Snow and Ice-Symposium-Neiges et Glaces (Proceedings of the Moscow Symposium, August 1971; Actes du Colloque de Moscou, août 1971): IAHS-AISH Publ. No. 104, 1975.

Metamorphism from snow to firn and ice in a small snow patch on Mt. Daisetsu, Hokkaido, Japan

G. Wakahama and H. Narita

Abstract. Fairly large single crystals of ice, 5-10 cm in diameter, have sometimes been found in perennial snow patches in high mountainous regions in Japan, in spite of the fact that the size of the snow patches is mostly less than 100 m x 100 m in area and their thickness does not exceed 10 m at the end of the ablation season. This suggests that the mechanism of grain growth of ice in a snow patch does not always depend on the overburden pressure or mechanical strains caused by flow. Based on glaciological studies made from 1962 to 1970 on snow patches in Japan, it is proposed tentatively that liquid water in a snow patch may accelerate both the transformation process from snow to ice and the formation of large single crystals of ice.

Résumé. Des monocristaux de glace de très grande taille (5-10 cm de diamètre) ont été trouvés parfois dans des névés permanents des hautes montagnes du Japon, et cela bien que l'étendue de ces névés soit généralement inférieure à un hectare et que leur épaisseur ne dépasse pas 10 m à la fin de la saison d'ablation. Cela fait penser que le mécanisme de croissance du grain dans un névé n'est pas toujours lié à la pression ou au fluage. En se basant sur des études glaciologiques poursuivies de 1962 à 1970 sur les névés du Japon, on émet l'hypothèse que l'eau liquide présente dans un névé accélère à la fois la transformation de la neige en glace et la formation de grands monocristaux.

INTRODUCTION A small snow patch in Honshu named 'Hamaguri-yuki' was investigated in 1962-63 (Yoshida, 1964). In this investigation, many large single crystals of ice, 5-10 cm in diameter, were found in the snow patch which consisted entirely of ice (density >0.85 g/cm3). Twenty-two annual snow boundaries were observed in the snow patch, which allowed us to estimate the age of the ice layers in which large single crystals were found as several tens of years old. These ice crystals are comparable in size with those found in temperate glaciers in Alaska and Canada. In order to inquire into the mechanisms of the grain growth of ice and transformation process from firn to ice occurring in such a small snow patch, glaciological studies have been made for 1964-70 on a snow patch in Hokkaido (Kinosita, 1965 and 1966; Wakahama, 1968 and 1969).

STUDIES OF A SNOW PATCH ON MT. DAISETSU, HOKKAIDO A small snow patch named 'Yukikabe' (1730 m above sea level) on Mt. Daisetsu has been investigated. The accumulation of snow on the snow patch attained 15-20 m in depth in winter. The cumulative air temperature and the average ablation rate on the snow patch were estimated as 1000°C days and 7 cm water/day. Flow measurements were made by means of a borehole in 1966, but no discernible flow was observed during the ablation season.

Core analysis A boring was made every year in the snow patch, and petrofabric studies of cores were made. The temperature of the snow observed in the borehole was 0°C throughout the entire mass. 348 G. Wakahama and H. Narita Stratigraphy, density The stratigraphies obtained in 1967-70 together with the density profiles are shown in Fig. 1. The solid lines, thick dark bands and dots indicate thin ice sheets, thick ice layers and snow grains respectively. The numbers I, II ... , VII marked at the left-hand side of each profile show the age of the snow, which are separated by the annual snow boundaries. These annual boundaries were identified by sawdust scattered on the surface of the snow patch at the end of each previous ablation season. It was confirmed in 1970 that the snow layers of 6 and 7 years old had almost transformed into ice whose density was 0.80-0.85 g/cm3.

sit 1967 1968 1969 1970 D

FIGURE 1. Stratigraphy and density profiles of 'Yukikabe' snow patch on Mt. Daisetsu from 1967 to 1970.

