45° 

42' 

METRIC CONVERSION FACTORS 

Multiply 

foot (ft) 

cubic yard (yd3 ) 

mile (mi) 

square mile (mi2 ) 

cubic foot per second (ft3 /sec) 

cubic mile (mi3) 

By 

0.3048 

0.7646 

1.609 

2.590 

.02832 

4.17 

To obtain 

meter (m) 

cubic meter (m3 ) 

kilometer (km) 

square kilometer (km 2 ) 

cubic meter per second (m3 /s) 

cubic kilometer (km3) 

105' 

120' 110' I 

~ 49'---..:;: ----- L 115' _j_ ---------~ ~~Q - ------,T_L__________ -- I , _ _.) ' ' \ I ! , 

Spokane o I \ \ 

' "'- MONTANA \ I '\ I 

·Seattle 

I I 

l •Mount St He~~----l '7 He~ena \ 

---------- ') (, \ Portland ~ __ J 

/ \ r--------------- 0 

Salem 

OREGON 

/ ,, ---1'~ 1 .. ,...- I 

I I 

r 0 Boise \ 

i IDAHO \ 

I . ---- , I -------------~--- ------------~ 

0 100 200 300 MILES 

0 100 200 300 KILOMETERS 

COVER: North Fork Toutle River, June 30, 1980. Volcanic mud flow breccia 

and debris from the May 18, 1980 eruption of Mount St. Helens (in upper 

right) are as much as several hundred feet thick in the reach shown. 

Photograph by Austin Post, U.S. Geological Survey. 
Effects of Volcanism on the Glaciers of 

Mount St. Helens 

By Melinda M. Brugman and Austin Post 

Hydrologic Effects of the Eruptions of 

Mount St. Helens, Washington, 1980 

GEOLOGICAL SURVEY CIRCULAR 850--D 

1981 
United States Department of the Interior 

JAMES G. WATT, Secretary 

Geological Survey 

Dallas L. Peck, Director 

First printing 1981 

Second printing 1983 

Free on application to Distribution Branch, Text Products Section, 

U. S. Geological Survey, 604 South Pickett Street, Alexandria, VA 22304 
FOREWORD 

On May 18, 1980, after more than a month of earthquakes and eruptions, Mount St. Helens, in 

southwestern Washington, exploded in a volcanic eruption more violent than any in the conterminous 

United States during the 20th century. A lateral blast of hot gas and rock particles devastated an 

area of about 150 square miles on the northern side of the mountain knocking down trees to a 

distance of 15 miles. Several minutes later, a giant ash cloud rose to about 60,000 feet. Winds then 

carried the ash cloud across the United States, with heavy fallout and deposition in eastern 

Washington and parts of Idaho and Montana. Earlier, smaller eruptions deposited ash in western 

Washington and parts of Oregon and Canada. 

The hydrologic effects of the May 18 eruption have been both widespread and intense. During the 

eruption, a massive debris avalanche moved down the north flank of the volcano depositing about 3 

billion cubic yards of rock, ice, and other materials in the upper 17 miles of the North Fork Toutle 

River valley. The debris deposits are about 600 feet thick in the upper reaches of the valley. Following the avalanche, runoff from the melted glaciers and snow, and possible outflow from Spirit Lake, 

caused an extraordinary mudflow in the North Fork Toutle River. The mudflow shattered and 

uprooted thousands of trees, destroyed most of the local bridges, and deposited an estimated 25,000 

acre-feet of sediment in the Cowlitz River channel. A considerable amount of additional sediment was 

conveyed through the lower Cowlitz into the Columbia River where it deposited and formed a shoal 

that blocked the shipping channel. Mudflows also occurred in the South Fork Toutle River and in 

tributaries on the east flank of Mount St. Helens which enter Swift Reservoir. 

As part of a concerted Geological Survey effort to study the volcanic event and to identify potential 

hazards, Survey hydrologists have mounted an intensive program to document the hydrologic effects 

of the eruptions. The major initial hydrologic findings are reported in this circular series. Quick, 

useful assessment was made possible only because the Survey has long conducted extensive waterresources investigations in the affected areas of Washington, Oregon, and Idaho. Hence, there was a 

well-defined basis for identification and documentation of the types and magnitudes of hydrologic 

changes. 

