| OCR Text |
Show underlying surface. In a bed of melting snow, the water runs over the dense layers, or over the granular layers, and in its spread raises the overlying layers and provides the basis for a slide, as happened in the case of an avalanche of May 7, 1934, already mentioned. It is possible for some avalanches to be caused by ground swelling ( frost-heaving). In the vicinity of Kirovsk, where the ground freezes to a depth of a meter and more, ground swelling is very common. Freezing of the ground water increases volume. One often observes ground swelling to a depth of 30 cm and more, often with lenses of pure ice appearing in the soil mass. Conditions for ground swelling are especially favorable on mountain slopes where water circulation is on such a large scale, and where impermeable rock is present to serve as an undersurface for the water- bearing snow layer. Where ground swelling takes place over not too large an area, break- away of the entire snowcover thickness may result, since the swelling acts with greatest force on the bottom or weakest snow layer. Even where snow stability is maintained, the snowcover on a slope does not remain immobile, but moves very slowly downslope. This downward displacement is imperceptible but is considerable enough to bend over trees and gauge poles on valley sides. One sometimes observes a small cleared space below a tree, where the snow has slipped completely away. Where a critical snow bed is stretched ( pulled apart) in the upper strata and compressed in the bottom layers, inner stresses are set up in both cases, particularly in snow on upper slopes. Internal stress by stretching often results in crevasse production. These crevasses appear at breaks in grade, i. e., at points of increase, or drops, parallel to the break line. Such crevasses open suddenly, with a characteristic sound like the deafening crack of breaking ice. They appear also in wet snow at temperatures of 0° C, showing that they result from stresses produced by stretching and not from temperature changes. In addition to the internal stresses of stretching and compression, strains of another order are set up in snow moving slowly downslope. The upper snow layers move more than the lower ones, a fact confirmed by the greater width of crevasses at the snow surface than at the bottom. As a consequence, the tendency to shear of a given layer is all the greater, especially so since the different layers vary in plasticity. And since the relative displacement of the upper and lower layers is greater than that between different parts of the same layer on different portions of the slope, these stresses are greater than those set up by surface stretching or deep compression in the critical snow bed. The layered structure of snowcover is the key factor in this connection. Windblown, and even hard, snow readily lays down layers. The shearing force is usually less in layer contact planes than in the layer mass. The ( interlayer) ice crusts not only provirJe a perfect slippage surface but conduce to avalanching because of the decrease in density of the snow in contact with them. - 44- |