Snow grains Almost all snow layers were composed of coarse-grained snow. The grain size was 0.5-1.5 mm in diameter.

Rate of crystal growth and ice fabrics Size of crystals, rate of crystal growth The size of ice crystals composing the snow grains was measured on microphotographs of thin sections from each snow layer. The size was less than 5 mm in diameter, and no large single crystal of ice as found in 'Hamaguri-yuki' was observed. The mean cross- sectional area of ice crystals plotted against the age of snow yields the average rate of crystal growth of 12 x 10~2 mm2/yr. Metamorphism from snow to firn and ice 349 Fabric analysis The crystallographic c-axes measurements were conducted on a Rigsby stage. Typical fabric patterns obtained from 6-year-old icy snow are illustrated in Fig. 2. The snow was composed of randomly orientated crystals in the upper portion of the layer. In the middle portion, however, a fairly strong vertical pole and another broad concentration at 70°-80° from the vertical were observed, while the vertical pole disappeared and the horizontal concentration became more remarkable in the lower portion. A similar fabric variation with depth was found in each annual snow layer.

FIGURE 2. Ice fabric patterns obtained from the upper (a), middle (b), and lower (c) portions of 6-year-old icy snow in 1970.

DISCUSSION The role of free water in transformation from snow to ice It may be considered that the degree of transition from firn to ice depends primarily on the snow temperature and overburden pressure in the snow. However, this argument is not always applicable, because the transition layer was found at 15-40 m in depth in a temperate glacier (Benson, 1962; Shimizu and Wakahama, 1965), while it was at 6-8 m in the 'Yukikabe' snow patch. This fact suggests that there may be another factor controlling the transformation process from snow to ice. According to our experiment, it was found that when the snow, being immersed in water of 0° C, was loaded with a weak stress of 100 g/cm2, it turned to ice within a week (Wakahama, 1967). The bottom layer of a snow patch is usually saturated with melt water, because water percolating through snow does not quickly permeate into bed-rock but remains on the bed. Such a strong water-impermeable boundary may not be observed in the deep firn of a temperate glacier. The plastic deformation of the bottom snow in a snow patch may be accelerated by the existence of free water, and it turns into a thick ice layer.

The growth rate of ice crystals in the snow patch Gow (1969) reported the temperature dependence of the growth rate of ice crystals in the firn in Antarctica. An extrapolation of his curve to the melting point of ice yields the growth rate of 8 x 10~2mm2/yr at 0°C, which agrees well with that obtained in our snow patch: 12 x 10~2mm2/yr. The agreement suggests that the driving force in ice crystal growth in our snow patch may be attributed to the interfacial free energy between ice crystals. If a large single crystal of ice found in 'Hamaguri-yuki' had grown at this rate, it would take more than 20,000 years, which is most unlikely. Another possible mechanism of ice crystal growth is that when a cold wave begins to penetrate into the snow patch, liquid water existing in pores and grain boundaries in icy layer may be sealed off by partial freezing. The isolated water exerts high pressure on the surrounding ice crystal lattice with decreasing temperature (more than 100 kg/cm2/—1°C). This may cause plastic deformation or internal fracture followed 350 G. Wakahama and H. Narita by the recrystallization within the ice mass, as in the case of the 'frost-shattering effect' in a rock. The following fact should be reported to emphasize the above argument on the mechanisms of ice crystal growth. On 8-9 March 1968, when a drill hole was made near the terminus of Menenhall Glacier in Alaska, it was observed that the drill hole was filled with liquid water percolating through the glacier body at the rate of 6 x l(T6cm/s, which is equivalent to the permeability of a compact silt (Takahashi and Wakahama, 1970). This fact is strong evidence for the existence of liquid water in the glacier ice mass; it must play an important role in producing large single crystals of ice near the surface of a temperate glacier. Experimental studies on this line are going on in our laboratory, and the preliminary results seem to support the possibility of this mechanism of grain growth.

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