The Geological Survey Circular 850, "Hydrologic Effects of the Eruptions of Mount St. Helens, 

Washington, 1980," consists of individually published short chapters that emphasize data collection 

activities, field observations, and initial comparisons of pre- and post-eruption conditions. The series 

will cover hydrologic events occurring on May 18 in the Toutle and Cowlitz River; physical alteration 

of the Toutle River system; the chemical and physical quality of precipitation, streams, and lakes 

affected by volcanic ash fall; ash-leaching studies; and Mount St. Helens glaciers. 

III 

Doyle G. Frederick 

Acting Director 

CONTENTS 

Page 

Abstract _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ D1 Specific Effects of Volcanic Activity on Glaciers-Cont. 

Introduction______________________________ 1 May 19 to October 6 ____________________ _ 

Background _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 1 Glacier Outburst Floods ____________________ _ 

Specific Effects of Volcanic Activity on Glaciers _ _ _ _ 4 Ash Blanket Effects ______________________ _ 

March 20 to May 17 _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 4 Discussion: Potential Hazards and Research 

May 18 _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 4 Opportunities __________________________ _ 

ILLUSTRATIONS 

Page 

FIGURES 1-3. Maps of Mount St. Helens: 

1. Before May 18, 1980, showing glaciers _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ D2 

2. After May 18, 1980, showing remains of glaciers_________________ 3 

3. Showing areas affected by blast - --------------------------- 4 

4-8. Photographs of: 

4. Mount St. Helens from the northeast before the 1980 eruptive activity _ 5 

5. Mount St. Helens from the northeast during period of deformation and 

minor eruptions preceding cataclysmic eruption of May 18, 1980--- 6 

6. Mount St. Helens from the northeast after the May 18, 1980, eruption-- 8 

7. Crater rim following the May 18, 1980, eruption, showing cross sections of 

glaciers _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 9 

8. Glaciers on Mount St. Helens following the May 18, 1980, eruption.____ 10 

TABLES 

Page 

TABLE 1. Glacier area, depth, and volume, before and after the May 18, 1980, eruption ________________________________________ _ D7 

2. Depths of glaciers on Mount St. Helens measured by radio-echo 

sounding ________________________________________ _ 7 

v 

Page 

D6 

7 

8 

10 

HYDROLOGIC EFFECTS OF THE ERUPTIONS OF MOUNT ST. HELENS, WASHINGTON, 1980 

EFFECTS OF VOLCANISM ON THE GLACIERS OF 

MOUNT ST. HELENS 

By MELINDA M. BRUGMAN and AusTIN PosT 

ABSTRACT 

The cataclysmic eruption of Mount St. Helens May 18, 1980, 

removed 2.9 km2 (about 0.13 km3

) of glacier snow and ice including a large part of Shoestring, Forsyth, Wishbone, Ape, Nelson, 

and all of Loowit and Leschi Glaciers. Minor eruptions and 

bulging of the volcano from March 27 to May 17 shattered 

glaciers which were on the deforming rock and deposited ash on 

other glaciers. Thick ash layers persisted after the May 18 

eruption through the summer on most of the remaining snow 

and ice, and protected winter snow from melting on Swift and 

Dryer Glaciers. Melting and recrystalization of snow and ice 

surviving on Mount St. Helens could cause and lubricate mudflows and generate outburst floods. Study of glaciers that remain on this active volcano may assist in recognizing potential 

hazards on other volcanoes and lead to new contributions to 

knowledge of the transient response of glaciers to changes in 

mass balance or geometry. 

INTRODUCTION 

This article briefly summarizes the effects 

which the 1980 eruptive activity of Mount St. Helens had on the snow and ice masses on the volcano. 

The volcanic activity from March 20 through October 6, 1980, is divided into three periods, and topics 

such as glacial outburst floods, mudflows, and ash 

blanket effects are discussed. Areas, depths, and 

volumes of ice before and after May 18, 1980, are 

displayed in two tables, indicating the magnitude 

of the explosion which removed 2.6 km3 (99 billion 

ft3

) of the volcanic cone and about 0.1 km3 (3.5 

billion ft3

) of glacier snow and ice within minutes. 

