Quaternary Research 61 (2004) 325 – 329
www.elsevier.com/locate/yqres

Short Paper

Posteruption glacier development within the crater
of Mount St. Helens, Washington, USA
Steve P. Schilling, a,* Paul E. Carrara, b Ren A. Thompson, b and Eugene Y. Iwatsubo a
b

a
U.S. Geological Survey, 1300 SE Cardinal Court, Vancouver, WA 98683, USA
U.S. Geological Survey, Mail Stop 980, Denver Federal Center, Denver, CO 80225, USA

Received 3 May 2003
Available online 23 January 2004

Abstract
The cataclysmic eruption of Mount St. Helens on May 18, 1980, resulted in a large, north-facing amphitheater, with a steep headwall
rising 700 m above the crater floor. In this deeply shaded niche a glacier, here named the Amphitheater glacier, has formed. Tongues of icecontaining crevasses extend from the main ice mass around both the east and the west sides of the lava dome that occupies the center of the
crater floor. Aerial photographs taken in September 1996 reveal a small glacier in the southwest portion of the amphitheater containing
several crevasses and a bergschrund-like feature at its head. The extent of the glacier at this time is probably about 0.1 km2. By September
2001, the debris-laden glacier had grown to about 1 km2 in area, with a maximum thickness of about 200 m, and contained an estimated
120,000,000 m3 of ice and rock debris. Approximately one-third of the volume of the glacier is thought to be rock debris derived mainly from
rock avalanches from the surrounding amphitheater walls. The newly formed Amphitheater glacier is not only the largest glacier on Mount
St. Helens but its aerial extent exceeds that of all other remaining glaciers combined.
Published by University of Washington.

Introduction
Prior to the cataclysmic eruption of May 18, 1980, Mount
St. Helens in south-central Washington was a symmetrical
cone with a summit elevation of about 2950 m. The volcano
supported 13 small glaciers with a combined surface area of
about 5 km2 (Fig. 1) (Brugman and Post, 1981). The eruption
removed about 70% of the ice volume on the volcano. It
completely destroyed the Loowit and Leschi Glaciers, most
of the Wishbone Glacier, and large headward regions of the
Shoestring, Ape, Nelson, and Forsyth Glaciers (Brugman and
Post, 1981). Much of this glacier ice was probably melted
immediately and contributed to the lahars that occurred
during the eruption (Brugman and Post, 1981).
The eruption resulted in the formation of a large, northfacing amphitheater, about 2 km wide, with a steep headwall rising about 700 m above the crater floor (Fig. 2).
The steep amphitheater walls are the source of rockfall and
rock and snow avalanches that commonly sweep across

* Corresponding author. Fax: (360) 993-8980.
E-mail address: sschilli@usgs.gov (S.P. Schilling).
0033-5894/$ - see front matter. Published by University of Washington.
doi:10.1016/j.yqres.2003.11.002

and cover large areas of the crater floor (Mills, 1992). The
elevation of the highest point along the crater rim is about
2550 m.
This report documents the formation of a new glacier,
here named the Amphitheater glacier, that has formed in the
crater of Mount St. Helens. Within the two decades following the May 18, 1980, eruption, the glacier has formed in
the deeply shaded niche at the base of the north-facing
amphitheater wall and two tongues of ice extend from the
main body and flow around the east and west sides of the
270-m-high lava dome (Fig. 2). Precipitation feeding the
glaciers in the Mount St. Helens area is dominated by
Pacific storms between November and March (accounting
for 70% of the annual precipitation.). During this period the
Aleutian Low directs storm tracks east across the continent
between about 45 and 50jN (Barry and Chorley, 1976).
Hence, moisture-laden winds moving inland from the Pacific Ocean rise on the flanks of Mount St. Helens and
release large amounts of orographic precipitation. Much of
this precipitation falls as snow because of the seasonal
distribution of the moisture.
Climatic records for the Mount St. Helens area are
lacking; however, precipitation is probably heavy on the

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S.P. Schilling et al. / Quaternary Research 61 (2004) 325–329

Fig. 1. Mount St. Helens before the May 18, 1980, eruption, showing location and aerial extent of glaciers (modified from Brugman and Post, 1981).

Fig. 2. Aerial view of the Mount St. Helens crater looking south showing the newly formed Amphitheater glacier at the base of the amphitheater wall. Note
rock glaciers near the eastern and western margins of the glacier. Photographed by Bergman Photographic Services (under contract to U.S. Geological Survey)
on October 5, 2000.

