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Show - 2- See Chapter 6 of " Snow Avalanches" ( FSH 2332.81) for explanation of observation methods and symbols used in plotting the profiles. Figures I and 2 illustrate a typical soft slab avalanche. Both profiles were taken on the same day on different avalanche paths to depict the uniformity of snow conditions which led to a cycle of artificial slide releases throughout I the Alta ski area. In each case the layer of new snow from a single storm slid: on a sun crust to which it was poorly attached due to the low January temperatures. In one case ( Figure 2) a thin layer of old snow was located between the new snow and crust, but this apparently did not play an essential role in the avalanche release. Although the base consisted of unstable depth hoar which became involved in the slide as it built up momentum, this did not influence the origin of the slide, which was released as a direct- action soft slab running on the thin sun crust. Note that the ram profile shows very little stratigraphy in such a situation, the only detectable layer being the crust. Figures 3 and 4 were also taken on the same day to illustrate a climax hard slab condition which extended' throughout the Alta area. In this case the sliding layer involved snow from several distinct storms. The sliding surface was a crust underlain by old snow which had undergone partial constructive metamorphism, but which was stable enough not to become involved in the avalanche. The immediate cause of this avalanche cycle was a very weak, thin layer at the bottom of the slab which offered low shear strength and served as the lubricating layer for the slab release. It consisted of very fragile depth hoar crystals just above the crust. Weather records do not show clearly the origin of this layer,: but it is presumed to be a sMllow deposit of fluffy snow in which temperature gradient and constructive metamorphism were subsequently enhanced in relation to the surrounding snow. The roles of sliding surface ( crust) and lubricating layer ( depth hoar) are clearly illustrated in this case. |