Studies of the role of snow and ice in the damaging 

effects of the 1980 eruptions is essential for three 

major reasons: scientific understanding, predictions of the effects of future eruptions of Mount St. 

Helens, and evaluating potential hazards presented by glaciers on other volcanoes. 

D1 

BACKGROUND 

The majestic cone of Mount St. Helens, which 

dominates the skyline of the southwestern Washington Cascade Range, has long been famous for 

its graceful symmetry. Prior to the 1980 volcanism, the mountain was covered with about 5 km2 

(1.9 mi2

) of glacier area, approximately 0.18 km3 

(0.043 mi3

) of snow and ice volume. Wishbone, 

Loowit, Leschi, Forsyth, Nelson, Ape, Shoestring, 

Swift, Dryer, Toutle, and Talus Glaciers formed 

the bulk of the ice cover, which also included a 

number of perennial snow and ice patches (fig. 1). 

Seasonal snow with depths from about 3 to 10m 

(10 to 33 ft) mantled the mountain. 

Earthquakes began shaking the volcano on 

March 20, 1980, increasing in frequency until 

March 26. On March 27, Mount St. Helens broke 

130 years of quiescence with a series of minor ash 

and steam eruptions. A small crater was formed 

near the summit, a large fracture split the mountain, and a large elliptical region on the north side 

of the volcano began to bulge at a rate of as much 

as 2 m per day. Steam and ash eruptions alternated with intervals of up to 2 weeks of little activity, while the swelling of the mountain's northern 

flank continued steadily. 

On the sunny morning of May 18, 1980, a 

magnitude-5 earthquake triggered a massive avalanche and eruption, and the northern portion of 

Mount St. Helens exploded. A horizontal blast devastated the countryside from almost 160° on the 

northwest, north, and east sides of the volcano, 

reaching the Green River Valley 24 km (15 mi) 

from the mountain. About 2.8 km3 of the mountain 

was removed (fig. 2). Blast and avalanche deposits, 

debris, and pyroclastic flow materials totalling 
Explanation 

[ill Glacier 

R Snow and 

LJSept.1975 air 

e Survey Point 

Out I i ne of moving 

--bulge (4/12- 5/1/80) 

t 

N 

0 

Contour interval 

FIGURE 1.-Mount :St. Helens before May 18, 1980, showing location of glaciers. 

about 2.5 km3 accumulated in the upper North 

Fork Toutle River valley. Mudflows, lubricated in 

part by the sudden melting of glaciers and seasonal snow, swept down the North and South 

Forks of the Toutle River and on down the Cowlitz 

River. Similar events occurred in the valleys 

draining the east flank of the mountain (fig. 3). 

The eruptive ash clouds reached heights of 20,000 

m (65,000 ft) piercing the troposphere, and dumping several centimeters of ash across eastern 

D2 
Explanation 

~Glacier ~ 

• Survey Point 

-·-Crater rim (6/80) 

t 

N 

0 

Contour 

" ( ...... L 

'- ...... tlimberline 

\ 

... --.-, 

(_, " I 

FIGURE 2.-Mount St. Helens after the May 18, 1980, volcanic blast. The locations of the remains of Wishbone Glacier, covered 

by debris, is indicated by query. 

Washington and parts of Idaho and Montana. A 

cloud of fine ash encircled the world. 

Subsequent eruptions on May 25, June 12, July 

22, and August 7 were much smaller in size than 

D3 

that of May 18. Plug domes rose in the main crater 

and were blown off during later eruptions. With 

these eruptions, new pyroclastic flows lapped over 

older flows which covered 7 km of slope between 
,-------::;.,-------,----------,---------------.---------------,--- 46°25 1 

FIGURE 3.-Mount St. Helens area, Washington, showing areas affected by the May 18, 1980, volcanic blast (shutter pattern) and 

areas covered by volcanic debris (stipple pattern). Heavy lines indicate areas of mudflows and flooding in stream valleys. 

the volcano and Spirit Lake (fig. 3). Minor spillovers of hot material fell on the upper slopes of the 

remaining glaciers during these events, generating mudflows of much smaller volume than those 

which occurred on May 18. 

