The Alaska Volcano Observatory (AVO) was established in 1988 to carry out volcano ... ·...

The Alaska Volcano Observatory (AVO) was established in 1988 to carry out volcano monitoring,eruption notification, and volcanic hazards assessments in Alaska. The cooperating agencies ofAVO are the U.S. Geological Survey (USGS), the University of Alaska Fairbanks GeophysicalInstitute (UAFGI), and the Alaska Division of Geological and Geophysical Surveys (ADGGS).AVO also plays a key role in notification and tracking of eruptions on the Kamchatka Peninsula ofRussia as part of a formal working relationship with the Kamchatkan Volcanic Eruptions ResponseTeam (KVERT).

Cover photo: Iliamna Volcano and Umbrella Glacier as viewed from the valley ofWest Glacier Creek. View is toward the northeast.

Preliminary Volcano-Hazard Assessment for Iliamna Volcano, Alaska

by Christopher F. Waythomas1 and Thomas P. Miller1

Open-File Report 99-373

1999

This report is preliminary and subject to revision as new data become available. Any use of trade, product or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Geological Survey

U.S. DEPARTMENT OF THE INTERIORU.S. GEOLOGICAL SURVEY

1Alaska Volcano Observatory, Anchorage, Alaska

U.S. DEPARTMENT OF THE INTERIORBRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEYCharles G. Groat, Director

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

U.S. Geological Survey U.S. Geological SurveyAlaska Volcano Observatory Branch of Information Services4200 University Drive Box 25286Anchorage, AK 99508 Denver, CO 80225-0286

http://www.avo.alaska.edu

CONTENTS

Summary of hazards at Iliamna Volcano. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Purpose and scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Physical setting of Iliamna Volcano . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5Prehistoric eruptive history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5Historical eruptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10Hazardous phenomena at Iliamna Volcano . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Volcanic hazards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11Volcanic ash clouds. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11Volcanic ash fallout and volcanic bombs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13Lahars, lahar-runout flows, and floods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15Debris avalanche . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18Pyroclastic flow and surge. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21Directed blasts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22Volcanic gases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25Lava flow. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Event frequency and risk at Iliamna Volcano . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25Hazard warning and mitigation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26References cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

ILLUSTRATIONS

PLATE

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

FIGURES

1. Map showing location of Iliamna Volcano and other volcanoes in the Aleutian Arc 3 2. Map showing location of Iliamna Volcano in Lake Clark National Park and

Wilderness and place names mentioned in text . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43. Photograph of Iliamna Volcano as viewed from the Johnson River Valley

toward the southwest . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54. Generalized geologic map of Iliamna Volcano . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65. Diagram showing known history of eruptive activity and other volcanic events

at Iliamna Volcano for the past 10,000 years . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96. Photograph of Iliamna Volcano and distinct steam plume trailing westward . . . . . . 10 7. Simplified sketch of a stratovolcano and associated hazardous phenomena . . . . . . . 128. Sketch maps of average wind direction and likely travel paths for volcanic ash

clouds from Iliamna Volcano . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

tic d Can-water.

9. Map showing the approximate extent of volcanic ash fallout for small to moderate eruptions of Iliamna Volcano . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

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

11. Photograph of noncohesive lahar deposit exposed along the coastline near the mouth of the Red River . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

12. Photograph of the southeast flank of Iliamna Volcano showing debris-avalanche deposits from 1997, the fumarole zone near the summit, and older avalanche scar at the head of Red Glacier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

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

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

15. Photograph of the north side of Red Glacier showing debris-avalanche deposits formed in 1997 and an older slightly more extensive deposit that is probably a few hundred years old or less . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

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

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

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

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

CONVERSION FACTORS

VERTICAL DATUM

Sea level: In this report, “sea level” refers to the National Geodetic Vertical Datum of 1929--A geodedatum derived from a general adjustsealvelment of the first-order level nets of the United States anada, formerly called Sea Level Datum of 1929. In the area of this report, datum is mean lower low

Multiply By To obtain

meter (m) 3.281 foot

kilometer (km) 0.6214 mile

cubic meter (m3) 35.3 cubic foot

cubic kilometer (km3) 0.2399 cubic mile

degree Celsius (°C) °F = 1.8 x °C + 32 degree Fahrenheit (°F)

Preliminary Volcano-Hazard Assessment for Iliamna Volcano, Alaska

By Christopher F. Waythomas and Thomas P. Miller

SUMMARY OF HAZARDS AT ILIAMNA VOLCANO

Iliamna Volcano is a 3,053-meter-high, ice- and snow-covered stratovolcano in the south-western Cook Inlet region about 225 kilometers southwest of Anchorage and about 100 kilome-ters northwest of Homer. Historical eruptions of Iliamna Volcano have not been positivelydocumented; however, the volcano regularly emits steam and gas, and small, shallow earthquakesare often detected beneath the summit area. The most recent eruptions of the volcano occurredabout 300 years ago, and possibly as recently as 90-140 years ago. Prehistoric eruptions have gen-erated plumes of volcanic ash, pyroclastic flows, and lahars that extended to the volcano flanksand beyond. Rock avalanches from the summit area have occurred numerous times in the past.These avalanches flowed several kilometers down the flanks and at least two large avalanchestransformed to cohesive lahars. The number and distribution of known volcanic ash deposits fromIliamna Volcano indicate that volcanic ash clouds from prehistoric eruptions were significantlyless voluminous and probably less common relative to ash clouds generated by eruptions of otherCook Inlet volcanoes. Plumes of volcanic ash from Iliamna Volcano would be a major hazard tojet aircraft using Anchorage International Airport and other local airports, and depending on winddirection, could drift at least as far as the Kenai Peninsula and beyond. Ashfall from future erup-tions could disrupt oil and gas operations and shipping activities in Cook Inlet. Because IliamnaVolcano has not erupted for several hundred years, a future eruption could involve significantamounts of ice and snow that could lead to the formation of large lahars and downstream flood-ing. The greatest hazards in order of importance are described below and shown on plate 1.

• Volcanic ash clouds

Clouds of fine volcanic ash will drift away from the volcano with the wind. These ash clouds are a haz-ard to all aircraft downwind. Airborne volcanic ash can drift thousands of kilometers from its sourcevolcano. Ash from future eruptions could interfere with air travel especially during a large sustainederuption.

• Volcanic ash fallout

Ash fallout from prehistoric eruptions of Iliamna Volcano reached parts of south-central Alaska whereaccumulations of several millimeters or more of fine ash are known. Fine ash is a nuisance and maycause respiratory problems in some humans and animals. Heavy ashfall can disrupt many humanactivities and may interfere with power generation, affect visibility, and could damage electrical com-ponents and equipment. Resuspension of ash by wind could extend the unpleasant effects of ash fallout.

Summary of Hazards at Iliamna Volcano 1

THE ALASKA VOLCANO HAZARD ASSESSMENT SERIES This report is part of a series of volcano hazard assessments being prepared by the Alaska Vol-cano Observatory. The reports are intended to describe the nature of volcanic hazards at Alaska volcanoes and show the extent of hazardous areas with maps, photographs, and other appropriate illustrations. The reports are preliminary and subject to revision as new data become available.

• Lahars and floods

Hot volcanic debris interacts with snow and ice to form fast-moving slurries of water, mud, rocks, andsand. These flows, called lahars, are expected to form during most future eruptions of Iliamna. Laharstend to follow streams and drainageways, and will probably flow to the coastline. Lahars could be haz-ardous to people and facilities in major valleys during an eruption.

•Debris avalanches

A debris avalanche is a rapidly moving mass of solid or incoherent blocks, boulders, and gravel initi-ated by a large-scale failure of the volcano flank. Multiple prehistoric and several small historicaldebris avalanches have occurred at Iliamna Volcano but did not extend very far beyond the base of thevolcano. Large debris avalanches could form that may involve a significant amount of the volcanicedifice. A major flank collapse could transform to a lahar that would inundate streams and drainage-ways on the distal slopes of the volcano.

• Pyroclastic flow and surge

Hot material expelled from the volcano may travel rapidly down the volcano flanks as flows of volca-nic debris called pyroclastic flows and surges. These flows will primarily travel along major valleysand are not expected to reach the coast. They pose little hazard except to people or facilities in majorvalleys during an eruption.

