The Crater of Mt. St. Helens -- 20 Years After

For

Mount St. Helens National Volcanic Monument

U. S. Forest Service

By

Charles H. Anderson, Jr.

Mark R. Vining

October 14, 1999

International Glaciospeleological Survey

547 SW 304 St., Federal Way, WA 98023

INTRODUCTION

After 20 years, nature has made incredible changes to the crater of Mount St. Helens.  It has come to possess active glaciers, snow and ice caves, deep erosion, geothermal springs, and bacterial colonies growing in a rainbow of colors. During the period of 1981 through 1999, members of the International Glaciospeleological Survey (IGS) in cooperation with the U.S. Forest Service studied and documented these changes. The glaciers have developed from repeated years of compacted snow and rock accumulation in the crater.  The caves are a system of melt passages in snow and ice that have collected through the 1980s and 1990s.  Precipitation water flowing across and through the crater fill material has both leached and transported chemicals and sediment, making the area into an array of dangerous canyons with scalding hotsprings that host weird and slimy (but beautiful) bacteria colonies.

Mount St. Helens is an active andesite-dacite volcano, which is currently in a semi-dormant state after a catastrophic explosive eruption in May 1980 and subsequent eruptions through 1986.  The crater is occupied by a dacite dome, which plugs the volcanic vent.  The crater floor has been progressively covered by a layer of snow, firn, and glacier ice since 1981.  Heat, steam, and volcanic gases from the crater fumaroles have melted over 2,415 m (7925 ft) of cave passage in the crater ice mass.

CRATER ICE

The shade of the steep crater walls to the east, south, and west largely protects the crater ice accumulation (Fig. 1). The ice body is a new glacier that continues to grow. It is not readily apparent from a distance that glacier ice is present in the crater, but bergschrunds and crevasses can be seen around the crater walls where snow and rockfall debris collect continually (Fig. 2). The snows stacking higher each year have locally compressed the lower layers into dense, crystalline glacier ice. The ice body shows signs of flow around both sides of the Lava Dome and is flowing out toward the north side of the dome.  Because of the severely limited quantity of ice density data, the mean ice density and therefore the total mass of ice can only be estimated (see graph, Fig. 3). The ice density at the base of the crevasses has been measured at 0.85 g/cc, corresponding to solid glacier ice.

CRATER FIRN CAVES

Bodies of ice exposed to conditions above freezing tend to develop internal systems of water drainage. Flow of warm air subsequently expands these conduits, forming interconnected sub-glacial cave passage networks.  The crater firn caves of Mount St. Helens are located on the east, south, and west flanks of the Lava Dome, in the crater floor ice body.  The passages form a circumferential pattern around the dome, with entrance passages on the dome flanks. Sub-glacial fumaroles and relatively warm air currents form and maintain the cave passages.

The cave system is dynamic, responding to competition between ice body growth and decay processes.  Ablation, caused by outside air circulation, gradually enlarges cave passages. Basal melting of the whole ice body tends to diminish the caves. Increases in geothermal activity in the crater are expressed by the rapid enlargement of steam cups, dome-shaped melt pockets localized near fumaroles. The Mount St. Helens caves are in balance with the present geothermal heat release, because they have reached an overall stable morphology. Individual passage can be observed to change over time, but the system as an whole remains much the same.

The cave floors consist of loose dome rock, and in places the dome surface. Room sizes range from 4.6 by 4.6 by 2.4 m high to 12 by 24 by 6 m high. Most caves occur in the presence of fumaroles. Other caves form adjacent to the crater and dome walls where melt water undermines the ice body (Fig. 4).

CRATER FLOOR ENVIRONMENT

The most active processes taking place at the surface are (1) continued landslides from the steep crater walls, (2) fluvial down-cutting in the stream courses that have established themselves across the crater floor, and (3) debris flows (lahars) developing from slope failure in the debris on the north crater floor. Perhaps the most significant subsurface process acting on the crater contents is percolation of meteoric water and consequent alteration and leaching of the volcanic minerals.

The present crater floor is underlain by porous and permeable, loose landslide debris from the 1980 eruption. Subsequent eruptions, including the later part of the May 1980 eruption, covered the landslide surface with new pumice and ash deposits, smoothing the landslide topography and creating what is known today as the Pumice Plain. The first lava domes formed at the top of the volcanic conduit, and were partially destroyed by explosions. After the October 1980 eruption, dome growth gradually covered the fringe areas of crater-filling rockfall talus cones, which are intermingled with accumulating snow.  The whole body is insulated and compacted by its own mass. Later eruptions have added only minor amounts to the pile. The most significant addition to the post-1986 crater floor environment, therefore, is the accumulated ice and rock debris.

Several small, intermittent surface streams flow from the crater ice body. Snowmelt and rain percolating through fractures in the Lava Dome and through the permeable crater fill, rise in geothermal springs that feed the crater streams.

Nearly two decades of precipitation and runoff have eroded and leached material from the thick, unconsolidated mass of volcanic and slide debris on the crater floor. Streams draining the crater have cut through this material and formed steep-walled canyons with unstable slopes (for example, Loowit Canyon on the northeast flank of the crater. Workers in the crater have observed repeated slope failures and small slides.

In August 1997, a slump developed at the Breach in a mass of saturated crater floor material, forming a scarp and debris flow lahar (Fig. 5). The semicircular scarp fed a debris flow extending downslope about 0.7 km. Geothermal springs rise from several places around the scarp walls and the dissected pumice plain below it.  Calcium carbonate is rapidly precipitating from water that rises from these springs. It has formed short-lived but beautiful deposits of flowstone, dripstone, helictites (cored by bacterial filaments), and cave pearls.(Fig. 6).  Water percolating through freshly exposed loose material in the slump debris supplies nutrients to the streams, supporting growths of red (sulfur), orange (iron), and minor green (chlorophyllic) bacterial slime. (Fig. 7).  Within a one-year period, calcite stalactites and stalagmites grew to a maximum observed size of 27 cm in diameter and 30 cm in length, and calcite cave pearls grew to 3.6 cm in diameter.

FIGURE CAPTIONS

 Figure 1. A view into the crater from the north in summer 1998. Firn reaches around both sides of the Lava Dome (center). Avalanche debris falls from the far crater walls onto the ice mass and becomes incorporated into it. Calcite deposits form in streams rising from thermal springs on the crater flats.   Debris flows below the dome can form in the unstable, loose crater floor (center foreground).

Figure 2. Sketch map on Mount St. Helens topographic base showing the location of the crater firn caves, crater ice body, and incipient headwall glaciers in Mount St. Helens Crater, October 2000.

Figure 3. Graph of ice volume accumulation in Mount St. Helens crater from 1980 through present. Figures are estimated from snowfield mapping, ice density readings, and direct thickness observations.

Figure 4. A typical ice cave passage in Mount St. Helens crater firn adjacent to the slopes of the Lava Dome. Dacite boulder debris forms talus. The scalloped ceiling and walls continually drip cold water during warm seasons, but ice stalactites form at these points during the winter.

Figure 5. View of the August 1997 slump scarp that generated a small lahar on the crater floor north of the dome. The calcite stream issues from springs in this feature.

Figure 6. Calcite deposits from a geothermal stream below the lahar tongues on the crater floor of Mount St. Helens crater. These stalactites form on bacterial strands hanging from rock projections in the stream. The water temperature is 24° C.

Figure 7. Pre-August-1997 geothermal stream issuing from Mount St. Helens crater. The white calcite coating of the stream highlights the streambed in the photo.  Red bacteria line the shores.