount St. Helens in 

Washington, Nevado del Ruiz in Colombia, and Augustine 

Manuscript approved for publication, October 31, 1990. 

Volcano in Alaska-enabled AVO to provide accurate information about Redoubt's activity and to issue advance warnings 

for several explosive episodes. 

This report summarizes the Alaska Volcano 

Observatory's observations during the first nine months of 

the 1989-90 eruption of Redoubt Volcano and discusses the 

hazards and effects of the eruption sequence. 

Highlights of the Eruption and Monitoring 

The activity of Redoubt during the first nine months of 

the 1989-90 eruption was not as violent-and did not produce as much erupted material-as some other eruptions in 

the world in this century; nor were effects of this activity 

unexpected. Nevertheless, monitoring the volcano's activity 

provided valuable lessons for mitigating volcanic hazards in 

the future, not only in Alaska but elsewhere in the United 

States and the world. Principal observations include the following: 

• The explosive onset of the eruption on December 14 

was preceded by only about 24 hours of intense seismic 

activity. Because the volcano was being monitored, this 

was sufficient time for AVO to warn officials and the 

public of Redoubt's restlessness, and for AVO and other 

Federal, State, and local government agencies to activate 

emergency plans before eruptive activity began. 

• Increases in the rate of seismicity beneath the volcano 

were the basis for short-term warnings issued by AVO 

before major explosive episodes on December 14 and 

January 2 and before moderate explosive episodes on 

March 23 and April 6. AVO monitored the seismic activity beneath Redoubt 24 hours a day in order to enable 

scientists both to alert aviation and public officials of the 

increased potential for ash and debris flows and to determine when such events were in progress, especially at 

night and when the volcano was obscured by clouds. 

Not all significant explosive episodes were forecast, 

however, because a seismic station on the volcano was 

destroyed early in the eruption sequence and also because some of the smaller explosions were not preceded 

by detectable changes in seismicity. 

• The size and type of volcanic activity that occurred during the Redoubt eruption were typical of the volcano's 

past activity as inferred from the geologic record (Till 

and others, 1990}. Analyses of hazards based on a 

Summary 
volcano's eruptive history can be used in long-term 

planning of land use and investment in volcanic areas to 

help minimize the damaging effects of future eruptions. 

The Drift River Terminal was built in 1967, before such 

a hazard assessment was initiated. 

• Volcanic ash is the most common and widespread volcanic hazard in Alaska; it is especially dangerous to aircraft. At least four commercial jet aircraft suffered damage from encounters with airborne ash during the first 

three months of the eruption. The most _serious occurred 

on December 15 when a Boeing 747 jetliner carrying 

231 passengers entered an ash cloud 240 km northeast 

of Redoubt. The jet lost power in all four engines and 

dropped about 4,000 m in altitude before the pilot succeeded in restarting the engines. The plane landed safely 

in Anchorage, but sustained an estimated $80 million in 

damage (Steenblik, 1990). Although existing technology 

cannot always track the movement of ash away from a 

volcano in real time, this near-tragic incident prompted 

AVO and other government agencies to search for ways 

to provide better information to the airline industry regarding forecast wind direction and speed and eruptive 

activity. 

• Numerous floods of water, ice, and volcanic-rock debris 

(debris flows) inundated the Drift River valley when explosive activity disturbed portions of the volcano's extensive cover of snow and ice. Pyroclastic flows swept 

across Drift glacier and eroded and melted several hundred million cubic meters of snow and glacial ice, generating water that combined with sediment to cause 

massive debris flows. The largest debris flows, on January 2 and February 15, entered the Drift River Terminal 

from Rust Slough and damaged its logistical support facilities. No oil, however, was spilled. Although the likelihood of flooding in the Drift River valley was recognized before the 1989-90 eruption, uncertainties in estimating various contributing factors prevented scientists 

from predicting the magnitude of the debris flows. These 

uncertainties included the type and magnitude of volcanic activity, the volume and rate of snowmelt and icemelt 

during different types of volcanic activity, and the volume of sediment that could be incorporated, transported, 

and deposited along the Drift River channel during a 

single flood event. 

• Recently developed seismic data acquisition and analysis systems using relatively low cost personal computers 

(PC's) proved extremely effective in monitoring seismicity beneath Redoubt during the eruption. These systems 

permitted scientists both to quantify the volcano's seismic 

activity on a minute-by-minute basis and to detect subtle 

changes in the character of the seismicity before some 

eruptions. Since these PC-based systems are easily portable, it will be possible to install similar seismic systems 

quickly during future volcanic crises. 

• New experimental systems were effective in detecting 

large ash plumes and volcanic debris flows. Lightning 

discharges associated with several large plumes of ash 

were detected with a system designed for locating cloudto-ground strikes, and debris flows in the upper Drift 

River valley were detected by an array of seismometers 

especially sensitive to high-frequency vibrations of the 

ground. 

INTRODUCTION 

Redoubt Volcano lies near the northeast end of 

the Aleutian volcanic arc, an active chain of volcanoes 

that extends 2,500 km from near Anchorage southwest 

along the Alaska Peninsula to the western Aleutian Islands (fig. 1). At least one volcano in the chain erupts, 

on average, each year (Simkin and others, 1981). Scientists have identified more than 40 historically active 

volcanic centers along the chain, and they are particularly concerned about volcanoes whose eruptions could 

affect the Cook Inlet region, where 60 percent of 

Alaska's population resides and which is the State's 

major supply, business, and financial center (fig. 2). 

Three volcanoes west of Cook Inlet-Mount Spurr, Redoubt, and Augustine-have erupted ash over Cook Inlet population centers seven times since 1900. Recently, 

geologists identified about 90 different layers of volcanic ash in the upper Cook Inlet region that formed during the past 10,000 years (Riehle, 1985). These layers 

record only a small percentage of the eruptions thought 

by scientists to have occurred during this time. At least 

30 of the layers are attributed to eruptions of Redoubt 

Volcano, and 35 layers are attributed to the Crater Peak 

vent on nearby Mount Spurr. 

The potential for future hazardous eruptions near 

Cook Inlet led Congress to support an expanded Alaska 

volcano monitoring program in 1988. The Alaska Volcano Observatory (AVO) was formally established in 

1988 to better monitor unrest at active volcanoes in 

Alaska and to assess the hazards they pose to life and 

property. AVO is a cooperative program consisting of 

scientists and facilities of the U.S. Geological Survey 

(USGS) in Anchorage, Alaska, Vancouver, Wash., Menlo 

Park, Calif., and Hawaii; the Geophysical Institute of 

the University of Alaska in Fairbanks (UAGI); and the 

Alaska Division of Geological and Geophysical Surveys (ADGGS) in Fairbanks (fig. 3). Although its participants are widely dispersed, the observatory serves 

as a coordination center for the study of Alaskan volcanoes and the dissemination of volcanic-hazard information. The goals of AVO are as follows: 

• to assess the nature, likelihood, and timing of volcanic activity 

• to evaluate potential volcanic hazards associated 

with future eruptions, including the types of activity, effects, and areas at risk 

• to provide timely and accurate information on volcanic hazards, and warnings of impending dangerous activity to local, State, and Federal officials and 

the public 

2 The &uption of Redoubt Volcano, Alaska, December 14, 1989-August 31, 1990 
Redoubt Volcano 

Redoubt Volcano is a large steep-sided composite volcano 3,108 m high. It is constructed of alternating layers of lava and ash and other fragmented volcanic 

rock debris formed by explosive eruptions over the past 

several hundred thousand years. Ten glaciers radiate 

from its summit region, which includes an ice-filled 

crater 1.8 km in diameter (see frontispiece). Breached 

to the north, the crater is the source of Redoubt's longest 

glacier, informally named Drift glacier. The glacier extends 8 km down a canyon incised in the volcano's 

north flank and across the Drift River valley to within 

50 m of the north valley wall, forming a broad, rounded 

piedmont lobe. The volcano is drained to the north by 

Drift River, to the east by Redoubt Creek, and to the 

0 

I 

0 

200 400 KILOMETERS 

I 

200 400 MILES 

BERING SEA 

i 

I 

I 

south by the Crescent River, all of which flow into 

Cook Inlet (fig. 4 ). 

Redoubt Volcano lies within the rugged Lake 

Clark National Park and Preserve and is visited infrequently, mostly by outdoor enthusiasts and scientists. 

The closest facility with year-round residents is an oilstorage terminal built in 1967 at the mouth of Drift 

River, 35 km downstream from the volcano, where the 

river flows across a broad delta (fig. 5). Oil is pumped 

from 10 oil-well platforms in western Cook Inlet via a 

buried pipeline to the storage terminal, then onto tankers at a loading platform just offshore. The terminal 

can store a total of 1.9 million barrels (83.6 million 

gallons) of oil in seven tanks. Prior to the eruption, 

the terminal was bordered on the north and east by an 

L-shaped levee 3.6 m tall that was designed to divert 

158° w 

I 

ALASKA 

150°W 

PACIFIC OCEAN 

EXPLANATION 

1 Mt. Wrangell 12 Ukinrek maars 23 Makushin Volcano 34 Kasatochi Island volcano 

2 Mt. Spurr 13 Mt. Chiginagak 24 Bogoslof Island volcano 35 Great Sitkin Volcano 

3 Redoubt Volcano 14 Aniakchak caldera 25 Okmok Caldera 36 Kanaga Volcano 

4 Iliamna Volcano 15 Mt. Veniaminof 26 Mt. Vsevidof 37 Tanaga Volcano 

5 Augustine Volcano 16 Pavlof Volcano 27 Kagamillsland volcano 38 Mt. Gareloi 

6 Mt. Katmai 17 Mt. Dutton 28 Carlisle Island volcano 39 Mt. Cerberus 

7 Novarupta 18 lsanotski Peaks 29 Mt. Cleveland 40 Little Sitkin caldera 

8 Mt. Trident 19 Shishaldin Volcano 30 Yunaska Island volcano 41 Segula Peak 

9 Mt. Mageik 20 Fisher Caldera 31 Amukta Island volcano 42 Kiska Volcano 

10 Mt. Martin 21 Westdahl Peak 32 Seguam volcano (Pyre Peak) 

11 Mt. Peulik 22 Akutan Peak 33 Korovin Volcano 

Figure 1. Historically active volcanoes in Alaska. 

Introduction 3 
Aleutian ls\!~ds o 0 o ,6>0 --------------....J -xl_. I 

• HAYES 

~ .~ ~ MT SPURR 

__.) LAKE CLARK 

I NATIONAL PARK 

I-, AND PRESERVE 

r 

• • CRATER 

PEAK 

Figure 2. Geographic features of Cook Inlet region. Solid triangles are volcanoes. 

4 The Eruption of Redoubt Volcano, Alaska, December 14, 1989-August 31, 1990 

0 

I 

0 

GULF 

0 F 

ALASKA 

50 Ml LES I 

I 

50 Kl LOMETERS 
60°30' 

600 15' 

ALASKA 

VOLCANO OBSERVATORY 

USGS supporting facilities: 

Cascades Volcano Observatory, Vancouver, Wash. 

Alaska Seismic Project, Menlo Park, Calif. 

Hawaiian Volcano Observatory, Hawaii 

Figure 3. Institutional components of the 

Alaska Volcano Observatory (AVO). 

