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Gigantic Debris Avalanche of 

Pleistocene Age From Ancestral 

Mount Shasta Volcano, California, and 

Debris-Avalanche Hazard Zonation 

By DWIGHT R. CRANDELL 

U.S. GEOLOGICAL SURVEY BULLETIN 1861 
DEPARTMENT OF THE INTERIOR 

DONALD PAUL HODEL, Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

UNITED STATES GOVERNMENT PRINTING OFFICE: 1989 

For sale by the 

Books and Open-File Reports Section 

U.S. Geological Survey 

Federal Center, Box 25425 

Denver, CO 80225 

Any use of trade names in this report is for descriptive purposes only and does 

not imply endorsement by the U.S. Government. 

Library of Congress Cataloging-in-Publication Data 

Crandell, Dwight Raymond, 1923-

Gigantic debris avalanche of Pleistocene age from ancestral Mount Shasta 

volcano, California, and debris-avalanche hazard zonation. 

(U.S. Geological Survey bulletin; 1861) 

Bibliography: p. 

Supt. of Docs. no.: I 19.3:1861 

1. Debris avalanches-California-Shasta, Mount, Region. 2. Geology, 

Stratigraphic-Pleistocene. 3. Geology, California-Shasta, Mount, 

Region. I. Title. II. Series. 

QE75.89 no. 1861 [QE599.U5] 557.3 s 88-600286 

[551.2' 1 '0979421] 
CONTENTS 

Abstract 1 

Introduction 1 

Topographic and geologic setting and age of the debris avalanche 3 

Previous interpretations of the geology and topography of Shasta Valley 3 

Description of the debris-avalanche deposits 4 

Matrix facies 6 

Block facies 8 

Fossils in the matrix facies 11 

Distribution of the debris-avalanche deposits 11 

Extent, thickness, and volume of the deposits 14 

Origin of morphology and drainage on the deposits 16 

Sources of water in the debris avalanche 19 

Cause of the debris avalanche 19 

Initiation of the debris avalanche and emplacement of deposits 21 

Comparison of wet and dry volcanic debris avalanches 22 

Hazard zonation for volcanic debris avalanches 23 

Examples of debris-avalanche hazard zonation 24 

Discussion 25 

Acknowledgments 25 

References cited 25 

Volcanic ash analysis 32 

FIGURES 

1. Map showing inferred minimum extent of debris-avalanche deposits in 

Shasta Valley 2 

2. Topographic map showing morphology of debris-avalanche deposits 5 

3-5. Photographs showing: 

3. Hills arid flat areas formed by the debris avalanche 6 

4. Morphology of the debris-avalanche deposits 7 

5. Matrix facies of debris-avalanche deposits 7 

6. Map showing localities at which pebbles in the matrix facies were 

identified and the inferred extent of the block facies 9 

7-12. Photographs showing: 

7. Mounds on the surface of the matrix facies 10 

8. Three lava flows on a hillside 11 

9. Block of volcaniclastic deposits in a borrow pit 12 

10. A buried soil on a pyroclastic-flow deposit in a borrow pit 13 

11. Block of volcaniclastic deposits exposed in a road cut 14 

12. Blocks of andesite from Mount Shasta enclosed within and resting 

on matrix facies 15 

13. Topographic profiles of the debris-avalanche deposits, and profile of 

the inferred upper surface of the avalanche during emplacement 15 

14-19. Maps showing: 

14. Subdivisions used in estimating average thickness and volume of 

debris-avalanche deposits 17 

15. Clusters of hills and ridges in the proximal part of the debrisavalanche deposits 18 

Contents Ill 
16. Morphology of the southwesternmost cluster of hills and ridges of 

the debris avalanche 20 

17. Possible future debris avalanches, Mount Shasta volcano 25 

18. Possible future debris avalanches, Mount Rainier volcano 26 

19. Possible extents of debris avalanches that could have originated at 

Mount St. Helens before the May 18, 1980 eruption 28 

TABLES 

1. Percentages of pebbles from Mount Shasta in the matrix facies at various 

distances from the summit of volcano, based on identification of 100 pebbles 

at each location 8 

2. Extent and thickness, inferred from well logs, and estimated volume of the 

debris avalanche in seven areas 16 

3. Potential lengths of debris avalanches in three directions from the summit of 

Mount Shasta, which has an altitude of 4,316 m 24 

4. Potential lengths of debris avalanches from the summit area of Mount Rainier, 

which has an altitude of 4,392 m 26 

5. Potential lengths of debris avalanches from the summit area of the pre-1980 

Mount St. Helens volcano, which had an altitude of about 2,949 m 27 

UNITS OF MEASURE AND THEIR EQUIVALENTS 

To convert metrics 

Micrometers 

Centimeters (em) 

Meters (m) 

Kilometers (km) 

Square kilometers (km2

) 

Cubic kilometers (km3

) 

Kilometers/hour (km/h) 

IV Contents 

Multiply by 

0.00003937 

.3937 

3.28 

.62 (about 2/3) 

.3861 

.2399 (about 1/4) 

.62 

To obtain 

Inches (in.) 

Inches (in.) 

Feet (ft) 

Miles (mi) 

Square miles ( mi2

) 

Cubic miles ( mi3

) 

Miles/hour (mi/h) 
Gigantic Debris Avalanche of Pleistocene Age From 

Ancestral Mount Shasta Volcano, California, and 

Debris-Avalanche Hazard Zonation 

By Dwight R. Crandell 

Abstract 

The deposits of an exceptionally large debris avalanche 

extend from the base of Mount Shasta volcano northward 

across the floor of Shasta Valley in northern California. The 

debris-avalanche deposits underlie an area of about 675 

km2 , and their estimated volume is at least 45 km3 . Radiometric limiting dates suggest that the debris avalanche 

occurred between about 300,000 and 380,000 yr ago. 

Hundreds of mounds, hills, and ridges formed by the 

avalanche deposits are separated by flat areas that slope 

generally northward at about 5 m/km. The hills and ridges are 

formed by the block facies of the deposits, which includes 

masses of andesite lava tens to hundreds of meters across 

as well as stratigraphic successions of unconsolidated 

deposits of pyroclastic flows, lahars, air-fall tephra, and 

alluvium, which were carried intact within the debris 

avalanche. The northern terminus of the block facies is near 

Montague, at a distance of about 49 km from the present 

summit of the volcano. The flat areas between hills and 

ridges are underlain by the matrix facies, which is an 

unsorted and unstratified mudflowlike deposit of sand, silt, 

clay, and rock fragments derived chiefly from the volcano. 

Boulders of volcanic rock from Mount Shasta are scattered 

along the west side of Shasta Valley and in the part of Shasta 

Valley that lies north of Montague, at heights of as much as 

100 m above the adjacent surface of the debris-avalanche 

deposits. The boulders represent a lag that was formed after 

the main body of the avalanche came to rest, when much of 

the still-fluid matrix facies drained away and flowed out of 

Shasta Valley down the Shasta River valley and into the 

Klamath River. 

The debris avalanche probably originated in a quick 

succession of huge landslides of water-saturated rock on the 

northwest flank of ancestral Mount Shasta, each of which cut 

progressively deeper into the volcano. Evidence is lacking of 

contemporaneous volcanic activity, and the cause of the 

landslides is not known. 

Manuscript approved for publication June 6, 1988. 

The possible lengths of future volcanic debris 

avalanches from Mount Shasta, as well as other volcanoes, 

with volumes of 1 km3 or more can be estimated from the 

equation L= H/f, where Lis length, His the altitude difference 

between the summit of the volcano and points 20 km 

downslope from it, and the value off is 0.075. This value off 

is based on the average height-to-length ratios of 11 

representative volcanic debris avalanches with volumes of at 

least 1 km3 • For the purpose of hazard assessment, a safety 

factor could be added to the calculated value that is 

commensurate with local conditions of topography, land use, 

and population at risk. 

INTRODUCTION 

The deposits of an exceptionally large prehistoric 

debris avalanche cover an area of at least 675 km2 north 

of Mount Shasta in north-central California (fig. 1 ). 

Their lithology shows that the deposits were derived 

chiefly from an andesitic stratovolcano, which is 

presumed to be an ancestor of Mount Shasta. The 

absence of a landslide scar on the modern volcano 

implies that most of the ancestral volcano has been 

covered by products of later eruptions. 

Some of the features of the avalanche deposits have 

been described previously (Crandell and others, 1984). 

My subsequent studies have revealed additional 

information, presented here, concerning the extent, 

volume, and mode of emplacement of the debris 

avalanche. A concluding section of this report discusses 

how the lengths of very large volcanic debris avalanches 

can be anticipated. 

The term debris avalanche is used here to refer to 

the sudden and very rapid movement of an incoherent, 

unsorted mass of rock and soil mobilized by gravity 

(Schuster and Crandell, 1984). Movement is 

characterized by flowage in a dry or wet state, or both. 

Debris avalanches commonly originate in massive 

Introduction 
' 

Area 

~ of figure 1 

D 

6 Yellow Butte 

--------......._ / ~ 

/ ~ 

/ ""' \ 

Shastina 6 6 Mount Shasta 

"" ~ / / .....___ ~ 

0 Mount Shasta ----

10 KILOMETERS 

5 MILES 

\ 

\ 

l 

I 

I 

I 

/ 

2 Debris Avalanche, Mt. Shasta, California, Debris-Avalanche Hazard Zonation 
rockslides which disintegrate during movement into 

fragments ranging in size from small particles to blocks 

hundreds of meters across. If the avalanche has a large 

content of water and fine material, this material may 

continue to flow downslope after the coarser parts of the 

avalanche come to rest. Thus, the texture and behavior of 

a debris avalanche may change between its inception and 

the final stages of emplacement. 

TOPOGRAPHIC AND GEOLOGIC 

SETTING AND AGE OF THE 

DEBRIS AVALANCHE 

The debris-avalanche deposits underlie the western 

two-thirds of Shasta Valley, which is a broad depression 

between the Klamath Mountains on the west and the 

Cascade Range on the east (fig. 1). The valley is drained 

by the Shasta River, which flows northward across the 

avalanche deposits and younger basaltic lava flows. The 

river enters a bedrock gorge in the Klamath Mountains 

northwest of Montague and joins the Klamath River 

about 10 km farther downstream. The floor of the 

southern part of Shasta Valley slopes northward from an 

altitude of a little over 900 m near Weed to about 760 m 

near Montague, and the part of the valley that lies north 

of Montague slopes gently southward. Mount Shasta 

volcano, which has a summit altitude of 4,316 m and an 

estimated volume of about 335 km3 (Williams, 1932), lies 

at the south end of the valley. 

Shasta Valley is flanked on the west and north by 

complexly folded and faulted metamorphic, sedimentary, 

volcanic, and ultramafic rocks of Paleozoic age and by 

eastward-dipping marine sandstone and conglomerate of 

the Upper Cretaceous Hornbrook Formation (Peck and 

others, 1956; Hotz, 1977; Nilsen, 1984). A recent geologic 

map (Nilsen and others, 1983), which includes the 

northern part of Shasta Valley, shows the Hornbrook to 

dip 5-17° generally easterly in the area east of Yreka. 

Volcanic rocks of Tertiary age border the north 

and east sides of Shasta Valley and also form a few hills 

that rise above the central part of the valley floor, such as 

Figure 1 (facing page). Inferred minimum extent of debrisavalanche deposits (shaded) in Shasta Valley. Dashed circle, 

which represents the approximate base of Mount Shasta, is 

at about the 4,400-ft contour on the northwest side of the 

volcano. Short-dashed line along east margin of deposits is 

the western edge of post-avalanche lava flows; dotted line 

farther east is at an altitude similar to the maximum inferred 

height of the debris avalanche on the west side of Shasta 

Valley. Solid squares are mounds of avalanche deposits 

surrounded by lava; solid circles are blocks of Mount Shasta 

andesite along west side of Shasta Valley (see text). Lines 

A-8 and C-D show locations of topographic profiles across 

the avalanche deposits. 

Steamboat Mountain and Owls Head. Gregory 

Mountain at Montague is a cylindrical neck or plug of 

hornblende andesite (Hotz, 1977). 

Steamboat Mountain (fig. 1) is shown on the 

geologic map of Nilsen and others (1983) to be underlain 

by volcanic rocks of Tertiary(?) age. However, 

sedimentary rocks of the Hornbrook Formation crop out 

on the eastern slopes of the mountain topographically 

lower than the volcanic rocks, which cap the mountain. 