Other hazardous phenomena that may occur but are uncommon during typical eruptions ofIliamna Volcano include:

• Directed blasts

A directed blast is a lateral explosion of the volcano caused by rapid release of internal pressure com-monly caused by a slope failure or landslide. Directed blasts are rare volcanic events, and evidence fora directed blast has not been identified at Iliamna Volcano.

• Volcanic gases

Some volcanoes emit gases in concentrations that are harmful to humans. However, the frequentlywindy conditions, lack of closed depressions that could collect gases, and steep terrain at Iliamna pre-vent the buildup of volcanic gases. Thus, the hazard from volcanic gases is minimal.

• Lava flow

Streams of molten rock (lava) may extend a few kilometers from the active vent. Lava flows moveslowly, only a few tens of meters per hour, and pose little hazard to humans. Some lava flows maydevelop steep, blocky fronts and avalanching of blocks could be hazardous to someone close to theflow front.

2 Preliminary Volcano-Hazard Assessment for Iliamna Volcano, Alaska

SUGGESTIONS FOR READING THIS REPORTReaders who want a brief overview of the hazards at Iliamna Volcano are encouraged to read the sum-mary and consult plate 1 and the illustrations. Individual sections of this report provide a slightly more comprehensive overview of the various hazards at Iliamna Volcano. A glossary of geologic terms is included and additional information about Iliamna Volcano can be obtained by consulting the refer-ences cited at the end of this report.

INTRODUCTION

Iliamna Volcano is an active snow- andice-covered stratovolcano located in the south-western Cook Inlet area (fig. 1). Apparently,the volcano has not erupted in historic time(about the past 150 years) and we know of noverifiable reports of historical eruptive activity.However, Iliamna exhibits all the signs of anactive volcano, including the emission ofsteam and gas from a prominent zone of fuma-roles near the summit, and small, shallowearthquakes that intermittently occur beneaththe volcano. Iliamna Volcano is located withina few hundred kilometers of the major popula-tion, commerce, and industrial centers ofsouth-central Alaska, and future eruptions,should they be explosive, could pose a hazardto the citizens and economy of the region. Thegeological record of eruptive activity atIliamna Volcano indicates that eruptions aregenerally effusive in nature and occur infre-quently. Large, rare eruptions could generatesubstantial volumes of volcanic ash that mayrise to more than 12,000 meters above sealevel. Clouds of volcanic ash, if they are pro-

duced, would be hazardous to jet aircraft in theCook Inlet region and for thousands of kilome-ters downwind from the volcano.

Most of the area around Iliamna Volcano iswithin Lake Clark National Park and Wilder-ness (fig. 2) and is uninhabited. Life and prop-erty are not at risk in the immediate vicinity ofthe volcano.

Purpose and Scope

This report summarizes the principal vol-canic hazards associated with eruptions ofIliamna Volcano. Hazardous volcanic phenom-ena that have occurred on the volcano as wellas distal effects of eruptions are described. Thepresent status of monitoring efforts to detectvolcanic unrest and the procedure for eruptionnotification and dissemination of informationalso are presented. A series of maps and illus-trations that show potentially hazardous areasare included. A glossary of geologic terms is atthe end of the report. Terms defined in theglossary are italicized at their first appearancein the text.

Introduction 3

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37. Tanaga38. Gareloi

39. Cerberus

40. Little Sitkin

41. Kiska

BERING SEA

GULF OF ALASKA

PACIFIC OCEAN

CookInlet

Figure 1. Location of Iliamna Volcano and other volcanoes in the Aleutian Arc. All of these volcanoes have exhibited some kind of eruptive activity in the past 200 years.


LAKE CLARKNATIONAL PARK

AND WILDERNESSBOUNDARY(approximate)

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Figure 2. Location of Iliamna Volcano in Lake Clark National Park and Wilderness and place names mentioned in text. Red dashed lines indicate two estimates of the proximal hazard zone based on H/L = 0.3 and H/L = 0.1 where H is the height of the volcano summit and L is the runout length of pyroclastic flows, lahars, and debris avalanches of comparable volume. Typical H/L values for other volcanoes range from 0.1 to 0.3.

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Acknowledgments

David Coray and the staff of the SilverSalmon Lodge are gratefully acknowledgedfor sharing their knowledge of the local areaand for logistical support during field activi-ties. We thank J.E. Begét, R.B. Moore, andT.E.C. Keith for review comments.

PHYSICAL SETTING OF ILIAMNAVOLCANO

Iliamna Volcano (fig. 3) is a cone-shapedstratovolcano with an extensive cover of gla-cier ice and snow. The volcano is 3,053 metershigh and several 5-to-10-kilometer-long valleyglaciers extend from the upper flanks of thevolcano (fig. 2). The volume of glacier ice onIliamna Volcano above 2,000 meters is about 1cubic kilometer. The four largest glaciers havea combined ice volume of about 15 cubic kilo-meters, which is about three times the totalvolume of ice and perennial snow on MountRainier, Washington (D.C. Trabant, USGS,written commun., 1999).

Iliamna Volcano consists primarily of astratified assemblage of andesite lava flowsand minor lahar, pyroclastic flow, and debris-avalanche deposits. A prominent zone of sul-furous fumaroles (fig. 3) is located on the southside of the volcano near the summit; steamfrom the fumaroles is common and is often

visible on calm cloudless days.

Iliamna Volcano is located in Lake ClarkNational Park and Wilderness and is onoccasionally visited by people. Because tvolcano is located in a national park and maof its drainages are important sport fisherierecreational use of bottomland areas near orthe volcano is common in summer. Residentdevelopments along the Cook Inlet coastlincould be at risk from ashfall, should a large brare explosive eruption occur.

PRE-HISTORIC ERUPTIVE HISTORY

The modern volcanic edifice of IliamnaVolcano consists of a stratified assemblageandesitic lava flows, lahar, pyroclastic-flowand debris-avalanche deposits. These depoare depicted on a preliminary geologic map the volcano (fig. 4) that was completed duringeologic studies in 1995-1997 (Miller and others, 1996). The oldest lava flows are of earPleistocene age (ca. 1 Ma) and were erupfrom a vent or vent complex associated witthe present edifice. Much of the volcano concealed by an extensive cover of ice asnow, and consequently, little is known abothe eruptive history prior to about 7,000 yeabefore present (B.P.). At various times durinthe Quaternary period (about the past 1.8 mlion years), Iliamna Volcano and the surrouning mountains were shrouded by glacier ic

Physical Setting of Iliamna Volcano 5

South Twin

fumarole zone Iliamna Volcano

Figure 3. Iliamna Volcano as viewed from the Johnson River Valley toward the southwest. Also shown is the fumarole zone near the summit and the South Twin volcanic center.


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Figure 4. Generalized geologic map of Iliamna Volcano (modified from Miller and others, 1996). Extent of debris cover on modern glaciers not shown.

Pre-Historic Eruptive History 7

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SURFICIAL DEPOSITS

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QUATERNARY

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EXPLANATION OF SYMBOLS

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

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

Rock-avalanche deposits.

Glacial deposits of Holocene age. Primarily ice-stagnation drift,outwash, and end moraine.

Lahar deposits of late Holocene age.

Johnson Glacier dome tephra deposits.

Johnson Glacier dome complex.

Lava flows from the present Iliamna volcanic center.

Lava flows from the North and South Twin volcanic center.

Granitic rocks of the Aleutian-Alaska Range batholith.

Sedimentary rocks, undifferentiated.

Metamorphic rocks, undifferentiated.

High-angle reverse fault; dotted where concealed.

Collapse scar

Major vent, location approximate

Former vent

Fumarole

Flow direction of lava flows

Figure 4. Continued.

and much of Cook Inlet and most of southernAlaska lay beneath a thick and areally exten-sive ice sheet. Early eruptions of Iliamna Vol-cano may have been subglacial which wouldlimit the preservation of proximal volcanicdeposits and volcanic ash, therefore making itdifficult to determine the early eruptive his-tory. Because Quaternary glaciation was epi-sodic, proximal volcanic deposits generatedduring times of relatively restricted ice covermay have been removed by erosion duringtimes of glacier expansion.

