OCR Text |
Show of 200 m^ and over occurred. The Origin of Avalanches There is no difficulty in detecting the movement of an avalanche of wet snow, but it is not always possible to follow the movement of an avalanche of dry snow, over which hangs a cloud of obscuring snow dust. This cloud may accompany the avalanche right to its terminus and disperse only after all movement has ceased. All the transformation stages between stable snowcover and terminal mound ( snow) may be observed in the avalanche whether the travel is 1- 2 m, 100- 200 m, or a kilometer or more. Avalanches are often observed of which the tail ( upper) end still holds, the slope and preserves the same properties as stable snowcover, and the fore ( lower) end of which consists of a loose, heterogeneous snow mixture. Examination of the break- away site of an avalanche provides much information, not only on the cause of the slide, but also on the nature of its primary movement. Of great value also is inspection of the avalanche track, and especially of the terminal mound, for features of avalanche movement and the final moments of its travel. Thus, it is not difficult to reconstruct in all its details the descent of avalanche snow. The character of the upper end of an avalanche depends on snow conditions, the lay of the snow, and the particular causes of the slide. When bound snow" slides, a rift appears and the detached snow descends, constituting an avalanche. The snow remaining behind is left in the form of a step rising from the bare ground. The vertical face of this step is a plane surface, even when the snow is multilayered. Separate layers seldom protrude. This step face is most often normal to the slope, but may also be vertical. The usual height of the breakaway step is 20- 70 cm, up to 2 m in large avalanches. The highest rupture face observed, 4 m, was left by a 10,000 m^ avalanche which slipped only 8 m, the shortest travel measured, on May 7, 1934, on the northwest slope of Mount Ukspor. Here the vertical face was smooth and even, as though cut by a knife. The breakaway for this slide occurred in deep snowcover where the snow had been washed out by stream action. Ordinarily the length of the rupture face is measured in tens of meters. Being of considerable length, then, the break- away nearly always consists of a line of separated step faces sometimes parted at layer floors. With a 285,000 m3 avalanche of December 22, 1936, on the north slope of Mount Aykuayventchorr, the 800 m rupture line consisted of a chain of separated steps, each one 20- 25 m long. The longest step was 500 m, observed with a 100,000 m3 avalanche of December 17, 1936, at Hackmann Pass. Seen from above, the breakaway step presents a straight, evenly curved, wavy, or zigzag front, sometimes an arch or " cape" where dictated by the relief. As a rule, steps appear above breaks or sharp declivities in the slope. In avalanches of strong, firm snow overlying loose granular or coarse granular snow, the rupture line usually shows above a break at the steepest part of the slope; in avalanches of loose windblown snow, on the other hand, the break- away occurs at more gently graded parts of the slope. The first case is explained by the fact that the break- away of strong, firm snow takes place on the borderline between parts of the snowcover that are subject to different gravitational force; in the second case the break- away is caused by the accumulation at the foot of steep slopes of windborne snow from above, to the point where the snow gives way under gravity. Thus, the answer to the old question argued by mountain climbers as to whether a steep broken or - 32- |