5 KI L OMETERS 

EXPLANATION 

~~~ :-... :~l Glaciers 

Lake Clark National Park 

and Preserve boundary 

Buried oil pipeline 

Redoubt Volcano topographic contours. 

Contour interval 1000 feet 

OF 

ALASKA 

Figure 4. Topography of Redoubt Volcano and location of drainages and the Drift River Terminal. 

Introduction 5 
high-water flows of the Drift River away from the facility. Individual storage tanks are surrounded by 2.4-

m-tall berms designed to contain oil in the event of a 

leak. In addition to the tanks, the Drift River Terminal 

includes an airstrip and several buildings for living 

quarters and maintenance and operational support. 

Eruptive History 

Volcanism at Redoubt began as early as about 

900,000 years ago, but the bulk of the cone was formed 

in the past 200,000 years (Till and others, 1990). An 

early cone-building period is marked by numerous thin 

(less than 6 m) basalt and basaltic-andesite lava flows, 

and layers of ash and coarser fragmented volcanic debris. The volcano's upper cone consists of thick (30 to 

60 m) andesite lava flows and a voluminous apron of 

pyroclastic debris. Redoubt has a history of intermittent explosive eruptions accompanied by pyroclastic 

flows and debris flows, some much larger than those 

that were generated during the 1989-90 eruption (Till 

and others, 1990; Riehle and others, 1981). These larger 

pyroclastic flows, however, occurred during the early 

history of the volcano when the composition of the 

magma was dacite, which tends to produce more explosive activity and more voluminous pyroclastic flows 

than the andesite magma that is currently being erupted. 

The recent history of the volcano suggests that a return 

to highly explosive eruptions and extensive pyroclastic 

flows is not very likely during the present activity. 

Before the current eruption, the largest known 

debris flows at Redoubt were those that swept down 

Crescent River valley all the way to Cook Inlet (see 

fig. 4) about 3,500 years ago and deposited rock debris that dammed Crescent Lake (Riehle and others, 

1981). Numerous small mounds near the lake are remarkably similar to mounds of the volcanic landslide 

deposit that formed during the eruption of Mount St. 

Helens on May 18, 1980. This similarity suggests that 

the debris flows probably accompanied a large landslide or partial collapse of Redoubt's south flank (J.R. 

Riehle, oral commun., 1990). Sedimentary deposits also 

suggest that at least four and possibly six major debris 

flows flowed through Drift River valley to Cook Inlet 

during the past 200 to 400 years (Till and others, 1990). 

Figure 5. Aerial photograph of Redoubt Volcano, Drift River valley, Rust Slough, Cannery Creek, and Drift River Terminal 

(between Rust Slough and Drift River). View to west Photograph by Steven R. Brantley on April 27, 1990. 

6 The Eruption of Redoubt Volcano, Alaska, December 14, 1989-August 31, 1990 
More recently, during eruptive activity in January 1966, 

a debris flow swept down Drift River valley to Cook 

Inlet and inundated the site where the oil terminal was 

subsequently located. A seismic-exploration crew 

trapped near the site had to be rescued. 

Regional Geologic Setting 

In recent decades, scientists have learned that the 

earth's crust is divided into many large "plates" that 

move relative to one another. An average of 80 km 

thick, these rigid plates are spreading apart from, sliding 

past, or colliding with each other in slow motion on 

top of the Earth's hot pliable interior. The volcanoes 

of southern Alaska, including Redoubt Volcano, lie just 

landward of the boundary between the Pacific and North 

American plates. Here the Pacific plate is thrust beneath 

the North American plate at a rate of about 6 to 8 em 

per year along a gently dipping geologic fault which 

forms the Aleutian trench and which extends in an arc 

from south-central Alaska to the west end of the Aleutian Islands (fig. 6). As the Pacific plate is thrust beneath the continent and the Aleutian Islands, it bends 

downward. Earthquakes, which accompany the downward movement of the Pacific plate, mark the top of 

the relatively cold, brittle downgoing plate (fig. 7). As 

the Pacific plate descends, rock in the overlying plate 

is partially melted. Some of this newly formed magma 

rises toward the Earth's surface to form the volcanoes 

west of Cook Inlet. 

CHRONOLOGY OF THE 1989-90 ERUPTION 

Pre-Eruption, November 20-December 13, 1989 

The first report of unusual activity at Redoubt was 

received by AVO late on the afternoon of December 8, 

1989, when a steam plume rising from high on 

Redoubt's north flank was visible from Anchorage. 

Seismicity at this time was at a low level, and scientists 

interpreted the plume to reflect a slight increase of 

geothermal heat in the summit crater. Clouds completely 

obscured the volcano between December 9 and 14, 

preventing scientists from making observations of the 

summit crater until after the eruption began on December 14. Scientists at AVO later learned that a pilot flying less than 1 km southwest of Redoubt on November 

20 had noticed a small steam "cloud" rising a few hundred meters above the summit crater and a strong sulfur odor downwind from the volcano. Another pilot 

observed a small steam plume on December 3. 

Elevated levels of seismicity were first observed 

on December 13, and the seismicity rapidly developed 

into an intense swarm of earthquakes. More than 4,000 

earthquakes smaller than magnitude 2 were recorded 

in the 24 hours before the first eruption on December 

14. The rapidly increasing seismicity beneath Redoubt 

prompted AVO to notify the Department of Emergency 

Services, the Federal Aviation Administration, the National Weather Service, the Drift River Terminal, and 

the news media of the increased volcanic hazard (fig. 

8). A statement was issued late in the afternoon on 

Figure 6. The Pacific plate being thrust beneath the North American plate along the Aleutian 

trench. Sketch by Robert F. Gilmore. 

Chronology of the 1989-90 &uption 7 
59°L..... ______ .l_ ___ ___ ..L-----' 

B 

0 

(f) 

a: 50 w A.._<v fw q_...:J" :::;: 

0 

_J 9;>CJ ~ 

:;;::: .ji ~ X.'?' 

:r:' 100 ~ f- ~a Cl... 0 w MAGNITUDES 0 

0 0 <2.0 

150 0 0 2.0-3.9 

8 0 :::4.0 ~0 Cbo 

0 50 100 150 

DISTANCE, IN KILOMETERS 

Figure 7. Location of earthquakes beneath Cook Inlet region 

between january 1980 and November 1989. A, Map showing 

location of cross section A-A', which lies above the boundary 

between the Pacific and North American plates. 8, Vertical 

section through the Earth showing the location of earthquakes 

within 50 km of cross section A-A'. Most of the earthquakes 

occurred in a landward-dipping zone that lies in the upper 

part of the descending Pacific plate. A zone of shallow earthquakes lies beneath the line of active volcanoes on the west 

side of Cook Inlet. 

December 13 that noted, "It is impossible to state at 

this time whether the seismic activity is a precursor to 

a volcanic eruption, but that is a possibility of some 

concern." 

Most of the earthquakes were located at depths 

less than 1 to 2 km beneath the crater and consisted of 

low-frequency events, one of two main types of earthquakes usually associated with volcanoes (fig. 9A). (The 

second type consists of high-frequency events at greater 

depths, fig. 9B, and will be discussed later). This intense 

seismicity was likely caused by the rise of magma 

(molten rock) inside the volcano and the magma's interaction with ground water to produce highly pressurized water and volcanic gases, predominantly steam. 

As the magma and gases moved through existing fractures and caused new fractures to form, the upper part 

of the volcanic cone was weakened, and a pathway for 

the magma was created. 

December 14-December 21, 1989 

A major explosive eruption on December 14 at 

9:47 a.m. AST (Alaska standard time) was the first of 

several explosive episodes that opened a new vent in 

the summit crater and repeatedly spread tephra 

throughout south-central Alaska (figs. 10 and 11). 

Tephra refers to all airborne products of an eruption 

which fall from a volcanic plume, including ash-size 

(less than 2mm in diameter) and larger rock fragments. 

During the next 5 days, intermittent tephra plumes carried by strong winds blanketed south-central Alaska with 

thin layers of gritty ash. The explosive activity also 

generated large debris flows in Drift River valley that 

swept past the Drift River Terminal. Though the terminal 

was not flooded, its vulnerability to larger debris flows 

was clearly demonstrated, and crews were evacuated 

on December 14. 

In the first 25 hours of the eruption, four explosive 

episodes produced giant columns of tephra as high as 

12,000 m above sea level, where strong winds blew 

the tephra several hundred kilometers to the northeast 

(table 1). Initially, the tephra consisted primarily of 

shattered rock and mineral fragments derived from 

preexisting rocks within the volcano. But by the morning of December 15, the character of the tephra changed: 

it became rich in fresh glass shards and pumice fragments, which form when new gas-rich magma is explosively expelled (fig. 12). This change indicated 

opening of a vent in the summit crater; the vent had 

been cleared of older rocks, and magma was now escaping through it to the surface. The majority of pumice 

was erupted during the most energetic episode, which 

began at 10:15 a.m. on December 15. Within 20 minutes, 

8 The &uption of Redoubt Volcano, Alaska, December 14, 1989-August 31, 1990 
Figure 8. Seismic center at the University of Alaska Geophysical Institute in Fairbanks. Seismic data are recorded 

digitally on a mainframe and personal computer and displayed on seismographs shown in foreground. AVO's warnings 

of eruptive activity at Redoubt Volcano were based on seismicity beneath the volcano. Photograph by James Coccia, 

University of Alaska Geophysical Institute. 

w 

0 

:::J 

f-

-' 

a. 

A 

:::;; B <( 

3 4 5 6 7 8 

TIME, IN SECONDS 

Figure 9. Typical seismograms illustrating the two main types of seismic events recorded at Redoubt Volcano. A seismogram is a graphical 

representation of the motion of the ground at the location of a seismometer-an instrument that measures ground motion. The amplitude of the ground shaking is represented by the vertical deviations 

of the peaks and troughs from the normal baseline. Time is recorded 

in the horizontal direction. These seismograms are from seismic station 

RDN on the north flank of Redoubt (see fig. 34). A, Low-frequency 

event originating near the summit of the volcano. B, High-frequency 

event occurring at a depth of several kilometers beneath the volcano. 

The frequency of the seismic signal is expressed in terms of the 

number of peaks or troughs that occur during one second of time. 

Note that for a given interval of time there are more peaks in the 

bottom seismogram; in other words, it is characterized by a higher 

dominant frequency. In this figure, the frequencies (number of peaks 

10 11 per second, or hertz) are about 2.3 and 5.0 hertz (Hz) for the upper 

and lower seismograms, respectively. 

Chronology of the 1989-90 Eruption 9 
Figure 10. Tephra plume rising from Redoubt's crater to about 5,000 m above sea level and drifting to the northeast on 

December 16, 1989. Photograph by Hollis Twitchell, National Park Service. 

10 The &uption of Redoubt Volcano, Alaska, December 14, 1989-August 31, 1990 

Figure 11. Nighttime Advanced Very 

High Resolution Radiometer image 

from a National Oceanic and Atmospheric Administration (NOM) satellite 

of Redoubt Volcano in eruption at 3:17 

a.m. AST (Alaska standard time) on 

December 16, 1989. The tephra plume 

extends downwind from Redoubt 

across Cook Inlet and over the Kenai 

Peninsula. Compare to figure 2. Snow, 

clouds, and plume are white; land and 

water are shades of gray to black. The 

image was processed by the EROS 

Alaska Field Office (U.S. Geological 

Survey) in Anchorage, Alaska. 
Table 1. Chronology of the 1989-90 eruption of Redoubt Volcano 

[X, volcanic activity occurred; na, not applicable;?, volcanic activity uncertain] 

Type of volcanic activity 

Dome Debris Plume height, 

Day Time1 Explosion2 collapse flow azimuth3 Remarks 

December 1989 

14 9:47am X(17) na X >10 km, NE First major explosive episode. 

15 1:40am X(12) na ? ? Seismicity consists of tremor, probably 

3:48am X(10) na ? ? accompanied by explosive activity. 

10:15 am X(40) na X >12 km, NNE Major explosive episode generates 

pyroclastic flow and avalanche of snow, 

ice, and rock down Drift glacier. Debris 

flow crests 3 m below top of levee at Drift 

River Terminal. 

16-18 X na ? <7 km, NE/SE Nearly continuous ejection of tephra from 

crater. 

19 6:30am X(9) na X >9 km,? 

January 1990 

2 5:48pm X(26) X X >12 km, NNE Explosions accompanied by collapse of 

7:27pm X(61) X X >12 km, NNE lava dome down north flank; debris flow 

causes flooding at Drift River Terminal. 

8 10:09 am X(15) X X >12 km, ESE No precursory seismicity detected. 

11 10:01 am X(5) ? ? >9 km,? 

1:42pm X(12) ? ? ? 

16 10:48 pm X(13) ? X >11 km, NNE Minor debris flow in upper Drift River. 

February 1990 

15 4:10am X(20) X X >10 km, SE Pyroclastic flow travels about 12 km NE 

of crater. 

21 12:32 am X(6) X X 9 km, ENE 

24 5:05am X(4) X X 9 km, NE 

28 9:47am X(5) X X 8 km,? 

March 1990 

4 8:39pm X(8) X X 12 km, NNE Drift River flows into Cannery Creek for 

the first time. 

9 9:51am X(10) X X 10 km, WSW 

14 9:47am X(14) X X 12 km, NE 

23 4:04am X(8) X X >10 km, NE 

29 10:33 am X(7) X ? ?, NNE Vigorous "steam plume" reported by 

pilots above 15 km. 

April1990 

6 5:23pm X(7) X X 9km, E 

15 2:49pm X(8) X X >10 km, NW 

21 6:11pm X(4) X X >8 km, NNW 

1 Alaska Standard Time. 2

Numbers in parentheses indicate explosive activity in minutes, based on duration of seismicity recorded 

at seismic stations on Mt. Spurr (see figs. 2 and 22). 

3

Estimated plume height above sea level; azimuth from Redoubt Volcano. 

pumice fragments as large as 10 em across were falling 

about 40 km downwind of Redoubt. 

The initial explosive episodes also triggered several debris flows consisting of water, ice, and rock debris in Drift River that swept past the oil terminal and 

into Cook Inlet. The largest debris flow crested about 

8 m above the river channel near the Drift River Terminal, 3 m below the top of the outer protection levee 

that borders the storage tanks. Blocks of ice probably 

covered the surface of the debris flow, for when the 

