スピッツベルゲンおよびわが国の高山・山地における凍結融解による斜面物質移動

抄録

In this study the author attempts to discuss slow mass-movement processes caused by freezing and thawing in relation to the compositions of slope materials and fluctuations of ground temperature, in Spitsbergen and in the Japanese high mountains.
Study sites in Spitsbergen are located in Adventdalen and Reindalen, Nordensiöldland, central Spitsbergen (Fig. 1). These sites are debris slopes with gradients ranging from 6° to 32°, mainly covered with Jurassic, Cretaceous, and Tertiary shale fragments.
The rates and deformation patterns of downslope movement of surface rubble are measured by eighteen painted stone-lines. Many grass fibre-tubes, furthermore, were inserted vertically into the ground in order to detect downslope movement in the active layer.
Year-round ground temperature measurement was achieved at a flat surface (480 m a.s.l.) of the Adventdalen site using data logged at 3-hour intervals in 1988-1989. Sensors were installed at 5, 15, 30, 50 and 80 cm depths.
Studies to slow mass-movement in the Japanese high mountains were performed in Mt. Shirouma, the Kitakami Mountains, and Mt. Hoh-o. Long-term ground temperature records were observed on the Kitakami Mountains, Mt. Hoh-o and Mt. Akaishi.
The results of this study can be summarized as follows.
The average movement rate of painted stones from 1988 to 1990 was 3.3 cm/yr. in Spitsbergen (Fig. 2). Deformation features of all painted stone-lines showed patterns parallel to the basement lines independently of the compositions of slope materials (Figs. 3. 4. 5. 6). The movement rates at the tops of grass fibre-tubes were similar to the rate of movement measured by painted stone-lines (Fig. 8). These imply that deformations of painted stone-lines and grass fibre-tubes occurred by the same processes, that is, frost creep and gelifluction. Furthermore, movement of slope materials by diurnal freeze-thaw cycles is negligible in Spitsbergen, because of the very low frequency of this cycle (Fig. 10; Table 3). Therefore it is considered that annual freeze-thaw cycles mostly contribute to the movement of the slope materials in this area.
Average movement rates of painted stones measured in the Japanese high mountains are 26.1 cm/yr. in Mt. Shirouma, 21.7 cm/yr. in the Kitakami Mountains, and 39.7 cm/yr. in Mt. Hoh-o (Fig. 2). These rates are one figure faster than those of Spitsbergen. Deformation features of painted stone-lines are classified mostly into two patterns, one a festoon type and the other a parallel type (Fig. 7). The parallel type occurs on slopes mantled with thick surface rubble of coarse size. The festoon type is recognized on slopes mantled with thin surface rubble and fine materials.
The vertical profiles of many flexible tubes from the above-mentioned mountains on which they were inserted to become almost negligible at about 30-40 cm in depth (Fig. 8). This depth is about one half as shallow as that of Spitsbergen.
Freeze-thaw cycles in the Japanese high mountains overlap with diurnal ones and annual ones, especially those occurring in very high frequency (Fig. 9; Table 3). Movements on the slopes covered with thin rubble and fine materials are caused by diurnal and annual cycles. On these slopes deformations of painted stone-lines show the festoon pattern, and the movement rates are faster than those on slopes consisting of coarse rubble, which almost all move in annualfreeze-thaw cycles.
Considering the above mentioned results, in Spitsbergen differences incompositions of slope materials do not reflect movement processes and the rate of movement, because movement of slope materials occurs in almost annual freeze-thaw cycles. On the other hand, in the Japanese high mountains movement of slope materials occurs in two different freeze-thaw cycles, which are annual and diurnal ones.