SPECIFIC EFFECTS OF VOLCANIC 

ACTIVITY ON GLACIERS 

The eruptions and other types of volcanic activity before, during, and after the cataclysmic May 

18 eruption had very different effects on the glaciers. Therefore, discussion of these effects is here 

divided into three periods: March 20 to May 17, 

May 18, and May 19 to October 6. 

MARCH 20 TO MAY 17 

Growth of the prominent volcanic bulge and 

continuing earthquakes during this period did not 

affect terminal positions or flow of the glaciers, 

except in the immediate vicinity of the growing 

bulge (fig. 1). Deformation, earthquakes, and oversteepening caused intricate crevassing and ice avalanches on the bulge which affected portions of 

Forsyth, Loowit, Leschi, Wishbone, Nelson, and 

upper Shoestring and Toutle Glaciers (fig. 1). Ice 

velocities measured on lower Shoestring Glacier 

from February 23 to May 16, 1980, disclosed no 

major changes from the pre-eruptive velocities of 

19 em (7.5 in.) per day. Ash layers deposited from 

March 27 to May 17 around the mountain generally ranged in thickness from a trace to 3 em (1.2 

in.) and were interbedded with snow. Photographs 

taken before and during this period show the extent of changes which occurred (figs. 4 and 5). 

MAY 18 

The explosive eruption of Mount St. Helens on 

May 18 removed about 70 percent of the ice volume 

on the volcano (table 1). Large parts of Wishbone, 

Forsyth, Nelson, Ape, and Shoestring Glaciers and 

all of Loowit and Leschi Glaciers (figs. 1 and 2) 

were blasted away, melted, or displaced downvalley as shattered blocks. Some iceblocks were 

carried by floods and mudflows down the Toutle 

River valley and many kilometers past the end of 

the debris pile (fig. 3). About 0.13 km3 (0.032 mi3

) of 

ice volume was lost (table 1); much of this was 

probably melted quickly and contributed to the 

lubrication of the Toutle River debris and 

mudflows. Ice fragments pelted survivors on the 

fringes of the initial blast. Two months later, some 

blocks of ice still remained buried in the debrisflow deposit on the North Fork Toutle River. 

D4 
Forsyth, Nelson, Ape, and Shoestring Glaciers 

were left beheaded, with their accumulation areas 

removed (fig. 2). 

In addition to the explosive removal of ice, pyroclastic flows melted or eroded snow, firn, and 

minor amounts of ice from the surface of the remaining glacier ice (table 2). Pyroclastic flows 

melted about 6 m (20ft) of snow and firn off Shoestring Glacier, generating large mudflows that 

swept down the valleys of the Muddy River and 

Pine Creek. Melting of portions of Nelson and Ape 

Glaciers caused mudflows which entered Smith 

Creek. Pine Creek, Muddy River, and Smith Creek 

mudflows deposited more than 14 million m3 

(11,000 acre-feet) of sediment and water in Swift 

Reservoir soon after the May 18 eruption began. 

This volume is about 3 times larger than the estimated surface melt on the parts of Nelson, Ape, 

and Shoestring Glaciers that remained following 

the blast, so much of the water must have been 

derived from melting of the upper removed portions, perhaps combined with melting of seasonal 

snow. Other large mudflows were generated from 

ice melted from Toutle and Talus Glaciers. Ash, 

pumice, and mudflow deposits approximately 1 to 

3 m (3 to 10 ft) thick covered the remaining glaciers on the volcano (figs. 6, 7, and 8). 

The changes in each glacier resulting from the 

May 18 eruption can be observed by comparing 

figures 1 and 2, and are summarized in tables 1 and 

2. Ice-volume data reported in these tables required estimates of ice thickness. Many of these 

Removed May 18, 1980 

FIGURE 4.- View of Mount St. Helens from the northeast before the 1980 eruptive activity. Dashed line marks boundary of area 

removed by the May 18, 1980, blast. U.S. Geological Survey photograph 67L7-14, by Austin Post, September 18, 1967. 