crews returned to the terminal on December 17 the 

channel was littered with ice blocks as large as 10 m 

Chronology of the 1989-90 Eruption 11 
across and lined with ice levees 3 m tall. These levees 

partly protected the terminal because they confined the 

debris flow to the main channel. The largest debris flow 

was most likely triggered by the explosive episode at 

10:15 a.m. on December 15, which was the most energetic of the early explosive episodes. 

The triggering mechanisms of these debris flows 

are not well understood because the precise timing of 

the debris flows relative to specific eruptive episodes 

is not known. However, the debris flows were probably caused by fast-moving pyroclastic flows (avalanches of hot pumice, ash, rock debris, and gas) that 

swept down Redoubt's north flank and across Drift 

glacier and melted massive volumes of snow and ice. 

When geologists investigated the upper Drift River 

valley at the base of Redoubt on December 26, they 

Figure 12. A, Scanning electron microscope (SEM) reveals the 

shape and type of ash particles erupted from Redoubt Volcano 

on December 15, 1989. Copyrighted photograph by Bill Roth, 

Anchorage Daily News, reprinted by permission. 8, SEM view 

of volcanic glass with many elongate, thin, delicate walls 

between vesicles (bubbles). SEM view by Maurice Lynch, 

Minerals Management Service, Anchorage, Alaska. 

discovered a multi-layered deposit as thick as 10 m 

consisting of snow, blocks of glacial ice as large as 

2.5 m across, rock debris, and some pumice. The deposit extended at least 4 km downstream from the Drift 

glacier piedmont lobe. Field evidence indicates that 

some layers were wet when they formed and were 

overridden and partly eroded by subsequent flows of 

water. The complex deposit was probably the result of 

several avalanches of snow, ice, and rock debris that 

were triggered by pyroclastic flows moving down the 

upper flanks of the volcano. 

Seismicity was highly variable from December 14 

to 21. It consisted of periods of quiet, periods of tremor, 

and periods of intense seismicity associated with explosive activity. (Tremor is a continuous and rhythmic 

signal associated with the movement of fluids-magma 

and water-or gas within a volcano.) The initial explosive episodes were preceded and accompanied by 

shallow earthquakes of a low-frequency type discussed 

earlier (fig. 9A). These earthquakes were followed on 

December 16 by deeper earthquakes between 4 and 10 

km beneath the crater (fig. 13). The deeper earthquakes 

consisted chiefly of a high-frequency type that is related to the brittle fracture of rock beneath a volcano 

(fig. 9B). They were likely associated with readjustment 

of stress in the rocks surrounding the magma conduit 

system as magma moved upward from deeper levels. 

December 22, 1989-January 2, 1990 

The character of the eruption changed on about 

December 22 from intermittent explosive activity to the 

extrusion of viscous lava. The lava gradually formed a 

steep-sided mound, called a lava dome, over the vent 

(fig. 14). Such domes are common features at composite volcanoes where lava extruded from a vent is too 

viscous to flow very far; lava of the Redoubt dome is 

andesite with a silica (SiOz) composition ranging from 

about 58 to 63 percent (see examples in table 2, p. 

21). Its relatively high silica content makes andesite a 

moderately viscous type of lava. Seismicity indicates 

that this first dome likely began growing as early as 

December 22, though it was not seen by scientists until December 26. Small plumes of ash accompanied 

frequent rockfalls that swept down the steep north side 

of the dome. By January 1, the dome was perched precariously above Drift glacier and had an estimated volume of about 25 million m3 (Alaska Volcano 

Observatory Staff, 1990). 

Seismicity during growth of the first lava dome 

was similar to seismicity observed during the extrusion of viscous magma at other volcanoes, notably 

Mount St. Helens in Washington, Augustine Volcano 

in Alaska, and Usu volcano in Japan. At the station 

12 The Eruption of Redoubt Volcano, Alaska, December 14, 1989-August 31, 1990 
55' 50' 45' 40' T I o 

®DFR -

I I 

-

MAGNITUDE 

0 < 1.0 0 

I 

r T 

0 ~ 1.0 0 5 KILOMETERS I I I I 

B 

0 

d' o OG'io 0 cP 

0 0 0 0 

0 0 0. 

0{;) 

5 

(f) 

a: 

w 

Iw10 

~ 0 5 10 15 20 _J 

::z DISTANCE, IN KILOMETERS 

~ 

::r:: 0 

I0... 

w 

0 0 

5 

15 20 25 30 

DECEMBER 1989 

Figure 13. Seismicity at Redoubt Volcano, December 13-

31, 1989. A, Spatial distribution of seismic events beneath Redoubt Volcano. Light lines are topographic contours, in feet. A fifth seismic station, RDT, is located east 

of this map (see fig. 34). B, Vertical cross section beneath 

Redoubt showing depth of seismic events shown in A. C, 

Diagram showing how depth of seismic events varied 

over time, from December 13-31. Note beginning of 

seismic events deeper than 4 km on December 16. 

closest to the crater, small shallow earthquakes dominated the seismic record during extrusion. The number 

of seismic events began increasing steadily about December 28, and the seismicity again changed character 

with the return of low-frequency events on December 

29 (fig. 9A). Scientists interpreted these changes as indications that the magma system beneath the dome was 

becoming increasingly pressurized, perhaps because of 

a "capping effect" of the growing dome. Accelerating 

seismic activity and the precarious position of the dome 

prompted AVO to issue warnings on January 1 and 2 

of the possibility of "moderate to strong explosive activity" associated with collapse of the dome. On January 2 at 3:50p.m., after discussions with AVO scientists, 

the Cook Inlet Pipeline Company evacuated its crews 

from the Drift River Terminal. 

January 2, 1990 

AVO's warnings were on the mark. Two powerful explosions at 5:48p.m. and 7:27p.m. on January 2 

sent tephra plumes to heights of more than 12,000 m 

and destroyed more than 80 percent of the dome. A 

few minutes after the first explosion began, a pilot 65 

km south of Redoubt described the activity as an intermittent "orange flame shooting straight up from the 

summit like a cannon" (Captain Jerry Friz, oral common., 1990). Lightning from the eruption damaged the 

only seismic station then located on the volcano (station RDN on fig. 34), which impaired seismic monitoring until the station was repaired on January 9. 

Both explosions were accompanied by partial 

collapse of the dome that generated fast-moving avalanches of hot blocks and ash down the north flank of 

Figure 14. Sketch of the eruption's first lava dome, which 

formed in Redoubt's summit crater between about December 22, 1989, and january 2, 1990. Steam rising from bottom 

of canyon below dome was caused by stream of hot water 

melted from Drift glacier. Most of the dome was destroyed 

on January 2. Sketch by Bobbie Meyers. 

Chronology of the 1989-90 Eruption 13 
Redoubt and across Drift glacier. The avalanches 

scoured and melted ice and snow from the glacier and 

caused the largest debris flow of the eruption (fig. 15). 

Field evidence indicates that the debris flow consisted 

of at least two pulses. Unlike earlier events, this debris flow contained a large proportion of hot rock debris 

from the dome, and it completely filled the 2-km-wide 

valley floor. Some rocks were as large as 8 m across. 

Trees as large as 0.8 m in diameter were toppled and 

whittled by the flowing debris into downstream-pointing bayonet-like shapes. The peak discharge of the debris flow about 22 km downstream from the crater was 

estimated to be 30,000-50,000 m3

/s, which is comparable to the peak discharge of the 1985 volcanic debris 

flow that destroyed Armero, Colombia (Alaska Volcano 

Observatory Staff, 1990), and slightly more than the 

average discharge of the Mississippi River. 

Because of the deposition of debris, the main 

channel of the Drift River became clogged with sediment about 8 km upstream from Cook Inlet, forcing 

most of the debris flow into adjacent Rust Slough (fig. 

16). Muddy water from Rust Slough entered the oil 

terminal from the west and south, inundating some 

buildings with water as deep as about 75 em and disabling the facility's power generators. Flooding of the 

terminal forced the Cook Inlet Pipeline Company to 

temporarily curtail its operation in order to repair damaged equipment and to reassess its operating procedures. 

. Tephra from the explosions spread northeast from 

the volcano but resulted in only light ash fall at the 

Beluga power plant west of Anchorage and in Anchorage itself (fig. 2). However, the ash fall was sufficient 

to cause airlines to cancel flights. Near the volcano, 

the tephra deposit was as thick as 10 em and consisted 

EXPLANATION 

~ Debris-flow inundation 

~_:_:·_·::::_:._·:_::_::_; Ponded or flowing water generated . at margins and distal end of debris flow 

l:t\:\:t\\\i\::~ Redoubt Volcano glaciers and ice fields 

of tiny aggregate balls of wet ash, known as accretionary lapilli. These lapilli form when moisture and electrical charges develop within an expanding tephra plume. 

The tephra deposit did not contain pumice, and dense 

rock fragments were no larger than a few millimeters 

in diameter. The size of the tephra and the absence of 

fragments embedded in snow on the volcano argue 

against the dome being blasted apart by a series of explosions. These characteristics and the distribution of 

large dome rocks throughout the Drift River valley 

suggest that most or all of the dome was destroyed by 

catastrophic collapse and avalanche down the volcano's 

north flank. 

january 8-February 14, 1990 

Removal of most of the lava dome from the summit crater on January 2 left an open vent that provided 

an unobstructed pathway for magma to rise with relative ease, and seismicity declined to the lowest levels 

since the eruption began in December. On January 8, 

with no seismic warning, a sudden explosion destroyed 

the remainder of the dome. A mushroom-shaped column of tephra quickly formed above the volcano and 

spread east over the Kenai Peninsula within about 1 

hour (fig. 17), turning day into night (fig. 18). Hot 

avalanches again swept down Drift glacier and triggered 

a relatively small debris flow containing dome rocks 

(fig. 19) and few ice fragments. The debris flow diminished in size within a few kilometers of the volcano, and no flooding occurred in the lower valley. 

Two small explosive episodes on January 11 and 

16 were followed by renewed extrusion of lava that 

.. .... 

Edge of valley floor ---~r:::;:9~~~~~?~~~~;.4.<~~~0 Stream channels, 

pre-1989 ~~~~~~::z;..~~~ 

5 KILOMETERS 

I 

I 

5 MILES 

Figure 15. Approximate boundaries of area inundated by debris flow in Drift River valley on January 2, 1990. Some 

buildings in Drift River Terminal (black rectangle) were flooded with water as deep as about 75 em. 

14 The Eruption of Redoubt Volcano, Alaska, December 14, 1989-August 31, 1990 
built a new lava dome, which attained about one-third 

the volume of the first dome. No explosive events accompanied growth of the dome, but numerous rockfalls 

down its steep flanks generated ash-rich plumes a few 

hundred meters high. Seismicity consisted primarily of 

small shallow earthquakes. 

By mid-January, more than 100 million m3 of ice 

had been removed from Drift glacier (Alaska Volcano 

Observatory, 1990). Pyroclastic flows and avalanches 

of hot debris from the dome eroded hundreds of gullies 

and enlarged crevasses on the glacier, forming a network 

of steep-walled channels tens of meters deep. Many of 

these merged with a narrow canyon that was carved 

along the axis of the glacier from the crater down to 

the piedmont lobe. Nearly all of the ice in the canyon 

was removed to bedrock; only vestiges of glacial ice 

remained on the canyon floor. 

February 15, 1990 

A weak increase in seismicity preceded by about 

4 days a large explosion and the collapse of the second 

dome on February 15 at 4:10 a.m. Because the increase 

was much less dramatic than those preceding the December 14 and January 2 explosions, seismologists 

overlooked its significance and no warning of an impending eruption was issued. About 75 percent of the 

dome was removed in an avalanche of hot blocks and 

ash that swept down the north flank of Redoubt and 

across Drift glacier. A fast-moving ash cloud swept 

across Drift River valley and climbed 700 m up the 

north valley wall, which faces the volcano. This hot 

mixture of gas, ash, and gravel-sized rock debris burned 

and abraded small willow trees on the north valley wall 

and drove willow branches endwise into the snow at 

least 1 m. The ash cloud was diverted eastward by the 

valley wall at least 4 km, and it deposited ash along 

the way. On Redoubt, the ash cloud spread across slopes 

to the east and west of Drift glacier that had been unaffected in earlier eruptions, causing numerous snow 

avalanches and again destroying the critical seismic 

station that was struck by lightning on January 2. 

The hot avalanche that swept across Drift glacier 

severely eroded its surface and generated a debris flow 

in Drift River that flowed to Cook Inlet. When geologists visited the glacier 10 hours after the event, the 

sandy deposits on the glacier still had a temperature of 

at least 230 °C, and the glacier surface was visibly more 

irregular and incised than it had been after the January 

8 episode (fig. 20). Many large ice blocks were partly 

detached from gully walls. The debris flow rafted hot 

Figure 16. Storage tanks of Drift River Terminal between Rust Slough (foreground) and main channel of Drift River. The debris 

flow on January 2 diverted Drift River south into Rust Slough. The tanks can store about 1.9 billion barrels of oil; when the 

eruption began on December 14, 1989, about 830,000 barrels were stored in the tanks. View to east. Copyrighted photograph 

by Erik Hill, Anchorage Daily News, January 8, 1990; reprinted by permission. 

Chronology of the 1989-90 &uption 1 S 
Figure 17. Huge, eerie, anvil-shaped ash cloud moving toward the Kenai Peninsula from Redoubt Volcano on January 8, 

1990. View to west. Copyrighted photograph by Roy Shapley, reprinted by permission. 

Figure 18. Daytime darkness in downtown Soldotna on the Kenai Peninsula at 12:56 p.m. during an ash fall from Redoubt 

Volcano. Copyrighted photograph by Thomas Weaver on january 8, 1990, reprinted by permission. 

16 The &uption of Redoubt Volcano, Alaska, December 14, 1989-August 31, 1990 
