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CURRENT DEVELOPMENTS IN THE CRATER OF
MOUNT ST. HELENS, WASHINGTON
1980 THROUGH 2,000
For Mount St. Helens Crater Permit Application
Prepared 12/10/2000
By
Charles H. Anderson, Jr.
Dr. Mark R. Vining
International Glaciospeleological Survey
547 SW 304 St., Federal Way, WA 98023
INTRODUCTION
This paper describes crater firn caves and recent developments in the crater of Mount St. Helens, Washington. The caves are a system of melt passages in firn ice that have collected since the mid-1980s. Glaciologists mentioned geothermal firn caves in other volcanic craters (Kiver and Mumma, 1975; Kiver and Steel, 1975) and sometimes have dealt with their origin. No one, however, has provided detailed observation of the evolution of a geothermal ice cave system.
Figure 1. Map The steams Caves Mount Rainier
This investigation involved systematic photo-reconnaissance, mapping, and sampling from 1981 through 2,000 by members of the International Glaciospeleological Survey (IGS) in cooperation with the U.S. Forest Service's Mount St. Helens National Volcanic Monument. This paper documents a unique opportunity to study the interaction of geothermal energy on the accumulation of alpine snowpack from its inception after a major eruption in 1980.
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 as early as 1986. 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. The caves are in approximate balance with the present geothermal heat release. Geothermal activity influences the dimensions, location, ceiling, wall, and wall ablation features of these caves. Cave passages are located above fumaroles and fractures in and adjacent to the dacite dome. Cave passages gradually enlarge by ablation, caused by outside air circulation and by geothermal sources beneath the ice. The passages form a circumferential pattern around the dome, with entrance passages on the dome flanks.
Figure 2. The catastrophic explosive eruption on May 18, 1980. Photo by USGS
GEOLOGICAL SETTING
Mount St. Helens Volcano
Mount St. Helens is located in the southwest Cascade Mountains of Washington State, U.S.A. The mountain is presently 2550 m (8365 ft) in elevation and is 9.7 km across at its baseThe surrounding ridges reach elevations up to 1340 m (4395 ft). Prior to the 1980 eruption it stood 2950 m (9675 ft) at its summit. It was the fifth highest mountain in Washington. The peak stands about 100 km (60 miles) from each of three other major Cascade volcanoes: Mt. Rainier and Mt. Adams in Washington, and Mt. Hood in Oregon. The mountain is drained by three river systems: Toutle River and Kalama River on the north, and Lewis River on the south. The flanks on the mountain receive about 360 cm (140 inches) of rain yearly (Foxworthy and Hill, 1982).
Mount St. Helens
Location: Washington
Latitude: 46.20 N
Longitude: 122.18 W
Height: 2,549 Meters (8,364 feet (9,677 feet before May 18, 1980))
Type:Stratovolcano
Number of eruptions in past 200 years: 2-3 2
Latest Eruptions: Between 1600 and 1700; about 1800-1802; 1831; 1835; 1842-44(?); about 1847-1854; 1857; 1980-? 3.
Present thermal activity: Strong steaming
Remarks: Continuous intermittent volcanic activity since 1980 2 ... Occasional eruptions of steam and ash; occasional pyroclastic flows; intermittent extrusion of dome-forming 3.
Geology of Mount St, Helens
The geology of Mount St. Helens has been summarized by Pringle (1993). It is a stratovolcano - lava interlayered with ash and pumice. The average lava composition is dacitic, but andesite and basalt are also present. Dacite is a fine-grained extrusive rock composed mostly of calcium-sodium plagioclase feldspar and quartz, with lesser quantities of ferromagnesian (dark) minerals and volcanic glass. Andesite and basalt have progressively more dark minerals and calcium feldspar, and less quartz, but are otherwise similar.
The crater walls expose a cross section of the volcano structure as it was before the 1980 eruption. The lower section of the crater wall consists of a dacite dome of Pine Creek age (approximately 3000-2500 yr B.P.). This dome is cut by dikes of the Castle Creek age (approximately 2500-1500 yr B.P.), that are feeder dikes for the Cave Basalt flows south of the mountain. Above this, the dacite summit dome of Kalama Age (approximately 500-200 yr B.P.) formed the pre-1980 summit of Mount St. Helens.
Figure 3. Mount St. Helens, Photo taken from the air in Dec. 1999.
Recent Activity
The most recent large eruption was that of May 18, 1980, a lateral blast followed by dacite tephra and pyroclastic flows. The eruption removed the entire north flank of the mountain, depositing the great majority of its mass as debris flows in the area north of Spirit Lake. The greatly enlarged and deepened crater was horseshoe-shaped, open to the north, with the south wall of the crater being the highest remaining part of the volcano averaging 2500 m (8200 ft) above sea level. The lip of the open crater on the north was estimated to be about 1900 m (6230 ft) in elevation. Since 1980, the mountain has experienced occasional eruptions of steam and ash, small pyroclastic flows, and intermittent extrusion of a dacite plug in the crater center.
On June 15, 1980, it became evident that a small lava dome was forming on the floor of the crater. The dome measured about 185 m in diameter and was less than 40 m in height. By June 23, 1980, it had grown to be 200 m in width and 60 m high. From May 1980, to October 1986, there was a series of 16 dome-building eruptions, constructing the new 915 m wide by 305 m high lava dome, in the crater formed by the May 18, 1980, eruption (Swanson and Holcomb, 1989; Holcomb and Colony, 1995). The 2-km wide, 3.2-km long, 600-m deep crater is so large it makes the lava dome seem small.
Sufficient time has elapsed since the last dome-building eruption (October 1986) for magma in the conduit beneath the dome to crystallize and form a plug. A future major eruption could be expected to match or exceed the explosive power of the massive 1980 eruption because of conduit blockage (USGS, 1994).