Prehistoric eruptions of Iliamna Volcanohave generally been from a central vent associ-ated with the present edifice, although an oldervent existed in the vicinity of South Twin peak(fig. 3). Flank vents present on the south sideof Iliamna Volcano and the southeast side ofSouth Twin experienced dome-building erup-tions of unknown size during the Pleistoceneepoch, but because they are inaccessible, theyhave not been studied in detail. An assemblageof nested lava domes on the northeast side ofIliamna near Mt. Nick (fig. 3) was the site oferuptive activity 4,000 to 5,000 years ago.

The history of eruptive activity since about7,000 yr B.P. (fig. 5) is determined by studyingthe stratigraphic relations of volcanic depositsexposed in riverbanks and gullies on the flanksof the volcano and by analyzing volcanic ashdeposits preserved in areas beyond the vol-cano. In general, volcanic activity is episodicand long periods of inactivity are punctuatedby periods of rapid deposition of volcanic sed-iment during eruptions. Vegetation growth andsoil development may occur during non-erup-tive periods which then are buried by volcanicdeposits during subsequent eruptions. Overtime, a stacked, vertical sequence of buriedsoils and vegetation, volcanic deposits, andvolcanic ash develops. Because the strati-graphic sequence evolves in this manner overmany thousands of years, it is possible todetermine the eruptive history of the volcanoby dating buried soils and plant remains asso-

ciated with volcanic deposits. This methodol-ogy has been applied at Iliamna Volcano and isthe basis for deciphering the eruptive historyof the volcano.

At least two fine-grained volcanic-ashdeposits from Iliamna Volcano have been iden-tified on the Kenai Peninsula. These ash layersare within a vertical sequence of peat and vol-canic-ash deposits from other Cook Inlet vol-canoes and have been radiocarbon dated andgeochemically correlated with a pumiceouslapilli tephra found on the proximal flanks ofIliamna Volcano. A discontinuous mantle ofpumiceous lapilli tephra is common on bed-rock pinnacles that extend above modern gla-ciers northeast of Iliamna Volcano. This tephradeposit is found as much as 50 kilometersnortheast of the present summit of IliamnaVolcano. The pumiceous lapilli tephra and fineash layer on the Kenai Peninsula are evidencefor a large plinian eruption of Iliamna Volcanofrom a vent on the northeast upper flank of thevolcano about 4,000 yr B.P.

The second of the two fine-grained ash lay-ers found on the Kenai Peninsula is geochemi-cally similar to the proximal lapilli tephra anddates to about 7,000 yr B.P. This ash bed isevidence for an older plinian(?) eruption ofIliamna Volcano but so far no associated vol-canic deposits have been identified on theproximal flanks of the volcano.

During the past 4,000 years, many of themajor valleys on the volcano were inundatedby volcanic mudflows called lahars (fig. 5). Inmost areas, the lahars were generated by thedynamic interaction of pyroclastic flows withice and snow and are the direct products oferuptive activity. However, a few lahars appearto have been generated from large-scale col-lapses of a discrete part of the volcanic cone.These lahars are slightly clay rich and containan abundance of hydrothermally altered rockdebris. Such lahars are called cohesive laharsand apparently, they may develop without anyrelation to an eruption.


Pre-Historic Eruptive History 9

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EXPLOSIVE ERUPTION

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

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

Johnson River Valley

Red River Valley

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

W. Glacier Creek

Johnson River Valley.Tephra fallout on Kenai Peninsula

EXPLOSIVE ERUPTION Tephra fallout on Kenai Peninsula

Ash emission & tephra fallout

Pyroclastic flow

Noncohesive lahar

Cohesive lahar

Lava dome

Debris avalanche

AGE, INYEARS B.P.

TYPE OF VOLCANIC ACTIVITY AREAS AFFECTED

EXPLANATION

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

Figure 5. Known history of eruptive activity and other volcanic events at Iliamna Volcano for the past 10,000 years.

eiv-

tha

- askesep-eds

HISTORICAL ERUPTIONS

Iliamna Volcano has had no documentedhistorical eruptions. References to historicaleruptions in the 1860’s and 1870’s are poorlydocumented and reports of “smoke” issuingfrom the volcano (fig. 6) do not clearly indi-cate eruptive activity or emission of volcanicash (Miller and others, 1998). Two lahars inun-dated the lower Johnson River Valley (fig. 2)in the past 150 years and one of these wasprobably related to an eruption. Another laharflowed down Red River to the coast about 300years ago. This lahar was initiated by a pyro-

clastic eruption of Iliamna Volcano and is thbest known evidence for recent eruptive actity.

Elevated levels of seismic activity beneaIliamna Volcano were recorded by the AlaskVolcano Observatory (AVO) in 1996 (McGimsey and Neal, 1997). Earthquakes as largemagnitude 3.2 and as many as 82 earthquaper day were recorded between May and Stember 1996. During this period, an increasflux of carbon dioxide and sulfur dioxide gawas detected over the volcano.


Figure 6. Iliamna Volcano and distinct steam plume trailing westward. Distant observations and photo-graphs of the volcano like this one, taken near Seldovia, resulted in several false reports of eruptive activity. A vigorous steam plume (composed primarily of water vapor) is often visible on clear calm days and does not indicate that an eruption is imminent or in progress. Date and name of photographer unknown but the photograph was probably taken sometime before 1950.

HAZARDOUS PHENOMENA AT ILIAMNA VOLCANO

A volcanic hazard (fig. 7) is any volcanicphenomenon that is potentially threatening tolife or property. In general, hazards associatedwith volcanic eruptions are grouped as proxi-mal or distal depending on the areas mostlikely to be affected by specific volcanic phe-nomena. The classification of hazardous phe-nomena at Iliamna Volcano as proximal ordistal is an approximate classification becausethe extent of a particular hazard is in partrelated to the scale of the eruption. Thus, alarge eruption may cause some phenomena toaffect areas well beyond the volcano, whereasduring a smaller eruption, the same phenom-ena may only affect areas in the immediatevicinity of the volcano.

Proximal hazards are those phenomenathat occur in the immediate vicinity of the vol-cano, typically within a few tens of kilometersof the active vent. The proximal hazard zone isdelineated by the ratio of the volcano summitheight (H) to the runout length (L) of on-ground hazardous phenomena such as pyro-clastic flows, debris avalanches, and lahars.Typical H/L values range from 0.1 to 0.3. Lifeand property within the proximal hazard zonemay be at risk during eruptions depending onthe eruptive style and duration of activity.Anyone in this zone would have little or notime to escape from the area in the advent of aneruption. Because most of the area aroundIliamna Volcano is uninhabited, only the occa-sional visitor is at risk from the various proxi-mal hazards. Residential areas along the CookInlet coastline are within the proximal hazardzone for H/L = 0.3 (fig. 2) but because of thelocal topography these areas are not likely tobe subjected to any volcanic hazards exceptash fallout.

Distal hazards pose less risk to peoplebecause there is usually adequate time forwarning and evacuation. This group of hazardsaffects people and structures that are more than

about 10-30 kilometers from the active vent.Volcanic ash, either in explosive eruption col-umns or ash clouds that drift far away from thevolcano, can be both a proximal and a distalhazard, especially to aircraft.

The geologic map of Iliamna Volcano (fig.4) depicts deposits formed by various volcanicphenomena. Most of these phenomena areconfined to the flanks of the volcano and themajor drainages that extend from the summit.Only volcanic ash clouds, ash fallout, pyro-clastic flow and surge, and unusually large vol-ume lahars could affect areas well beyondIliamna Volcano (fig. 2).

VOLCANIC HAZARDS

Volcanic Ash Clouds

Because volcanic ash deposits fromIliamna Volcano are uncommon in the geo-logic record, it appears that large-volume,explosive, ash-forming eruptions of Iliamnaare probably unlikely in the future but signifi-cant disruptive small eruptions could occur.Several times in the past 10,000 years, explo-sive eruptions of Iliamna propelled significantquantities of fine ash particles or tephra intothe atmosphere forming an eruption cloud (fig.7) that drifts away from the volcano with thewind (fig. 8). The fine ash particles mayremain in the atmosphere for days to weeksdepending on the size of the eruption. Volcanicash clouds are a hazard to all aircraft down-wind from the volcano (Casadevall, 1994).