dome rocks, as large as 1 m across, and blocks of ice 

and snow down the Drift River valley. Field measurements made after the event suggest the debris flow had 

a velocity of about 60 km/hr at the base of the glacier 

and about 30 km/hr midway down the valley. Most of 

the debris flow went down Rust Slough, and muddy 

water inundated part of the oil terminal's runway. For 

the first and only time, water and debris topped the 

inner containment berm surrounding the tank closest 

to Rust Slough. No oil was spilled. 

February 21-April 20, 1990 

During the next two months, lava was extruded 

almost continuously to form a succession of domes that 

were mostly destroyed by explosions every 4 to 8 days. 

The explosions produced tephra plumes as high as 

12,000 m (fig. 21) and hot avalanches of dome blocks 

and ash that triggered relatively small debris flows in 

Drift River valley. The avalanches continued to ravage 

the surface of Drift glacier, and the debris flows transported significant quantities of sediment to the upper 

Drift River. On the delta, the river changed its course 

several times between Rust Slough, Cannery Creek, and 

its original channel (fig. 4). Tephra was blown in almost every direction from the volcano between February and April (table 1), but only traces of ash were 

deposited in Anchorage and on the Kenai Peninsula. 

Earthquakes continued beneath the volcano at depths 

less than 10 km at approximately the same rate that 

had characterized the activity since mid-December. 

The explosive episodes during this period were 

Figure 19. Block of the first lava dome, deposited in Drift 

River valley 5 km downstream from Redoubt Volcano. The 

block was transported downstream by a debris flow caused 

by an explosive episode on january 8, 1990. When measured 

6 days later, the temperature of the rock was still 145 "C. 

Photograph by C. Dan Miller on january 14, 1990. 

Figure 20. A, North flank of Redoubt Volcano looking across 

Drift glacier piedmont lobe (see fig. 4). Note channels eroded 

in Drift glacier. Dark areas on glacier are pyroclastic-flow 

deposits. 8, Close view of channels in Drift glacier looking in 

direction indicated by arrow in A. The highly irregular surface 

of Drift glacier was eroded and melted by repeated pyroclastic flows generated by dome collapses in the summit crater. 

Photographs by Richard B. Waitt on March 4, 1990. 

Chronology of the 1989-90 &uption 17 
less energetic and of shorter duration than earlier explosive events and followed one another with remarkable 

regularity, averaging about 4.5 days apart until March 

14, and 8 days apart through April (fig. 22). Between 

February 15 and March 4, there were no seismic stations 

operating on the cone of the volcano, which precluded 

the detection of precursory changes in seismicity during this interval. After a new station was established 

on the volcano, however, clear seismic precursors 

prompted A YO to issue advance warning before the 

explosions on March 23 and April 6. In addition to 

patterns of seismic activity, AVO scientists used the 

regularity of eruptions after February 15 to define time 

intervals within which explosions were most likely to 

occur. This statistical forecasting method was only 

partially successful. On this basis, advance warning was 

provided for an explosion on April 15, which occurred 

within a 3~-day forecast window announced by A YO 

on April 12; no seismic precursors were detected prior 

to this explosion. However, no explosion occurred during a similar forecast window between April 22 and 

26. 

While the eruption of Redoubt settled into a familiar pattern of recurrent dome growth and partial destruction, geologists took advantage of longer daylight 

hours to spend more time investigating volcanic deposits 

on Drift glacier. By March 1, the glacier's surface had 

Figure 21. Redoubt Volcano during a moderate explosive 

episode on April 21, 1990. The mushroom-shaped tephra 

plume rises from a pyroclastic flow that swept down the 

volcano's north flank. A smaller, light-toned plume rises from 

the summit crater. View is northwest across Cook Inlet. Copyrighted photograph by Robert j. Clucas, reprinted by 

permission. 

become highly irregular, marked by deep gullies, closed 

basins, and isolated ice buttes. A gully as much as 30 

m deep and 25 m wide formed a central ice canyon 

along the east side of the glacier (fig. 23). Both sides 

of this deep canyon were covered with hot pyroclasticflow deposits as thick as 10 m (fig. 24). The deposits 

contained dome rocks as large as 1 m in diameter in a 

loose sandy matrix and retained their heat for several 

months; temperatures as high as 400 oc were measured 

in these deposits in August. As some of the loose material slid into the canyon, temporarily damming the 

channel, clouds of hot ash rose several hundred meters 

above the gorge. When the dams failed, small floods 

scoured the canyon floor. On April 10, a series of explosions at the 890-m level of Drift glacier formed an 

elongate crater in the deposits about 300 m long and 

sent an ash and steam plume as high as 1,000 m. Similar events sometimes resulted in reports of eruptive activity from passing pilots and residents on the Kenai 

Peninsula. The explosions were probably caused by the 

interaction between the hot deposits and water flowing 

on the glacier surface. 

April 21-August 31, 1990 

A small explosion on April 21 destroyed the latest dome; a new lava dome was observed in the crater 

five days later and continued to grow through early June 

(fig. 25). The new dome's north flank quickly became 

oversteepened and was the source of several rockfalls 

that generated ash-laden clouds a few thousand meters 

above the volcano's north flank; the largest rockfalls 

(f) 

w 

f--

::::> 

z 

~ 

z 

u.i 

~ 

f= 

70 

60 

50 

40 

30 

20 

10 

0 

DEC 

1989 

JAN FEB MAR APR 

1990 

Figure 22. Duration of Redoubt's explosive episodes determined by the duration of seismic signals recorded by seismic 

station SPU, located on Mount Spurr about 100 km north of 

Redoubt Volcano (see fig. 2). The relative size of each eruptive episode generally corresponds to the duration of the seismicity that accompanied each episode. 

18 The &uption of Redoubt Volcano, Alaska, December 14, 1989-August 31, 1990 
occurred on April 26 and May 26. The dome has remained intact as of the time of this writing (October 

1990) and is estimated to be the third-largest dome in 

the current eruption sequence. Seismic activity declined 

between June and August to the lowest levels observed 

since the eruption started, but scientists still characterized the volcano's status as restless (color-code yellow 

in table 3). 

EFFECTS OF THE 1989-90 ERUPTION 

Introduction 

The eruption of Redoubt affected the Cook Inlet 

region by significantly interrupting air traffic, causing 

ash fall over populated areas, and triggering debris flows 

down Drift River valley. Downwind from the volcano, 

ash fall produced darkness during daylight hours and 

caused local power outages (fig. 26) and school closures. Sports and cultural events normally held in winter were canceled, including an invitational collegiate 

ice-hockey tournament in Anchorage, and industries and 

commumttes were forced to face the possibility of 

sudden ash fall throughout the spring and summer of 

1990. Downstream from Redoubt, debris flows spread 

over the floor of Drift River valley, inundated part of 

Drift River Terminal, and raised channel beds, diverting the river from its original course to Rust Slough, 

Cannery Creek, and Montana Bill Creek (fig. 4). Oil 

production in Cook Inlet was severely reduced for several weeks. These effects were temporary and no lives 

were lost, but preliminary estimates of damage to 

property and loss of revenue attributed to the eruption 

exceed $100 million. Protective measures at the Drift 

River Terminal alone cost the Cook Inlet Pipeline 

Company more than $20 million. 

Tephra 

Explosive eruptions blast shattered fragments of 

magma (molten rock) and old rock into the air. Collectively, these airborne fragments are called tephra; 

they range in size from particles larger than 5 em in 

diameter to fragments of crystals and rapidly quenched 

Figure 23. Ice canyon eroded into Drift glacier by pyroclastic flows generated by dome collapses. Glacier is mantled with 

pyroclastic-flow deposits. Note helicopter in foreground for scale. Photograph by Thomas P. Miller on june 19, 1990. 

Effects ofthe 1989-90 Eruption 19 
Figure 24. Pyroclastic-flow deposits atop Drift glacier. These 

two layers were deposited during different eruptive episodes 

when domes collapsed down the north flank of Redoubt as 

hot, fast-moving avalanches of rock debris, or pyroclastic flows. 

Deposits in this view are about 2 m thick; other flow deposits 

were as much as 1 0 m thick. Photograph by Cynthia A. Gardner 

on May 24, 1990. 

Figure 25. Third-largest dome of the eruption, which formed after the explosive episode on April 21, 1990. The dome stopped 

growing in june. Below the dome, the upper part of Drift glacier has been completely removed down to bedrock. View to 

southeast. Photograph by David E. Wieprecht on July 17, 1990. 

20 The Eruption of Redoubt Volcano, Alaska, December 14, 1989-August 31, 1990 
fragments of volcanic glass less than 0.01 mm in length. 

Tephra consisting of particles less than 2 mm in diameter is called ash. During a major explosive eruption, 

tephra can be carried more than 20,000 m into the air 

and fall over very large areas. For example, the area 

covered by tephra from the eruption of Mount St. Helens 

on May 18, 1980, was about 150,000 km2• For comparison, the area covered by tephra from the eruption 

of Redoubt on December 15, 1989, was estimated as 

less than 20,000 km2 • The composition of tephra varies 

with each volcano and sometimes during an eruption 

sequence (table 2). 

With increasing distance downwind from Redoubt, 

both the size of the tephra fragments (fig. 27) and the 

thickness of the resulting deposits generally decreased. 

Communities bordering Cook Inlet on the Kenai Peninsula were the hardest hit by ash fall during the eruption. The ash was more of a disruption and nuisance 

than a danger to public health, but during brief periods 

of heavy ash fall, as a precautionary measure, officials 

closed schools and encouraged people to stay indoors 

or to wear dust filter masks. Researchers studying the 

health effects of ash from the eruption of Mount St. 

Helens concluded that ash is an irritant at low to moderate concentrations, but exposures experienced downwind of Mount St. Helens were too low to pose a serious 

Figure 26. Workers repair an electric transformer that shortcircuited because of ash fall in Soldotna on December 16, 

1989. Snow is covered by ash in foreground. Copyrighted 

photograph by Roy Shapley, reprinted by permission. 

Table 2. Chemical composition, in percent, of rock samples 

from several recent eruptions in Alaska 

[ -- , not reported] 

Volcano ..... Redoubt1 

Eruption date .. 11/15/89 

Si02 

Al20 3 

Fe20 3 

FeO 

MgO 

CaO 

Na20 

K20 

H20-

H20+ 

Ti02 

P20s 

MnO 

s 

Cl 

6022 

18.22 

5.35 

2.08 

6.60 

4.08 

1.48 

52 

21 

.14 

1

Pumice sample. 2

Lava sample. 

Redoubt2 Mt.Sp~ Augustine4 

1/1/90 7/9/53 4/3/86 

62.14 55.2 645 

17.31 17.2 10.2 

3.1 1.5 

458 42 2.0 

1.73 3.7 2.1 

5.68 7.4 3.0 

4.09 3.4 3.6 

1.65 1.0 12 

24 .14 

.93 .19 

.74 .55 

.18 .30 29 

.13 .16 .09 

.60 

.10 

3Wilcox, R.E., 1959, Some effects of recent volcanic 

ash falls, with especial reference to Alaska: U.S. Geological 

Survey Bulletin 1028-N, p. 409-476. 4Yount, M.E., Miller, T.P., Gamble, B.M., 1987, The 

1986 eruptions of Augustine Volcano, Alaska--Hazards and 

effects, in Hamilton, 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. 

threat for the development of silicosis (a chronic lung 

disease) (Buist and others, 1986). The volume and duration of ash fall resulting from the eruption of Redoubt was much less than that resulting from the 

eruption of Mount St. Helens. The layers of ash that 

blanketed the Kenai Peninsula were less than 1 em thick, 

and they were covered with snow within hours to days 

of each eruptive episode. The snow cover significantly 

reduced or eliminated the concentration of airborne ash 

particles. However, when the snow began to melt in 

April, residents and health officials reported unusually 

high dust levels that may have been related to the ash 

(Rinehart, 1990). At the Beluga power plant (fig. 2), 

the main. power source for Anchorage, a brief period 

of heavy ash fall on February 24 forced all but one of 

the gas turbines to be shut down as a precautionary 

measure. 

Following several of Redoubt's explosive episodes 

in December and early January, domestic and international airline traffic above south-central Alaska was 

brought to a standstill. Hundreds of scheduled flights 

were canceled, most of them during the Christmas 

holiday season (fig. 28), and many international flights 

were routed around the State. The Anchorage International Airport {AlA) is a major hub of domestic and 

Effects of the 1989-90 Eruption 21 
international air traffic (fig. 29), and as the eruption of 

Redoubt demonstrated, flights within the State and between the Orient and Europe are susceptible to disruption by eruptions that produce significant tephra plumes. 

In December 1989 and January 1990, AlA experienced 

an estimated revenue shortfall of $2.62 million (Ken 

Burdette, written commun., 1990) because of canceled 

flights into the airport. Between December and February, 

four commercial jet aircraft encountered ash downwind 

of the volcano (Steenblik, 1990). The abrasive quality 

of the ash "sandblasted" the cockpit windshields and 

the leading edges of wings; the windows had to be replaced and the wings polished. All of the engines required thorough inspection. 

The most serious incident occurred on December 

15 when a Boeing 747 enroute from Amsterdam and 

carrying 231 passengers and 13 crew members began 

its descent into Anchorage. Though the pilot was following the path taken by another Boeing 747 only 20 

minutes earlier, the plane encountered a cloud of volcanic ash 240 km downwind from Redoubt at an altitude of about 7,500 m. The volcano had erupted 90 

minutes earlier. As the pilot attempted to climb out of 

the cloud, ash particles melted by the heat of the engines began to solidify and formed a glassy coating in 