Current Activity
Seismic activity in the crater of Mount St. Helens was observed in 1989 through 1991. Although these explosions were relatively small, they did throw dome rock material up to 30 cm in diameter at least as far as 0.8 km from the dome and sent ash plumes as high as 6100 m (20,000 ft) elevation.
The number of small magnitude earthquakes (less then magnitude 1) beneath Mount St. Helens Crater increased slowly and steadily from less than 10 events per month in January 1995 to about 100 events per month in September 1995. There were no steam explosions during this period in 1995. The following table shows the frequency of small earthquakes in the crater in 1998.
Period Number of quakes
Jan - Mar 1998 60
May 1998 165
June 1998 318
July 1998 445
Aug. 1998 48
Figure 4. Graft of seismic events in 1998.
These quakes were so small, however, that the total seismic energy release was the same as the month of May, USGS AND UW open files report.
A survey of volcanic gases showed that magmatic carbon dioxide is present above the volcano, USGS open file report. As magma rises towards the surface, carbon dioxide exsolves from it and escapes through the fractured overburden. Areas in the crater may have appreciable concentrations of carbon dioxide at the surface. Potential hazards exist for people who may be in confined spaces such as caves. The increase in earthquakes and release of carbon dioxide from Mount St. Helens suggests that the magma chamber beneath the crater may be refilling. Sulfurous fumes can be observed near fumaroles on the lava dome, but their general presence in the crater area is not noticeable. Sulfur crusts can be found crystallized in fumarole vents.
MOUNT ST. HELENS INFORMATION STATEMENT
The rate of earthquake activity, which accelerated markedly from May through mid July, has returned to a level similar to that of last winter. The number of well located earthquakes in July was 445, compared to 318 in June, but most of July earthquakes occurred during the first three weeks of the month. The average rate for the past two weeks has been only about 4 well-located earthquakes per day. There have been several temporary increases in earthquake activity since the last dome-building eruption in October 1986. This recent episode was the most intense.
Airborne surveys of volcanic gases reveal that levels of carbon dioxide have decreased since June. However carbon dioxide is still present and measurable. The carbon dioxide is probably being released from magma that entered the magma reservoir during the past few months. The top of the magma chamber is about 7 kilometers (4 miles) below the crater. Because carbon dioxide is heavier than air, it can concentrate in surface depressions on the dome or crater floor, especially under calm conditions, and pose an asphyxiation hazard. Poorly ventilated cavities, such as caves in the mass of snow and ice behind the dome, couldalso be hazardous.
In 2000, we explored the caves and took samples of the volcanic gases in the crater of Mount St. Helens. The main fumaroles in this crater are located at the North Side of the crater. Air circulated downwards in the eastern branch and upwards in the northern and southern branches. Very few fumaroles were observed deep within the cave, and the air circulation kept the at-mosphere safe to breathe. The C02 content measured in the fumaroles was around I percent and the C02 concentration in the cave atmosphere was close to 300 pp m. No sulfur was detected in the gases. Fumaroles with sulfur crystal formation at a temperature of 86'C were located and sampled in several cave entrances. The atmostphere in this cave contained 0.3 percent C02 and 2 to 5 pp m fl2S, giving it a rotten egg odor. These concentrations are be- low the toxic admitted concentrations. Samples of soil minerals resulting from rock alteration by the volcanic gases were taken in the caves.
The crater ice body has been expanding since 1986, and its mean density is gradually increasing. It has at least two active crevasses. Changes in geothermal activity in the crater have been expressed as changes in cave passage morphology, rapid deposition of calcium carbonate in thermal streams emanating from the crater, and episodic occurrence of small gas explosions and debris flows. Presently, the danger of large debris flows and associated catastrophic flooding presents the greatest hazard to human developments near the mountain (Wolfe and Pierson, 1995).
The first observations of geothermal streams in the crater floor area were made in the mid-1980s in Loowit Canyon and to its west (D. A. Swanson, U.S.G.S., 1999, written comm.). In 1995 we first observed green bacteria growing on streambeds. In the summer of 1996, a hot spring and creek north of the Lava Dome at 5300 ft (1615 m) elevation began to deposit calcite along its channel bed, where no mineral deposition had been noted previously.
This landmark became so prominent that it was used for crater navigation. At that time, red bacteria began growing on the streambed, dominating over the green bacteria. By mid-1997, thick calcite crust covered the entire streambed. Sulfur bacteria continued to grow beside the streambed up to 10 m away from it on either side.
CRATER ICE
A permanent body of ice has accumulated in the crater of Mount St. Helens. The shade of the steep crater walls to the east, south, and west largely protects the accumulation. The crater ice body is an incipient glacier that continues to grow. It is not readily apparent from a distance that glacier ice is present in the crater, but small bodies of ice on the south crater wall behind the Lava Dome have crevasses and flow features, indicating their transformation into glaciers. 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.
Figure 5. 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). Photo By Charles H. Anderson Jr.
The crater headwall rises to 2550 m (8365 ft) on the south (Fig. 4). The contiguous crater floor ice body extends from a maximum elevation of 2000 m (6560 ft) south of the Lava Dome, downward to the northeast and north around both sides of the dome. The crater floor north of the dome (elevation 1800 m, 5900 ft) hosts only seasonal snow accumulations.
Figture 6. Charlie Anderson examining a ice crevasses on the north west
side of the lava dome. Photo By Blair Binnar
At least two radial (relative to the crater center) crevasses are present in the crater floor ice body. Glacier movement over an uneven surface causes crevasses to form. A crevasse is conclusive evidence that plastic deformation is taking place in glacier ice (Sharp, 1960). One crevasse is located on the northwest side and the other is on the northeast side of the crater near the Lava Dome. Both crevasses penetrate through the lowermost layers of the ice body. The crevasse on the northwest side was revealed in September 1994, when the roof of an ice cave collapsed.