At one point on the eastern side of Steamboat Mountain 

these sedimentary rocks strike nearly due north and dip 

about 15° east. The presence of the Hornbrook near the 

center of Shasta Valley suggests that the valley south of 

Montague is largely underlain by that formation. 

The avalanche deposits are overlain on the east by 

basaltic lava flows and on the south by andesitic lava 

flows, lahars, and alluvium from Mount Shasta. 

Modern Mount Shasta is made up of at least four 

overlapping cones, the oldest of which is more than 

100,000 yr old and forms the south flank of the volcano 

(Christiansen and others, 1977). Even this oldest cone, 

however, probably postdates the debris-avalanche 

deposits in Shasta Valley and may itself represent more 

than one "evolutionary cycle" in the growth of Mount 

Shasta (Christiansen and others, 1977, p. 15; oral commun., 1986). Volcanic rocks that evidently are older than 

this cone, and that predate the debris avalanche, crop out 

at the west base of the volcano (Crandell and others, 

1984). A sample of these rocks collected by R.L. Christiansen and potassium-argon dated by G.B. Dalrymple of 

the U.S. Geological Survey had an age of 

593,000±41,000 yr. 

The age of the avalanche is roughly bracketed by 

radiometric dates on rocks within the avalanche deposits 

and on a postavalanche lava flow. Charles Chesterman of 

the California Division of Mines and Geology collected 

samples for potassium-argon dating from two andesite 

blocks within the avalanche deposit. These rocks were 

analyzed by James Aronson of Case Western Reserve 

University, who reported ages of 360,000 ± 40,000 and 

380,000±60,000 yr (Crandell and others, 1984). A basalt 

lava flow that locally overlies the avalanche deposits was 

dated by Aronson as 300,000± 100,000 yr old. Thus, the 

debris avalanche seems to be between roughly 300,000 

and 380,000 yr old. This age is consistent with the 

presence, within the avalanche deposits, of clasts of a 

volcanic ash, described subsequently, that is about 

400,000 yr old. 

PREVIOUS INTERPRETATIONS OF THE 

GEOLOGY AND TOPOGRAPHY OF 

SHASTA VALLEY 

The numerous mounds, hills, and ridges of Shasta 

Valley (figs. 2, 3, 4) have puzzled geologists for many 

Previous Interpretations of the Geology and Topography of Shasta Valley 3 
years. Diller and others (1915) noted that the hills consist 

of volcanic rock and stated that they "appear to be, in 

part at least, the products of minor and local eruptions 

that broke through the Cretaceous beds, each vent 

contributing its little pile of material." Fenner (1923) 

subsequently proposed that a shallow sill had been 

intruded beneath Shasta Valley and that small bodies of 

magma rose from the sill to the surface to form the 

individual hills. These geologists evidently did not 

recognize the Mount Shasta provenance of the rocks that 

form the hills and their cogenetic relation to the mudflowlike material that veneers the hills and underlies the 

flat surfaces between them. 

Williams (1949, pl. 1) mapped the hills and ridges 

between the base of Mount Shasta and Lake Shastina as 

moraines formed by a glacier of Tioga age, which moved 

into Shasta Valley from the northwestern slopes of the 

volcano. His geologic map shows the northernmost 

moraine to coincide with the hills along the south side of 

Lake Shastina, and flat areas between the hills are shown 

as "fluvioglacial outwash" of Pleistocene age. The long 

ridge north of Lake Shastina is mapped as "Western 

Cascade flows and tuff-breccias" of Tertiary age, as are 

the hills and ridges to the north. Flat areas between these 

hills are shown as alluvium. The region north of Lake 

Shastina was aptly described by Williams (1949, p. 11) as 

"volcanic rocks eroded into a myriad of hillocks that 

range from a few feet to 200 and rarely to 300 feet in 

height. Most of these hillocks are domical, some are 

conical, others are mesas, and a few are long hogback 

ridges. Together they form a strange landscape, 

deceptively like the products of recent volcanic activity." 

Although Williams inferred that the Western 

Cascade volcanic rocks of Shasta Valley dip generally 

eastward, he (Williams, 1949, p. 53) noted that "the 

trouble is that the Western Cascade lavas [blocks in the 

debris-avalanche deposits] in the western half of Shasta 

Valley show no well-defined dips, and in the other half 

are buried by Recent flows." 

Williams' interpretations were accepted by Mack 

(1960), whose geologic map shows the area south of 

Lake Shastina to be underlain by "morainal deposits" 

and the hills to the north to be "volcanic rocks of the 

Western Cascades." Mack's map shows most flat areas 

between hills and ridges as ''younger alluvium" of 

"Recent" age, although terraces northwest of Weed are 

shown as "fluvioglacial deposits." Mack (1960, p. 44) 

described an outcrop in the ridge bordering the 

northwest side of Lake Shastina as "deposits of stratified 

drift which were probably laid down in contact with the 

wasting glacial ice." Mack's "stratified drift" consists of 

the deposits of pyroclastic flows and lahars, alluvium, and 

beds of air-fall tephra (Crandell and others, 1984; this 

report, fig. 9). 

Hotz (1977, p. 55-56) also agreed with Williams' 

interpretation of the Shasta Valley landscape and stated, 

"The many small conical hills in Shasta Valley***are 

erosional remnants of an extensive blanket of andesitic 

tuff breccia" which apparently represent a continuation 

of the Western Cascade volcanic series of Tertiary age 

mapped by Williams (1949). Hotz noted that most 

outcrops in the hills reveal "a jumbled accumulation of 

blocks of volcanic rock as much as 1.5 m across," and he 

went on to describe the lithology of the volcanic rocks. 

Hotz mapped the interhill areas as "alluvium of Shasta 

Valley," and noted that the alluvium does not extend into 

the valleys to the west. He suggested that the alluvium is 

of glaciofluvial origin and was deposited by meltwater 

from glaciers in the Klamath Mountains southwest of 

Weed and on Mount Shasta. 

Christiansen (1982) briefly mentioned the unusual 

topography of Shasta Valley and suggested that it 

originated from a large debris avalanche from Mount 

Shasta. Subsequent field studies by myself and my 

colleagues (Crandell and others, 1983, 1984) verified the 

debris-avalanche origin of the topography. Evidence that 

bears on this conclusion is as follows. 

(1) The unusual morphology of Shasta Valley is 

unlike that normally produced by stream 

erosion, and local closed depressions among 

the hills and ridges seem, in fact, to rule out 

this mode of origin. 

(2) The presence of stratigraphic successions of 

volcaniclastic deposits within hills and ridges, 

and their widely variable structure from place 

to place, suggest that these deposits are not in 

place. 

(3) Most rocks in the hills and ridges are pyroxene 

andesites similar to those of modern Mount 

Shasta volcano. 

( 4) The presence between hills of a mudflowlike 

deposit that contains rocks of similar lithology 

to those in the hills suggests that all these 

deposits had a similar origin. 

(5) The morphology of the deposits on the Shasta 

Valley floor resembles that of known debris 

avalanches elsewhere. 

DESCRIPTION OF THE 

DEBRIS-AVALANCHE DEPOSITS 

The debris-avalanche deposits of Shasta Valley 

include two texturally distinct parts, which are referred to 

as the block facies and the matrix facies (Crandell and 

others, 1984). The block facies is responsible for the 

hundreds of mounds, hills, and ridges that dot the surface 

of the avalanche deposits; the matrix facies veneers these 

4 Debris Avalanche, Mt. Shasta, California, Debris-Avalanche Hazard Zonation 
2 MILES 

Figure 2. Topographic map of the southwestern part of the Lake Shastina 15' quadrangle and the northwestern part of the 

Weed 15' quadrangle, both published by the U.S. Geological Survey in 1954, showing the morphology of the debris-avalanche 

deposits. The hills are formed by the block facies, and flat areas are underlain by the matrix facies. The summit of Mount Shasta 

is about 20 km southeast of Lake Shastina. The approximate location of U.S. Interstate 5 is shown by the dotted line. 

Description of the Debris-Avalanche Deposits 5 
Figure 3. View southward from top of Gregory Mountain toward hills and flat areas formed by the debris avalanche. Black 

Butte, at the west base of Mount Shasta, is a volcanic dome formed about 9,500 yr ago (Miller, 1978). 

landforms, underlies flat areas between them, and is 

inferred to enclose the lower parts of the blocks that 

characterize the block facies. 

Matrix Facies 

The matrix facies consists of an unsorted and 

unstratified mixture of pebbles, cobbles, and boulders in 

compact silty sand; texturally it resembles the deposit of 

a mudflow (fig. 5). A single sample of the matrix from a 

locality about 0.5 km west of Edgewood contained about 

54 percent sand, 20 percent silt, and 12 percent clay, and 

the remainder consisted of material coarser than sand. 

Virtually all boulders and most cobbles in the matrix 

facies are pyroxene andesites similar to those that make 

up the present Mount Shasta volcano. At the Edgewood 

locality, nearly all clasts are angular to subangular 

regardless of their lithology. 

At a locality about 32 km farther north, near 

Montague, most pebbles and cobbles of Mount Shasta 

rocks are angular to subangular and have rough surfaces, 

whereas nearly all clasts of non-Shasta rocks are round to 

subround and have smooth surfaces suggestive of 

abrasion during stream transport. These non-Shasta 

clasts presumably were picked up by the avalanche from 

alluvium on the floor of Shasta Valley. 

Pebbles in the matrix facies consist of rocks wholly 

or chiefly derived from ancestral Mount Shasta within a 

distance of 22 km from the summit of the present 

volcano, and include smaller percentages at greater 

distances (table 1, fig. 6). Samples 1, 3, and 4, which seem 

to contain only rocks derived from ancestral Mount 

Shasta, were each taken from the matrix facies on ridges 

near the base of the volcano. Sample 10 was obtained 

from the matrix facies at an outcrop in the Shasta River 

gorge; only 25 percent of the pebbles in it are of Mount 

Shasta rock types. I attribute the northward increase of 

percentages of non-Shasta rocks to scour and 

incorporation of preexisting deposits by the avalanche. 

6 Debris Avalanche, Mt. Shasta, California, Debris-Avalanche Hazard Zonation 
Figure 4. View southeastward from a hill about 6 km northwest of Lake Shastina, showing the morphology of the 

debris-avalanche deposits. The hills in the middle distance are formed by the block facies, and the valley floor of Parks Creek 

in the foreground is underlain by the matrix facies. The hill at the left is about 190m high. Black Butte is at the far right. 

Figure 5. Matrix facies of debris-avalanche deposits 

exposed in gully west of Edgewood (see fig. 7 for aerial view 

of location). 

In addition to pebbles, cobbles, and boulders of 

volcanic rock from Mount Shasta, and a variety of rocks 

from bedrock formations of Cretaceous and preCretaceous age, the matrix facies of the debris avalanche 

in the Montague area locally contains clasts of compact 

but unconsolidated diatomite, laminated silt, and 

volcanic ash. A sample of volcanic ash taken from a clast 

within the matrix facies at a small quarry 1.3 km 

northwest of Montague was examined by Andrei SarnaWojcicki and his associates Charles Meyer and Robert 

Fehr, all of the U.S. Geological Survey (A. SarnaWojcicki, written commun., 1987) (see appendix 

"Volcanic Ash Analysis"). From their examination, 

Sarna-Wojcicki concluded that the ash is 

petrographically and chemically similar to an ash bed that 

was found at a depth of about 59 m in a core at Tule 

Lake, Calif. That ash is correlated with the Rockland ash, 

which was erupted from a vent in the Lassen Peak area of 

northern California, about 160 km southeast of Montague. The probable age of the Rockland ash is about 

400,000 yr, based on fission-track dating of zircons in it 

(Sarna-Wojcicki and others, 1985). 