parts of the turbine section of the engine, which retz 

w 

(.) 

a: 

w 

a.. 

~ 

z 

<( 

:::c 

ta: 

w 

z 

ii 

en 

en 

<( 

~ 

100 

90 

80 

70 

60 

50 

40 

30 

20 

10 

0 

• December 15, 1989 

• January 8, 1990 

• February 15, 1990 

• February 21, 1990 

~--~--~~---L--~--L_~ __ _L __ ~ 

1.0 0.5 .25 .125 .062 .032 .016 .008 .004 .002 

GRAIN SIZE, IN MILLIMETERS 

Figure 27. Particle-size distribution of ash that fell downwind 

of Redoubt Volcano. Locations where ash samples were collected for particle-size analysis are as follows: Dec. 15, 1989, 

115 km north of Anchorage along Highway 3; jan. 8, 1990, 

30 km southwest of Soldotna along Highway 1; Feb. 15, 

1990, 45 km southwest of Soldotna along Highway 1; Feb. 

21, 1990, 15 km southeast of Anchorage along Highway 1. 

stricted the engines' intake of air. All four engines shut 

down. For the next 8 minutes, the plane glided steeply, 

losing 4,000 m of altitude before the pilot succeeded 

in restarting the engines with less than 2,000 m of 

clearance between the plane and the ground. The cold 

Alaska air and repeated attempts to restart the engines 

Figure 28. Numerous explosive episodes of Redoubt Volcano 

resulted in hundreds of canceled flights during the busy 

Christmas holiday season. Copyrighted photograph by Michael 

Dinneen, Anchorage Times, reprinted by permission. 

22 The Eruption of Redoubt Volcano, Alaska, December 14, 1989-August 31, 1990 
resulted in a partial breakup of the glass coating, and 

the plane landed safely in Anchorage about 38 minutes 

after the incident began. All four engines had to be 

replaced (fig. 30), as well as the electrical and avionics 

systems, and the entire aircraft had to be thoroughly 

cleaned of fine ash. These repairs cost an estimated 

$80 million (Steenblik, 1990). 

This near-tragic encounter underscored the importance of notifying pilots of wind conditions and 

volcanic eruptions that generate significant tephra 

plumes only minutes after they occur. Such notification 

requires quick, reliable communication between AVO, 

the Federal Aviation Administration, the National 

Weather Service, airline officials, and pilots during 

eruptive activity. In an effort to better characterize the 

volcano's activity, AVO adopted a color-code scheme 

(see below) in February, and several airlines developed 

emergency plans that specified a certain response for 

each color. Beginning in early January, AVO provided 

airline officials with maps predicting the paths along 

which an ash cloud would move away from Redoubt at 

different altitudes and times (fig. 32). In order to more 

accurately measure wind speed and direction, the 

National Weather Service installed a state-of-the-art 

Wind Profiler radar system near Homer during the 

summer and has developed plans for additional installations in south-central Alaska in 1991. 

Figure 29. Anchorage International 

Airport. The geographic location of 

Anchorage makes it an ideal fueling 

and distribution center for flights between the Orient and Europe. In 1989, 

the total landed cargo weight was 3.63 

million metric tonnes (8 billion 

pounds), the highest in North America 

and more than that handled by john F. 

Kennedy International Airport (New 

York) and Los Angeles International 

Airport combined. View to east. Photograph by Mark Skok, Anchorage 

International Airport; reprinted by permission of the Alaska International Airport System. 

Figure 30. Crew examines one of the Boeing 747 jet engines 

that suffered extensive damage when the jet encountered an 

ash cloud on December 15, 1989. Copyrighted photograph 

by Bob Hamilton, Anchorage Daily News, reprinted by permission. 

Effects of the 1989-90 Eruption 23 
Debris Flows 

Debris flows, rapidly flowing mixtures of water 

and rock debris, are commonly produced at steep-sided, 

snow- and ice-covered volcanoes. Debris flows vary 

considerably in the concentration of water and rock 

debris, ranging from very water-rich flows containing 

less than 20 percent sediment by volume to sedimentrich flows containing more than 60 percent sediment. 

Volcanic debris flows are generated when large volumes of water mix with loose material on a volcano or 

in river valleys that drain the volcano. This mixing can 

occur during the explosive ejection of water from a 

crater lake, during the erosion of volcanic rock debris 

by heavy rain, or during the rapid melting of snow and 

ice by hot volcanic debris. Some of the world's most 

destructive debris flows result from eruptions of volcanoes mantled by snow and ice--the 1985 eruption of 

Nevado del Ruiz in Colombia, for example, generated 

debris flows that claimed at least 22,000 lives (Naranjo 

and others, 1986). 

The largest debris flow of the Redoubt eruption 

was triggered by the major explosions and dome collapse 

on January 2. The debris flow flooded part of the Drift 

River Terminal, and the Alaska Department of Environmental Conservation (DEC) issued an emergency 

order on January 5 calling for a reduction in the amount 

of oil stored at the terminal to 150,000 barrels (Petroleum Information, 1990a). Later, the U.S. Coast Guard 

requested that this limit be reduced to 50,000 barrels, 

which led industry officials to suspend tanker loadings 

because of the increased danger of a spill resulting from 

longer loading times. Additional debris flows in subsequent episodes prompted the Cook Inlet Pipeline Company and DEC to suspend operations at the terminal 

on March 17, to remove most of the oil from the terminal, and to develop alternative plans for its future 

long-term operation. 

Meanwhile, 10 platforms in Cook Inlet that supplied oil to the terminal were forced to suspend production, which totaled about 30,000 barrels of oil a day 

before the eruption (Petroleum Information, 1990b). On 

the basis of November 1989 royalty figures, the halt in 

production cost the State about $39,000 a day. The 

economic consequences of a long-term shutdown of oil 

production in western Cook Inlet would be serious, especially to the Kenai Peninsula. For example, the total 

revenue from Cook Inlet oil in 1989 was $168.5 million or about $14 million a month, and royalty payments 

to the State averaged $1.7 million per month. About 

500 industry jobs with an estimated payroll of $27.5 

million are tied to oil production in western Cook Inlet 

(Wayne Haerer, written commun., 1990). 

In order to protect the storage tanks from future 

volcanic debris flows and floods, the Cook Inlet Pipeline Company constructed new levees 6 m tall around 

much of the terminal and raised critical electrical 

equipment at least 1 m above the ground. For the protection of its crews, a safe house was built atop an 

enormous platform 3 m above the ground. These measures cost about $20 million (Harold Mouser, oral common., 1990). Under an interim agreement with the DEC, 

the levees will permit the company to store up to 

338,000 barrels of oil at the terminal as long as Redoubt Volcano remains active. 

Deposition of sediment in Drift River valley and 

on the delta at its mouth during the 1989-90 eruption 

upset the normal course of the river, forcing the river 

south into Rust Slough and Cannery Creek and north 

into Montana Bill Creek. During the summer, the river 

changed its course frequently, sometimes flowing back 

in its original channel on the north side of the terminal. 

Significantly increased water flow in Rust Slough caused 

severe bank erosion near the terminal. To monitor future changes in the river system, the U.S. Geological 

Survey initiated a surveillance program in the spring 

of 1990. Surveillance now includes repeated surveys 

of the river channel, aerial photography, detailed mapping of the delta area, and water and sediment discharge 

measurements. 

HAZARD INFORMATION PROVIDED BY 

ALASKA VOLCANO OBSERVATORY 

Introduction 

The emergency caused by the eruption of Redoubt 

created an enormous demand for volcano information 

that quickly overwhelmed A YO's small staff. Scientists 

from other facilities of the USGS were brought to Anchorage and Fairbanks to help monitor and interpret 

the volcano's activity and to disseminate information 

as needed, and a Public Information Scientist was 

identified to facilitate the release of information to the 

news media and public. Also, many scientists from the 

UAGI and ADGGS (fig. 3) were diverted from their 

normal tasks to aid in the response. Hazard information was disseminated by AVO through the immediate 

notification of key government and industry officials 

(for example, officials of the Department of Emergency 

Services and the Federal Aviation Administration) when 

warranted by volcanic activity, through printed volcano 

updates, and through briefings. Briefings were given 

to ( 1) representatives of private industry; Federal, State, 

and local government agencies; and congressional 

committees; (2) the Governor of Alaska; (3) Senators 

and their staffs; (4) news media; and (5) groups of 

citizens. Before key officials were notified of a volcanic event, scientists at the crisis center in Anchorage 

24 The &uption of Redoubt Volcano, Alaska, December 14, 1989-August 31, 1990 
and the seismic center in Fairbanks first confirmed the 

nature of the event that was occurring at the volcano. 

Volcano Update 

Seven days after the eruption started, AVO began 

releasing a printed "Redoubt Volcano Update" one or 

more times each day which described eruptions, earthquake activity, important observations, and noteworthy 

events during the preceding 24 hours; beginning on July 

6, the updates were issued once a week. The updates 

also included a general summary of potential volcanic 

hazards over a period of days to weeks and, when possible, specific predictions about future events (fig. 31). 

This ·information was also provided on a recorded phone 

message that wa.s continually updated. 

Color Code 

In early F~ebruary, a scheme of color codes was 

developed to represent the level of concern based on 

the seismic activity beneath Redoubt or confirmed by 

visual observations by AVO field crews or the airline 

industry (table 3). This system was implemented in order to more consistently describe the status of the volcano when a pot·entially hazardous event was in progress 

or was expected. Four colors were chosen to represent 

different levels of activity that were either presently 

occurring or expected to occur at the volcano. Green, 

for example, meant the volcano was in a "dormant" 

REDOUBT VOLCANO UPDATE 

Thursday, March 22, 1990; 11:15 AST 

Level of concern about the state of the volcano 

1-------------------------------------------------------------------------------------- I 

I Current level is: I At this level since: March 22, 1990 I 

I ORANGE I Last level was YELLOW I 

I I For definitions see update of 2/23/90 I 

1-------------------------------------------------------------------------------------- I 

Seismic activity at stations nearest the summit has steadily increased 

during the last 12 hours, and is presently at a higher level than it has 

been in the last few days. We have upgraded the level of concern 

color code to ORANGE. Based on projected wind data below 50,000 

feet, we would expect a plume, if emitted today, to move to the west 

and northwest of thH volcano. 

An AVO field crew yesterday noted a continuous steam plume 

emanating from the dome, but this plume seldom reached elevations 

higher than the summit of the mountain. The crew had a clear look at 

the dome and it appeared to have grown since Sunday. Gases from 

the steam plume wore analyzed during an overflight of the mountain. 

Given the current level of seismic activity, continued dome-building or 

explosive activity remain likely over the next few days or weeks. AVO 

will continue to monitor the volcano 24 hours a day looking for any 

subtle changes in the character of the seismicity which may be 

indicative of increased activity. The level of concern color code will be 

updated as appropriate. 

Figure 31. Text of Redoubt Volcano Update, March 22, 1990. 

state. Red indicated either the occurrence or the imminence of a large explosion or series of explosions likely 

to send ash higher than about 8 km above sea level. 

(In practice, many of the episodes in March and April 

that produced plumes between 8 and 10 km high were 

characterized as "orange.") 

Forecast Plume Trajectory 

In early January, AVO arranged with the National 

Weather Service (NWS) and National Oceanic and Atmospheric Administration (NOAA) to receive forecast 

wind data above south-central Alaska from which potential ashfall areas downwind from Redoubt could be 

determined (Miller and others, 1981). The predicted 

paths of parcels of air originating at the volcano at 

various times and altitudes were plotted on maps of 

Alaska and the Cook Inlet region (fig. 32). The maps 

were used by various agencies and private industries, 

especially the Alaska Department of Emergency Services and both domestic and international airlines, to 

anticipate probable ash-fall-hazard zones following an 

eruption. 

0 

I 

0 

EXPLANATION 

Elevation, in feet 

• 5,000 

• 10,000 

50 MILES + 18,000 

... 23,000 

0 30,000 

0 39,000 

6. 53,000 I 

I 

100 KILOMETERS 

Figure 32. Plume trajectory forecast for 3:00 a.m. on February 15, 1990. Symbols represent the location of parcels 

of air originating above Redoubt Volcano at successive 3-

hour intervals for specific altitudes. An explosive episode 

at 4:10 a.m. generated a tephra plume that blew from 

Redoubt to the southeast, as predicted by this forecast. 

Hazard Information Provided by Alaska Volcano Observatory 25 
Table 3. Color-code scheme developed by Alaska Volcano 

Observatory to characterize eruptive status of Redoubt 

Color Description 

Green Volcano is in its nonnal "donnant" state. 

Yellow Volcano is restless. Earthquake activity is 

elevated, and a plume of gas and steam may 

be rising a few km above the crater. Activity 

may include extrusion of lava and 

emplacement of a lava dome. 

Orange Small ash eruptions are expected or confinned. 

Plume(s) not likely to rise above 8 km (25,000 

ft) above sea level. Minor flooding on the Drift 

River is possible. Seismic disturbance recorded 

on Redoubt seismic stations but not recorded 

at more distant locations. 

Red Large ash eruptions are expected or confinned. 

Plume likely to rise above 8 km (25,000 ft) 

above sea level. Significant flooding in Drift 

River possible; flooding in the Crescent River 