The crater ice body has been expanding since as early as 1982 (P. T. Pringle, Washington Dept. of Natural Resources, 1999, written comm.). The volume of the crater ice body increased from approximately 28 million m} of uncompacted snow and firn in 1988 to over 55 million m in 1995. These figures are derived from published U.S. Geological Survey (USGS) figures (Mills and Keating, 1993), modified in this study byconsideration of thickness information collected from direct cave observation. The computation considers the volume of included rock debris derived from the crater walls, calculated from changes in topographic maps. As of late 1999, the crater was estimated to have 79 million m} of ice, firn, and snow. The thickness in places
reaches as much as 60 m.
Figure 7. 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 8. A simplified topographic base showing the cone of Mount St. Helens, the crater ice body (delineating firn and ice areas), the 1997 debris flows and other fitures in and around the crater. Note Loowit canyon, which is located inside the the breach to the east of the 1997 debris flow.
Because of the severely limited quantity of ice density data, the mean ice density and therefore the total mass of ice can only be crudely estimated. The ice density at the base of the crevasses was measured at 0.85 g/cc in September of 1994. The ice density in the lowest cave passage was measured at 0.86 g/cc in September of 1996. We obtained density measurements by using a cylindrical saw and weighing and measuring the cut samples in the field.
In the spring of 1997, an ice and rock debris avalanche formed a tongue about 25 m in height on the ice surface near the north side of the dome. We estimated the deposit to be composed of about 40 percent rock debris. The tongue froze into hard firn and lasted through the summer of 1997. Dark-colored rock debris around the tongue focused surface ablation, because of concentrated absorption of solar radiation.
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 well known ice caves of Mount Rainier occur in stagnant ice bodies such as Paradise Glacier (Anderson and others, 1994), the summit crater ice body (Kiver and Steel, 1975), and active glaciers such as the Carbon River Glacier (Halliday and Anderson, 1970).
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. Cave passages are located above fumaroles and fractures in and adjacent to the dacite dome. 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. Heat, steam, fumarole gases, and air circulation have melted more than 2415 m (7925 ft) of cave passages in the crater ice body.
Figure 6. 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. Photo By Charles H. Anderson Jr.
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 ldblquote steam cups, rdbluote dome-shaped melt pockets localized near fumaroles (Kiver and Steel, 1975).
Air circulation converts these to the typical scalloped ceiling and wall forms seen in ice caves ubiquitously (Anderson and others, 1994). We interpret the Mount St. Helens caves to be approximately 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. Changes in the geothermal activity or climate would be expected to affect the dimensions, location, ceiling, wall, and wall ablation features of these caves.
Cave Description
We mapped The Mount St. Helens caves by compass and steel tape survey. All gear was carried on foot. We recorded our observations on the surface and inside the caves by videotape and still camera. We visually estimated the physical dimensions of rooms and cave features.
We found entrances to and mapped 15 firn caves around the perimeter of the Lava Dome in the period from 1996 through 1999. Some have spectacular large rooms. Most have small rooms and crawlways. Cave features include scalloped surfaces of ceilings and walls, moulins in the ceiling, multiple domes connected by crawlways, and skylights. In winter, short-lived ice stalactites, stalagmites, and helictites form inside the caves from water dripping from protrusions on the cave ceiling.
Figure 7. Bill Greninger look up at the cave ceiling in the lowest passage. Photo By Charles H. Anderson Jr.
The cave floors consist exclusively of talus, 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.
Six main entrances and numerous smaller ones lead down the 40 degree sloping crater floor immediately adjacent to the dome. Passages around the dome perimeter are surprisingly horizontal. Without geothermal control, passage patterns would be dendritic and follow the crater slope. The arcuate distribution of caves and their association with thermal anomalies localized around the perimeter of the Lava Dome, suggest that future cave exploration should follow a circumferential trend. Descending passages have vertical sides and ceilings that are convex upward. Passages paralleling the slope contours are often shaped like right triangles with the 90-degree angle located at the junction of the downslope ice wall and the ice ceiling. Floors composed of mud to boulder-size volcanic rubble slope at about 30 degrees, and against the Lava Dome surface they may exceed 40 degrees.
Ridge-like accumulations of rock debris from the Lava Dome occur in many places on the floor against or near the ice wall of the passage. They are composed of unsorted, unstratified mud and rock debris derived from the upslope portion of the cave floor. In some places, they occur toward the center of the floor and, in others, closer to or in contact with the downslope ice wall. They probably represent talus caught against the wall. The walls appear to retreat in response to the production of warm geothermal gas emanations. Fluctuations in the rate of retreat could be due to seasonal meteorological changes or to changes in volcanic thermal activity.
Progressive Recrystallization of Crater Ice
Generally in the ice caves, older firn is distinguished from recrystallized recent snow by textural differences and stratigraphic relationships. Winter snowpacks from multiple years have persisted and provided the pressure increase necessary to convert snowfall into a permanent ice body. As recrystallization continues in the deepest layers, the individual ice crystals grow together to form a rigid fabric with limited permeability (glacier ice). From 1986 to present, we observed the gradual change from snow to firn to glacier rice in cave passages (ice densities were not measured systematically). An apparently abrupt decrease in percolating water occurred in the final stage of the transition. We interpret this as the result of bulk freezing in intergranular pores.
A permanent glacial core has developed and grown on the Mount St. Helens crater floor. Through the series of heavy winters and mild summers of the 1980s and 1990s, a continued sequence of yearly net snow accumulations enabled the ice body to persist. The ablation process coupled with geothermal energy supply has not been sufficient to remove it.
Geothermal Activity in the Caves
The Mount St. Helens Lava Dome is the locus of the active volcanic vent. It therefore is a source of volcanic gas emanations throughout the crater area. The caves are a primary result of the concentration of heat. They are localized at active fumaroles, and form as conduits of venting for the heated gases. They are further enhanced by the drainage of heated surface water from the dome directly into the ice body.