Description of the Debris-Avalanche Deposits 7 
Table 1. Percentages of pebbles from Mount Shasta in the matrix 

facies at various distances from the summit of the volcano, based on 

identification of 100 pebbles at each location 

[The sample sites are shown on figure 6) 

Sample Distance Rocks from Other Not 

site from Mount Shasta rocks identified 

summit (km) 

1 ............. 17 100 0 0 

2 ..... ..... ... 21 84 11 5 

3 .. ... ........ 21 98 0 2 

4 ............. 22 98 0 2 

5 ... ... . .. .... 28 35 62 3 

6 ·· ·· ··· ······ 42 40 60 0 

7 ..... ........ 44 41 50 9 

8 ···· ···· ·· ··· 47 47 53 0 

9 .......... ... 50 45 53 2 

10 ............. 55 25 71 4 

At a road cut on the west side of the Shasta River, 

5 km northwest of Montague, the matrix facies overlies 

an unconsolidated deposit of oxidized sand and gravel 

which seems to be part of an alluvial fan. Incorporated in 

the matrix facies here are masses several meters in 

diameter of similar sand and gravel. A few clasts consist 

of diatomite, laminated silt, and volcanic ash. Fossils of 

aquatic organisms, which are described subsequently, 

were noted in the matrix facies here and elsewhere in the 

Montague area. 

A nearly horizontal 40 x 80 x 110-cm clast of 

volcanic ash in this road cut was examined in 1987. The 

clast lies 35 em above the base of the avalanche deposit. 

Ash in the basal 19 em of the clast is of fine to very fine 

sand size, whereas the upper part is of silt and possibly 

also of clay size. The upward decrease in grain size 

suggests that the basal part of the clast includes the lower 

part of the original ash bed. Faint bedding in the upper 10 

em of the ash indicates that the smallest dimension of the 

clast, 40 em, represents a minimum thickness of the 

original ash layer. The ash is enclosed by the matrix 

facies, and its original base and top may not be present. 

Fossil gastropods are abundant along a horizon 1 em 

below the top of the ash. In mineralogy and texture the 

ash resembles that from the quarry near Montague, and 

it probably is also correlative with the Rockland ash. The 

bedding and fossils in the ash suggest that it was 

deposited in a lake on the floor of Shasta Valley. 

The fine-grained component of the matrix 

evidently is made up of several constituents, including 

pulverized volcanic rock as well as silt and sand derived 

from the Hornbrook Formation and alluvial and lacustrine sediments incorporated by the avalanche as it 

traversed the floor of Shasta Valley. 

The matrix facies forms nearly flat surfaces which 

surround hills and ridges and which also extend westward 

beyond the hills to the side of Shasta Valley. These 

surfaces slope northward about 5 m/km in the western 

part of Shasta Valley, and represent the original 

depositional surface of the matrix facies rather than 

postavalanche stream erosion. Deposits of the matrix 

facies are inferred to be present along the east side of the 

avalanche but have been buried to depths of 20-90 min 

that area by postavalanche lava. 

The surface of the matrix facies locally displays 

areas of circular mounds, the most prominent of which is 

1-2 km west and northwest of Edgewood (fig. 7). In that 

area, mounds roughly 10-30 min diameter and as much 

as 60 em high cover the nearly flat surface of the matrix 

facies in an area adjacent to and some 22 m higher than 

the Shasta River. Few large clasts are present on the 

mounds, but accumulations of cobbles and boulders 

occur around the perimeter of each mound. Masson 

(1949) attributed the formation of the mounds to the 

freeze and thaw of water-saturated soil; they probably 

were formed during a climate colder than that of the 

present. 

Block Facies 

The block facies of the debris-avalanche deposits 

occurs in a band as much as 12.5 km wide which extends 

north-northwestward to the vicinity of Montague (fig. 6), 

where the northernmost large block from Mount Shasta 

is at a distance of about 49 km from the present summit 

of the volcano. The block facies includes individual 

andesite blocks ranging in size from tens to hundreds of 

meters in maximum dimension, many of which are 

shattered, and masses of similar size of coherent but 

unconsolidated volcaniclastic deposits. Chemical 

analyses reported by Hotz (1977, p. 57) of rocks in blocks 

at seven localities near Montague show a range of 

61-62.8 percent SiOz. 

Some hills are formed by one or more large blocks 

of a single rock type, and other hills are formed by 

lithologically dissimilar blocks. Continuous stratigraphic 

successions of lava flows can be recognized on the flanks 

of some hills (fig. 8). Other hills and ridges contain 

unconsolidated volcaniclastic deposits which are in their 

original stratigraphic succession and which remained 

coherent during transportation despite their low shear 

strength. 

One such succession, exposed in a pit on the east 

side of the north-trending ridge adjacent to the west 

abutment of Lake Shastina dam (fig. 9), includes the 

deposits of pyroclastic flows, lahars, air-fall tephra, and 

alluvium. Buried soils in these units are right side up (fig. 

10). Most units in the succession exposed in the pit dip 

8 Debris Avalanche, Mt. Shasta, California, Debris-Avalanche Hazard Zonation 
Area of figure 6 

Owls Head 

6 Yellow Butte 

~------.......____ 

/ ~ 

/ ~ 

/ ""' I \ 

I \ 

I \ 

I Shastina 6 \ 6 Mount Shasta 

\ I 

\ I 

Bla\k Butte I 

"" / ""' ~ / / 

--- ____..... O Mount Shasta ---

10 KILOMETERS 

5 MILES 

Figure 6. Map showing localities (numbered dots) at which pebbles in the matrix facies were identified (table 1) and 

the inferred extent (shaded) of the block facies. 

Description of the Debris-Avalanche Deposits 9 
Figure 7. Aerial view eastward of mounds on the surface of the matrix facies, 0.5 km west of the community of Edgewood, 

which is at upper right. U.S. Interstate 5 is in the center of the photograph, and the gully at right center is the location of 

figure 5. 

20--25° westward. They are displaced as much as 20 m by 

normal faults which strike roughly parallel to the 

northward trend of the ridge. Most faults that are east of 

the axis of the ridge dip steeply eastward, and those that 

are west of the axis dip 50--60° westward. The northerly 

strike of most of the faults, parallel to the ridge crest, and 

the dips and downward displacements toward the flanks 

of the ridge suggest that the faulting occurred after the 

ridge-forming block was emplaced. Faulting may have 

resulted from adjustment of the settling block to the 

underlying surface. 

Similar successions of unconsolidated deposits are 

exposed in road cuts through ridges west of Weed along 

U.S. Interstate 5. Most units exposed in the westernmost 

cut are east-dipping lithic pyroclastic-flow deposits which 

are broken by steeply west dipping normal faults (fig. 11). 

The deposits in the next road cut to the east are of similar 

origin but dip westward. 

Individual hills formed by the avalanche deposits 

become smaller northward, probably as a consequence of 

the progressive disaggregation of blocks during 

transport. The northernmost part of the block facies is 

represented by scattered large blocks of gray andesite in 

the area near Montague. Some of these blocks are more 

than 100m across and more than 10m thick, and some 

have been used as sources of crushed rock. 

Blocks of andesite enclosed by matrix facies were 

exposed prior to 1987 in the walls of the quarry 1.3 km 

northwest of Montague (fig. 12). Some of these blocks 

were broken by cracks less than one to several tens of 

centimeters wide, which were filled with material of the 

matrix facies. From their study of cracks within blocks of 

the Shasta Valley avalanche deposits, Ui and Glicken 

(1986) concluded that most cracks like these did not 

result from transport within the avalanche, but had been 

formed primarily on or near the volcano, and were then 

progressively opened during transport. 

This quarry is near the terminus of the block facies 

of the debris avalanche, in an area where the matrix 

facies of the avalanche had been at least 50 m deep 

before most of it drained away. An outcrop of preavalanche gravel a few hundred meters east of the pit 

10 Debris Avalanche, Mt. Shasta, California, Debris-Avalanche Hazard Zonation 
Figure 8. View of north side of a hill 7.5 km northwest of 

Lake Shastina. A succession of three lava flows crops out 

high on the hillside. The hill is about 100 m high. 

indicates that the matrix facies between mounds in that 

area is now locally less than 1 m thick. 

The transportation of intact stratigraphic 

successions of lava flows and volcaniclastic deposits has 

been described in other volcanic debris-avalanche 

deposits. Internally coherent blocks as much as 2.5 km 

long and 1 km wide are present in the deposit of a debris 

avalanche from Socompa volcano in northern Chile, 

which occurred about 7,500 yr ago (Francis and others, 

1985; Francis and Self, 1987). These blocks apparently 

slumped and rotated without internal disruption during 

their movement from near the summit of the volcano 

5-10 km to its base and now lie at the mouth of the 

amphitheater produced by the debris avalanche. 

Likewise, some blocks in a very large debris-avalanche 

deposit at Popocatepetl in Mexico are formed by lava 

flows in their original stratigraphic succession (Robin 

and Boudal, 1987). Megablocks also were transported 

intact in the prehistoric Nirasaki debris avalanche in 

Japan for distances of more than 20 km (Mimura and 

others, 1982). Measurements of remanent magnetism on 

units within these blocks indicate that the blocks had 

rotated in a horizontal, but not vertical, plane during 

transport. Sketches of outcrops of these blocks show that 

they are broken by many vertical faults of small 

displacement (Mimura and others, 1982, fig. 3). Many 

blocks in the May 18, 1980 debris avalanche from Mount 

St. Helens display fragile stratigraphic or structural 

features that had been formed on or within the volcano, 

and their preservation during avalanche transport has 

been attributed to laminar "plug" flow (Voight and 

others, 1981, p. 360). Davies (1982) postulated that the 

main zone of shearing in a moving sturzstrom (debris 

avalanche) is close to its base; consequently, rocks 

shattered during the initial fall from the source can be 

carried in the main body of the avalanche long distances 

without becoming widely separated. 

FOSSILS IN THE MATRIX FACIES 

Fossils of aquatic organisms were found in the 

matrix facies of the debris-avalanche deposits at eight 

localities in the Montague area. A sample of this facies 

obtained from about 0.5 m above the base of the deposit 

at a road cut on the west side of the Shasta River valley 

5 km northwest of Montague contained ostracodes, 

gastropods, bivalves, bones and scales of fish, and 

diatoms. According to Richard M. Forester, U.S. 

Geological Survey, the diatoms are present both in the 

matrix of the debris avalanche and in fragments of 

diatomite within the sample. Forester reported identifying the following ostracode taxa: Candona patzcuaro 

Tressler, Limnocythere itasca Cole, Limnocythere sappaensis Stapin, and Candona candata?. In addition, 

Cytheromorpha, sp. undetermined, was identified; this is 

an estuarine or shallow-marine ostracode of unknown 

age. Forester reported that the first four ostracode taxa 

are common inhabitants of shallow, often ephemeral, 

lacustrine environments. The species are not usually 

found in fluvial environments but could live in a lowenergy fluvial setting. They have a broad tolerance for 

temperature differences and could exist in temperate 

continental climate. These fossils, as well as those of 

gastropods, fish, and bivalves, probably were present in 

soft lacustrine and fluvial sediments on the floor of 

Shasta Valley and were incorporated when the debris 

avalanche moved across these deposits. 

The presence of these fossils in the matrix facies 

suggests that when the debris avalanche occurred, the 

floor of Shasta Valley was of low relief and locally 

occupied by small lakes and low-gradient streams. The 

valley floor then, as now, probably was flanked by alluvial 

fans debouching from adjacent bedrock hills. 

DISTRIBUTION OF THE 

DEBRIS-AVALANCHE DEPOSITS 

The debris-avalanche deposits were initially 

recognized only in the central part of Shasta Valley 

upstream from the Shasta River gorge, although it was 

inferred that the matrix facies of the avalanche moved 

downstream to the Klamath River (Crandell and others, 

1984). Subsequent investigations have revealed 

avalanche deposits in three additional areas: along the 

Distribution of the Debris-Avalanche Deposits 11 
Figure 9. Block of volcaniclastic deposits exposed in the south side of a borrow pit west of Lake Shastina dam. The block 

includes the deposits of pyroclastic flows, lahars, air-fall tephra, and alluvium. These units dip 2Q-25° west (toward the right), 

and are displaced by faults (dashed lines) that dip toward the east and west sides of the ridge. Arrows show the direction of 

displacement along the faults. 

lower west flank of Shasta Valley, within the Shasta River 

gorge, and in the part of Shasta Valley that lies north of 

Montague. 

Boulders of volcanic rock from Mount Shasta from 

less than a meter to several meters in diameter are 

scattered along the west side of Shasta Valley in areas 

adjacent to and upslope from the main body of the 

avalanche deposits. These boulders were noted at 17 sites 

at heights of as much as 95 m above the adjacent Shasta 

Valley floor, which is underlain by the matrix facies of the 

avalanche, but the matrix facies was not found in 

conjunction with any of the boulders. Although some 

blocks could have been carried to their present location 

by man, others obviously were not, inasmuch as they 

weigh many tons. The boulders are not a lag left by 

stream erosion and mass-wasting processes that removed 

a deposit that was originally tens of meters thick; 

evidently the boulders were left when the matrix facies 

that transported them drained away downslope. 

