Posteruption glacier development within the crater of Mount St. Helens, Washington, USA

Transcription

1 Quaternary Research 61 (2004) 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 a U.S. Geological Survey, 1300 SE Cardinal Court, Vancouver, WA 98683, USA b U.S. Geological Survey, Mail Stop 980, Denver Federal Center, Denver, CO 80225, USA Received 3 May 2003 Available online 23 January 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 km 2. By September 2001, the debris-laden glacier had grown to about 1 km 2 in area, with a maximum thickness of about 200 m, and contained an estimated 120,000,000 m 3 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 km 2 (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) address: sschilli@usgs.gov (S.P. Schilling). 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 /$ - see front matter. Published by University of Washington. doi: /j.yqres

2 326 S.P. Schilling et al. / Quaternary Research 61 (2004) 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.

3 S.P. Schilling et al. / Quaternary Research 61 (2004) 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 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 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 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 Glacier area, inside the crater, is about 1 km 2, whereas the glacier area outside the crater is 0.52 km 2.

4 328 S.P. Schilling et al. / Quaternary Research 61 (2004) 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 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 km 2. By September 2001 the debris-laden glacier had grown to about 1 km 2 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 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 m 2 ), yielding an estimate of total erosion and total deposition volumes for the span of time between 1980 and 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 m 3 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 1980 a (km 2 ) Forsyth Wishbone Shoestring Ape Loowit Swift Nelson Toutle Leschi Talus Unnamed Dryer Unnamed Amphitheater 1.00 Total a Data from Brugman and Post (1981). September 2001 (km 2 )

5 S.P. Schilling et al. / Quaternary Research 61 (2004) Conclusions 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 km 2 in area with a maximum thickness of about 200 m and contained an estimated 120,000,000 m 3 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. Acknowledgments 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). References Barry, R.G., Chorley, R.J., Atmosphere, Weather, and Climate, third ed. Methuen, London. Brugman, M.M., Post, A., Effects of volcanism on the glaciers of Mount St. Helens. U.S. Geological Survey Circular 850-D. Lundstrom, S.C., McCafferty, A.E., Coe, J.A., Photogrammetric analysis of surface altitude change of the partially debriscovered Eliot Glacier, Mount Hood, Oregon, U.S.A. Annals of Glaciology 17, Mills, H.H., Post-eruption erosion and deposition in the 1980 crater of Mount St. Helens, Washington, determined from digital maps. Earth Surface Processes and Landforms 17, Muller, E.H., Coulter, H.W., Incipient glacier development within Katmai Caldera, Alaska. Journal of Glaciology 3, Swanson, D.A., Holcomb, R.T., Regularities in growth of the Mount St. Helens dacite dome, In: Fink, J. (Ed.), The Mechanics of Lava Flow Emplacement and Dome Growth. International Association of Volcanology and Chemistry of the Earth s Interior. Proceedings in Volcanology, vol. 2. Springer-Verlag, Berlin, pp Thompson, R.S., Schilling, S.P., in press. Photogrammetry. In: Dzurisin, D. (Ed.), Volcano Geodesy: Exploring Unstable Ground, Praxis, Chichester, UK.