Hundreds of small fumaroles emit considerable quantities of steam that frequently impair visibility in the firn caves and make mapping, photography, and other observations difficult. Some of these fumaroles make audible hissing and gurgling noises. Sulfurous fumes occur locally in the caves. Although the rising heat and steam cause the ice walls and ceilings to drip constantly, we have not observed appreciable quantities of standing or flowing water in the caves. Changes in passage dimensions and location (from periodic observations and resurveys of the caves) indicate changes in heat-flow and the locations of volcanic emanations.
Gases from the numerous fumaroles and slowly circulating surface air mix throughout the cave passages. The presence of breathable air throughout the known cave system indicates that volcanic gases are rapidly removed from the caves and mixed with fresh air. Some accumulations of carbon dioxide have been reported in the caves (D. A. Swanson, U.S.G.S., 1999, written comm.), although we have not observed passages with either stagnant or poisonous air compositions. Many of the larger cave rooms provide a protected environment for monitoring volcanic gas composition. These rooms would be reasonable sites for prolonged monitoring of changes in volcanic emanations, because they are relatively easy to find and their narrow connections with the upslope cave passages prevent rapid mixing with outside air.
Cave Ablation
Within the caves, evaporation, sublimation, and heat conduction are the major ablative processes (Anderson and others, 1994). Since the caves are sheltered from sunlight, radiation from the sun has no direct influence on cave ablation, but energy from heated ground and fumaroles has an appreciable effect. The main control of cave ablation is the amount of air flow against the cave walls. Trunk passages tend to form and remain dominant in the vertically developed cave networks, because air circulation is enhanced by convection.
As cave ablation and surface ablation continue through a summer season, it is normal for the cave ceiling to approach and intersect the ice surface progressively over time. If the ice is fractured, or perhaps after winter snow adds weight to the ceiling, a cave passage may experience ceiling failure. In either case, the cave system suddenly gains a vent to outside air. The effect of venting in summer is to allow cold cave air out and warm outside air in. The effect in winter is reversed. The importance of ablation vents is exaggerated when there is any superimposed restriction in the system, such as winter snow or rockfall blocking other entrances. In these cases, the vent entrance becomes a major means of communication with outside air. When all vents to the surface are closed, the ordinary glacier cave becomes dormant.
In crater firn caves that contain internal heat sources, the ablation process can continue by convection even when all external openings are blocked. The system is therefore less seasonally dependent, and may evolve faster than an ordinary glacier cave.
GEOMORPHIC CONDITIONS IN THE CRATER
Crater Floor Environment
The present crater floor is underlain by porous and permeable, loose landslide debris from the 1980 eruption that caused the upper third of the volcano to collapse. The bulk of this material shifted downward and to the north,filling the valley of the North Fork Toutle River. Subsequent eruptions, including the later part of the May 1980 eruption, covered the landslide surface with juvenile pumice and tephra 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 (Holcomb and Colony, 1995). After the October 1980 eruption, dome growth gradually covered the fringe areas of crater-filling rockfall talus cones (Mills, 1992). These cones are intercalated with accumulating snow. The whole body is insulated and compacted by its own mass. Later tephra eruptions have added only minor amounts to the sediment pile. The most volumetrically significant addition to the post-1986 crater floor environment, therefore, is the accumulated ice and rock debris. Through 1988, the rock debris fraction of post-1980 crater fill gradually dropped from 100 percent to about 65 percent of the total (Mills, 1992).
The most active processes taking place at the surface are (1) continued landslides from the steep crater walls, (2) fluvial downcutting 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.
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.
During the summer of 1996 a little known hot spring and creek that it forms in the upper pumice plain began to have mineral deposition along its channel bed. This became very interesting, because the month before no mineral deposition was taking place. The spring forms a creek that varies from 100 feet to 400 feet long before it converges with a colder stream. The stream has changed channels from the summer of 1996 to the summer of 1997 where the stream channel has shortened and flows over a small falls into the cold stream channel. The streams mineral deposition has also increased during the summer of 1997. Calcite mineralization has formed thick flowstone along the creek channel bed. Where the stream flows through deeper pools in the channel, calcite pearls (cave pearls) form. Calcite helictites, stalactites, and stalagmites have formed on small waterfalls.
Figure 8. 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. Photo By Charles H. Anderson Jr.
One of the primary issues investigated during the September 1999 visit to the crater was the white calcite precipitate accumulating along what has become known as the “Calcite Stream.” The deposit was noted to begin precipitation at the outlet of a geothermal area (CS1.1). The material consists of a pillowy-textured, soft (hardness <2), crystalline material with noticeable layering (stratification of white and green bands to 1-2 mm) and an iridescent sheen produced by multiple parallel cleavage surfaces, indicating that large volumes of the material are held in crystallographic continuity, despite the highly porous nature of it. Some blue green staining is present in some hand specimens collected; indicating possible impurities (copper?) are present. The material forms as a veneer over native stream rock, and was found in layers to 4 cm thick. Approximately 35 m downstream from the initial occurrence of the material, algae appear in the stream as well, and the integrity of the precipitate declines. At this point, the material becomes very fragile and powdery, with a hardness significantly less than 1.
Degradation of the Crater Floor
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; Shevenell and Goff, 1995). These canyons have a dangerous reputation as conduits of crater access (Anderson and others, 1998). 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. The semicircular, steep-walled scarp was originally 25 to 30 m deep and about 0.2 km in diameter. The debris flow extended from the basal breach of the scarp for about 0.7 km downslope, from 1700 m (5580 ft) at the scarp brink to 1550 m (5085 ft near the toe of the lahar tongues. A geothermal stream rises from the scarp floor, forming clear, heated pools that appear to be free of living matter. Two streams exit the scarp mouth and flow through lahar debris in recently excavated gullies. One tongue of the debris flow followed the original stream, producing a lahar deposit that now fills that stream course. Continued discharge was diverted around its sides and through the deposit itself, producing additional springs and seeps all along its length.