D5 
Dogs Head 

FIGURE 5.-View of Mount St. Helens from the northeast during the period of deformation and minor eruptions preceding the 

cataclysmic eruption of May 18, 1980. The area of mountain bulging to the north is outlined. Note the extensive crevassing 

of glaciers on this bulge. Much of the snow and ice is darkened by ash fall. U.S. Geological Survey photograph 80SZ-140 by 

David Hirst, May 1, 1980. 

estimates were based on comparison with results 

obtained at other glaciers. Some glacier thickness 

measurements were obtained during June and 

July 1979 and 1980 using radio-echo sounding 

measurements; these are summarized in table 2. 

MAY 19 TO OCTOBER 6 

During this period of the Mount St. Helens volcanic activity, deep rills and channels formed on 

the remaining glaciers and snow patches. These 

erosional features were the result of small streams 

cutting into and undermining the insulating ash 

deposits, commonly eroding through the snow and 

D6 

into the ice below. The rills may have been formed 

by rainfall on the easily eroded ash deposits. Alternatively, rills may have resulted from the melting of snow and ice owing to heat transfer from 

pyroclastic flow deposits on portions of the glaciers, some of which were still warm in August 

1980. With the removal of the insulating ash, deep 

rills quickly developed on the glacier surfaces. 

Rills are particularly prominent on Toutle and Talus Glaciers (fig. 7). 

Following the May 18 eruption, large ice avalanches frequently fell into the crater where the 

pulverized ice soon melted. The remaining ice cliffs 
TABLE 1.-Glacier area, depth, and volume, before and after the May 18, 1980, eruption 

AREA 

Before Removed Percent 

Glacier (km2

) (km2) removed 

Forsyth - ------ 0.87 0.65 75 

Wishbone------ .83 .79 95 

Shoestring - - - -- .64 .43 68 

Ape---------- .45 .11 24 

Loowi t - - - - - - - - .43 .43 100 

Swift-- - ------ .36 .03 7 

Nelson-------- .29 .12 41 

Toutle -------- .28 .02 4 

Leschi -------- .26 .26 100 

Talus--------- .23 0 0 

Unnamed (1) --- .16 0 0 

Dryer --- - ---- .14 .02 14 

Unnamed (2) --- .08 0 0 

Total ---- 5.02 2.86 

TABLE 2.-Depths of glaciers on Mount St. Helens 

measured by radio-echo sounding 

Number of Average 

Altitude measure- centerline 

Glacier (m) Date ments depth (m) 1 

Shoestring -- 1630-1880 06/08179 14 56 

1560 07/20/80 2 29 

1610 07/20/80 2 42 

Forsyth - --- 1640 07/20/80 1 24 

Swift------ 2270 07/20/80 3 21 

1 Depths were estimated by comparison with other glaciers and by radio-echo 

sounding measurements attempted on Shoestring, Forsyth, and Swift Glaciers. Safe 

working places for radio-echo sounding were limited. Shallow debris-entrained ice, 

numerous crevasses, and a thick ash cover caused many spurious reflections. Therefore, most of the radio-echo sounding depths are rather ambiguous. 

slowly moved away from the crater rim due to 

glacier flow and melting. The removal of the accumulation areas of glaciers such as Forsyth and 

Shoestring may result in stagnation and retreat, 

but the insulating effect of the thick ash and pyroclastic materials will allow some of the remaining 

ice to persist for several years. 

GLACIER OUTBURST FLOODS 

Outburst floods may be caused by the sudden 

release of water accumulating in crevasses, potholes, or caverns in glaciers. An example observed 

in August 1980 of such ponding on Shoestring Glacier was a small lake at an altitude of 2,000 m 

(6,550 ft) with a volume of approximately 3,500 rna. 

Volcanic heating similar to that which generates 

jr,kulhlaups (outburst floods) in Iceland may occur, 

but has not yet been documented on Mount St. 