 S.P. Schilling et al. / Quaternary Research 61 (2004) 325–329

flanks of the volcano. Climatic records from the Paradise
Ranger Station in Mount Rainier National Park, about 75
km north-northeast of Mount St. Helens and at an elevation (1690 m) similar to the crater floor, serve as the
closest proxy of climate data for Mount St. Helens. The
Paradise Ranger Station records show an average annual
precipitation of about 295 cm and an average annual
snowfall of 1730 cm (Western Regional Climate Center,
unpublished data accessed Nov. 16, 2001, on the World
Wide Web at URL http://www.wrcc.dri.edu/index.html).
Precipitation at higher elevations on both Mount St.
Helens and Mount Rainier may be greater than that
indicated by the Paradise records due to the orographic
effect.

Development of the Amphitheater glacier within the
Mount St. Helens crater
The Amphitheater glacier probably formed because of
its location at the base of the deeply shaded, north-facing
amphitheater wall. In addition, snow accumulation on the
glacier is probably significantly augmented by snow
avalanches from the amphitheater walls and by blowing
snow from the flanks of the volcano. Furthermore,
melting of the glacier is probably inhibited by its debris
cover. In a study of the effects of superglacial debris on
the Eliot Glacier of Mt. Hood, Oregon (about 100 km

327

south-southeast of Mount St. Helens), Lundstrom et al.
(1993) concluded that the debris thickness at which the
ablation rate is less than for that clean ice can be as little
as 2 cm.
The development of the Amphitheater glacier within the
Mount St. Helens crater is documented by a series of aerial
photographs taken at various times between July 1980 and
September 2001. The photos consist of prints and (or)
diapositives taken in color or black and white that range
in average scale between 1:7000 and 1:16,000. These
photos were inspected visually using a stereo viewer with
3  eyepieces and a photogrammetric stereoplotter.
Chronology of glacier development
Subsequent to the May 18, 1980, eruption, episodic
dome growth and associated pyroclastic eruptions continued until 1986 (Swanson and Holcomb, 1990). The elevated temperatures associated with these eruptions
inhibited the buildup of snow and ice on the crater floor
for several years. For instance, aerial photos from March
1981 indicate that snow was present on the ground outside
the crater, but little snow was present on the crater floor.
The few patches of snow that were present were deposited
by frequent dirty-snow avalanches off the amphitheater
walls (Mills, 1992) and were probably quickly melted.
Aerial photos from August 1985 indicate that a small
snowbank may be present against the southwestern am-

Fig. 3. Extent of ice on Mount St. Helens. Area in white shows location and extent of glaciers as of September 2001. Glacier area, inside the crater, is about 1
km2, whereas the glacier area outside the crater is 0.52 km2.

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S.P. Schilling et al. / Quaternary Research 61 (2004) 325–329

phitheater wall; however, the debris cover makes identification uncertain. Aerial photos from August 1986 indicate
a small ice patch in this same part of the amphitheater. No
crevasses can be distinguished in these photos. A large
snow bank is present on the crater floor between the south
amphitheater wall and the dome and extends along the east
and west sides of the dome as seen on aerial photos taken
in 1989 and 1990. No crevasses can be distinguished in
these photos. Aerial photos of September 1996 reveal a
small glacier in the southwest portion of the amphitheater
containing several crevasses and a bergschrund-like feature
at the head of the ice body. The boundary of the glacier at
this time is hard to distinguish because of a light snow
cover but probably is about 0.1 km2. By September 2001
the debris-laden glacier had grown to about 1 km2 in area,
with a maximum thickness of about 200 m. Ice-containing
crevasses extended from the area south of the lava dome
along the amphitheater wall and around both the east and
the west sides of the lava dome (Fig. 2). Rock glaciers
from the amphitheater walls encroach upon these ice
tongues.
Estimation of glacier volume
In order to estimate the volume of the Amphitheater
glacier, digital elevation models (DEMs) were constructed
from 1:24,000 scale digital contours compiled from aerial
photographs taken in 1980 (published USGS topographic
map) and from 1:12,000 scale aerial photographs of September 2000. Global positioning system (GPS) instruments
were used to occupy control points of an established
deformation GPS network both within the crater and on
the flanks of the volcano in order to reference the aerial
photographs to a ground coordinate system.
We use a methodology that Mills (1992) established
while calculating estimates of snow volume within the
crater. This methodology accounts for dome growth and
debris accumulation from the nearly continuous rock fall
from the amphitheater walls in summer and fall. Our volume
estimate of the Amphitheater glacier was obtained by
subtracting the 1980 posteruption DEM surface from the
2000 DEM surface using geographic information system
(GIS) software (Thompson and Schilling, in press), summing separately the absolute value of the negative values
(erosion), where the 2000 surface lies beneath the 1980
surface, and the positive values (deposition), where the 2000
surface lies above the 1980 surface. Each summed result
was multiplied by the area of a single grid cell (100 m2),
yielding an estimate of total erosion and total deposition
volumes for the span of time between 1980 and 2000. An
estimate of the dome volume (Swanson and Holcomb,
1990) was subtracted from the total positive summed value.
The volume of rock eroded from the crater walls (total
negative summed value), assuming a closed system, was
then subtracted from the remaining positive volume. The
remaining volume is an estimate of the volume of snow and