The altitudes of the highest boulders at 13 localities 

along the west side of Shasta Valley, projected eastward 

to the axis of the debris-avalanche deposits, form a 

reasonably straight north-sloping longitudinal profile 

from a point northwest of Weed to a point northwest of 

Montague (fig. 13, profile A-B). The slope of this profile 

roughly parallels the present surface slope of the matrix 

facies of the debris-avalanche deposits. North of the 

position of the Shasta River indicated on the profile, the 

tops of hills formed by the block facies are no higher than 

the crest of the moving debris avalanche, but to the south 

some hills and ridges extend as much as 90 m above the 

profile (fig. 13, profilesA-B and C-D). ProfilesA-B and 

C-D were constructed by assuming that the top of the 

moving debris avalanche was flat; however, it may have 

been somewhat convex. The heights of the profiles imply 

that many blocks were carried completely submerged, 

whereas others were only partly submerged and 

"floating" in the matrix facies. 

12 Debris Avalanche, Mt. Shasta, California, Debris-Avalanche Hazard Zonation 
Figure 10. A buried soil (behind the pick) is represented by 

yellowish-brown oxidation in the upper part of a pyroclasticflow deposit exposed in the borrow pit west of Lake Shastina 

dam. The deposit contains abundant breadcrusted bombs. 

ProfileA-B seems to record the crest of the matrix 

facies during flowage. The profile probably does not 

represent a static condition-that is, a time when the 

entire valley was simultaneously filled with mud and rock 

debris to that height. Instead, the profile probably marks 

the crest of a wave which left behind a veneer of matrix or 

scattered boulders, or both, after it passed downvalley. 

Such deposits are analogous to those left tens to 

hundreds of meters high on valley walls after the downvalley flowage of very large lahars at Mount Rainier, 

which originated in debris avalanches from the volcano 

(Crandell, 1971, p. 7). 

Although the block facies of the debris avalanche 

from ancestral Mount Shasta reached only to the vicinity 

of Montague, two lobes of the matrix facies continued 

farther. One extended northwestward into the Shasta 

River gorge, and the other moved into the northern part 

of Shasta Valley. A remnant of the matrix facies was 

found in a road cut on the west side of the Shasta River 

gorge about 3.5 km downstream from the upper end of 

the gorge. Upslope from the road, boulders of Mount 

Shasta rocks litter the side of the gorge to a height of 

about 43 m above the Shasta River. About 1 km farther 

downstream, similar boulders on the east side of the 

gorge extend from near river level up to a maximum 

height of about 88 m above the river. At this height, the 

gorge is about 500 m wide. The boulders here, some of 

which are as much as 1 min diameter, veneer the inside 

of a U-shaped bend in the gorge. 

The presence of these remnants of the matrix facies 

within the Shasta River gorge indicates that the gorge was 

virtually as deep when the debris avalanche occurred as it 

is today, and that the matrix facies was locally at least 88 

m deep as it flowed down the gorge. The absence in the 

gorge of a fill terrace formed by the matrix facies suggests 

that after the crest of the flow passed, most of it drained 

on downstream, leaving behind only bouldery veneers on 

the gorge walls. 

The northern lobe of the matrix facies moved into 

the part of Shasta Valley that lies north of Montague, 

crossed a low divide, and drained into a tributary of the 

Klamath River. The avalanche in this area is represented 

by abundant scattered boulders of volcanic rock from 

Mount Shasta. Although outcrops of the matrix facies 

were not recognized in that part of Shasta Valley, drillers' 

logs of water wells record the presence of 6-20 m of 

deposits which seem to represent that facies. No moundor hill-forming blocks of the avalanche were recognized 

in that area. The highest boulders in that part of Shasta 

Valley are at an altitude of about 823 m (2,700 ft), which 

is 15-30 m higher than the valley floor and about 8 m 

lower than the inferred maximum crest height of the 

moving matrix facies near Montague (fig. 13). At the 

north end of Shasta Valley, the matrix facies crossed a 

divide at an altitude of about 808 m (2,650 ft) and spilled 

into the north-trending valley of Willow Creek. Scattered 

boulders are present along Willow Creek at least as far as 

downst£·eam as a point about 2 km from the Klamath 

River. At that point, boulders of volcanic rock from 

Mount Shasta are as high as 17 m above Willow Creek. 

This location is about 52 km from the northern base of 

Mount Shasta and about 64 km from its summit; the 

boulders are at an altitude of about 670 m (about 2,200 

ft), which is about 3,600 m lower than the present summit 

of the volcano. 

The vertical distribution of the boulders in the 

northern part of Shasta Valley and the deposits 

encountered in drill holes indicate that the matrix facies 

was at least 50 m deep when it flowed northward across 

that area. Northward flowage ceased when the crest of 

the matrix facies progressed down Shasta Valley beyond 

Montague and moved into the Shasta River gorge; some 

of the matrix facies in the northern part of the valley then 

drained back to the southwest. North of Montague, the 

floor of Shasta Val1ey now slopes about 4 m/km 

Distribution of the Debris-Avalanche Deposits 13 
Figure 11. Block of volcaniclastic deposits exposed in a road cut along U.S. Interstate 5 west of Weed. The deposits dip east 

and are displaced along west-dipping faults. The pinkish-gray zone in the left center of the photograph is the thermally altered 

top of a pyroclastic-flow deposit. 

southwestward, which is a slightly lower gradient than 

that in the valley south of Montague (about 5 m/km). 

Although the matrix facies must have moved into 

the Klamath River along Willow Creek as well as the 

Shasta River, a road reconnaissance of the Klamath 

River valley between the mouth of the Shasta River and 

the Pacific Ocean failed to identify any deposits that 

contain rocks frpm Mount Shasta. 

EXTENT, THICKNESS, AND 

VOLUME OF THE DEPOSITS 

The area covered by the debris-avalanche deposits 

was initially estimated to be at least 450 km2 between the 

base of Mount Shasta, at an assumed altitude of 1,220 m 

(about 4,000 ft), and the head of the Shasta River gorge 

(Crandell and others, 1984). Subsequent investigations, 

however, show that the debris-avalanche deposits 

underlie a total area of at least 675 km2

• 

No remnants of the debris-avalanche deposits have 

been recognized on the lower slopes of the volcano 

northeast of Weed. The last blocks of the debris 

avalanche probably had so much kinetic energy that they 

moved to or beyond the foot of the volcano. However, 

postavalanche lava flows, domes, and deposits of pyroclastic flows and lahars blanket areas upslope from the 

southernmost blocks and may cover avalanche deposits 

in those areas. 

The eastward extent of the avalanche deposits is 

poorly known because of the presence of one or more 

postavalanche lava flows. Although hummocks of the 

debris avalanche protrude through the lava at a few 

places, none of these are more than a few kilometers 

from the boundary between the avalanche deposits and 

the lava. Nevertheless, the deposits are inferred to extend 

beneath the lava for two reasons. First, the scattered 

boulders of rock from Mount Shasta along the west side 

of Shasta Valley indicate that the avalanche was thicker 

during movement than it is now, and should have spread 

eastward to a height roughly similar to that on its west 

side (fig. 1). Second, logs of water wells drilled through 

the lava describe deposits, discussed subsequently, that 

14 Debris Avalanche, Mt. Shasta, California, Debris-Avalanche Hazard Zonation 
Figure 12. Blocks of andesite from Mount Shasta enclosed within and resting on matrix facies (yellow). Note the pervasive 

shattering of the blocks of red and gray andesite. This outcrop in a small quarry 1.3 km northwest of Montague was destroyed 

during quarrying operations in 1987. 

ALTITUDE 

I hi 

3400 

3000 

2600 

A 

Debris-avalanche 

Hill projected into 

Shasta hne of profile Parks 

Slough Little Shasta River Rwer j Creek 

------------- -- - -- - -------- - ------------------------- - - ---- -t -\-,---- - ------ --- - - -------

20 km from summit 

of Mt. Shasta 

B 

2200 ~~---------------------------------------------------------------------------------------L

Pre-Cretaceous rocks 

c D 

ALTITUDE 

lit) 

3800 

3400 

3000 

2600 

2200 ~-f---------------------'-------------___ B..:.... as--'a_lt_f_lo_w_,_( s_!_) --------------'---

Pre-Cretaceous rocks 

10 KILOMETERS 

5 MILES 

Figure 13. Longitudinal (A-8) and transverse (C-D) topographic profiles of the debris-avalanche deposits, and profile of the 

inferred upper surface of the avalanche (dotted line) during emplacement. The profiles are located on figure 1 and havb a 

vertical exaggeration of 5x. On profile A-8, note that the tops of hills north (to the left) of the Shasta River are all below the 

dotted line, but some to the south are above the line. 

Extent, Thickness, and Volume of the Deposits 15 
Table 2. Extent and thickness, inferred from well logs, and estimated volume of the debris 

avalanche in seven areas 

Area Extent (km2

) Maximum Average Volume (km8

) 

thickness ( m) thickness ( m) 

A ..................... 118.3 > 120 75 8.87 

B .. .................... 146.3 >105 100 14.63 

C ...................... 100.2 

D .......... ........... 132.1 

£ ...................... 8.0 

F ...................... 127.3 

G ..................... 34.0 

Totals ............. 676.2 

are inferred to represent the debris avalanche. Boulders 

derived from Mount Shasta were not found on surfaces 

east of the lava flows. 

An average thickness of 50 m was originally 

estimated from logs of water wells reported by Mack 

(1960), and the volume of the deposits was calculated to 

be at least 26 km3 (Crandell and others, 1984). This 

figure included a volume of 4 km 3 that is represented by 

the hills and ridges of the avalanche. 

In 1985, I inspected more than 200 drillers' logs of 

water wells in Shasta Valley at the Red Bluff office of the 

California Department of Water Resources. Most of 

these wells were completed after 1960, so they 

supplement those reported by Mack. Each well log was 

examined for possible deposits of the debris avalanche, 

and the well site was plotted on a base map of Shasta 

Valley. The debris avalanche is recorded in these logs by 

such terms as clay and gravel, clay and boulders, clay and 

rock, and lava and clay, and in some logs by interbedding 

of these materials with "lava rock." The "lava rock" 

recorded in some drill holes probably consists of blocks 

from Mount Shasta which are suspended in the matrix 

facies. 

The debris-avalanche deposits were divided into 

seven areas (fig. 14), and the extent, maximum and 

average thicknesses, and volume were estimated for each 

area (table 2). The area of greatest uncertainty is area F, 

much of which is directly underlain by postavalanche 

lava. One driller's log of a well in the northern part of 

area F records the debris avalanche ("Volcanic cong. and 

boulders") overlying Hornbrook Formation ("brown 

sandstone") at a depth of 143ft ( 43.6 m), and other wells 

in area F, after penetrating 100-359 ft (30.5-109.5 m) of 

lavas, evidently bottomed in debris-avalanche deposits 

more than 85 ft (26 m) thick. Some of the "lavas" 

penetrated in these wells probably are blocks within the 

debris-avalanche deposit. An average thickness of the 

deposit of 25 m in this area probably is a conservative 

estimate. Some drill holes in area F ended in "clay and 

gravel" or "clay and rock," terms that are inferred to 

represent debris-avalanche deposits. Other drill holes 

>77 75 7.52 

>61 50 6.61 

>26 25 0.45 

>26 25 3.18 

>21 15 0.51 

41.77 

ended in "clay," but it is not known whether this term 

refers to the matrix facies of the debris avalanche or to 

lacustrine deposits on the floor of the preavalanche 

Shasta Valley. 

The estimated volume of the debris-avalanche 

deposits beneath the present floor of Shasta Valley is 

41.77 km3

, to which is added a calculated volume of 4 

km3 represented by the hills and ridges formed by the 

block facies; thus, the total volume of the debrisavalanche deposits is estimated to be at least 45 km3

• This 

is the largest of the volcanic debris avalanches tabulated 

by Siebert and others (1987) and is also larger than a 

debris avalanche (described as a debris flow) from 

Popocatepetl, which has recently been described by 

Robin and Boudal (1987). That deposit, which is inferred 

to be 30,000--50,000 yr old, has an estimated volume of 

about 28 km3

, similar to that of the Socompa debris 

avalanche in Chile (Francis and Self, 1987). 