also possible but less likely. Strong seismic 

signal recorded on all Redoubt stations and 

commonly on more distant stations. 

MONITORING THE 1989-90 ERUPTION 

Introduction 

Volcano monitoring is undertaken to assess the 

current status and near-term eruptive potential of a 

volcano and to document the occurrence and character 

of eruptions. Volcano monitoring involves a variety of 

measurements and observations designed to detect 

Figure 33. Scientific methods used to monitor volcanoes. A, Seismic station detects vibration of the ground caused by earthquakes 

and volcanic activity, and transmits data by 

radio to a volcano observatory. 8, Correlation spectrometer (COSPEC) measures sulfurdioxide emission and is used either from the 

ground or an airplane. C, Precise surveying 

instruments measure a volcano's changing 

shape caused by magma rising into its cone. 

D, Geologists observe volcanic deposits to 

understand eruptive processes, and collect 

lava and tephra samples to determine their 

chemical composition. Sketch by Bobbie 

Meyers. 

changes at a volcano that reflect increasing stress caused 

by the movement of magma within or beneath the volcanic cone (fig. 33). Experience gained at well-monitored volcanoes indicates that most eruptions are 

preceded by measurable changes in the state of the 

volcano. The challenge faced by scientists is not only 

to recognize the signs of restlessness that may foretell 

possible eruptive activity but also to interpret the 

monitoring information in order to specify a time window within which an eruption is likely to occur. Not 

all signs of unrest lead to an eruption. An even more 

difficult task that scientists face is to determine when 

an eruption is over so that people can resume their 

normal activities. Volcano monitoring also involves 

detecting the onset of eruptive activity and determining the type and magnitude of volcanic events during 

an eruption, including the speed and areal extent of 

tephra plumes, lava flows, debris flows, and pyroclastic flows. 

Monitoring Redoubt Volcano during the winter 

was extremely difficult. Visual observations of eruptive 

activity, investigation of the resulting deposits, and installation and repair of monitoring instruments were 

limited by the generally poor weather conditions, short 

daylight (less than 6 hours per day in December), and 

long travel times between Anchorage and the volcano 

(1 hour by helicopter). Snowfall obscured the details 

of pyroclastic- and debris-flow deposits, and extremely 

cold temperatures caused deposits containing water to 

freeze, making them difficult to examine and interpret. 

In one regard, the winter weather aided researchers: 

snowfall temporarily preserved tephra-fall deposits for 

further study and made it possible to map areas covered with thin tephra layers from an airplane. 

26 The &uption of Redoubt Volcano, Alaska, December 14, 1989-August 31, 1990 
Seismicity 

Seismic monitoring is the primary means of volcano surveillance, and it provided the basis for warnings 

issued by AVO during the Redoubt crisis. The Redoubt 

experience clearly demonstrates the tremendous value 

of maintaining seismic networks consisting of several 

seismometers within 10 to 20 km of historically active 

volcanoes, including one or more located on each volcano. In October 1989, just two months before Redoubt 

began to erupt on December 14, a five-station seismic 

array (fig. 34) centered on Redoubt became fully operational. This array provided critical information for 

gauging the status of the volcano and for providing 

warnings of impending eruptions. During most of the 

eruption sequence, earthquake activity beneath the volcano changed constantly, requiring seismologists to work 

24 hours a day to detect subtle variations in the type 

and intensity of seismic activity and to determine when 

a potentially hazardous eruptive episode was occurring. 

The network of seismometers around Redoubt and new 

seismic-recording and seismic-analysis systems that run 

on small, powerful personal computers (Lee and others, 1989; Rogers, 1989; Murray and Endo, 1989) were 

critical for accurate and timely interpretations of the 

seismicity. The computers record the seismic data in 

digital form and perform several different analyses every few minutes. 

An essential tool for monitoring the overall level 

of seismicity during the Redoubt crisis was the Realtime Seismic Amplitude Measurement system (RSAM). 

152' 50' 152' 40' 

5 KI LOMETERS 

0 5 MILES JFP. 60' .~ 35' Driff River ./' 

~CT 

152' 30' 

/./;:::::' 

"7// )/. 

/ JJV C~c;.'Oel'l (;le V . 

/ ROT 

... 

EXPLANATION 

• Redoubt Volcano 

glaciers and ice fields 

v_y Pre-1 989 crater 

.& Seismic station 

e Debris· flow sensor 

Figure 34. Location of seismic stations and debris-flow sensors around Redoubt Volcano. Seismic stations DFR, NCT, 

RDN, RDT, and RED became fully operational in October 

1989. The other stations and sensors were installed at various 

times after the eruption began. 

The RSAM was developed by the Cascades Volcano 

Observatory of the USGS in 1985 for monitoring seismicity at Mount St. Helens, Washington (Murray and 

Endo, 1989). Unlike other seismic data acquisition 

systems, which continuously record the oscillation of 

the ground at a seismic station, the RSAM system 

computes and stores the average amplitude of the ground 

oscillation over 1 0-minute intervals. As either the 

magnitude or number of earthquakes increases, and as 

volcanic tremor increases, the average amplitude of 

ground oscillation also increases. A small increase in 

the number of earthquakes, however, has only a small 

effect on the RSAM averages because the averages are 

computed over 10-minute intervals. The RSAM averages must be interpreted in conjunction with standard 

seismograms since ground shaking can be caused by 

different processes, including earthquakes, wind, volcanic explosions, or debris flows. 

The advantage of the RSAM system is its ability 

to continuously measure the level of seismicity during 

intense activity, especially during eruptions and volcanic tremor, and when earthquakes are so numerous that 

individual events cannot be distinguished and counted. 

The pronounced increases in the rate and amplitude of 

seismic events at Redoubt that prompted A YO's warnings of possible eruptive activity before December 14, 

1989 (fig. 35), and January 2, 1990 (fig. 36), are clearly 

demonstrated in the RSAM data. In the last few hours 

leading up to these eruptive episodes, seismologists 

could not recognize and count individual earthquakes, 

and they relied on the RSAM system to quantify the 

level of seismic activity at Redoubt. 

Recently, seismologists have developed a new 

system for continuously monitoring both the amplitude 

and frequency of a seismic signal. The seismic signal 

corresponds to vibrations of the ground and consists of 

a series of waves that vary in amplitude and frequency, 

measured in cycles per second (hertz, abbreviated Hz), 

as a function of time (fig. 9). The Seismic Spectral 

Amplitude Measurement (SSAM) system measures the 

relative amplitude of the seismic signal in specific frequency bands, permitting seismologists to determine 

which frequencies dominate a signal from each seismometer. Different types of seismic events generate 

signals with different characteristics. By examining the 

frequency of a signal, seismologists can infer its possible 

origin, or classes of origin. 

The SSAM system aided AVO seismologists in 

recognizing patterns of subtle seismic changes before 

several moderately explosive episodes in March and 

April; these subtle patterns were not always apparent 

on standard seismic records. Beginning as many as 27 

hours before an eruption, seismologists observed seismic 

signals dominated by frequencies between 1.9 and 3 

Hz at stations on the volcano (fig. 37). The SSAM sysMonitoring the 1989-90 &uption 27 
tern played an increasingly important role in forecasting explosive episodes as the eruption sequence continued. Between March 5 and April 21, when at least 

one seismometer was operating within 3 km of the 

crater, clear signals preceded five of the eight moderate explosive episodes that occurred during this time 

period. On the basis of these signals, the level-of-concern color code was elevated from yellow to orange 

several hours before the onset of eruptive episodes on 

March 23 (fig. 31) and April 6. 

Field Observation 

Visual observations of Redoubt's eruptive activity and investigation of volcanic deposits both on the 

volcano and in Drift River valley were essential for 

assessing the volcano's activity and the hazards associated with it. As the eruption progressed, field observations provided critical information needed for relating 

the character of the seismicity to the volcano's actual 

eruptive activity. Geologic observation of debris-flow 

deposits was critical for interpreting the signals from 

sensors of the debris-flow detection system (see below) and for advising State officials about flood hazards 

at the Drift River Terminal. As often as weather permitted, geologists made observation flights over the 

volcano and Drift River valley, evaluated growth and 

stability of the lava dome, mapped volcanic deposits 

w 

0 

:::::> 

f-

::J 

a... 

:::2: 

<t: 

(.) 

Seismic station RON 

~ ~--~--~--~~--~--~--~~--~-J ffi rT---r--,---.---r--.--~---r--,---~, 

(/) 

w 

> 

i= 

:5 

w 

a: 

Seismic station RED 

13 14 15 16 17 18 19 20 21 22 

DECEMBER 1989 

Figure 35. Seismicity at Redoubt Volcano between December 

13 and 22, 1989, expressed in average amplitude values 

(RSAM) for seismic stations RDN and RED (see fig. 34 for 

station locations). Note increase in amplitude beginning late 

on December 13 before the first explosive event at 9:47 a.m. 

AST on December 14. Peaks in amplitude correspond generally to explosive episodes (see table 1 ). 

from the volcano to Cook Inlet, and documented changes 

occurring on Drift glacier. 

Sulfur-Dioxide Emission 

Redoubt Volcano continuously emits volcanic gas 

from its summit crater and lava dome. Most of the gas 

is water vapor, but emissions also include sulfur dioxide, 

carbon dioxide, hydrogen, and lesser amounts of other 

gases such as carbon monoxide and hydrogen chloride. 

The concentration of volcanic gases is difficult to 

measure, and during eruptions, gas is extremely dangerous to collect. Scientists have learned, however, that 

the emission rate of sulfur dioxide gas generally reflects the total gas release from an active volcano. Sulfur 

dioxide gas can be measured safely from an airplane 

using an instrument originally designed for pollution 

studies. The correlation spectrometer (COSPEC) measures the amount of solar ultraviolet light absorbed by 

sulfur dioxide and compares it to an internal standard. 

Several traverses are made beneath the plume at right 

angles to the plume path to calculate an average daily 

emission rate. 

Sulfur dioxide emission rates since March 20 

ranged from about 800 to 6,600 metric tonnes per day 

(fig. 38). Measurements could not be made before March 

because of the low angle of the sun above Alaska during 

winter. Emission rates decreased beginning in late May 

and remained relatively low through August, which 

corresponds generally to the time period during which 

new magma was not rising into the dome. The relationship between emission rates and eruptive episodes is 

not clear. Before the March 23 episode, emission rates 

decreased from 4,600 to 1,600 tonnes per day at a time 

when the number of small seismic events was increasing. Such a decrease may reflect the sealing of the dome 

26 27 28 29 30 31 

DEC 1989 

2 3 

JAN 1990 

Figure 36. Cumulative energy release recorded by seismic station RDN, December 26, 1989-january 3, 1990, calculated 

from RSAM values (see fig. 34 for station location). The rapid 

upturn in the curve reflects the accelerating seismic activity 

beneath the volcano prior to the january 2 explosions and 

was the basis for warnings issued by AVO before the explosions occurred. 

28 The Eruption of Redoubt Volcano, Alaska, December 14, 1989-August 31, 1990 
to gas loss. As the magma system beneath the dome 

became pressurized, seismicity increased until the dome 

was ruptured by explosive activity or collapsed down 

Redoubt's north flank. A similar decrease in emission 

rates, however, was not measured before the subsequent 

eruptive episodes. 

Slow-Scan Video Monitor 

Within 48 hours of the first explosive episode on 

December 14, a video camera was -installed at Kasilof 

(fig. 2), 80 km east of Redoubt on the Kenai Peninsula, to provide images of the volcano during clear 

weather for scientists at the seismic center in Fairbanks. 

The camera is nearly 3,500 times more sensitive to light 

than most commercial home-video cameras, and thus it 

can record images of the volcano at night. An image is 

transmitted every 35 seconds to the seismic center and 

displayed on a black-and-white television monitor. This 

video system helped seismologists to correlate seismic 

events with volcanic activity. Five explosive episodes 

(two of which occurred at night), lightning, and brief 

periods of incandescence were successfully observed. 

Clouds prevented observations of most of the explosive 

episodes. 

Lightning-Detection System 

Spectacular lightning displays were observed 

during Redoubt's explosive episodes, even from places 

as far away as Homer, 120 km to the southeast. The 

7,000 I I 

+ 6,000 + -

+ 

2 - ~ 5,000 -

00 + + 

Ci5a: 

enw + + ~ a.. 4,000 \ -

wen + + 

oz 

ww ++ - z + + + ~ ~ 3,000- + -

+ + 0(.) + + + + c±a: + + + ~ tu 2,000 r- -

+ 5~ + ++ + en~ + + + + 1,000 1- ++ 

++ + 

0 I 

March April May June July August 

Figure 38. Daily sulfur-dioxide emission rates of Redoubt Volcano. 

cause of eruption lightning is uncertain, but it may result from friction between tephra particles and steam 

and other gases within a tephra plume. Eruption lightning discharges include both cloud-to-ground and cloudto-cloud strikes. During a large seismic event when the 

volcano is not visible, the presence of lightning dispels the uncertainty in interpreting the seismicity-a 

tephra plume is almost certainly present when sustained 

seismic signals and lightning occur together. AVO requested the help of the Bureau of Land Management's 

Alaska Fire Service to deploy a lightning-detection 

Eruption 4:04 AM ~ 

N 

I 

~PTil8--F-~==-~=--illl >--~ 0~----~T.---------~----~~ 

z 