Figure 9. View of the August 1997 slump scarp that generated a small lahar of the
crater floor north of the dome. The calcite stream issues from springs in this feature.
Photo By Charles H. Anderson Jr.
We measured water temperatures up to 80 ) in pools in the slump scarp, and temperatures of ( 50 F) or greater downstream of the lahar tongues. Another slide occurred in September 1997 on the north slope of Mount St. Helens, passing down the Loowit drainage. Most of the sediment from it was deposited upstream of Johnston Ridge.
Calcite and Bacterial Growth in Geothermal Streams
Calcium carbonate is actively precipitating from solution in the stream water that rises from the scarp floor noted above. It has formed deposits of travertine and tufa as flowstone, dripstone, helictites (cored by bacterial filament aggregates), and cave pearls. These coatings have formed on the streambed and hang from steps and waterfalls. Samples of the calcite coating exhibit compact, coalesced fan-shaped aggregates of acicular to bladed, rhombohedral-terminated crystals as much as 1 mm in cross section. These appear to be pseudomorphs after aragonite bundles. Figure 12 is a scanning electron microscope (SEM) photomicrograph of a flowstone surface from a waterfall overhang. We previously (1996 and 1997) observed and filmed CaCO3 growth in thermal stream beds in area now covered by lahar deposits.
Water percolating through freshly exposed loose material in the slump debris supplies nutrients and mineral components to the streams. Red (sulfur), orange (iron), and minor green(chlorophyllic) bacterial slime (Folk, 1993) coat the streambed and accumulate into masses in streambed pockets in gullies cut in to the lahar deposits. We observed (summer 1999) abundant water seeping from gully walls in the presence of flourishing bacterial growths. Downstream of the 1997 lahar deposits for a distance of about 0.5 km, heavy CaCO3 coatings had grown on streambed rocks and had encapsulated bacterial growths.
Figure 10. 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. Photo By Charles H. Anderson Jr.
These encrustations actively grow in the flowing water and splash zone along the stream banks. Helictites grow as thin CaCO3 coatings on strands of red bacteria that hang from rock projections on the streambed. Carbonate coatings continue to grow on and engulf the bacterial colonies. Lithified remains of the bacterial growths can be found inside hollow flowstone crusts. SEM microscopy indicates the presence of bacteria and nannobacteria, similar to those described by Folk (1993), in the carbonate growths. Only incipient, very thin carbonate coating grew in the scarp pools and streams leading out of the scarp mouth.
Calcite deposition in streams of the Breach area has been rapid and continuous. At least two episodes of calcite deposition took place (and are recorded) in a gully cut into the 1997 lahar deposits during the period September 1997 through August 1999. Terraces of an older calcite-coated streambed are preserved on the walls of the gully 1 to 2 meters in elevation above the present calcite-coated streambed, indicating that the newest coatings developed after the most recent gully-deepening erosion. Within a one-year period, calcite stalactites and stalagmites grew to a maximum observed size of 27 cm diameter and 30 cm length, and calcite cave pearls grew to 3.6 cm diameter.
Figure 11. Cave Pearls in the Crater of Mount St. Helens. Photo By Charles H. Anderson Jr.
Nutrients derived from decomposition of volcanic material appear to support the bacterial population of crater streams. The presence of red sulfur bacteria indicates that sulfur is a prominent source of acidity in the water acting to digest crater rocks.
Chemical Environment in Geothermal Streams of the Crater
We believe the supply of calcium to thermal streams derives from leaching of fresh, porous dacite in the crater upon exposure to percolating meteoric water. The chief process affecting the chemistry of crater runoff has evolved from degassing of newly injected magma (waning to insignificance about 1985) to passage of meteoric water through the crater floor deposits in a manner too fast to attain equilibrium (Shevenell and Goff, 1995). Such undersaturated groundwater conditions could leach mobile components from a large volume of crater deposits.
Figure 12. Pre-August-1997 geothermal stream issuing from Mount St.Helens crater. The white calcite coating of the stream high lights the
streambed in the photo. Photo By Charles H. Anderson Jr.
The high precipitation rate produces a high flux of water through the dome area and out of the crater mouth. Heated groundwater resurges where the local unconfined water table intersects the crater floor. Further downstream, calcite precipitates in the rapidly cooling surface streams. Consequently, the local ground-water flow has been able to transport calcium salts by aqueous solution from the dome area northward. We did not quantitatively evaluate the rate of mass loss from the crater environment by this process, but it could be significant in comparison to the changes in the crater ice volume and crater topography.
The Appendix provides field measurements of water chemistry in streams flowing from the Breach out of the crater.
FAUNA OF THE CRATER AND CRATER CAVES
There is little direct evidence of animals inhabiting the crater floor area, with one possible exception: Mice have been reported the crater floor north of the dome in 1982 (D. A. Swanson, U.S.G.S., 1999, written comm.). Deer have visited the lower part of the crater on occasion, leaving only tracks for the careful observer to notice. We have seen insects including honeybees, ladybird beetles, and carpenter ants in the crater environs, presumably blown in by winds.
We also found a mountain beaver skull, probably left by a predatory bird. Fauna observed during ice cave exploration include insects and ice worms that are presently inhabiting the caves and snowfield environment. These same types are known from ice caves at Mount Rainier (Anderson and Halliday, 1969; Anderson and others, 1994).
Biologists have long sought the primitive, cold-adapted insects of the genus Grylloblatta in the glaciers and craters of Mount Rainier, Mount Baker, Mount Hood, and Mount St. Helens, because of especially favorable conditions. We found an undescribed species of grylloblatta in September 1997 on the ice surface on the northwest side of the Lava Dome. Grylloblattids are known from the Paradise and Stevens Glacier Caves of Mount Rainier (Halliday and Anderson, 1970).