ORIGIN OF MORPHOLOGY AND 

DRAINAGE ON THE DEPOSITS 

The morphology of the Shasta Valley debrisavalanche deposits evidently has changed little since their 

formation at least 300,000 yr ago. The Shasta River has 

cut down about 22 m into the flat-topped matrix facies 

near Edgewood, and a similar amount northwest of 

Montague. The lack of a well-integrated drainage system 

and absence of deep and widespread dissection of the 

deposits are due to the gently sloping surface of the 

matrix facies and to the presence of resistant rock in the 

bed of the Shasta River at the head of the gorge 8.5 km 

northwest of Montague. This bedrock threshold is the 

base level for the upstream part of the Shasta River 

drainage basin. The relatively dry climate of the area, 

with average annual precipitation ranging from about 51 

em at Edgewood to about 30 em at Montague (Mack, 

1960, p. 14 ), also inhibits erosional modification of the 

deposits. 

16 Debris Avalanche, Mt. Shasta, California, Debris-Avalanche Hazard Zonation 
41° 

30' 

122°30' 

F 

8 

I 

A 

I 

I 

I 

~ \ 

10 KILOMETERS 

5 MILES 

MOUNT 

SHASTA 

Figure 14. Subdivisions used in estimating average 

thickness and volume of debris-avalanche deposits. The 

long dashed line is the approximate base of Mount Shasta; 

the dotted line is the approximate eastern margin of the 

avalanche deposits inferred from an eastward projection of 

maximum altitudes of boulders on the west side of Shasta 

Valley (see text). Shaded areas were not covered by the 

debris avalanche. 

The hundreds of hills and ridges of the debris 

avalanche are its most conspicuous morphologic 

features. Many hills and ridges seem to occur in clusters 

whose long axes are transverse to the direction of 

avalanche movement (fig. 15). The clusters are 

distributed along the axial part of the debris avalanche 

and are partly or wholly bordered by flat areas underlain 

by the matrix facies. Ridges within some clusters have 

trends that are transverse to the direction of avalanche 

movement, ridges in other clusters are mostly aligned in 

the direction of movement, and those in still other 

clusters seem to have random trends. 

A cluster of hills and ridges southwest of Weed 

seems to be an avalanche-marginal feature (fig. 16). 

Although most ridges in the cluster trend northward, a 

northeast-trending ridge borders the south edge of the 

cluster. The crest of this ridge is a little higher than 1,292 

m and is the highest point anywhere on the avalanche 

deposits. Drillers' logs indicate that the debris-avalanche 

deposits are about 67 m thick adjacent to the ridge. This 

cluster at the southwest margin of the debris avalanche 

may have been formed by part of the initial slide block 

from the volcano. A similar cluster of hills and ridges is 

not present near the base of the volcano at the northeast 

margin of the debris avalanche. 

The presence, within ridges, of intact stratigraphic 

successions of unconsolidated volcaniclastic deposits as 

well as large blocks of andesite indicates that the hills and 

ridges were not formed by the bulldozing up of 

accumulations of rock debris, but are masses that 

remained reasonably coherent as they moved downslope 

from the volcano. The local presence of veneers of the 

matrix facies on these blocks indicates that they were 

partly or wholly enclosed in matrix when the blocks were 

emplaced. The blocks now stand in relief not because of 

postavalanche erosion, but because after they came to 

rest, the more mobile matrix facies drained away from 

them. 

The drainage pattern on the surface of the debrisavalanche deposits in Shasta Valley probably originated 

during the final stages of emplacement as the matrix 

facies drained northward. One drainage route is 

represented by the flat, north-sloping surface of the 

matrix facies along the west side of Shasta Valley north of 

Gazelle, which has been modified locally by alluvial fans 

at the mouths of some east-flowing streams. A similar 

north-sloping surface is inferred to exist beneath postavalanche lava along the east side. 

The Shasta River flows northward along the west 

side of the block facies as far as a point about 3 km north 

of Edgewood, then turns and follows a northeastward 

course between parallel ridges formed by the block 

facies, as does Parks Creek. Beyond Lake Shastina, the 

Shasta River probably originJlly flowed northward along 

a matrix-drainage channel, perhaps joining Little Shasta 

River east of Steamboat Mountain. The Shasta River 

probably was diverted from this northward course by the 

Origin of Morphology and Drainage on the Deposits 17 
~Ovvls Head 

/ 

/ 

/ 

Lake Shastina 

- - --- Base of volcano 

----1..... ~ 

""' 

~ / ~ / ........____ __........-

------ 10 KILOMETERS 

5 MILES 

""' ~ 

\ 

\ 

I 

/ 

/ 

\ 

\ 

I 

I 

I 

I 

I 

Figure 15. Clusters of hills and ridges, indicated by dash-and-dot lines, in the proximal part of the debris-avalanche 

deposits. The short-dashed line indicates the western edge of postavalanche lava flows, and the dotted line is at a height 

similar to the inferred maximum height of the debris avalanche on the west side of Shasta Valley. The shaded part of Mount 

Shasta represents a possible source area of the debris avalanche on an ancestral volcano. 

postavalanche lava flows, which caused the river to find a 

new course diagonally back to the northwest across the 

block facies. East of Grenada, a matrix-drainage channel 

was followed by a northwesterly trending lobe of postavalanche lava. Farther north, Little Shasta River foiiows a 

westerly course that apparently also was established 

during the draining away of the matrix facies. 

Closed depressions locally are present on the 

surface of the debris-avalanche deposits. At least five of 

these that occur along the ridges that border the 

18 Debris Avalanche, Mt. Shasta, California, Debris-Avalanche Hazard Zonation 
northwest side of Lake Shastina are shown on the U.S. 

Geological Survey topographic map of the Lake Shastina 

15' quadrangle. Some of these depressions have closures 

of more than 20 m and are as much as 2 km long and 0.5 

km wide. Only one contains a lake. Other basins, some of 

which contain lakes, are present 3-11 km north and 6 km 

west of Lake Shastina. Lakes farther north and northwest 

are in man-made reservoirs or in depressions on the 

surface of a postavalanchc lava flow. 

The origin of the closed depressions is problematic. 

Debris avalanches that move in a dry or nearly dry state 

typically form deposits that have very rough surfaces, and 

closed depressions are common as part of the original 

constructional topography of the deposits. Thus, the 

presence of closed depressions in the deposits north and 

west of Lake Shastina would not be considered unusual 

except for evidence that the matrix of the avalanche was 

fluid and very mobile. Such a matrix seemingly should 

have filled depressions that are lower than the crestal 

height of the avalanche. 

If, as I believe, these depressions were formed after 

the bulk of the avalanche came to rest and the still-fluid 

matrix facies drained away, they probably originated in 

one or more ways. Some could have been formed by 

differential compaction of the avalanche deposits, 

although if this was an important process, the 

depressions seemingly should be more numerous and 

widespread than they are. Other closed depressions may 

be kettles left by the melting of blocks of glacier ice 

transported from the slopes of the volcano as part of the 

debris avalanche. In addition, some depressions could 

have been formed when blocks shifted apart from one 

another as they adjusted to local conditions of basal and 

lateral support after the matrix facies drained away. 

SOURCES OF WATER IN THE 

DEBRIS AVALANCHE 

Several lines of evidence indicate that the matrix 

facies of the debris avalanche was wet during movement. 

The amount of water required to mobilize this facies is 

suggested by a liquid limit of 22.5 percent, which was 

determined on a sample of matrix obtained from an 

outcrop 0.5 km west of Edgewood (fig. 5). If it is assumed 

that a water content of 22.5 percent was necessary for the 

matrix to move as a fluid, and that half of the debrisavalanche deposits is made up of the matrix facies, at 

least 5 km3 of water would have been required to 

mobilize the mass. This figure may be correct only within 

an order of magnitude, but it does suggest the 

involvement of a very large amount of water. 

Rock debris in the avalanche probably was wet 

from the start, and as the avalanche moved across the 

floor of Shasta Valley it could have incorporated bodies 

of surface water as well as saturated fluvial and lacustrine 

sediments. The ridges formed by the avalanche deposits 

adjacent to Lake Shastina are veneered with matrix 

facies, which locally surrounds blocks of coherent rock 

tens of meters in diameter. The presence of the matrix 

facies on those ridges, which include some of the last 

material to move from the volcano, implies that the 

source material was wet when the debris avalanche 

occurred. 

Several sources of water presumably existed on and 

within the pre-avalanche volcano. Probably the most 

important was subsurface water of meteoric origin, some 

of which could have been part of an active hydrothermal 

system. Water also could have been contributed by snow 

and ice to the parts of the avalanche that originated on 

the flanks of the volcano. If ancestral Mount Shasta had 

a crater lake, it also could have provided water to the 

initial part of the avalanche. However, a crater lake 

seemingly would have been drained during or shortly 

after the initial failure of the volcanic edifice, and 

therefore would not have contributed water to rock 

masses involved in subsequent failures. The presence of 

matrix facies on the proximal parts of the avalanche and 

evidence that much of this facies drained away at a late 

stage of avalanche emplacement suggest that the 

proximal parts were just as wet as the distal parts. If so, 

a crater lake evidently was not the only source of water. 

A large water content may not have been essential 

to the initial landslides from the volcano, or to the 

movement of the debris avalanche beyond the base of the 

volcano, but it clearly was instrumental in determining 

the distance of movement and the draining away of much 

of the matrix facies during the final stages of avalanche 

emplacement. 

CAUSE OF THE DEBRIS AVALANCHE 

Slope failures on volcanoes can be triggered by 

steam or magma explosions and by earthquakes, and 

such failures can trigger eruptions if magma is present 

within a volcano. Rocks representing juvenile magma 

were not recognized in the avalanche deposits, and no 

evidence has been found that the debris avalanche was 

associated with magmatic activity. The summit of Yellow 

Butte, at the north base of the volcano, was searched 

without success for the presence of directed-blast 

deposits that might have been genetically associated with 

the formation of the debris avalanche. 

The avalanche at Mount Shasta could have been 

triggered by one or more steam explosions within the 

volcano, like those that have triggered debris avalanches 

at other volcanoes during historic time. For example, at 

Bandai-san volcano in Japan, laterally directed steam 

explosions in 1888 caused a massive slope failure which 

Cause of the Debris Avalanche 19 
2 MILES 

20 Debris Avalanche, Mt. Shasta, California, Debris-Avalanche Hazard Zonation 
resulted in a debris avalanche with a volume of 1.5 km? 

(Nakamura, 1978). At Shiveluch volcano, U.S.S.R., a 

debris avalanche of similar volume in 1964 probably was 

caused by either an earthquake or volcanic explosion 

(Miller and Bogoyavlenskaya, 1983; Siebert, 1984). 

It is also possible that the debris avalanche at 

Mount Shasta was caused by an earthquake. At Mount 

St. Helens, a magnitude-5.2 quake was the direct cause of 

a debris avalanche in 1980, but the stability of the north 

flank of the volcano had been greatly reduced during the 

preceding two months by the intrusion of a dacite 

cryptodome. The north flank of ancestral Mount Shasta 

could have become unstable for a similar reason, but 

evidence is lacking. 

The stability of ancestral Mount Shasta could also 

have been affected by glacial erosion. The debris 

avalanche originated on the north flank of the volcano, 

where glaciers would have been largest, but this relation 

may only have been fortuitous. The extent of glaciers on 

ancestral Mount Shasta is not known, nor is it clear 

whether the debris avalanche occurred during a glacial or 

an interglacial period. The age limits of the debris 

avalanche are so broad that it could have occurred either 

during isotope stage 10 of the marine stratigraphic 

record (glacial, between 356,000 and 336,000 yr ago) or 

stage 9 (interglacial, between 336,000 and 276,000 yr 

ago )(Hays and others, 1976). 

INITIATION OF THE DEBRIS AVALANCHE 

AND EMPLACEMENT OF DEPOSITS 

The debris avalanche probably started as a rockslide or quickly spaced series of rockslides on the 

northwest flank of ancestral Mount Shasta. As the rock 

masses disintegrated during downslope movement they 

were transformed into one or more debris avalanches. 

Beyond the base of the volcano, the debris avalanches 

seem to have merged into a single very large mass which 

moved northward along the floor of Shasta Valley. 