Airborne ash from Iliamna Volcano hastraveled at least as far as the Kenai Peninsuladuring Holocene eruptions. During futureeruptions, ash could be dispersed over theCook Inlet area and beyond. Variable windsover Cook Inlet could temporarily detain adrifting ash cloud causing dusty ash-laden airto linger in the region. High winds after a sig-nificant ashfall can resuspend ash and prolongthe foul air condition.

Hazardous Phenomena at Iliamna Volcano 11


magma

pyroclastic flow

eruption column

lahar (debris flow)

lava flow

prevailing wind

magma reservoir

fumaroles

eruption cloud

ash (tephra) fallacid rain

bombs

conduit

crack

ground water

landslide(debris avalanche)

fumaroles

Volcanoes generate a wide variety of phenomena that can alter the Earth's surface and atmosphere and endanger people and property. While most of the natural hazards illustrated and described in this diagram are associated with eruptions, some, like landslides, can occur even when a volcano is quiet. Small events may pose a hazard only within a few kilometers of a volcano, while large events can directly or indirectly endanger people and property tens to hundreds of kilometers away.

Figure 7. Simplified sketch of a stratovolcano and associated hazardous phenomena (modified from Myers and others, 1997)

ifn

slles,re,--

NN

N

N

Anchorag eKodiakKing

Salmon ILIAMNAVOLCANO

Cold Bay

13,000

11,000

9,000

5,000

3,000

1,000

7,000

ALTITUDE, IN METERS

Figure 8. Average wind direction and likely travel paths for volcanic ash clouds from Iliamna Volcano. Cold Bay data from 1945-57; King Salmon data from 1953-60; Kodiak data from 1949-62; Anchorage data from 1948-72. Original data obtained from the National Climatic Data Center, Asheville, NC. Windrose lengths are proportional to wind frequency determined by annual percent.

Ash clouds from other eruptions of CookInlet volcanoes have drifted to the southeastover western Canada and over parts of theUnited States and eventually out across theAtlantic Ocean, indicating that long-distancetransport of volcanic ash is possible duringfuture eruptions of Iliamna Volcano.

Volcanic Ash Fallout and Volcanic Bombs

As clouds of volcanic ash drift from thevolcano, a steady rain or fallout of ash usuallyoccurs. Volcanic ash is one of the most trouble-some and hazardous products of explosive vol-canism. Because it may be transported longdistances, it has the potential to affect areasmany hundreds of kilometers from the vol-

cano. Few people have been killed by fallingash, but the weight of a thick ashfall couldcause structures to collapse and inhaling ashparticles is a health hazard and can be lifethreatening to some people with respiratoryproblems. Sometimes a “mud rain” results airborne volcanic ash mixes with falling raior snow.

Blocks or bombs of volcanic rock debrimay be ejected as ballistic projectiles that faor strike areas near the vent. In extreme casblocks may be ejected distances of 10 to mothan 30 kilometers from the vent. Typicallythe zone of bomb fallout is within a few kilometers of the vent. People or low-flying aircraft would be at risk only within a fewkilometers of the vent.

Volcanic Hazards 13

Ashfall from eruptions of Iliamna Volcanomay be a public health concern for parts ofsouth-central Alaska, especially the KenaiPeninsula. The approximate extent of ash fall-out from future eruptions is shown on figure 9.Because wind direction and speed will control

the movement of the ash plume, the areas mostlikely to receive ashfall are those in the zone ofprevailing winds. The strongest and most con-sistent winds are from the west, southwest, andnorthwest (fig. 8). The thickness of ash falloutwill decrease in a downwind direction but it is


IliamnaVolcano

Mount Spurr

Crater Peak

RedoubtVolcano

IliamnaVolcano

Mount Douglas

Seward

Kasilof

Hope

Anchorage

60o

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Penin

sula

152o 150o

Homer

AugustineVolcano

Drift RiverOil Terminal

Tyonek60o

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Nikiski

SterlingCooperLanding

Soldotna

Ninilchik

Kenai

EXPLANATION

59o

30'

Anchor Point

COMPOSITE AREA OF POSSIBLE ASH FALLOUT IN UPPER COOK INLET FOR A FUTURE ERUPTION OF ILIAMNA VOLCANO(For any single erupt ive event theash plume may af fect only anarrow sector of the st ippled area depending on wind direct ion)

decreasing probability

Cook In

let

SilverSalmon

Seldovia

Airport

Oil platforms

Oil tankerloading dock

Power plant

Oil refinery

Known deposits oftephra and fine ashfrom Iliamna Volcano

Approximate extentof volcanic ash falloutexpected during smallto moderate eruptionsof Iliamna Volcano.

Stippled pattern indicatesareas likely to receiveash fallout because ofprevailing wind direction.

154o

100 KILOMETERS

60 MILES

50

30

0

0

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

impossible to predict how much ash will beemitted during an individual eruption, thoughany amount of ashfall could be disruptive.Industrial facilities in the upper Cook Inletregion such as oil drilling rigs, oil refineries,manufacturing plants, and power plants couldbe affected by ashfall.

Lahars, Lahar-Runout Flows, and Floods

Most of the volcanoes in Alaska supportglaciers or are snow-covered most of the year.During typical eruptions, hot pyroclasticdebris expelled from the volcano interactsdynamically with the snowpack or glaciercover causing extensive melting and waterproduction. As meltwater mixes with availableunconsolidated sediment, various types offlowage phenomena may occur on the volcanoflanks and in stream channels and drainagesdownstream from the volcano. Most of thesephenomena are categorized as debris flows(fig. 7) or more specifically as noncohesivelahars. Lahars consist of a poorly sorted mix-ture of boulders, sand, and silt. Noncohesivelahars typically undergo a downstream trans-formation to finer grained, watery flows,called hyperconcentrated flows or lahar-runout flows. If enough sediment is lost from alahar during flowage, the lahar may transforminto a normal streamflow or flood and consistmostly of water.

Lahars also may form directly from water-saturated, clay-rich volcanic rock avalanches(Hoblitt and others, 1995; Vallance and Scott,1997) that may or may not be triggered by aneruption. Such lahars are called cohesivelahars because the matrix sediment of typicaldeposits contains more than about 3 percentclay. Cohesive lahars are thought to developfrom large volcanic avalanches that remove asignificant amount of the volcanic edifice.Hydrothermal alteration of volcanic bedrock

within the volcano decreases the strength ofthe rock mass and eventually a sector of thevolcano becomes gravitationally unstable, col-lapses, and forms a large rock avalanche.Because the weathered volcanic rock is watersaturated and contains clay minerals, the ava-lanche debris may evolve into a cohesive laharthat can flow for several to tens of kilometersdownstream from the volcano. Cohesive laharscan be more hazardous than their noncohesivecounterparts because they tend to be moredense, and may develop from rock avalanchesthat are unrelated to volcanic eruptions andthus may occur without the usual precursoryvolcanic activity that heralds an eruption.

Lahar deposits of Holocene age have beenidentified in several of the drainages onIliamna Volcano (fig. 10) and cohesive lahardeposits are present in the Johnson River andRed Creek Valleys and at one location alongWest Glacier Creek. The cohesive lahar depos-its in the Red Creek- Johnson River area areabout 90 years old and indicate that a major(?)flank collapse that transformed to a lahar hasoccurred in the recent past. Noncohesive lahardeposits also are present in the RedCreek/Johnson River area and these depositsformed during an eruption of Iliamna Volcanoabout 140 years ago.

The distribution and age of lahar depositsin the Red Creek Valley and in the lowerJohnson River Valley indicate that this area hasbeen inundated by lahars at least twice in aboutthe past 150 years. The lahar flows were atleast several meters deep and probablyextended across the entire width of the lowerRed Creek Valley and the central part of theJohnson River Valley. The noncohesive flowsmay have transformed to sediment-laden waterfloods by the time they reached the coast northof Silver Salmon (fig. 10), a residential area. Itwould take approximately 45-90 minutes formoderate-to-large-volume lahars (>10 millioncubic meters) to travel this far.