DEPTH 

Estimated 

Average VOLUME 

Before After Before Removed Percent 

(m) (m) (km3

) (km3) removed 

45 20 0.039 0.035 90 

45 5 .037 .037 99 

40 30 .026 .020 77 

35 30 .016 .006 37 

30 0 .013 .013 100 

25 25 .009 .001 11 

35 30 .010 .005 50 

30 15 .008 .004 50 

30 0 .008 .008 100 

30 15 .007 .003 43 

20 20 .003 0 0 

15 15 .002 0 0 

15 15 .001 0 0 

0.179 0.132 

Helens. Outburst floods on Shoestring Glacier during the summer of 1980 did not appear to be as 

large as those during the previous July and August, but they occurred more frequently, generating small mud and rock flows about every 5 to 10 

minutes. Some mudflows were initiated by watersaturated ash decoupling from steep snow and ice 

slopes. The mudflows totaling about 800 rna (28,000 

fta) of muddy water, ash, pumice, and rock debris, 

were about 1 m deep during rock flow. 

By late August through early October 1980, at 

least one set of mud and rock flows, depths 2 to 3 

m and total volume about 4,800 rna (170~000 fta), 

was observed on Shoestring Glacier each day. 

These travelled at speeds of about 3 km/hr (2 

mi/hr) and carried large amounts of volcanic debris down the headwaters of the Muddy River. 

Mudflows were also observed coming from other 

glaciers on the volcano, especially the Toutle, Talus, and Swift Glaciers. These presented hazards 

throughout the summer in the narrow canyons below these glaciers. 

The rate of discharge of the Shoestring Glacier 

outlet streams during July 1980 was quite similar 

to that observed during July 1979, when the flow 

was 0.23 rna/sec (8 fta/sec); in July 1980, the flow 

was 0.18 rna/sec (6 fta/sec). Although the area of 

the glacier was reduced about 70 percent by the 

May 18 eruption, the stream discharge was reduced only slightly. Perhaps this was because the 

area removed was at high altitude where less ablation normally occurs. Another possibility is that 

D7 
the terminus stream only reflected local melt, 

while most of the upper glacier melt was discharged as ground water flow or outburst floods. 

Increased melt due to thin volcanic ash cover and 

the scouring of surface streams would also increase the runoff from the remaining ice. 

Both before and after the eruptions, the Shoestring Glacier outlet streams did not emerge from 

the end of the glacier, in contrast to most other 

glaciers. The streams begin above the altitude of 

2,000 m (6,500 ft), and follow the sides of the glacier, joining about 200m (650ft) downvalley from 

the terminus which lies at the altitude of 1,520 m 

(5,000 ft). The bedload in the outlet streams was 

high before and after the eruption, filling approximately 50 percent of the stream depth. The glaciers on Mount St. Helens typically flow over lahar, 

pumice, and other volcanic deposits that are 

highly permeable; thus, most of the glacier melt 

water becomes ground water flow, except in areas 

where the streambed is relatively impermeable 

glacier ice or old lava flows. 

ASH BLANKET EFFECTS 

Preliminary results from ash studies by Carolyn 

Driedger, U.S. Geological Survey, indicate that a 

trace to 1 mm (.04 in.) of ash increases the rate of 

snowmelt by as much as 20 to 30 percent, but an 

Former Summit f 2950m 

,,-------.... , ... , 

/New Summit ~ 

" 

... ~ 2549 m 

FIGURE 6.-View of Mount St. Helens from the northeast following the May 18, 1980, eruption. All remaining snow and glacier 

ice has been covered by ash and other volcanic materials, but the tongue of Forsyth Glacier is clearly discernible below and 

to the right of Dogs Head. Timberline road and parking areas, deeply buried under ash and debris, are outlined. U.S. 

Geological Survey photograph 8059-227, by Austin Post, July 24, 1980. 

D8 
ash thickness of 10 to 25 mm (.4 to 1.0 in.) or more 

insulated the snow, reducing melt. Ash depths on 

the Swift Glacier average 1,300 mm (51 in.). Most 

of the glaciers remained hidden from view until 

late summer when crevasses in the snow and ice 

finally broke through the overlying ash cover; a 

good example of the difficulty in locating the glaciers is shown in figure 8. The ash formed a strong 

insulating blanket; 5 m (197 in.) of previous winter 

snow remained on the lower glacier in August 

1980. Where deposited on snow or ice, ash deposits 

thinner than a few centimeters (or about an inch) 

tended to collect in small clumps. Thin ash distributions over snow and ice became progressively 

more discontinuous or uneven in thickness with 

time. 