Deceleration of this mass led to emplacement of the 

block facies, but the wet matrix continued to move 

downslope as a mudflow. 

A gradual transition probably occurred from rockslide to debris avalanche and from debris avalanche to 

Figure 16 (facing page). Topographic map of the 

northwestern part of the Weed 15' quadrangle, published in 

1954 by the U.S. Geological Survey, showing the 

morphology of the southwestern most cluster (dashed line) of 

hills and ridges of the debris avalanche . Note that most 

ridges in the cluster trend northerly, whereas those in the 

adjacent cluster, north of Boles Creek, trend westerly. The 

summit of the volcano is about 16 km east of Weed. The 

location of U.S. Interstate 5 west of Weed is shown by the 

dotted line. 

mudflow. Such transitions occurred in the debris 

avalanche at Mount St. Helens in 1980, which originated 

in three large slump blocks on the north flank of the 

volcano. These broke up during downslope movement to 

form debris avalanches. Even though the rock debris was 

wet, it was emplaced as a closely spaced series of debris 

avalanches, and the muddy matrix did not drain away 

immediately. Five hours after the debris avalanche came 

to rest, however, water began flowing from the deposits 

and, cutting down through and remobilizing the 

incoherent material, formed a very large mudflow which 

extended more than 120 km farther downvalley (Janda 

and others, 1981). At Mount St. Helens, less than 30 

seconds elapsed between the first and second collapses 

(Voight and others, 1981, 1983) and all the retrogressive 

collapses may have taken place within no more than a few 

minutes. The rockslides left a broad deep depression in 

the center of the volcano, open to the north, which 

resembles the so-called avalanche calderas (Siebert, 

1984) created by similar events at many other volcanoes. 

The debris avalanche at Mount Shasta probably 

also originated in a rapid succession of rockslides, each of 

which extended progressively deeper into the volcanic 

edifice as lateral support was removed. The 

northernmost blocks probably originated on the north 

flank of the ancestral volcano, and the blocks that are 

now near the base of the volcano may include parts of the 

southern flank. If so, the block facies of the avalanche 

deposits, from Montague to the base of the volcano, 

probably includes a north-to-south cross section through 

ancestral Mount Shasta. 

The clusters of hills and ridges that characterize the 

avalanche surface may each represent the collapse of a 

very large landslide block which subsequently broke up 

during downslope transport. An especially large block 

that remained almost intact is represented by the ridge 9 

km long that borders the northwest side of Lake Shastina 

(fig. 2). The slightly sinuous trend of this transverse ridge 

seems to indicate that velocity distribution in this part of 

the debris avalanche was almost uniform, despite an 

overall transport distance of more than 20 km from its 

probable source area on the volcano. The ridge may 

represent one of the last large collapses of the succession 

and probably came from what was then the south side of 

the volcano. The length of the ridge seems to be a 

minimum figure for the linear extent of the headwall left 

by the landslides. It is not known whether the slight 

sinuosity of the ridge reflects the shape of the headwall or 

was caused by deformation of the long slide block during 

movement. The proximity of the ridge to the base of the 

volcano implies that this part of the debris avalanche had 

less kinetic energy than preceding parts, perhaps because 

it did not descend from as great a height as the parts of 

the avalanche that preceded it. 

Initiation of the Debris Avalanche and Emplacement of Deposits 21 
The avalanche moved initially northwestward from 

the base of the volcano and thence generally northward 

along the floor of Shasta Valley in a massive wave 

consisting of water-saturated matrix carrying huge blocks 

of rock and poorly consolidated sediments. Volcaniclastic 

deposits that form ridges northwest of Lake Shastina and 

west of Weed are tilted and faulted but are right-side up, 

which suggests that these blocks were not overturned 

during transport. The avalanche slowed as it moved 

northward across the gently sloping valley floor, and the 

blocks were dropped as the carrying ability of the 

avalanche decreased, or when they became "grounded" 

on the underlying surface. The more mobile matrix facies 

continued northward along two routes into the Klamath 

River. Further drainage of the matrix downvalley 

lowered the surface of the debris avalanche in Shasta 

Valley, locally by at least as much as 95 m, so that blocks 

were left standing higher than the adjacent deposits of 

the matrix facies. 

No unequivocal evidence was found that, because 

of its velocity, the debris avalanche ran up onto 

topographic obstacles in its path. The maximum height of 

boulders along the west side of Shasta Valley, about 95 

m, could have been reached by run up if the avalanche 

had been traveling at a speed of 43 m/sec (about 155 

km/h). Those boulders, however, cannot be identified 

specifically as products of avalanche run up, and the fact 

that the highest of them seem to form an even, northsloping profile suggests that they probably represent the 

crest of a northward-moving wave. Furthermore, the 

principal direction of movement of the debris avalanche 

evidently was northward, coincident with the axis of the 

block facies, and velocities should have been greatest in 

this direction. The southern slope of Gregory Mountain 

was searched for topographically high boulders of Mount 

Shasta rock, which would constitute evidence of high run 

up and thereby a high speed. The highest boulders of 

Mount Shasta rock found on this slope, however, are only 

about 33 m above the adjacent surface of the matrix 

facies and are lower than the inferred maximum height of 

the matrix facies in this area during movement. 

The shape, size, and height of ancestral Mount 

Shasta either before or after the debris avalanche are not 

known. However, the altitude of the ancestral volcano 

can be approximated by making certain assumptions. It is 

pointed out in a subsequent section that representative 

debris avalanches with volumes of at least 1 km3 have an 

average height-to-length ratio of 0.075. If it is assumed 

that the end of the avalanche proper was at a distance of 

49 km from the summit, use of this ratio to calculate 

height indicates that the altitude of the ancestral volcano 

would have been about 4,453 m, only 39m higher than 

that of today. 

The approximate altitude of the source of the 

ridge-forming block at Lake Shastina can be calculated if 

it is assumed that this block is 25 km from its source, and 

that the base of the ridge is at an altitude of 723 m (the 

topographic base of the ridge, less 100 m for the inferred 

thickness of the avalanche deposit at that site). A 

calculation based on these values suggests that the source 

area of the block was at an altitude of about 2,600 m. 

COMPARISON OF WET AND DRY 

VOLCANIC DEBRIS AVALANCHES 

A wide variety in the proportion of water to rock 

debris probably exists among debris avalanches. At one 

end of the scale, some debris avalanches seem to have 

been virtually dry; with the addition of hot rock debris, 

these grade into pyroclastic flows. For example, a quick 

succession of three avalanches occurred about 300 yr ago 

at the Chaos Crags dacite domes in Lassen Volcanic 

National Park, Calif. (Crandell and others, 1974). The 

local presence of abundant prismatically jointed blocks of 

dacite in those debris-avalanche deposits, as well as rock 

debris that was thermally altered after it came to rest, 

suggests that some of the source material was hot (Crandell, unpub. data, 1972). Likewise, Kawachi (1983, p. 

173) inferred that the A.D. 170 Ohtsukigawa volcanic 

debris avalanche from Mount Tengudake in Japan 

moved as a "low temperature, dry particulate flow." 

A debris avalanche in 1984 from Ontake volcano in 

central Japan was triggered by an earthquake. Matsuda 

and Ariyama (1985) described the avalanche as "dry" 

and specified that "the interspace of blocks during 

movement was not water but air." A prehistoric debris 

avalanche from Lastarria volcano in Chile apparently was 

also dry but was highly mobile due to the entrainment of 

air (Naranjo and Francis, 1987). 

The 1888 debris avalanche at Bandai-san volcano 

in Japan contained water but probably was not saturated 

(Sekiya and Kikuchi, 1889). Nakamura (1978) referred 

to it as a volcanic dry avalanche and noted that the water 

present in the avalanche was not essential to movement. 

The 1980 debris avalanche at Mount St. Helens also 

contained water, but it, too, was not saturated (Voight 

and others, 1981, 1983). Wet debris avalanches like these 

can grade directly into lahars at their distal ends, as at 

Bandai-san; or the rock debris in them can be eroded and 

remobilized by water expelled after the avalanche comes 

to rest, as at Mount St. Helens. 

Other debris avalanches seem to have been so wet 

that water was an important factor in determining their 

behavior and mode of emplacement. The role of water in 

the debris avalanche from Mount Shasta has been 

discussed in earlier sections of this report. Another 

debris avalanche in which the fluid matrix drained away 

after the coarse component was emplaced occurred at 

Galunggung volcano in Indonesia. Jagger (1930) 

quoted Escher (1925) as follows: 

22 Debris Avalanche, Mt. Shasta, California, Debris-Avalanche Hazard Zonation 
"The hillocks represent the fixation of the last 

material of the slide, clots that remained standing higher, 

the principal mass having slowed down because of 

increasing bottom friction as it spread out fanwise and 

lowered, and as it lost part of its water. The finer mud 

and surface material flowed on down the slope, leaving 

the clots in relief where irregular clusters of boulders 

clothed with mud made the remnant hills. The largest of 

these were in the middle of the landslide fan and opposite 

the middle of the crater valley." 

Mounds on the Pungarehu lahar from Mount 

Egmont volcano on the North Island of New Zealand 

(Neall, 1979), which is now regarded as the deposit of a 

debris avalanche (Ui and others, 1986), also seem to have 

been left in relief when the wet matrix drained away 

(V.E. Neall, Massey University, New Zealand, written 

commun., 1987). 

Debris avalanches typically form hummockysurfaced deposits, but avalanches in which large blocks 

are absent may result in deposits with flat surfaces. The 

absence of such blocks may be due to pervasive 

fragmentation of rock during movement as a result of its 

low strength. In addition, some debris avalanches that 

originated in saturated rock, or became saturated during 

movement, have transformed directly into lahars whose 

deposits now have flat or nearly flat surfaces. The Osceola and Electron lahars, which originated at Mount 

Rainier, have such surfaces. Both are thought to have 

originated in avalanches of saturated, hydrothermally 

altered weak rock from the summit and upper slopes of 

the volcano (Crandell, 1971). Other lahars probably have 

formed in the same way, such as many of those at Mount 

Baker volcano (Hyde and Crandell, 1978) and at Mount 

Adams volcano (Vallance, 1986). 

Wet debris avalanches may be no more mobile 

than dry ones during their initial emplacement, but wet 

avalanches have the potential for extending as wet flows 

(mudflows) far beyond the end of the debris avalanche 

proper. Thus, debris avalanches from volcanoes in which 

rocks are saturated endanger larger areas than do those 

in which rocks are not saturated. Stratovolcanoes of the 

Cascade Range that bear glaciers and extensive snowfields are all presumed to be capable of producing wet 

debris avalanches, and such events have indeed occurred 

at some of these volcanoes in the past. 

HAZARD ZONATION FOR 

VOLCANIC DEBRIS AVALANCHES 

Debris avalanches are potential hazards at most if 

not all high, steep-sided volcanoes. The great height at 

which a falling or sliding rock mass can originate provides 

tremendous kinetic energy, which can cause the resulting 

debris to move far beyond the base of a volcano so swiftly 

that areas in its path can not be successfully evacuated 

after the debris avalanche begins. Some such events in 

the past evidently were not preceded by recognized 

warning signs or accompanied by volcanism. Likewise, 

future debris avalanches may occur without warning 

during intervals of volcano dormancy. 

The need to identify and evacuate threatened areas 

before debris avalanches occur suggests that these 

potential hazards should be assessed at volcanoes where 

they may occur. Hazard zonation for volcanic debris 

avalanches seemingly can best be accomplished by 

making analogies with similar events in the past. 

Scheidegger (1973) found a correlation between the 

volumes of 33 debris avalanches of nonvolcanic origin 

and their so-called "coefficients of friction" (f), which are 

derived from their height-to-length (H!L) ratios. Hsii 

(1975) subsequently referred to these ratios as 

"equivalent coefficients of friction." If an estimated 

volume is plotted on Scheidegger's curve, the 

corresponding value of f can be found and used in the 

equation: L = H/f. A disadvantage of this approach is 

that the maximum volume of a potentially unstable rock 

mass on a volcano is rarely possible to determine. 

A more convenient approach to determining 

possible avalanche length is based on the vertical distance 

through which a rock mass can be expected to drop, 

because length is proportional to this distance (Hsii, 

1975; Schuster and Crandell, 1984; Siebert and others, 

1987). On the basis of the H/L ratios of 11 debris 

avalanches with volumes greater than 1 km3

, Schuster 

and Crandell suggested that the length of an anticipated 

debris avalanche can be estimated by dividing its 

potential vertical drop (H) by an f value of 0.075. 