Figure 13. Grylloblattid in the ice mass in the Crater of Mount St. Helens. Photo By CharlesH. Anderson Jr.
Mountain climbers have observed ice worms (Oligochaeta: Plesiopora Enchytraeidae) of the species Mesenchytraeus solifugus rainierensis in snowfields of several Cascade mountains, especially Mount Rainier (R. Crawford, U. of Washington, 1998, oral comm.). We collected one specimen from an ice wall in the largest of the St. Helens crater firn caves in August 1996, actively crawling approximately 1 cm beneath the ice surface. These worms are thought to migrate through the ice in a diurnal cycle, taking advantage of pore spaces between ice crystals to move about. They have been recovered from cores up to 1.5 m below the surface in Alaska glaciers. The species was originally collected in 1915 and first recorded from the Paradise Glacier on Mount Rainier in the 1930s.
We collected Nymph and adult stoneflies (Plecoptera: Perlodidae) of the species Rickera sorpta on the surface and in cave interiors in the crater of Mount St. Helens (R. Crawford, U. of Washington, 1998, oral comm.) and from the Paradise and Stevens Glacier Caves of Mount Rainier (Anderson and others, 1994). Stonefly nymphs are aquatic. The near-mature state of specimens collected at Mount St. Helens indicates that they crawled out of water for the molt to adulthood. The dark coloration of nymphs makes them almost invisible against the dark bottom of a cave pool. Nymphs are extremely sensitive to warmth: one collected specimen expired after approximately fifteen seconds of exposure to human body heat.
Figure 14. Plecoptera Perlodidane, fount on the Ice Field in the Crater of Mount St.
Helens. Photo By Charles H. Anderson Jr.
Figure 15. Rickera Sorpta Found on the Ice Field in the Crater of Mount
St. Helens. Photo By Charles H. Anderson Jr.
Recent inspection of conditions in the crater of Mount St. Helens indicates potential landslide danger in the areas of Loowit Falls and below, as well as the large unnamed waterfall several hundred meters west and along its drainage. Open fissures were seen in the walls of this canyon (see attached picture), which has highly oversteepened slopes in coarse granular uncemented crater detritus. Comparison with existing topographic maps and photos from the 1980s indicate rapid erosion in both this canyon and Loowit canyon (recall the recent landslide in Loowit canyon, marked on picture).
The potential for a large landslide is greater in the unnamed canyon, because of a greater amount of exposed unstable material.
Figure 16. Mount St. Helens Crater and the Dangers of Debris Flows are in this picture. Photo By Charles H. Anderson Jr.
We have observed an active glacier snout at the northwest base of the lava dome (marked). This glacier is almost entirely covered by rock debris, and is not readily apparent to casual view. A substantial stream can be seenissuing from the glacier at the marked glacier cave. It re-enters firn nearly at the same point and disappears into the unconsolidated crater detritus under the firn field north of the dome.
Figure 17. Glacier Front in the crater of Mount St. Helens.
Figure 18. Glacier front in the Crater of Mount St. Helens with the Lava Dome. Photo By Charles H. Anderson Jr.
 Figure 19. Water resurges out of a glacier cave in the Glacier Front. Photo By Charles H. Anderson Jr.
Water resurges in many places across the crater plateau as hot springs (several marked). We believe the flow of water through the crater sediments, together with latent geothermal heat and concomitant chemical alteration, maintains an unstable condition along the plateau front. The 1997 slump and associated debris flow are indicated on the picture. This flow crosses the scars of several others in the plateau front slopes, and should remind us of the imminent landslide potential.
CONCLUSIONS
An ice body is presently forming and expanding in the Mount St. Helens crater. Its mean density is increasing with each passing year, and the transition from snow to firn to glacier ice (with active crevasses) is taking place presently. Net ice mass budget balances have been positive in the crater since 1986, when the snow pack was first subjectively recognized to be growing.
Ice caves form above fumaroles that are located along fractures in the Lava Dome and the surrounding crater floor. Cave passages are gradually enlarged by ablation, that is caused both by geothermal sources beneath the ice and by outside air circulation. Passages grew laterally and vertically toward the surface, spawning ceiling collapse. The network of fumaroles has produced a ring of relatively horizontal passages that are connected to the surface by a number of ascending entrance passages.
Changes in geothermal activity in the crater of Mount St. Helens have become noticeable through cave passage observation and remapping. Calcium carbonate from geothermal streams rising from the crater floor, produces coatings up to 15 cm thick in a single year. Chlorophyllic and later sulfur and iron bacteria are associated with these streams. A small slump scarp and minor lahar developed in the crater north of the do me in summer 1997, and later the same year, another flow occurred in Loowit Canyon. Increased thermal activity could mobilize crater ice to produce damaging debris flows that could affect the discharge and sediment load in Toutle River. Our detailed mapping and investigations of the crater environment could furnish a additional indicators of geothermal activity and incipient geomorphic changes that could augment information provided by remote surveys.
ACCESS TO THE CRATER
The U.S. Forest Service strictly regulates access to the crater of Mount St. Helens. The area is part of the Mount St. Helens National Volcanic Monument, and special permits are required for any activities other than visitation of public facilities. The authors have a crater access permit for the purpose of scientific study. No one should attempt to approach Mount St. Helens by foot or by air without written clearance from the Forest Service.
ACKNOWLEDGMENTS
The authors are grateful to IGS members and to staff of the U.S. Forest Service of Mount St. Helens National Volcanic Monument. We thank Peter Frenzen (Monument Scientist), James Quiring (U.S. Forest Service), and Donald Swanson (USGS Hawaiian Volcano Observatory), Daniel Dzurisin, Edward Klimasauskas, and Willie Scott (USGS Cascade Volcano Observatory), Wendy Gerstel and Patrick Pringle (Washington Dept. of Natural Resources) for critical review and editorial assistance in preparing this paper. Robert Folk of the University of Texas at Austin, Texas provided SEM photos and bacterial components. R. Crawford of Burke Museum, University of Washington identified our insect and worm specimens.