At the ash/snow interface, an unstable, watersaturated layer had developed by July 1980. Snow 

had recrystallized in the wet equitemperature environment into rounded ice pellets, 1 to 3 mm (.04 

to .12 in.) in size under the insulating ash layer. 

The ice pellets and layers were overlain by about 

100 mm (4 in.) of water-saturated volcanic ash 

with little or no internal cohesion. During July, 

above the saturated layer, there was about a meter 

of relatively dry ash and pumice with deeper deposits in some areas. The ash remained throughout the summer except where deeply gullied by 

surface streams or removed by minor mudflows. 

The surface ash generally formed a well-cemented, 

somewhat impermeable barrier to vertical water 

transport. In many places, the ash bridged crevasses as they opened in the ice beneath. 

By early October 1980, ice grains just beneath 

thick ash deposits had grown to unusually large 

sizes of 4 mm to 10 mm (0.2 to 0.4 in.). A layer a few 

FIGURE 7.-View of crater rim following the May 18 eruption. Arrows above the far rim indicate exposed cross sections (left to 

right) of Forsyth, Nelson, Ape, and Shoestring Glaciers. The horizontal arrow on the near slope indicates typical rills and 

channels through ash deposits into Talus Glacier (note crevasse further down the glacier). Dark (wet) streaks to the left 

indicate the ash-covered Toutle Glacier. U.S. Geological Survey photograph 80S9-115, by Austin Post, July 24, 1980. 

D9 
FIGURE 8.-Glaciers on Mount St. Helens following the May 18, 1980, eruption are difficult to detect because of the heavy cover 

of ash and other volcanic debris. The arrow points to typical glacier crevasses, penetrating through the ash from Swift Glacier 

on the south side of the mountain. U.S. Geological Survey photograph by Robert Krimmel, 80S9-303, August 7, 1980. 

centimeters (about an inch) thick consisting of 

faceted ice crystals developed as the ice grains 

grew together during late summer and early fall. 

The uppermost ice crystals with hexagonal cross 

sections averaged 10 mm (.4 in.) in diameter and 

were very clear. 

DISCUSSION: POTENTIAL HAZARDS 

AND RESEARCH OPPORTUNITIES 

The thick volcanic deposits overlying the glaciers of Mount St. Helens provide a large mass of 

well-lubricated, unstable debris which may slide 

DlO 
off the glacier surface, generating mudflows downvalley. Small mudflows occurred throughout the 

summer of 1980. In addition, large mudflows could 

be generated by movement of pyroclastic or lava 

flows over deep winter snow, firn, or glacier surfaces; the opening of a fumarole under a glacier; or 

the release of a glacier outburst flood. Recrystallizing ice layers may form unstable zones beneath 

thick ash deposits, possibly causing slides and 

mudflows. Glacier outburst floods are potentially 

damaging. The potential hazard for flooding downvalley will increase in winter months as snow and 

rime accumulate on the volcano. In particular, 

Swift and Yale Reservoirs, about 15 km (9 mi) 

from the south flank of the volcano, could be subject to large floods and mudflows lubricated by 

melt water from deep winter snow and glacier ice. 

Glaciers on volcanoes may provide precursory 

information on volcanic activity through evidence 

of changed patterns of crevasses, velocities, thicknesses, outburst floods, or glacier runoff. Studies 

on the effects of Mount St. Helens eruptions will 

greatly aid in the understanding of potential hazards presented by glacier-covered volcanoes such 

as Mount Hood, Mount Shasta, Mount Rainier, 

Mount Lassen, Mount Baker, Mount Adams, Glacier Peak, and others. 

Research topics of hydrologic or natural-hazard 

importance can now be addressed on Mount St. 

Helens. These include understanding the generation of volcanic mudflows and floods, and transient flow dynamics of glaciers with grossly 

changed geometries and mass balances. In particular, the flow of glaciers over volcanic beds can be 

modelled more realistically using data from cross 

sections of glacier beds along the crater rim and 

along deeply dissected glacier margins. 

D11