ice. Hence, the volume of the Amphitheater glacier is
estimated at 120,000,000 m3 of ice and rock debris. Approximately one-third of the volume of this glacier is
thought to be rock debris, derived mainly from rock
avalanches from the surrounding amphitheater walls
(Thompson and Schilling, in press).

Discussion
Two glaciers are known to have formed within several
decades in the crater that resulted from the 1912 cataclysmic
eruption of Mt. Katmai on the Alaskan Peninsula (Muller
and Coulter, 1957), similar to the events and formation of
the Amphitheater glacier in the crater of Mount St. Helens.
The Katmai eruption destroyed the summit of the glacierclad volcano and created a crater 4 km wide and 800 m
deep. Because parts of the new crater wall were still above
snowline two small glaciers were formed between 1923 and
1930 on landslide benches, below the north and south sides
of the crater rim. Within about 30 years these two glaciers
were 50 to 80 m thick and the largest was about 1 km in
length (Muller and Coulter, 1957).
As previously noted, the eruption of May 18, 1980,
completely destroyed the Loowit and Leschi Glaciers, most
of the Wishbone Glacier, and large headward regions of the
Shoestring, Ape, Nelson, and Forsyth Glaciers (Brugman
and Post, 1981). By September 2001 the Shoestring,
Nelson, Forsyth, and Dryer Glaciers no longer existed,
while the Ape Glacier shrank significantly from its preeruption size (Fig. 3; Table 1). It is interesting to note that in
the brief period since the eruption the newly formed
Amphitheater glacier is not only the largest glacier on
Mount St. Helens, but its aerial extent exceeds that of all
the other remaining glaciers combined.

Table 1
Areas of glaciers on Mount St. Helens before the May 18, 1980, eruption
and on September 2001
Glacier

Before May 1980a
(km2)

September 2001
(km2)

Forsyth
Wishbone
Shoestring
Ape
Loowit
Swift
Nelson
Toutle
Leschi
Talus
Unnamed 1
Dryer
Unnamed 2
Amphitheater
Total

0.87
0.83
0.64
0.45
0.43
0.36
0.29
0.28
0.26
0.23
0.16
0.14
0.08
—
5.02

0
0
0
0.12
0
0.10
0
0.07
0
0.14
0.06
0
0.03
1.00
1.52

a

Data from Brugman and Post (1981).

 S.P. Schilling et al. / Quaternary Research 61 (2004) 325–329

329

Conclusions

Acknowledgments

During the 20 years following the cataclysmic eruption
of May 18, 1980, a new glacier, here named the
Amphitheater glacier, has formed within the crater of
Mount St. Helens. The glacier has formed at the base
of the north-facing amphitheater wall and two tongues of
ice-containing crevasses extend from the main body
around the east and west sides of the lava dome. The
glacier probably formed as a result of its location at the
base of the deeply shaded amphitheater wall, and the
large annual snowfall that is augmented by both avalanches from the amphitheater walls and by blowing
snow from the flanks of the volcano. In addition, the
ablation rate on the glacier is reduced by its debris cover.
Development of the glacier was delayed as the crater
floor remained warm for several years after the eruption.
By September 2001 the debris-laden Amphitheater glacier
was about 1 km2 in area with a maximum thickness of
about 200 m and contained an estimated 120,000,000 m3
of ice and rock debris. The Amphitheater glacier is now
the largest on Mount St. Helens. Future work will involve
mass-balance studies of the Amphitheater glacier by
developing DEMs from new photography, and comparing
it to the DEM developed from the September 2000
photography. In addition, GPS receivers planted in the
ice will help constrain ice-flow models and surfaces
created from aerial photos taken between 1980 and
2000 will provide estimates of rates of accumulation of
rock and ice.

The authors thank J.A. Messerich, T.J. Casadevall, R.B.
Waitt, and E.W. Wolfe (U.S. Geological Survey) whose
knowledge and ideas contributed to this paper. Previous
versions of this manuscript benefited substantially from
reviews by J.S. Walder and C.L. Driedger (U.S. Geological
Survey) and N.J. Finnegan and G. Roe (University of
Washington).
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