Introduction
In the field of avalanche safety and control the association of depth hoar with some of our most treacherous avalanches was first reported by Reference PaulkePaulke (1932). Reference SeligmanSeligman (1936, p. 72) considered the requirements for the formation of depth hoar to be: (1) a temperature gradient, (2) some space available for crystal growth, and (3) a stream of vapor to nourish the growth. Neher (in Reference Bader, von Bader, Haefeli, Bucher, Neher., Eckel, Thams and NiggliBader and others (1939), English translation, p. 237) noted that depth hoar is not always lacking in strength, and he considered strong depth hoar to be a consequence of load hardening. In recent studies, Reference AkitayaAkitaya (1974) made exhaustive laboratory examinations of depth hoar development and defined various physical realms for different crystal types and strength changes. He related “hard depth hoar" to a high initial density and a fine texture of the snow.
In nature, the required temperature gradient is normally a function of the amount of heat stored in the ground during the summer. In areas of high geothermal flux, such as the Yellowstone National Park, depth hoar can result from the geothermal gradient. Many rammsonde profiles, pit studies, and most resistograms which involve depth hoar in the pack have suggested that the zone of minimum strength may be several centimeters above ground level, whereas the best euhedral depth hoar is often found in the basal layer of the snowpack. A coordinated laboratory and field study has been undertaken to examine this relationship and to discover whether the regimen which produces euhedral depth hoar is indeed different from that which produces the zone of minimum strength. This paper mainly discusses some preliminary results from the field study.
Field Site And Methodology
The site selected for the field investigation was a sector of snow-pack around the orifice of Old Faithful geyser in Yellowstone National Park. Previous studies in that area (Reference Bradley and AlfordBradley and Alford, 1967: Reference Alford and BradleyAlford and Bradley, 1968), indicated that the combination of abnormal geothermal gradient, cold winter climate, and thin snowpack produces euhedral depth hoar and collapsing snow virtually every winter. The observations reported here took place during December 1974.
Three thermistor stacks were installed on a radial line from the Old Faithful vent in order to monitor the thermal gradient in the ground. They were placed so that the stack nearest to the vent would fall just within the edge of the snowpack, whilst the furthest from the vent would be in an essentially normal portion of the pack. These positions were a matter of experience. The thermistors were set at 100 mm and 250 mm depth below ground level. The temperature profile in the snow was measured by the use of a portable thermistor probe and was recorded on a “Rustrack" recorder. This system is capable of giving temperature readings accurate to within 0.2°C. Other observations included (1) the daily weather (especially maximum-minimum temperatures, the snow fall, and snow accumulation on the ground); (2) frequent temperature readings from below ground moving up through the snowpack and eventually into the air; and (3) frequent snow strength readings made with a resistometer. (The resistometer is a probe having a conical point and a spring mounted handle which is coupled to a sensitive displacement dial indicator. It is capable of giving accurate spot strength measurements of very weak snow at depth.) Snow-pit studies of metamorphic zoning, including a photographic record of grain size, crystal development, density distribution and whisk broom abrasion profiles were also made.
Observations
By the time the seasonal snowpack was 280 mm deep, depth hoar appeared at the base of the pack in one or two days during a period of cold weather. Three well defined zones emerged as gradient metamorphosis proceeded. These zones had, in order, euhedral, subhedral, and anhedral structures, with the euhedral snow at the base. As expected, crystal size was related to euhedral development, with the coarsest snow at the base. Both whisk-broom abrasion tests on the pit face and resistometer readings showed a well defined band of weakness in the subhedral section of the metamorphosed layer (this occupied the middle third) with the strongest snow being the basal euhedral zone. Densities in the metamorphosed layer ranged from 0.16 to 0.20 Mg m-3. There was a very slight, but persistent, high value for the density of the intermediate weak zone with the basal strong zone having the lowest density in general. This suggests that upward mass transfer has taken place between the basal and intermediate layers.
The thermal profiles in nearly every case showed a marked increase in the temperature gradient associated with the intermediate weak zone, and thus there was a good correlation between snow weakness, ambient thermal gradient, and the subhedral phase of metamorphism. After the portion of the pack which had undergone gradient metamorphosis had attained a thickness of about 180 mm, depth hoar was well formed and the three zones were delineated. There was little evidence of the widening of the zones with time. There was, however, a general strengthening and perhaps a slight densification of all layers during this intermediate period. During the final period of observation, when the pack had attained a depth of 400 mm, both the anomaly in the thermal gradient and the anomaly in the strength had largely disappeared, even though the textural zoning was still clearly present. Throughout the period of zonal weakness there was a strong tendency for the snowpack to collapse onto the weak zone.
The laboratory portion of this study is incomplete, but so far it tends to support the field observations.
Conclusions
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Gradient metamorphism provides an advancing wave of change moving through the pack, usually up from the ground. First, there is a weakening of the snow, later, a period of strengthening occurs as recrystallization modifies the texture from finer anhedral snow to coarser euhedral snow.
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It seems likely that the metamorphic process itself generates the thermal gradient anomaly in the mid-zone, which in turn becomes the region of most rapid change accompanied by weakening. The state of minimum strength may be ephemeral in that it tends to disappear with the decreased thermal gradient as the pack deepens and mass transport diminishes.
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We suspect that the thermal gradient anomaly is in part a function of zonal differences in thermal conductivity and in part a function of sublimative heat exchange. The basal euhedral depth hoar probably represents a quasi-stable end product of gradient metamorphism. Its well sintered bonds and high permeability would favor conduction, convection, and the observed low thermal gradient. The subhedral zone on the other hand, represents the active front of metamorphism. Sublimative mass transfer from this zone to the upper zone would cause heat loss in the middle zone and heat gain in the upper zone which in turn could account for the observed anomalous inflections in the thermal gradient through those zones.
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Since a slight extra loading of the snowpack produced compressive failure and collapse of the pack on the weak zone above the euhedral zone, it seems unlikely that the strength of the dihedral zone can be attributed to load hardening.
A full report of this study is planned after the conclusion of the laboratory investigation of (he phenomenon.
Acknowledgements
This research was supported jointly by the office of Army Research, Durham under Grant No. DA-ARO-D-31-124-71-G59 and the National Science Foundation under Grant No. NSF FA-3943. The authors wish to thank the Yellowstone Park Service for making the Old Faithful area available for us to conduct the field research, as well as furnishing housing for the observer.