~~~~~+-~~~~~~~~tt 

@~----~L-~~~~4--~~~~ 

~~----~~~--~~~~~~~ 

Figure 37. Record from Seismic Spectral Amplitude Measurement system (SSAM) showing signature of a swarm of lowfrequency events that preceded the moderate explosion on March 23, 1990. On the basis of this signature, AVO issued a 

warning of future eruptive activity by upgrading the color code from yellow to orange (see fig. 31 and table 3) 17 hours 

before the explosion. The record shows the relative strength of the ground motion in specific frequency bands for seismic 

station RSO (see fig. 34 for station location). The strength of the motion is represented by the relative darkness of the dot 

pattern and, in this record, is most intense in the 1.9-2.3 hertz (Hz) frequency band. The precursory seismic activity began at 

about 1 a.m. on March 22 and continued until about 2 a.m. on March 23. 

Monitoring the 1989-90 Eruption 29 
network around Redoubt as an experiment. The Alaska 

Fire Service maintains a lightning-detection network in 

Alaska during summer months to help locate and prevent the spread of lightning-caused forest fires. A network capable of both detecting and locating 

cloud-to-ground lightning strikes was set up around 

Redoubt and became operational on February 13. The 

results are promising: lightning was detected every time 

an explosive episode formed a tephra plume. For some 

episodes, scientists were able to conclude within minutes that a tephra plume was forming above the volcano even though the plume could not be seen (fig. 

39). The results also suggest that the number of lightning 

strikes detected is proportional to the size of a plume. 

Because of these promising results, AVO will soon install a permanent network to detect and locate eruption 

lightning at Redoubt and other volcanoes on the west 

side of Cook Inlet. 

Debris-Flow Detection System 

During almost every eruptive episode, debris flows 

were generated in Drift River valley by the melting of 

snow and ice by pyroclastic flows and dome collapses. 

Many of the debris flows traveled all the way to Cook 

Inlet. This repetitive activity was ideal for testing an 

experimental flow-detection system designed to track 

debris flows down river valleys. The flow-detection 

system consists of three stations located adjacent to Drift 

60° 

30' 

60° 

00' 

::.:;;=·,~;. .. :··.>::·''EXPLANATION 

.... 

··.· + Lightning strike 

25 MILES 

Figure 39. Location of lightning strikes during the February 

15, 1990, explosive episode. 

River at increasing distances from the volcano (fig. 34). 

Each station consists of a seismometer sensitive to highfrequency (10-300 Hz) ground vibrations caused by 

flowing mixtures of water and rock debris, and a radio 

to send the data to AVO facilities in Anchorage. The 

signals are separated into frequency ranges and continuously analyzed by a small computer. Flow events 

can be detected on the basis of their high-frequency 

character even during volcanic activity and earthquakes 

(fig. 40). When the flow-detection system was operating, 

all debris flows triggered by volcanic activity were detected. In the future, this system could provide important information on the speed and size of debris flows 

Cl 

z 

0 

(.) 

w 500 _(f) 

~a: -w (.)a_ 

0(/) 

.....Ja:: 

ww >IClW 

Z::::!: 

5~ 0 a::Z 

<!)~ 100 b 

~ 

6:00 

TIME, PM 

A 

Seismic station DFR 

B 

Flow sensor #1 

c 

Flow sensor #2 

D 

Flow sensor #3 

6:45 

Figure 40. A debris-flow detection system using seismometers 

proved effective in detecting and tracking debris flows during 

the eruption of Redoubt The debris flow triggered by an 

explosive episode at 5:23 p.m. on April 6, 1990, was detected as shown here (see fig. 34 for location of seismic station and flow sensors). A, Amplitude of the seismic signal that 

accompanied the explosive episode, measured from seismogram recorded at seismic station DFR. 8-D, Ground velocity of 

signals from flow sensors 1-3. (Amplitude and ground velocity 

are two different measures of ground motion.) The time delay 

between peak ground velocities recorded by the flow sensors 

can be used to calculate the speed of the flow, and the ground 

velocities can be used to evaluate relative size of the flow 

with increasing distance from the volcano. Note change in 

scale in 8-D. 

30 The Eruption of Redoubt Volcano, Alaska, December 14, 1989-August 31, 1990 
and could be used to warn people downstream of an 

advancing debris flow. 

FUTURE ERUPTIONS IN ALASKA 

The long history of frequent eruptions in Alaska 

and the State's geologic setting clearly imply that future eruptions are inevitable. In the past 10,000 years, 

nearly 10 percent of the Earth's eruptions occurred in 

Alaska (Simkin and others, 1981). Although scientists 

cannot predict which of Alaska's volcanoes will erupt 

next, the types of volcanic activity that can be expected 

from future eruptions are well known (Scott, 1989; 

UNDRO, 1985; Blong, 1984) and enable us to construct 

the following list of hazardous volcanic events that can 

be produced by Alaskan eruptions: debris flows, lateral blasts, lava flows and domes, pyroclastic flows and 

surges, tephra, tsunamis, volcanic gases, and volcanic 

landslides. Since 1800, most eruptions in Alaska have 

generated little cause for concern, but like the 1989-

90 eruption of Redoubt, some have had direct and dramatic effects on the lives of people and communities 

surrounding Cook Inlet. Historical eruptions demonstrate 

that tephra is the most common and widespread volcanic hazard in Alaska, followed to a lesser extent by 

debris flows, pyroclastic flows, and tsunamis (giant sea 

waves). Future eruptions are certain to produce tephra 

that will affect Alaska in ways similar to the recent 

activity at Redoubt. Larger eruptions could deposit 

considerably more ash in a downwind direction, which 

in the Cook Inlet area varies generally between north 

and southeast (fig. 41). 