Volcanic Hazards 15




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Red Glacier

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rescent River

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Glacier

ILIAMNAVOLCANO

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UmbrellaGlacier

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

Areas susceptible to inundation by lahars, lahar-runout flows, and floods duringmost eruptions but less likely to be affected than zone L1.

Areas that could be inundated by lahars, lahar-runout flows, and floods during large sustained eruptions but less likely to be affected than zones L1 or L2.

Known lahar deposits of Holocene age.

Boulders of volcanic rock transported by lahars.

L1

L2

L3

Greater

Lesser

Degreeof

Hazard

EXPLANATION

L3

L3L3

L2

L3

L1

L1

L2

L3

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

bygesbyisL1 Ifatyldasutd

har

dersk-

ifenreererss,teaser-

Noncohesive lahar deposits also are pre-served along the Cook Inlet coastline near themouth of the Red River (fig. 11). These depos-its contain cobble- and boulder-size clasts ofjuvenile andesite that were probably still hotwhen the lahar flowed to the sea. Spruce treesgrowing on the lahar deposit are about 300years old and indicate that the lahar and anassociated eruption of Iliamna Volcanooccurred at about this time (Begét, 1996). Sev-eral large unweathered boulders and blocks ofandesite from Iliamna Volcano are present onthe alluvial fan of the Red River (fig. 10).These blocks are part of the same lahar depositexposed along the coast and further documentan extensive lahar that must have swept downRed Glacier and inundated the Red River Val-ley and the Red River alluvial fan. Moderate-to-large-volume lahars could reach the coast atthe mouth of the Red River in as little as 30minutes.

Hazard zones for lahars, lahar-runoutflows, and floods are shown on figure 10 andon plate 1. Three hazard zones (L1, L2, andL3) that depict differing degrees of hazard areindicated. Hazard zone L1 indicates areas thatare likely to be inundated by lahars, lahar-runout flows, and floods during most eruptionsof any magnitude. Hazard zone L1 also

includes areas that have been affected lahars, lahar-runout flows, and floods durinthe past 300 years. Hazard zone L2 includareas that are susceptible to inundation lahars, lahar-runout flows, and floods but less likely to be affected than hazard zone during most eruptions from the present vent.the location of the vent changes such thactivity is more likely in a drainage presentlin hazard zone L2, the degree of hazard wouchange to L1. Hazard zone L3 includes arethat could be affected by lahars, lahar-runoflows, and floods only during large, sustaineeruptions. These areas contain no known ladeposits.

Because lahars, lahar-runout flows, anfloods can move rapidly, can be several metdeep, and can transport boulder- and blocsize particles, they would be hazardous to land property in the flow path. The distributioand age of lahar deposits indicate that futudevelopments in the Johnson and Red RivValleys, as well as the areas where these riventer Cook Inlet, could be at risk from laharlahar-runout flows, and floods. At presenhowever, lahars pose no hazard in these arbecause they are uninhabited and have no pmanent structures or facilities.

Volcanic Hazards 17

Figure 11. Noncohesive lahar deposit exposed along the coastline near the mouth of the Red River. The maximum age of spruce trees growing on the lahar deposit is about 300 years indicating the lahar and associated eruption occurred at about this time.

Debris Avalanche

Volcanic rock or debris avalanches (fig. 7)typically form by structural collapse of theupper part of the volcano. The ensuing ava-lanche moves rapidly down the volcano flankand forms a bouldery gravel deposit manykilometers from the source that may exhibit acharacteristic hummocky surface and broadareal extent. Most debris-avalanche depositsare traceable up the slopes of the volcano to anarcuate-shaped scar at or near the volcanosummit that marks the zone of collapse andorigin of the avalanche (fig. 12).

Although some debris avalanches occurduring an eruption, large-scale collapse of avolcanic cone may occur during a distinctlynon-eruptive period, sometimes as a result oflong-term chemical alteration of volcanic rockby hot, acidic ground water. As the interior

structure of the volcano becomes weakened,the flank may collapse and produce a debrisavalanche. Debris avalanches that form thisway produce deposits that contain a significantamount of matrix clay (usually more than 3percent) and the avalanche itself may trans-form into a cohesive lahar if it contains oracquires sufficient water. Numerous debris-avalanche deposits are present on the flanks ofIliamna Volcano but it is uncertain whetherthese deposits formed during eruptions or wereinitiated by other mechanisms, such as largeearthquakes or simple gravity-driven collapse.

Debris-avalanche deposits have been iden-tified in the Red River Valley and on Red Gla-cier, on Umbrella Glacier and in West GlacierCreek Valley, on Lateral Glacier, and on Tux-edni Glacier (fig. 13). All of these deposits arefresh appearing and relatively young (less than


Avalanchescar

1997 debris-avalanche

deposit

Fumarolezone

Figure 12. View of the southeast flank of Iliamna Volcano showing debris-avalanche deposits from 1997 (solid line), the fumarole zone near the summit (yellow dashed line), and older avalanche scar at the head of Red Glacier (red dashed line).

Volcanic Hazards 19



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153˚ 153˚30'

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EXPLANATIONDebris-avalanche deposits associated with Iliamna Volcano.

Debris-avalanche and rockfall deposits from Iliamna Volcano reworked by glaciers.

Rock-avalanche deposits not associated with Iliamna Volcano.

Maximum likely extent of a debris avalanche from Iliamna Volcano for H/L = 0.2

Maximum likely extent of a debris avalanche from Iliamna Volcano for H/L = 0.4

Probable travel path of debris avalanches from Iliamna Volcano.

Generalized extent of rockfall debris on glacier surfaces.

0 5

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MILES


Red Glacier

C O O K I N L E T

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iver

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laci

er

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dni

River

Red River

Johnson

River

Creek

West

Glacier

ILIAMNAVOLCANO

CHISIKISLAND

Red

Cre

ek

Silver Salmon

Glacier

Lateral

UmbrellaGlacier (residential area)

Snug HarborCannery (abandoned)airstrip

Figure 13. Hazard zonation map for debris avalanche.

about 1,000 years old) and some may be ofhistoric age (<200 years). The debris-ava-lanche deposits consist of angular gravel that iscomposed of poorly sorted mixtures of sand,cobbles, and boulders. All deposits containclasts of grey andesitic lava and yellow-orangehydrothermally altered rock debris.

Most of the exposed summit of IliamnaVolcano consists of hydrothermally altered andweakened volcanic bedrock. Periodically,small volumes of rock break loose from theupper part of the volcano and form minordebris avalanches (fig. 12), especially in springwhen the snowpack is melting. These ava-lanches incorporate snow and ice and in somecases may begin as snowslides or ice falls thatscour exposed altered bedrock. In 1994, 1996,

and 1997, several such avalanches originatedfrom the edifice of Iliamna Volcano (fig. 14)and flowed down upper Red and UmbrellaGlaciers for a distance of about 3-8 kilometers.The characteristic rocky debris cover on Red,Lateral, and Umbrella Glaciers is mostlydebris avalanche material and indicates thatthese glaciers are frequently swept by rockavalanches (fig. 15).

Hazard zonation for debris avalanche atIliamna Volcano is based in part on the runoutdistance of recent avalanches. The maximumlikely runout distance of a future debris ava-lanche was estimated using the ratio of the fallheight (H) to runout distance (L) of knownavalanche deposits. The H/L ratio for the 1994,1996, and 1997 debris avalanche deposits and


Area of hydrothermallyaltered bedrock likely to bethe source of a futuredebris avalanche. Volume ofaltered, weakened rockis about 0.4 cubic kilometers.

Scars of formerdebris-avalanches


0 1 Mile

1 Kilometer0

153o

60o

153o05'

60o

02'

Figure 14. Topographic map of summit area of Iliamna Volcano. Zone of hydrothermally altered, weakened bedrock and likely source area for future debris avalanches indicated by dashed red line. Probable flow paths for avalanches indicated by arrows.