REFERENCES CITED
Anderson, C. H. Jr. ; Vining, M, Dec. 1999, Observations of Glacial, Geomorphic, Biologic, and Mineralogic Deveolopments in the Crater of Mount St. Helens, Washington, v.27, no. 2/3/4, p. 9-19.
Anderson, C. H., Jr.; Behrens, C. J.; Floyd, G. A.; Vining, M. R., 1998, Crater firn caves of Mount St. Helens, Washington: Journal of Cave and Karst Studies, v. 60, p. 44-50.
Anderson, C. H. Jr.; Vining, M. R., 1997, Progress Report on Crater Environment Development, Mount St. Helens, Washington: presented to Mount St. Helens National Volcanic Monument, International Glaciospeleological Survey, Seattle, Washington, 16 p.
Anderson, C. H., Jr.; Vining, M. R.; Nichols, C. M., 1994, Evolution of the Paradise/Stevens glacier ice caves: Journal of Cave and Karst Studies, v. 56, p. 70-81.
Anderson, C. H.; Halliday, W. R., 1969, The Paradise ice caves, Washington--An extensive glacier cave system: National Speleological Society Bulletin, v. 31, p. 55-72.
Folk, R. L., 1993, SEM imaging of bacteria and nannobacteria in carbonate sediments and rocks: Journal of Sedimentary Petrology, v. 63, p. 990-999.
Halliday, W. R.; Anderson, C. H., Jr., 1970, Glacier caves: Studies in Speleology, v. 2, pt. 2, p. 53-59.
Kiver, E. P.; Mumma, M. D., 1975, Mount Baker firn caves, Washington: The Explorers Journal, p. 84-87.
Kiver, E. P.; Steel, W. K., 1975, Firn Caves in the volcanic craters of Mount Rainier, Washington: Journal of Cave and Karst Studies, v. 37, p. 45-55.
Mills, H. H., 1992, Post-eruption erosion and deposition in the 1980 crater of Mount St. Helens, Washington, determined from digital maps: Earth Surface Processes and Landforms, v. 17, p. 739-754.
Mills, H. H.; Keating, G. N., 1993, Maps showing posteruption erosion, deposition, and dome growth in Mount St. Helens crater, Washington, determined by a geographic information system: U.S. Geological Survey Miscellaneous Investigation Series, unpaginated.
Sharp, R. P., 1960, Glaciers: University of Oregon Books [Eugene, Ore.], 78 p.
Shevenell, L.; Goff, F., 1995, Evolution of hydrothermal waters at Mount St. Helens, Washington, USA: Journal of Volcanology and Geothermal Research, v. 69, p. 73-94.
GLOSSARY
cave pearl - a spherical calcite concretion formed in splashing water, usually deposited on a sand particle or rock fragment nucleus.
firn - naturally accumulated ice that has reached a density in excess of 0.55 g/cc, but not greater than 0.82 g/cc. It possesses granular crystal texture in which the grains are in contact but not intergrown.
flowstone - a mineral coating (usually calcite) deposited by precipitation from water flowing over an exposed surface, usually but not only found in caves.
glacier ice - naturally accumulated ice that has reached a density in excess of 0.82 g/cc. It possesses an intergrown crystalline matrix and flows plastically under its own weight.
helictite - a curved, angular, or dendritic twig-like flowstone deposit.
ice, ice body - a general term for any form of frozen water. In the context of Molunt St. Helens, it refers to an accumulated body of ice regardless of its density, texture, or fraction of non-ice content (air and rock debris).
moulin - a circular, nearly vertical hole or shaft in a glacier, formed by percolating surface water.
skylight - an opening to outside light in the ceiling of a cave.
stalactite - a cylindrical or conical flowstone deposit that hangs from the ceiling of a cave.
stalagmite - a cylindrical or conical flowstone deposit that rises from the floor of a cave.
APPENDIX - Water Chemistry
Project Activities:
Calcite Stream Information needs:
. Measure all streams in crater and survey them. Put all streams in crater on map.
· Measure length, width, and depth of stream bearing calcite.
· Measure length of stream bearing all kinds of algae and bacteria.
· Estimate percentages of different organisms.
· Collect samples of algae and bacteria.
· Collect water samples for analysis.
· Measure parameters of water (pH, Eh, DO, Temp, Turbidity, Conductivity, total dissolved solids, salinity, and ionic content (CO3, HCO3, S, SO3, SO4).
· Where does stream exit debris flow.
· Map debris flows, are they different.
· Map old calcite streams.
· Does stream appear to originate from snow field melt near the dome (i.e. is
there a stream that disappears into the debris flow? Is there buried snow under
the debris flow? Is there a thermal source under the debris flow?
· Measure the calcite growth rate using native materials as substrates.
. Pumice
. Rhyloite
. Dacite
. Compare the different growth rates on the different substrates.
· Use calibrated GPS unit to survey/perform measurements.
Develop map and record changes? make graphic of changing debris flow/
stream flow/snow levels.
Measurements:
JULY 31,1999
Reading 1 ( collected in a cup). 2 ( in stream). 3 ( in stream).
Location: Point of origin of stream calcite deposit area downstream from calcite area in
at foot of pyroclastic flow. algae area with very little calcite
Temp C 52 60 50.7
Turb NTUs 10 6 6
Cond ms/cm 3.43 3.4 3.3
pH 7.25 7.4 7.2
DO mg/L 2.75 2.3 3.3
Sal mg/L 0.16 0.16 0.16
AUGUST. 28, 1999
Reading 1 2 3
Location Start of Calcite Stream. 2 3
Temp C 39.3 43.9 34.6 Turb NTUs 6 -10 -1
Cond ms/cm 3.5 0.34 0.131
pH 7.22 6.93 5.11
DO mg/L 3.5 3.5 3.5
Sal mg/L 0.2 .02 0.2
Tbs 2.2 2.1 2.0
Orp mb 131 164 143
Flow GPMin. 5 20 50
Reading 4 5
Location 4 5
Temp C 44 41.1
Turb NTUs -10 -10
Cond ms/cm 0.34 0.35
pH 7.75 8.25
DO mg/L 3.9 4.6
Sal mg/L 0.2 ?