Alaskan volcanoes are capable of producing extremely large eruptions. The largest eruption on Earth 

this century occurred in 1912 at Novarupta in Katmai 

National Park on the Alaska Peninsula (fig. 2). This awesome eruption expelled an estimated 12-15 km3 of 

magma (Hildreth, 1987), about 30 times the volume 

erupted by Mount St. Helens, Washington, in 1980. Pyroclastic flows buried a river valley with as much as 200 m 

of debris to produce the famous Valley of Ten Thousand 

Smokes. A huge tephra plume deposited most of its material in the sea, but about 25 em of tephra fell at Kodiak 

village 160 km to the southeast, severely damaging many 

buildings. Some ash also fell in the Cook Inlet region. So 

much magma and old rock was expelled from beneath 

the Novarupta vent that a large depression called a caldera was created. The caldera was subsequently filled 

with volcanic debris (Hildreth, 1987). The eruption apparently triggered the collapse of Mount Katmai volcano 

10 km away, which also formed a caldera. At least 20 

similar catastrophic caldera-forming eruptions have occurred in Alaska during the past 10,000 years, averaging 

one every 500 years (Miller and Smith, 1987). 

Since 1988, AVO has improved seismic monitoring of Mount Spurr and Augustine and Redoubt Volcanoes by installing new seismometers and by upgrading 

the computer recording and analysis systems at the 

University of Alaska seismic center in Fairbanks. Other 

types of monitoring activities were also initiated at 

Augustine Volcano, and several geologic mapping 

studies are underway to decipher the eruptive history 

of Alaskan volcanoes (table 4). 

CONCLUSION 

To date, the 1989-90 eruption of Redoubt Volcano has cost more than $100 million in damage to 

aircraft and loss of oil-production revenue, making it 

the most costly of Redoubt's four eruptions this century. Numerous explosive episodes sent ash plumes 

higher than 8,000 m above sea level, and wind carried 

the ash across air-traffic routes. The ash clouds severely 

disrupted domestic and international air service and resulted in the near-tragic encounter of a Boeing T47 jet 

with ash 90 minutes after a major explosive episode on 

December 15, 1989. The explosive episodes also caused 

massive debris flows in Drift River valley when hot, 

ANCHORAGE KING SALMON 

Winter Summer Winter Summer 

and spring and autumn and spring and autumn 

12,000m .-Q 4)~-f) 

9,100m .f) ·@-~qj 

5,~m ~ .q) -~qj 

3,000m~, <J) qf.Qj 

Figure 41. Average wind direction at four altitudes above Anchorage and King Salmon, Alaska (see fig. 2). Data from National Climatic Data Center, Asheville, North Carolina. Anchorage data cover period 1948-72; King Salmon data cover 

period 1953-60. Compass-rose diagrams show percentage of 

time winds blew toward a given direction. Modified from Till 

and others (1990). 

Conclusion 31 
Table 4. Status of monitoring and geologic studies at Alaskan volcanoes 

Scale of Hazard Monitoring 

Volcano geologic map assessment Seismic Geodetic Gas 

Cook Inlet 

Hayes No mae No No No No 

Mt. Spurr 1:63,3601 In progress Yes No No 

1:63,3601 Redoubt Yes Yes Planned Yes 

Iliamna 1:63,3601 No No No No 

Augustine 1:24,0001 Preliminary Yes Yes Yes 

Alaska Peninsula 

Katmai group2 1:250,000 No Yes No No 

Ugashik caldera-Mt. Peulik 1:63,360'3 No No No No 

Yantami-Mt. Chiginagak 1:63,~ No No No No 

Aniakchak caldera 1:125,0001 No No No No 

Black Peak 1:63,36<f No No No No 

Mt. Veniaminof 1 :12S,ooo4 No No No No 

Mt. Dana 1:63,~ No No No No 

Emmons Lake caldera-Pavlof 1:12S,ooo4 No Yes No No 

Mt. Dutton 1:63,~ No Yes No No 

Aleutian Islands 

Reconniassance geologic maps exist for about one-third of the Aleutian volcanoes, but these lack the details 

necessary for deciphering eruptive history. Only a few monitoring and geologic studies have been made of 

Aleutian volcanoes. 

1

Begun. 

2

Includes Novarupta and Mounts Katmai, Martin, Mageik, and Trident. No studies of individual 

volcanoes. 

3

Compiled. 

4

Completed. 

pyroclastic flows and debris from dome collapses cascaded down the north flank of Redoubt and melted snow 

and ice on Drift glacier. The largest debris flows, on 

January 2 and February 15, invaded the Drift River 

Terminal near the mouth of the river, but no oil was 

spilled. These floods and the continued activity of Redoubt prompted State and industry officials to modify 

the operating procedures of the oil terminal, which led 

to the temporary shutdown of 10 oil platforms in Cook 

Inlet. 

The Alaska Volcano Observatory, established in 

1988 to improve monitoring of volcanoes near Cook 

Inlet, issued warnings of potential eruptive activity at 

Redoubt Volcano before several eruptive episodes, including those on December 14, 1989, and January 2, 

March 23, and April 6, 1990. These warnings were 

based on seismicity beneath the volcano, and they 

demonstrate the benefits of maintaining a seismic 

monitoring system on active volcanoes. Experimental 

systems for detecting significant ash plumes and debris 

flows also proved effective during the eruption sequence. 

Most volcanic eruptions are preceded by measurable changes in the state of the volcano. The explosive 

episode that marked the beginning of the 1989-90 

eruption of Redoubt was preceded by only about 24 

hours of intense seismicity. This period of time was 

sufficient for AVO to warn officials and the public about 

the volcano's potential for eruptive activity. Emergency 

response plans, however, require far more time to prepare. The briefness of the seismicity that preceded 

Redoubt's eruption underscores the importance of 

planning for future volcanic emergencies before the first 

signs of restlessness appear. 

The size and type of volcanic activity that occurred during this eruption were anticipated on the basis 

of an analysis of the volcano's eruptive history. Similar analyses are underway at Mount Spurr and at Augustine, Iliamna, and Hayes Volcanoes. Hazard 

assessments resulting from such geologic investigations 

show areas around volcanoes where there are risks to 

life and property, and discuss the character of volcanic 

hazards. 

ACKNOWLEDGMENTS 

This report draws upon the work of many individuals who participated in the response to the eruption 

through their documentation of volcanic deposits, interpretation of seismicity, analyses of eruptive products, 

installation and maintenance of instruments and computers, and communication of hazard information to the 

32 The Eruption of Redoubt Volcano, Alaska, December 14, 1989-August 31, 1990 
public and government and industry officials. Also, 

many members of the Branch of Alaskan Geology, U.S. 

Geological Survey, and students of the University of 

Alaska in Fairbanks spent long hours on 24-hour 

watches. In addition to those who contributed directly 

to the writing of this report (listed on title page), the 

following individuals contributed indirectly through their 

work, informal reviews, and preparation of illustrations. 

U.S. Geological Survey 

C.A. Aviles, T.P. Brabets, J.E. Costa, M.R. Couchman, 

P.B. Dawson, L.D. Dietz, J.M. Dorava, M.P. Doukas, 

E.T. Endo, J.W. Ewert, F.G. Fischer, R.F. Gilmore, D.H. 

Harlow, D.P. Hill, C.G. Janda, R.J. Janda, F.W. Klein, 

J.W. Kleinman, J.N. Marso, Michael McKinnon, Bobbie 

Meyers, C.G. Newhall, R.D. Norris, J.F. Paskievitch, 

D.H. Richter, J.R. Riehle, J.A. Rogers, W.E. Scott, C.M. 

Stine, D.C. Trabant, J.D. Unger, J.W. Vallance, D.E. 

Wieprecht, E.W. Wolfe, T.S. Yelin, and M.E. Yount. 

University of Alaska Geophysical Institute 

V.F. Avery, John Benevento, D.H. Christensen, K.G. 

Dean, Juergen Kienle, J.W. Knox, K.M. Lawson, B.N. 

McKibben, Asta Miklius, D.S. Pinney, C.A. Rowe, 

G.H.C. Sonafrank, S.D. Stiler, D.B. Stone, and G.C. 

Tytgat 

Alaska Division of Geological and Geophysical Surveys 

G.D. March and R.D. Reger. 

Michigan Technological University 

W.I. Rose and D.J. Schnieder 

Technical Reviewers 

T.J. Casadevall and C.D. Miller 

REFERENCES 

Alaska Volcano Observatory Staff, 1990, The 1989-1990 

eruption of Redoubt Volcano: EOS, Transactions, 

American Geophysical Union, v. 71, no. 7, p. 265-275. 

Blong, R.J., 1984, Volcanic hazards-A sourcebook on the 

effects of eruptions: Orlando, Fla., Academic Press, 424 

p. 

Buist, A.S., Bernstein, R.S., Johnson, L.R., and Vollmer, 

W.M., 1986, Evaluation of physical health effects due 

to volcanic hazards-Human studies, in Buist, A.S., and 

Bernstein, R.S., eds., Health effects of volcanoes-An 

approach to evaluating the health effects of an environmental hazard: American Journal of Public Health, v. 

76, p. 66-75. 

Hildreth, Wes, 1987, New perspectives on the eruption of 

1912 in the Valley of Ten Thousand Smokes, Katmai 

National Park, Alaska: Bulletin of Volcanology, v. 49, 

p. 680-693. 

Lee, W.H.K., Tottingham, D.M., and Ellis, T.O., 1989, Design and implementation of a PC-based seismic data acquisition, processing, and analysis system: IASPEI Software Library, v. 1, p. 21-46. 

Miller, C.D., Mullineaux, D.R., and Crandell, D.R., 1981, 

Hazards assessments at Mount St. Helens, in Lipman, 

P.W., and Mullineaux, D.R., eds., The 1980 eruptions of 

Mount St. Helens, Washington: U.S. Geological Survey 

Professional Paper 1250, p. 789-802. 

Miller, T.P., and Smith, R.L., 1987, Late Quaternary calderaforming eruptions in the eastern Aleutian arc, Alaska: 

Geology, v. 15, p. 434-438. 

Murray, T.L., and Endo, E.T., 1989, A real-time seismic amplitude measurement system (RSAM): U.S. Geological 

Survey Open-File Report 89-684, 21 p. 

Naranjo, J.L., Sigurdsson, Haraldur, Carey, S.N., and Fritz, 

W.J., 1986, Eruption of Nevado del Ruiz volcano, Colombia, on 13 November 1985-Tephra fall and lahars: 

Science, v. 233, p. 961-963. 

Petroleum Information, 1990a, Volcano disrupts Cook Inlet 

production: Alaska Report, v. 36, no. 12, p. 1. 

Petroleum Information, 1990b, Volcano halts most Cook Inlet 

production: Alaska Report, v. 36, no. 3, p. 1. 

Riehle, J.R., 1985, A reconnaissance of the major Holocene 

tephra deposits in the upper Cook Inlet region, Alaska: 

Journal of Volcanology and Geothermal Research, v. 26, 

p. 37-74. 

Riehle, J.R., Kienle, Juergen, and Emmel, K.S., 1981, Lahars 

in Crescent River valley, lower Cook Inlet, Alaska: 

Alaska Division of Geological and Geophysical Surveys 

Geologic Report 53, 10 p. 

Rinehart, Steve, 1990, Health experts think ash fouls city air: 

Anchorage, Alaska, Anchorage Daily News, April 13, 

p. B3. 

Rogers, J.A., 1989, Frequency-domain detection of seismic 

signals using a DSP coprocessor board: IASPEI Software Library, v. 1, p. 151-172. 

Scott, W.E., 1989, Volcanic and related hazards, in Tilling, 

R.I., ed., Volcanic hazards: Washington D.C., American Geophysical Union, p. 9-23. 

Simkin, Tom, Siebert, Lee, McClelland, Lindsay, Bridge, 

David, Newhall, C.G., and Latter, J.H., 1981, Volcanoes of the World: Stroudsburg, Pa., Hutchinson Ross, 

242p. 

Steenblik, J.W., 1990, Volcanic ash-A rain of terra: Airline 

Pilot, June/July, p. 9-15. 

Till, A.B., Yount, M.E., and Riehle, J.R., 1990, Redoubt 

Volcano, Cook Inlet, Alaska--A hazard assessment based 

on eruptive activity through 1968: U.S. Geological Survey Open-File Report 90-246, 34 p. 

UNDRO (Office of United Nations Disaster Relief Coordinator), 1985, Volcanic emergency management: New York, 

United Nations, 86 p. 

References 3 3 



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National Center, Reston, VA 22092. 

Note.--Prices of Government publications listed in older catalogs, 

announcements, and publications may be incorrect. Therefore, the 

prices charged may differ from the prices in catalogs, announcements, 

and publications.