Siebert and others (1987) tabulated the volume, 

distance, and vertical drop of all reported volcanic debris 

avalanches and found that f values of avalanches with 

volumes greater than 1 km3 are 0.05-0.13 and average 

0.09. They proposed that the value of 0.05 be used for 

hazard zonation for a ''worst case scenario" for ''very 

large avalanches," but they did not discuss how the 

potential for such avalanches could be recognized. Their 

tabulation shows that only two reported volcanic debris 

avalanches in the world have so low a value of f. 

Subsequently, an f value of 0.03 was reported by Francis 

and Self (1987) for a debris avalanche from Socompa 

volcano. They attributed its unusually high mobility to 

reduction of friction caused by seismic ground shaking 

while the avalanche was moving. 

Most volcanic debris avalanches are quickly 

channeled into the heads of valleys that drain the volcano 

or extend into basins beyond the foot of the volcano. The 

distances to which debris avalanches can move down 

these depressions can be estimated from the equation L 

= H/f, in which H is the difference in altitude between 

the summit of a volcano and a point at an arbitrary 

Hazard Zonation for Volcanic Debris Avalanches 23 
Table 3. Potential lengths of debris avalanches in three directions from the summit of Mount 

Shasta, which has an altitude of 4,316 m 

[Values for height are the differences in altitude between the summit and points 20 km from it. Potential 

lengths shown in columns A, B, and C are based on f values of 0.05, 0.075, and 0.09, respectively] 

Direction from volcano Height (m) 

Northwest. .......................... 3,462 

South ................................ 3,316 

Southwest ........................... 3,487 

distance of 20 km downvalley or downslope. This 

distance is the median length of the 41 debris avalanches 

tabulated by Siebert and others (1987) that have volumes 

greater than 1 km3

• Various values off can be used. As 

noted above, a value of 0.05 is that recommended by 

Siebert and his colleagues for a "worst -case scenario;" a 

value of 0.075 was recommended by Schuster and Crandell (1984); and the value of 0.09 is the average of a 

group of 11 representative debris avalanches chosen by 

Schuster and Crandell, all of which have volumes greater 

than 1 km3

• The value of 0.09 also is the average of the 41 

known debris avalanches of Quaternary age (Siebert and 

others, 1987). Siebert and his colleagues noted that the 

average f value of 20 volcanic debris avalanches with 

volumes under 1 km3 is 0.13. This value is not used here 

because a conservative approach to hazard assessment 

would dictate the assumption that the volume of an 

anticipated debris avalanche will be at least 1 km 3

, unless 

a smaller potential volume could be unequivocally 

determined. The desirability of such a conservative 

approach is illustrated by the 1980 debris avalanche at 

Mount St. Helens, which had a volume at least two orders 

of magnitude larger than that anticipated (Miller and 

others, 1981, p. 798; Schuster and Crandell, 1984). 

In this connection, a practical problem in 

anticipating the volume of a debris avalanche is in 

determining not only the size of an initial slope failure 

but also the possible sizes of closely following failures 

that could result from removal of support by the initial 

collapse. For example, the initial failure at Mount St. 

Helens on May 18, 1980, was immediately followed by 

two more debris avalanches whose combined volume was 

more than double that of the first one (Glicken, 1986). 

Examples of Debris-Avalanche 

Hazard Zonation 

The approach used here to anticipate lengths of 

debris avalanches is applied, for the purpose of 

illustration, to Mount Shasta, Mount Rainier, and the 

pre-1980 Mount St. Helens. 

Potential avalanche lengths (km) 

A B c 

69 46 38 

66 44 37 

70 46 39 

Table 3 shows the distances to which future debris 

avalanches could travel at Mount Shasta, based on three 

values of f and on heights determined from the altitude 

difference between the summit and points 20 km downslope to the northwest, south, and southwest. The only 

previous debris avalanche recognized at Mount Shasta is 

the one described here. It is assumed that the length ( 49 

km) of that debris avalanche from the present summit of 

the volcano is represented by the most distant mound of 

rock debris from the volcano, which is 2.2 km northwest 

of Montague, even though the fluid matrix of the 

avalanche extended far beyond this point. The distance of 

49 km is 3 km greater than the predicted length of a 

debris avalanche if an f value of 0.075 is used (fig. 17), 

but is 23 km shorter than the predicted distance based on 

an f value of 0.05. 

Possible lengths of future debris avalanches at 

Mount Rainier, determined in the same way as that 

explained above, are shown in table 4. Debris avalanches 

in the past at Mount Rainier have quickly transformed to 

lahars which have been funneled down one or more of 

the deep canyons that head on the volcano, and the 

avalanche deposits themselves have not been identified. 

Although Siebert and others (1987, Appendix B, p. 455) 

listed the Greenwater lahar from Mount Rainier as a 

debris avalanche 70 km in length, I (Crandell, 1971, p. 

18) believe the deposits to have been emplaced by a 

lahar. 

At Mount St. Helens, the debris avalanche of May 

18, 1980 traveled 29 km from the summit of the volcano 

down the valley of the North Fork Toutle River (fig. 19). 

Table 5 shows that an f value of 0.075 predicts a 

debris-avalanche length in that valley of 33 km, assuming 

pre-1980 topography. Table 5 indicates that a debris 

avalanche with a volume of at least 1 km3 moving from 

the pre-1980 volcano southeastward down the Pine 

Creek valley could have extended into Swift Reservoir, 

and one moving southward down the Swift Creek valley 

could have reached both Swift Reservoir and Yale Lake. 

Although this approach would have permitted us to 

anticipate the length of the 1980 avalanche in the valley 

of the North Fork Toutle River, this distance is no longer 

24 Debris Avalanche, Mt. Shasta, California, Debris-Avalanche Hazard Zonation 
w 

Inferred end of block facies of 

debris avalanche from ancestral volcano 

o I 

Yreka 0 Montague 

Lake Shastina 

/~-----......~ 

oWeed I Mo""' Shasta \\ 

\ 6 / 

""' Mt Shasta0 '------

0 10 20 KILOMETERS 

~------'--,-------.! 

0 10 MILES 

122° 

Figure 17. Map of Mount Shasta volcano and adjacent 

area. Semicircles show the distal ends of possible future 

debris avalanches, which have volumes of at least 1 km3

, in 

three valleys heading on the volcano. The lengths are based 

on differences in height between the summit and points 20 

km down each valley and on a height-to-length ratio of 0.075. 

The inferred end of the block facies of the debris avalanche 

in Shasta Valley is shown for comparison. 

applicable because the summit of the volcano is lower. 

The present topography and geology of the volcano 

suggest that debris avalanches of very large volume will 

not descend its east, south, or west sides in the 

foreseeable future. 

DISCUSSION 

Factors that can be considered in choosing a value 

of f for hazard assessment at a volcano include the 

number of lives and value of property at risk downvalley, 

the possible volume range of future debris avalanches, 

and the topography of areas beyond the base of the 

volcano. A very conservative approach to predicting 

avalanche length would be to use an f value of 0.05. A 

somewhat less conservative approach, but one that 

perhaps is more pragmatic for long-term planning, would 

be to use an f value of 0.075, plus a safety factor that is 

commensurate with local conditions. A debris-avalanche 

hazard zonation map of areas downslope from a volcano 

could show various degrees of hazard that are based on 

different values of f. 

Factors that are not considered in this approach, 

but which will influence the lengths of debris avalanches, 

include width and shape of valleys and the volume and 

per haps also the nature of the rock debris. The water 

content of the rock debris probably also will determine 

whether the debris avalanche (1) stops abruptly, (2) stops 

but is soon remobilized in part to form a mudflow that 

extends farther downvalley, or (3) becomes transformed 

directly into a mudflow as it moves downvalley. The 

length that can be anticipated from the L = H/f formula 

represents only a downvalley distance that might be 

affected directly by a debris avalanche. For purposes of 

hazard assessment it should be assumed that a debris 

avalanche from a snow-covered stratovolcano will 

generate a mudflow, in one way or another, which will 

endanger valley floors tens if not hundreds of kilometers 

beyond the end of the debris avalanche proper. 

Mudflows represent a different and less dangerous 

kind of hazard than debris avalanches. Mudflows 

typically travel down valley floors or other depressions at 

speeds ranging from a few kilometers to a few tens of 

kilometers per hour, and, with adequate warning, people 

on valley floors can avoid mudflows by moving to higher 

ground. 

ACKNOWLEDGMENTS 

I am indebted to Robert L. Christiansen and C. 

Dan Miller for their encouragement and advice during 

this study and to Robert L. Schuster for helpful 

consultation in the field. Personnel of the State of California Water Resources Division in Red Bluff kindly 

offered the facilities of their office while I was inspecting 

logs of water wells. I also appreciate the interest and 

hospitality of Mr. Don Meamber of Montague, Calif., 

who permitted my repeated access to a critical exposure 

on his property. 

REFERENCES CITED 

Christiansen, 1982, Volcanic hazard potential in the California 

Cascades, in Martin, R.J., and Davis, J.F., eds., Status of 

References Cited 25 
Table 4. Potential lengths of debris avalanches from the summit area of Mount Rainier, 

which has an altitude of 4,392 m 

(Values for height are the differences in altitude between the summit and points on valley floors 20 km 

from it. Potential lengths shown in columns A, 8, and Care based on f values of 0.05, 0.075, and 0.09, 

respectively] 

47" 

Alder 

Reservoir 

Valley 

White River ....................... . 

Carbon River .................... .. 

Puyallup River .................... . 

Nisqually River ................... . 

Cowlitz River ..................... . 

0 Enumclaw 

Height (m) 

3,438 

3,779 

3,842 

3,718 

3,779 

Opackwood 

10 20 KILOMETERS 

10 MILES 

Figure 18. Map of Mount Rainier volcano and adjacent 

area. Semicircles show the possible len~ths of debris 

avalanches that have volumes of at least 1 km , in five valleys 

heading on the volcano. The lengths are based on 

differences in height between the summit and points 20 km 

down each valley and on a height-to-length ratio of 0.075. 

Potential avalanche lengths (km) 

A 8 c 

69 46 38 

76 50 42 

77 51 43 

74 50 41 

76 50 42 

volcanic prediction and emergency response capabilities 

in volcanic hazard zones of California: California 

Division of Mines and Geology Special Publication 63, p. 

II-43- II-65. 

Christiansen, R.L., Kleinhampl, F.J., Blakely, R.J., Tuchek, 

E.T., Johnson, F.L., and Conyac, M.D., 1977, Resource 

appraisal of the Mt. Shasta Wilderness study area, 

Siskiyou County, California: U.S. Geological Survey 

Open-File Report 77-250, 53 p. 

Crandell, D.R., 1971, Postglacial lahars from Mount Rainier 

volcano, Washington: U.S. Geological Survey 

Professional Paper 677, 73 p. 

Crandell, D.R., Miller, C.D., Glicken, H.X., Christiansen, R.L., 

and Newhall, C.G., 1983, Huge debris avalanche of 

Pleistocene age from ancestral Mount Shasta volcano, 

California: Geological Society of America, Abstracts 

with Programs, v. 15, p. 330. 

Crandell, D.R., Miller, C.D., Glicken, H.X., Christiansen, R.L., 

and Newhall, C.G., 1984, Catastrophic debris avalanche 

from ancestral Mount Shasta volcano, California: 

Geology, v. 12, p. 143-146. 

Crandell, D.R., Mullineaux, D.R., Sigafoos, R.S., and Rubin, 

Meyer, 1974, Chaos Crags eruptions and rockfallavalanches, Lassen Volcanic National Park, California: 

U.S. Geological Survey Journal of Research, v. 2, no. 1, p. 

49-59. 

Davies, T.R.H., 1982, Spreading of rock avalanche debris by 

mechanical fluidization: Rock Mechanics, v. 15, p. 9-24. 

Diller, J.S., and others, 1915, Guidebook of the western United 

States, Part D, The Shasta route and coast line: U.S. 

Geological Survey Bulletin 614, 142 p. 

Escher, B.G., 1925, L'eboulement prehistorique de Tasikmalaja et le Volcan Galunggung: Leidische Geologische Medeelingen, deel 1, aflevering 1, p. 8--21. 