Tbs 2.2 20.6
Orp mb 128 ?
Flow GPMin. 80 ?
SEPTEMBER 11, 1999
Station Drainage from South Drainage from South 0+16
Description (if staked) - End of Morain End of Morain -
Sample Name Calibration of U-22 MOR-1 (sample 1)MOR-1 (sample 2) CS-1.1
Date Time 9/11/99 12:12 9/11/99 12:29 9/11/99 12:45 9/11/99 14:56
Actual Temperature (deg C) 17.6 < 1.8 < 2.1 >54.7
Estimated Flow (gpm) NA 50 50
Horiba U-22 Measurements:
pH 4 5.63 5.51 6.89
Specific Conductivity (S/m) 0.45 2 mS/cm 3 mS/cm 0.33
Turbidity (NTU) -0.8 687 739 -7.8
Dissolved Oxygen 9.8 13.9 13.8 3
Sample Temp. (deg C) 17.6 1.8 2.1 54.7
Salinity (%) 0.2 0 0 0.2
TDS (g/L) 2.9 0.02 0.02 2.2
delta t 1 0 0 0
ORP (mV) 286 237 270 106
Station 0+45.0 1+00.0 -
Description (if staked) - - 5 feet below confluence
Sample Name CS-2.1 CS-3 CS-3.1
Date Time 9/11/99 15:10 9/11/99 15:10 9/11/99 15:16
Actual Temperature (deg C) >51.3 32.5 >55
Estimated Flow (gpm) NA 50 50
Horiba U-22 Measurements:
pH 6.95 7.8 7.57
Specific Conductivity (S/m) 0.34 0.35 0.35
Turbidity (NTU) -9.8 2.1 -9.2
Dissolved Oxygen 3.1 10.5 3.8
Sample Temp. (deg C) 51.3 31.9 51
Salinity (%) 0.2 0.2 0.2
TDS (g/L) 2.2 2.2 2.3
delta t 0 0 0
ORP (mV) 129 117 116
Station 2+00.0 2+20 2+55 2+75
Description (if staked) - - - -
Sample Name CS-4 CS-4.1 CS-4.2 CS-5
Date Time 9/11/99 15:21 9/11/99 15:27 9/11/99 15:33 9/11/99 15:40
Actual Temperature (deg C) >49.7 >45.0 47.1 46.3
Estimated Flow (gpm)
Horiba U-22 Measurements:
pH 7.96 8.24 8.21 8.22
Specific Conductivity (S/m) 0.36 0.36 0.35 0.35
Turbidity (NTU) -9 -6 3 -4.3
Dissolved Oxygen 4.4 5.1 4.8 5.4
Sample Temp. (deg C) 49.7 45 47.1 46.3
Salinity (%) 0.2 0.2 0.2 0.2
TDS (g/L) 2.3 2.3 2.2 2.2
delta t 0 0 0 0
ORP (mV) 122 135 122 120
Station 3+00.0 4+00.0 5+00.0
Description (if staked) 3 bl. tape 4 bl. tape 5 bl. tape
Sample Name CS-6 CS-7 CS-8
Date Time 9/11/99 15:46 9/11/99 16:03 9/11/99 16:08
Actual Temperature (deg C) 46.2 44.6 39.8
Estimated Flow (gpm)
Horiba U-22 Measurements:
pH 8.25 8.36 8.39
Specific Conductivity (S/m) 0.34 0.34 0.35
Turbidity (NTU) -3.4 -4 -0.5
Dissolved Oxygen 5.9 5.9 5.9
Sample Temp. (deg C) 46.2 44.6 39.8
Salinity (%) 0.2 0.2 0.2
TDS (g/L) 2.2 2.2 2.2
delta t 0 0 0
ORP (mV) 115 102 120
Station 6+00 7+00 8+00 9+00
Description (if staked) 6 bl tape no rebar no rebar no rebar
Sample Name CS-9 CS-10 CS-11 CS-12
Date Time 9/11/99 16:12 9/11/99 16:15 9/11/99 16:18 9/11/99 16:20
Actual Temperature (deg C) 39.3 39.3 36.9 34.2
Estimated Flow (gpm)
Horiba U-22 Measurements:
pH 8.51 8.52 8.56 8.64
Specific Conductivity (S/m) 0.34 0.34 0.34 0.35
Turbidity (NTU) -0.9 -2 -0.4 0.3
Dissolved Oxygen 6.8 7.1 7.1 7.4
Sample Temp. (deg C) 39.3 39.3 36.1 34.2
Salinity (%) 0.2 0.2 0.2 0.2
TDS (g/L) 2.2 2.2 2.2 2.2
delta t 0 0 0 0
ORP (mV) 105 97 103 105
Aug.12, 2000
Horiba U-22 Measurements
LOCATION New Stream 1998-2000
Station 1 pH 7.5 Temp. 59c Cond. –77.2 mv
Station 2 pH 8.6 Temp. 48c Cond. –84.3 mv
Station 3 pH 8.4 Temp. 43c Cond. –82.3 mv
Station 4 pH 8.6 Temp. 38c Cond. –87.4 mv
Station 5 pH 8.2 Temp. 36c Cond. –90 mv
Station 6 pH 8.7 Temp. 32c Cond. –92 mv
Station 7 pH 8.7 Temp. 30c Cond. –94 mv
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