Red Glacier

1997 avalanche

debris

late Holoceneavalanche debris

Figure 15. View down the north side of Red Glacier showing debris-avalanche deposits formed in 1997 and an older slightly more extensive deposit that is probably a few hundred years old or less.

the debris avalanche deposits on the northernmargins of Red and Umbrella Glaciers is about0.4. Future similar debris avalanches areexpected to extend several kilometers from thevolcano summit but not far beyond the upperreaches of the major glaciers on the volcano(fig. 13). The runout distance of a debris ava-lanche is proportional to its volume, and iflarger avalanches occur at Iliamna Volcanothey will travel a greater distance down themajor valleys. For debris avalanches havingvolumes of 0.5-1.0 cubic kilometers, an H/Lratio of 0.2 gives a maximum runout distanceof about 15 kilometers (fig. 13). For all drain-ages except Red Creek, the avalanches will begenerally unconfined and able to spread andthin laterally which will limit their mobility.For drainages like Red Creek, the avalanchewill be directed down a discrete flow pathcausing it to behave like a channelized flow. Inthese settings, the avalanche may transform toa cohesive lahar that could travel a consider-able distance beyond the hazard zone bound-ary for debris avalanche.

About 0.4 cubic kilometer of hydrother-mally altered volcanic bedrock makes up thesummit of Iliamna Volcano. If this amount ofrock were to break loose and form a debrisavalanche, it would most likely inundate theRed or Umbrella Glacier Valleys and couldextend approximately 10-15 kilometers downthese glaciated valleys. If an avalanche thissize transformed to a cohesive lahar thatentered the Johnson River Valley via RedCreek, the lahar would probably inundate thelower Johnson River Valley across the entirewidth of the present valley and would probablyextend to the coastline.

Pyroclastic Flow and Surge

A pyroclastic flow is a hot, dry mixture ofvolcanic rock debris and gas that flows rapidlydownslope (fig. 7). A pyroclastic surge is simi-lar to a pyroclastic flow but has a higher gascontent. Because it is mostly gas, a pyroclasticsurge moves more rapidly than a pyroclasticflow and may not be confined by topography,

Volcanic Hazards 21

ry.st

up- isc

- ors-he-il-

farenw-amentn

e”mthens.t,

and therefore may climb up and over ridges.Pyroclastic flows are relatively dense and willgenerally follow topographically low areassuch as stream valleys. Any of the major drain-ages that head on the volcano could beengulfed by pyroclastic flows even duringmodest eruptions. Because they are hot andfast-moving, both pyroclastic flows and surgescould be lethal to anyone on the proximalflanks of Iliamna Volcano during an eruption.

Pyroclastic flows and surges at IliamnaVolcano will most likely form by collapse of acooling lava dome. Should a large, explosive,and rare plinian-style eruption occur, the erup-tion column would collapse and fall backtoward the volcano forming a fast-movingpyroclastic flow. It is likely that pyroclasticflows and surges generated by lava-dome col-lapse were associated with former eruptions ofIliamna Volcano, and these probably led to thedevelopment of noncohesive lahars. To datehowever, pyroclastic-flow or surge depositshave not been identified on the flanks ofIliamna Volcano.

Pyroclastic flows and surges from mosteruptions would be expected to reach at leastseveral kilometers beyond the vent and couldtravel in almost any direction. The runout dis-tance of pyroclastic flows is estimated withH/L values of 0.2 and 0.3 (fig. 16). These val-ues give runout distances of 10-15 kilometersand are most relevant to pyroclastic flows gen-erated by a collapsing eruption column. Pyro-clastic flows initiated by lava-dome collapseusually affect only a discrete sector of the vol-cano. During large eruptions pyroclastic flowscould reach the sea in the Red River Valley andpossibly in West Glacier Creek Valley (fig.16). Pyroclastic flows and surges from erup-tions of Iliamna Volcano that reach Cook Inletwould be unlikely except for a rare, extremeeruption.

It is difficult to accurately predict theextent of a pyroclastic surge. However,because of their genetic relation to pyroclasticflows, they have a slightly greater lateralextent. Thus, the extent of the hazard boundaryis uncertain (fig. 16). Because surges are hot(300 to 800 °C) and gaseous, death or injufrom asphyxiation and burning is likelyBecause the surge cloud may travel very fa(at least tens of meters per second), pre-ertion evacuation of the area near the volcanothe only way to eliminate risk from pyroclastisurges.

Directed Blasts

A directed blast is a large-scale lateral volcanic explosion caused by a major landslideslope failure that uncaps the internal vent sytem of the volcano. Such an event is rare in thistory of a volcano. Although geologic evidence indicates that landslides and slope faures have occurred at Iliamna Volcano, thus evidence for a directed blast has not yet bediscovered. The hazard zone boundary shoing the area most likely to be affected by directed blast (fig. 17) is based on data frothe 1980 eruption of Mount St. Helens. Thdirected blast associated with the 1980 MouSt. Helens eruption is one of the largest knowhistorical events and thus is a “worst casexample. If a directed blast were to occur frothe summit of Iliamna Volcano, it could affeca broad area, possibly a 180° sector from tvent. A directed blast will usually happen ithe first few minutes of an eruption and thuthere is no time for warning or evacuationLiving things in the path of a directed blaswill be killed or destroyed by impact, burningabrasion, burial, and heat.


Volcanic Hazards 23



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

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

Most likely flow path for pyroclastic flows and surges. During moderate to large eruptions,pyroclastic flows and surges could be directed along topographically low areas such asvalleys and drainages and could extend beyond the indicated hazard-zone boundaries inthese areas.

EXPLANATION

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




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Extent of area that could be affected by a directed blast similar to the blast generated during the 1980 eruption of Mt. St. Helens, Washington. This hazard-zone boundary is a worst-case condition for Iliamna Volcano. If a directed blast were to occur, it could engulf a large portion of the indicated hazard zone.

EXPLANATION

Figure 17. Hazard zonation map for directed blast. Directed blasts are uncommon and deposits formed by a directed blast have not been found at Iliamna Volcano. A directed blast is most likely to be directed down one or more of the major drainages and less likely to the west.

Volcanic Gases

Gases are emitted by most active volca-noes, because magma contains dissolved gasesand boils off shallow ground water that is typi-cally present within volcanoes. The most com-mon volcanic gases are water vapor, carbondioxide, carbon monoxide, sulfur dioxide, andhydrogen sulfide. Volcanic sulfur and halidegases that encounter water form large amountsof sulfuric acid (H2SO4) and minor amounts ofhydrochloric (HCl) and hydrofluoric acid (HF)as aerosols or droplets. Both carbon monoxideand carbon dioxide are colorless and odorlessand thus impossible to detect without a mea-suring device. Carbon dioxide is heavier thanair and may displace the available oxygen inconfined spaces or low-lying areas causingsuffocation. In high concentrations, carbondioxide, hydrogen sulfide, and sulfur dioxidemay be harmful or toxic to humans and maydamage vegetation downwind from the vol-cano. Acid precipitation may develop from themixing of snow or rain with acidic volcanicaerosols, which may cause various types ofskin and respiratory irritations and cause cor-rosive damage to paint, fabric, and structures.Wind tends to disperse volcanic gas and it istypically not found near the ground in concen-trations hazardous to humans or animals morethan about 10 kilometers from the volcano.During large eruptions, significant volumes ofgas can travel high in the atmosphere down-wind from the volcano for days and thousandsof kilometers.

The hazard from volcanic gases at IliamnaVolcano is extremely unlikely to be greaterthan that posed by other volcanic phenomena.Fumaroles located near the summit of the vol-cano produce a nearly constant plume of steamand carbon dioxide and sulfur dioxide gas andon calm days this plume is readily apparent. Attimes, the steam plume is especially vigorousand has been mistaken for an eruption cloud.

Volcanic gas may pose a health concern tosomeone near the active fumarole area; how-ever, frequent windy conditions at IliamnaVolcano and the absence of an efficient trap-ping mechanism inhibit localized buildup ofvolcanic gas. Therefore, the hazard from vol-canic gases is minor.

Lava Flow

Narrow streams of molten rock or lavamay form during a future eruption of IliamnaVolcano and commonly, lava flows (fig. 7)develop after explosive activity at the volcanodeclines. Most of Iliamna Volcano is com-posed of bedded lava flows (Qiv, Qtv, fig. 4).Typical Iliamna flows are andesitic in compo-sition and, when molten, are relatively viscous.Future eruptions will probably generate lavaflows similar to those preserved on the vol-cano. The lava flows are expected to moveslowly downslope, probably not more than afew tens of meters per hour. Lava flows of thistype pose little hazard to people who couldeasily walk from them; however, lava flows atIliamna may develop steep fronts and couldshed blocks and debris downslope. Lava flowsthat reach snow and ice could generate local-ized flooding.