Fenner, C.N., 1923, The origin and mode of emplacement of 

the great tuff deposit in the Valley of Ten Thousand 

Smokes: National Geographic Society Contributed 

Technical Papers, Katmai Series, no. 1, 74 p. 

Francis, P.W., Gardeweg, M., Ramirez, C.F., and Rothery, 

D.A., 1985, Catastrophic debris avalanche deposit of 

Socompa volcano, northern Chile: Geology, v. 13, p. 

600-603. 

Francis, Peter, and Self, Stephen, 1987, Collapsing volcanoes: 

Scientific American, v. 256, no. 6, p. 90-97. 

26 Debris Avalanche, Mt. Shasta, California, Debris-Avalanche Hazard Zonation 
Table 5. Potential lengths of debris avalanches from the summit area of the pre-1980 

Mount St. Helens volcano, which had an altitude of about 2,949 m 

rvalues for height are the differences in altitude between the summit and points 20 km downvalley 

from it. Potential lengths shown in columns A, 8, and Care based on f values of 0.05, 0.075, and 0.09, 

respectively] 

Valley Height (m) 

North Fork Toutle River ........ 2,462 

South Fork Toutle River ....... 2,571 

Kalama River ...................... 2,584 

Swift Creek-Lewis River ........ 2,797 

Pine Creek-Lewis River ........ 2,639 

Glicken, Harry, 1986, Rockslide-debris avalanche of May 18, 

1980, Mount St. Helens volcano, Washington: Santa 

Barbara, University of California, Ph. D. thesis, 303 p. 

Hays, J.D., Imbrie, John, and Shackelton, N.J., 1976, Variations 

in the Earth's orbit-pacemaker ofthe Ice Ages: Science, 

v. 194, no. 4270, p. 1121-1132. 

Hotz, P.E., 1977, Geology of the Yreka quadrangle, Siskiyou 

County, California: U.S. Geological Survey Bulletin 

1436, 72 p. 

Hsu, K.J., 1975, Catastrophic debris streams (sturzstroms) 

generated by rockfalls: Geological Society of America 

Bulletin, v. 86, p. 129-140. 

Hyde, J.H., and Crandell, D.R., 1978, Postglacial volcanic 

deposits at Mount Baker, Washington, and potential 

hazards from future eruptions: U.S. Geological Survey 

Professional Paper 1022-C, 17 p. 

Jagger, T.A., 1930, The crater of Galounggoung: The Volcano 

Letter, no. 286, p. 1-2. 

Janda, R.J., Scott, K.M., Nolan, K.M., and Martinson, H.A., 

1981, Lahar movement, effects, and deposits, in Lipman, 

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

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

Survey Professional Paper 1250, p. 461-486. 

Kawachi, Shimpei, 1983, Ohtsukigawa debris avalanche, Yatsugatake volcanic chain, central Japan: Geological 

Society of Japan Journal, v. 89, no. 3, p. 173-182. 

Mack, Seymour, 1960, Geology and ground-water resources of 

Shasta Valley, Siskiyou County, California: U.S. 

Geological Survey Water-Supply Paper 1484, 115 p. 

Masson, P.H., 1949, Circular soil structures in northeastern 

California: California Division of Mines Bulletin 151, p. 

61-71. 

Matsuda, Tokihiko, and Ariyama, Tomoo, 1985, Debris 

avalanche deposits of Ontake volcano, caused by the 

1984 western Nagano Prefecture earthquake-on the 

debris-spray zone: University of Tok.1'o Earthquake 

Research Institute Bulletin v. 60, pt. 2, p. 281-316 [in 

Japanese with English abstract]. 

Miller, C.D., 1978, Holocene pyroclastic-flow deposits from 

Shastina and Black Butte, west of Mt. Shasta, California: 

U.S. Geological Survey Journal of Research, v. 6, no. 5, p. 

611-624. 

Potential avalanche lengths (km) 

A 8 c 

49 33 27 

51 34 29 

52 34 29 

56 37 31 

53 35 29 

Miller, C.D., and Bogoyavlenskaya, G.E., 1983, Catastrophic 

directed blasts at Mount St. Helens volcano, Washington, 

and at Bezymianny and Shiveluch volcanoes, Kamchatka, 

U.S.S.R.: International Association of Volcanology and 

Chemistry of the Earth's Interior, XVIII General 

Assembly, Hamburg, Federal Republic of Germany, 

15-27 August, 1983, Programmes and Abstracts, p. 30. 

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. 

Mimura, Koji, Kawachi, Shimpei, Fujimoto, Ushio, Taneichi, 

Mizuho, Hyuga, Tadahiko, Ichikawa, Shigenori, and Koizumi, Mitsuaki, 1982, Debris, avalanche hills and their 

natural remanent magnetism-Nirasaki debris 

avalanche, central Japan: Geological Society of Japan 

Journal, v. 88, no. 8, p. 653-663 [in Japanese with English 

abstract]. 

Nakamura, Youichi, 1978, Geology and petrology of Bandai 

and Nekoma volcanoes: Sendai, Japan, Tohoku 

University Science Reports, ser. III, v. 14, no. 1, p. 

67-119. 

Naranjo, J.A., and Francis, Peter, 1987, High velocity debris 

avalanche at Lastarria volcano in the north Chilean 

Andes: Bulletin of Volcanology, v. 49, p. 509-514. 

Neall, V.E., 1979, Sheet P19, P20, and P21, New Plymouth, 

Egmont and Manaia, Geologic map of New Zealand: 

Wellington, New Zealand Department of Scientific 

Research, scale 1:50,000. 

Nilsen, T.H., 1984, Stratigraphy, sedimentology, and tectonic 

framework of the Upper Cretaceous Hornbrook 

Formation, Oregon and California, in Nilsen, T.H., ed., 

Geology of the Upper Cretaceous Hornbrook 

Formation, Oregon and California: Pacific Section, 

Society of Economic Paleontologists and Mineralogists, 

v. 42, p. 51-88. 

Nilsen, T.H., Barats, G.M., Elliott, M.A., and Jones, D.L., 1983, 

Geologic map of the outcrop area of the Hornbrook 

Formation, Oregon and California: U.S. Geological 

Survey Open-File Report 83-373, scale 1:62,500. 

References Cited 27 
Debris avalanche from Mount St Helens 

extended to this point in 1980 

10 MILES 

Spirit Lake 

Figure 19. Map of the pre-1980 Mount St. Helens volcano and adjacent area. Semicircles in five valleys show the possible 

lengths of debris avalanches, with volumes of at least 1 km3

, that could have originated at the volcano before the eruption on 

May 18, 1980 . The lengths are based on differences in height between the summit and points 20 km down each valley and 

on a height-to-length ratio of 0.075. The end of the May 18, 1980 debris avalanche in the North Fork Toutle River valley is shown 

for comparison. 

Peck, D.L., Imlay, R.W., and Popenoe, W.P., 1956, Upper 

Cretaceous rocks of parts of southwestern Oregon and 

northern California: American Associaton of Petroleum 

Geologists Bulletin, v. 40, p. 1968-1984. 

Robin, Claude, and Boudal, Christian, 1987, A gigantic 

Bezymianny-type event at the beginning of modern volcan Popocatepetl: Journal of Volcanology and Geothermal Research, v. 31, p. 115-130. 

Sarna-Wojcicki, A.M., Meyer, C.E., Bowman, H.R., Hill, N.T., 

Russell, P.C., Woodward, M.J., and Slate, J.L., 1985, 

Correlation of the Rockland ash bed, a 400,000-year-old 

stratigraphic marker in northern California and western 

Nevada, and implications for middle Pleistocene 

paleography of central California: Quaternary Research, 

v. 23, p. 236-257. 

Scheidegger, A.E., 1973, On the prediction of the reach and 

velocity of catastrophic landslides: Rock Mechanics, 

Springer-Verlag, v. 5, p. 231-236. 

Schuster, R.L., and Crandell, D.R., 1984, Catastrophic debris 

avalanches from volcanoes: Proceedings IV Symposium 

on Landslides, Toronto, v. 1, p. 567-572. 

28 Debris Avalanche, Mt. Shasta, California, Debris-Avalanche Hazard Zonation 
Sekiya, S., and Kikuchi, Y., 1889, The eruption of Bandai-san: 

Tokyo, Japan, Imperial University College of Science 

Journal, v. 3, pt. 2, p. 91-172. 

Siebert, Lee, 1984, Large volcanic debris avalanches-characteristics of source areas, deposits, and associated 

eruptions: Journal of Volcanology and Geothermal 

Research, v. 22, p. 163-197. 

Siebert, Lee, Glicken, Harry, and Ui, Tadahide, 1987, Volcanic 

hazards from Bezymianny- and Bandai-type eruptions: 

Bulletin of Volcanology, v. 49, p. 435-459. 

Ui, Tadahide, and Glicken, Harry, 1986, Internal structural 

variations in a debris avalanche deposit from ancestral 

Mount Shasta, California, USA: Bulletin of Volcanology, 

v. 48, p. 189-194. 

Ui, Tadahide, Kawachi, Shimpei, and Neall, V.E., 1986, 

Fragmentation of debris avalanche material during flowage-evidence from the Pungarehu Formation, Mount 

Egmont, New Zealand: Journal of Volcanology and 

Geothermal Research, v. 27, p. 255-264. 

Vallance, J.W., 1986, Late Quaternary volcanic stratigraphy on 

the southwestern flank of Mount Adams volcano, Washington: Boulder, University of Colorado, M.S. thesis, 

121 p. 

Voight, Barry, Glicken, Harry, Janda, R.J., and Douglass, P.M., 

1981, Catastrophic rockslide avalanche of May 18, 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. 347-377. 

Voight, Barry, Janda, R.J., Glicken, H., and Douglass, P.M., 

1983, Nature and mechanics of the Mount St. Helens 

rock-slide avalanche of 18 May 1980: Geotechnique, v. 

33,no.3,p.243-273. 

Williams, Rowel, 1932, Mount Shasta, a Cascade volcano: 

Journal of Geology, v. 40, p. 417-429. 

___ 1949, Geology of the Macdoel quadrangle: California 

Division of Mines Bulletin 151, p. 7-60. 

References Cited 29 

VOLCANIC ASH ANALYSIS 
VOLCANIC ASH ANALYSIS 

A sample of volcanic ash from a clast within the 

matrix facies of the debris avalanche at a locality 1.3 km 

northwest of Montague was examined by Andrei SarnaWojcicki (written commun., 1987) and his associates 

Charles Meyer and Robert Fehr, who reported the 

following results. Minerals identified in the whole sample 

include green hornblende, green to brown pleochroic 

hornblende, oxyhornblende, hypersthene, apatite (small 

crystals within glass shards), brown to reddish-brown 

biotite, clinopyroxene (?), and plagioclase feldspar. The 

green hornblende, biotite, hypersthene, and plagioclase 

feldspar phenocrysts are partly coated with glass. 

A heavy-mineral separate of the sample included 

the following: 

Hornblende 

Hypersthene 

Clinopyroxene 

Total 

Percent 

73 

24 

2 

99 

Glass shards in the 30-80-micrometer fraction of 

the sample are mostly clear, with angular bubble walls 

and bubble-wall junctions and straight ribs. Shards in the 

80-150-micrometer fraction are mostly tubular pumice 

with cylinder-, capsule-, or spindle-shaped vesicles. Some 

spindle-shaped vesicles are partly filled with fluid. Some 

shards have equant or spherical vesicles, and one shard of 

brown glass was noted. 

A probe analysis of the glass by Charles Meyer 

indicated the following chemical composition 

(recalculated to 100 percent): 

Percent 

Si02 78.35 

Al20s 12.33 

Fe20s 0.89 

MgO 0.18 

MnO 0.04 

CaO 0.87 

Ti02 0.14 

Na20 3.69 

K20 3.51 

Total 100.00 

Sarna-Wojcicki stated that the composition of the 

volcanic ash is closely similar to a sample of the Rockland 

ash bed obtained at a depth of about 59 m in a core at 

Tule Lake, Calif. He noted that samples of the ash have 

little or no biotite at most localities, but that a small 

amount of this mineral is present at the Tule Lake site. 

Sarna-Wojckicki suggested that the biotite is oogenetic 

and represents the most highly differentiated part of the 

magma that produced the Rockland tephra. 

32 Debris Avalanche, Mt. Shasta, California, Debris-Avalanche Hazard Zonation 

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