EVENT FREQUENCY AND RISK AT ILIAMNA VOLCANO

An eruption of Iliamna Volcano can beexpected in the future, but the timing of thenext eruption is uncertain. The primary proxi-mal hazard during a future eruption will belahars, lahar-runout flows, and floods thatcould inundate significant portions of majordrainages on the volcano. Thick accumulationsof sediment in affected valleys and drainageswill occur and sediment-laden runoff couldpersist for months to years after the eruption.

Event Frequency and Risk at Iliamna Volcano 25

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Because the immediate area aroundIliamna Volcano is uninhabited and no perma-nent structures or facilities are present, nothingwithin about 20 kilometers of the volcano is atrisk from future eruptions. During any futureeruption, ash could fall on residential areasalong the Cook Inlet coastline, such as SilverSalmon (fig. 2), and several millimeters of ashcould accumulate. The major drainages on thevolcano, especially the Johnson River, RedRiver, and West Glacier Creek, are vulnerableto inundation by lahars, lahar-runout flows,and floods. In the event of a large, explosiveeruption, pyroclastic flows could also engulfthese valleys.

Should a sustained explosive eruptionoccur, clouds of volcanic ash would be gener-ated that could drift thousands of kilometersdownwind. All aircraft, some facilities, andliving things—including humans—downwindfrom the volcano are at risk from effects ofvolcanic ash clouds and ash fallout. Ashclouds from Iliamna Volcano could rise to alti-tudes of 12,000 meters or more and move intothe flight paths of jet aircraft using AnchorageInternational and other airports in south-cen-tral and central Alaska. Aircraft using airroutes over the North Pacific Ocean and otherareas downwind of Iliamna, especially theGulf of Alaska and Pacific Northwest region,could encounter clouds of volcanic ash. Thefrequency at which dangerous clouds of volca-nic ash are produced and the amount of ashfallcannot be estimated with certainty. Signs ofvolcanic unrest will precede an eruption andpermit reasonable estimates of the likelihoodof volcanic ash emission once an eruptivephase is detected. However, it is not possible todetermine the characteristics of an ash cloudbefore an eruption occurs, except that it islikely to be similar to those generated by erup-tions of other Cook Inlet volcanoes.

HAZARD WARNING AND MITIGATION

Typically, eruptions at Cook Inlet volcanoes are preceded by a period of precursearthquake activity giving some degree of semic warning prior to an eruption. Many eruptions are preceded by at least several weeksincreased gas emission from the summit arWhen volcanic unrest is detected, other motoring techniques, such as satellite observtions, measurement of volcanic gas fluremote observation with real-time video otime-lapse cameras, and geodetic surveyiare used to develop a comprehensive assement of the likelihood of an eruption and itpotential effects.

The AVO monitors Iliamna Volcano with areal-time seismic network (fig. 18) equippewith an alarm system that is triggered by elvated levels of seismic (earthquake) activiindicating volcanic unrest. A network of sixradio-telemetered seismometers sends retime radio signals to AVO offices in Anchorage and Fairbanks. In addition, AVO maintaina field-based data collection program thincludes geodetic, temperature, and gas msurements.

One of the primary roles of the Alaska Vocano Observatory (AVO) is to communicattimely warnings of volcanic unrest and potetial eruptions (Eichelberger and others, 199p. 4). The AVO distributes by fax and electronic mail a weekly update of volcanic activity that summarizes the status of the more th40 historically active volcanoes along thAleutian Arc. During periods of unrest or volcanic crises, updates are issued more fquently to advise the public of significanchanges in activity. Recipients of these updainclude the Federal Aviation Administrationair carriers, the National Weather Service, tAlaska Department of Emergency Servicelocal military bases, the Governor’s officevarious State offices, television and radio sttions, news wire services, and others. Upda


Hazard Warning and Mitigation 27



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EXPLANATION

Figure 18. Location of seismic-monitoring stations on Iliamna Volcano.

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also are distributed by electronic mail to vari-ous volcano information networks and areposted on the AVO world-wide web site(www.avo.alaska.edu).

During the 1989-90 eruptions of RedoubtVolcano, the AVO developed a “level of con-cern color code” (Brantley, 1990; fig. 19). Thiscode provides efficient and simple informationabout the status of volcanic activity or unrestand conveys the AVO’s interpretation of thatactivity or unrest in terms of the potential foran eruption and its likely effects. In the adventof a volcanic crisis, various Federal, State, andlocal officials are contacted by telephone,advised of the situation, and the level of con-cern color code is established while an updateis being prepared. This approach has been usedsuccessfully during recent periods of volcanicunrest, such as the 1989-90 eruptions ofRedoubt Volcano, the 1992 eruptions of Mount

Spurr Volcano (Miller and Chouet, 1994Keith, 1995), and the 1996-97 eruption of Palof Volcano.

Minimizing the risks posed by eruptions oIliamna Volcano is possible through adequawarning of potential hazards, and by avoidindevelopment or utilization of areas likely to baffected by future eruptions (plate 1). Areawithin about 10-20 kilometers of Iliamna Volcano are at risk from all hazardous volcanphenomena. If for some reason, developmeis unavoidable in hazardous areas, engineermeasures may be employed to minimize prevent undesirable consequences.

Knowledge of potential hazards is requireto assess the risk associated with a speclocation on or near the volcano and to assewhether or not movement to another locatiowould be safer. Recreational users of La


LEVEL OF CONCERN COLOR CODEVolcano is in its normal "dormant" state. Volcano is restless.Seismic activity is elevated. Potential for eruptive activity is increased. A plume of gas and steam may rise several thousand feet above the volcano which may contain minor amounts of ash. Small ash eruption expected or confirmed. Plume(s) not likelyto rise above 25,000 feet above sea level. Seismic disturbance recorded on local seismic stations, but not recorded at more distantlocations.

Large ash eruptions expected or confirmed. Plume(s) likelyto rise above 25,000 feet above sea level. Strong seismic signal recorded on all local and commonly on more distant stations.

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LEVEL OF CONCERN COLOR CODE

Figure 19. Level of concern color code for volcanic eruptions.

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Clark National Park and Wilderness in thevicinity of Iliamna Volcano should recognizethat low-lying terrain along streams and gulliesthat extend toward the summit is subject topyroclastic flow and surge, lahars, lahar-runoutflows, floods, and avalanches. Given thepresent configuration of the summit area, adebris avalanche is most likely on the south-eastern flank at the head of Red Glacier, or onthe southwestern flank at the head of UmbrellaGlacier. During an eruption, access closer thanabout 10 kilometers from the volcano could beimpossible and the risks to human life great.Small planes and helicopters seeking a view ofan eruption could be at risk from intermittentand unpredictable discharge of ballistic projec-tiles (volcanic bombs) or sudden changes inthe travel direction of the eruption plume.

People and facilities located farther awayfrom the volcano may have additional time toprepare for the adverse effects of an eruption;however, an emergency plan developed andready prior to the onset of an eruption is use-ful. The planning for volcanic emergencies issimilar to that for other emergencies, such asflooding or extreme weather. The sources ofemergency information are often the same andthe usual interruption of essential services mayresult. Thus, planning for interruptions in elec-trical service, transportation (especially airtravel), and outdoor activities is appropriatefor volcanic emergencies.

REFERENCES CITED

Begét, J.E., 1996, Dendrochronologic, archeologic,and radiocarbon data on recent eruptions ofIliamna and Augustine Volcanoes, Alaska: Eos,Transactions of the American GeophysicalUnion, v. 77, no. 46, p. F814.

Blong, R.J., 1984, Volcanic hazards: Sydney, Aca-demic Press, 424 p.

Brantley, S.R., ed., 1990, The eruption of RedoubtVolcano, Alaska, December 14, 1989—August31, 1990: U.S. Geological Survey Circular 1061,33 p.

Casadevall, T.J., ed., 1994, Volcanic ash and aviatsafety—Proceedings of the First InternationSymposium on Volcanic Ash and AviationSafety: U.S. Geological Survey Bulletin 2047450 p.

Eichelberger, J.C., Keith, T.E.C., Miller, T.P., anNye, C.J., 1995, The 1992 eruptions of CratPeak vent, Mount Spurr Volcano, Alaska—Chronology and summary, in Keith, T.E.C., ed., The1992 eruptions of Crater Peak vent, Mount SpuVolcano, Alaska: U.S. Geological Survey Bulleti2139, p. 1-18.

Hoblitt, R.P., Walder, J.S., Driedger, C.L., ScotK.M., Pringle, P.T., and Vallance, J.W., 1995, Vocano hazards from Mount Rainier, WashingtoU.S. Geological Survey Open-File Report 98273, 12 p.

Keith, T.E.C., ed., 1995, The 1992 eruptions of CraPeak Vent, Mount Spurr Volcano, Alaska: U.SGeological Survey Bulletin 2139, 220 p.

McGimsey, R.G., and Neal, C.A., 1997, Volcaniactivity in Alaska and Kamchatka—Summary oevents and response of the Alaska Volcano Obsvatory 1996: U.S. Geological Survey Open-FilReport 97-433, 34 p.

Miller, T.P., Begét, J.E., Stephens, C.D., and MooR.B., 1996, Geology and hazards of Iliamna Vocano, Alaska: Eos, Transactions, American Gephysical Union, v. 77, p. F815.

Miller, T.P., and Chouet, B.A, 1994, The 1989-199eruptions of Redoubt Volcano—An introductionJournal of Volcanology and GeothermaResearch, v. 62, p. 1-10.

Miller, T.P., McGimsey, R.G., Richter, D.H., RiehleJ.R., Nye, C.J., Yount, M.E., and Dumoulin, J.A1998, Catalog of the historically active volcanoeof Alaska: U.S. Geological Survey Open-FilReport 98-582, 104 p.

Myers, B., Brantley, S.R., Stauffer, P., and HendleJ.W., II, 1997, What are volcano hazards?: U.Geological Survey Fact Sheet 002-97.

Vallance, J.M., and Scott, K.M., 1997, The Osceomudflow from Mount Rainier—Sedimentologyand hazard implications of a huge clay-rich debrflow: Geological Society of America Bulletin, v.109, p. 143-163.

References Cited 29

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GLOSSARY

Andesite. A fine-grained volcanic rock made up offeldspars and ferromagnesian minerals; typicallyhas a Si02 content of 54 to about 62 percent.

Ash. Fine fragments (less than 2 millimeters across)of lava or rock formed in an explosive volcaniceruption.

Cohesive. As applied to lahars, any lahar that containsmore than about 3-5 percent clay in the depositmatrix.

Debris avalanche. Rapidly moving, dry flows of dis-saggregated rock debris, sand, and silt. Volcanicdebris avalanches often form by some type ofstructural collapse of the volcano, usually thesteep front of the cooled lava dome, or other partsof the upper edifice. A large portion of the vol-cano may become unstable, break away from thevolcanic massif, and become an avalanche. Adebris avalanche may be triggered by an eruptionor earthquake. Debris avalanches move at veloci-ties ranging from a few tens of meters per secondto more than 100 meters per second and behavelike complex granular flows or slide flows. Oftenthey are quite voluminous (greater than 10 cubickilometers) and may run out considerable dis-tances (up to 85 kilometers) from their source.The resulting debris avalanche deposit usuallyexhibits hummocky surface morphology.

Directed blast. Large-scale volcanic explosionscaused by a major landslide or slope failure thatresults in a rapid drop in the pressure of theintruding magma near the surface of the volcanicedifice. The 1980 eruption of Mt. St. Helens wastriggered by a massive slope failure and the sub-sequent laterally directed blast affected a 180°sector north of the volcano and extended for sev-eral tens of kilometers outward. A directed blasttypically travels away from the volcano at a lowangle and may not be deflected by ridges or othertopographic barriers. Rock debris propelled by adirected blast moves much faster than typicallandslides and rockfalls. For example, at Mt. St.Helens, the initial velocity of the directed blastcloud was about 600 kilometers per hour decreas-ing to about 100 kilometers per hour at a distance25 kilometers from the volcano.

Edifice. The upper part of the volcanic cone, includ-ing the vent and summit areas.

Eruption cloud. Cloud of gas, ash, and other fragments that forms during an explosive volcaneruption and travels long distances with the prvailing winds.

Eruption column. The vertical portion of the erup-tion cloud that rises above a volcanic vent.

Fallout. A general term for debris that falls to thEarth from an eruption cloud.

Fumarole. Small, vent-like crack or opening ofescaping gas and steam.

Lahar. An Indonesian term for a debris flow containing angular clasts of volcanic material. For thpurposes of this report, a lahar is any type of sement/water mixture originating on or from thevolcano. Most lahars move rapidly down thslopes of a volcano as channelized flows adeliver large amounts of sediment to the riveand streams that drain the volcano. The flovelocity of some lahars may be as high as 20 tometers per second (Blong, 1984) and sedimeconcentrations of >750,000 parts per million anot uncommon. Large volume lahars can travgreat distances if they have an appreciable ccontent (> 3 to 5 percent), remain confined tostream channel, and do not significantly gain seiment while losing water. Thus, they may affecareas many tens to hundreds of kilometers dowstream from a volcano.

Lahar-runout flow. The downstream or distal component of a lahar. Lahar-runout flows are finegrained and more watery than a typical lahaMost noncohesive lahars transform to laharunout flows as they travel downstream.

Lapilli. Ejected rock or pumice fragments betweenand 64 millimeters in diameter.

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

Lava dome. A steep-sided mass of viscous and ofteblocky lava extruded from a vent; typically has rounded top and roughly circular outline.

Magma. Molten rock beneath the Earth’s surface.

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

Pleistocene epoch. The period of Earth historybetween 1.8 million and 10 thousand years befopresent.


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Plinian. Volcanic eruptions associated with highlyexplosive ejection of tephra and large-volumeemissions of ash. Ash plumes from plinian erup-tions usually reach 10,000 meters altitude or moreabove the vent.

Pumice. Highly vesicular volcanic ejecta; due to itsextremely low density, it often floats on water.

Pyroclastic. General term applied to volcanic prod-ucts or processes that involve explosive ejectionand fragmentation of erupting material.

Pyroclastic flow. A dense, hot, chaotic avalanche ofrock fragments, gas, and ash that travels rapidlyaway from an explosive eruption column, oftendown the flanks of the volcano (synonymous with“ash flow”). Pyroclastic flows move at speedranging from 10 to several hundred meters psecond and are typically at temperatures betwe300 and 800 °C (Blong, 1984). Pyroclastic flowform either by collapse of the eruption column, oby failure of the front of a cooling lava domeOnce these flows are initiated, they may travdistances of several kilometers or more and easoverride topographic obstacles in the flow path. person could not outrun an advancing pyroclasflow.

Pyroclastic surge. A low-density, turbulent flow offine-grained volcanic rock debris and hot gaPyroclastic surges differ from pyroclastic flows i

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that they are less dense and tend to travel as a ground-hugging, but highly mobile cloud that casurmount topographic barriers. Surges ofteaffect areas beyond the limits of pyroclastflows.

Sulfurous. Sulfur bearing or rich in sulfur com-pounds.

Stratovolcano. (also called a stratocone or composicone) A steep-sided volcano, usually conical shape, built of lava flows and fragmental deposfrom explosive eruptions.

Tephra. Any type of rock fragment that is forciblyejected from the volcano during an eruptionTephra may be fine-grained dust or “ash” (0.062to 2 millimeter diameter—silt to sand sized)coarser “lapilli” (2 to 64 millimeter diameter—sand to pebble sized), or consist of large blocksbombs (>64 millimeter—cobble to bouldesized). When tephra is airborne, the coarsest frtion will be deposited close to the volcano, but thfine fraction may be transported long distancand can stay suspended in the atmosphere many months. Tephra particles are typicalsharp, angular, and abrasive, and are composevolcanic glass, mineral, and rock fragments.

Vent. An opening in the Earth's surface through whicmagma erupts or volcanic gasses are emitted.

GLOSSARY 31

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