| OCR Text |
Show - 3- Figure 5 shows a climax hard slab running on a stable base of old snow. The origin of this slide represents a condition peculiar to the Alta area, the occasional deposition of graupel ( pellet snow) in large quantities. Layers of graupel 20 to 30 cm thick are not uncommon, and these are an excellent source of slab avalanches by virtue of their high density and stiffness ( high viscosity). A 32- cm layer of graupel at the bottom of the Figure 5 slab was probably a principal source of the instability. More interesting, though, is the formation of the lubricating layer by a shallow deposit of very poorly consolidated graupel, literally a case of natural " ball bearings." Stratigraphic weaknesses, other than a general low strength of the slab layer, are not suggested by the ram profile. ;. Figure 6 represents a climax slab condition which clearly originated from poor support by depth hoar in the lower snow cover. The sliding layer was built up from several snowfalls, the earlier of which suffered loss of tensile strength through constructive metamorphism. Even here, where the weakness in the snow cover is obviously generated by extensive depth hoar development, the actual fracture has run back to a distinct lubricating layer, a shallow layer of depth hoar which was noticeably more fragile than the rest below it. In this case the very weak lower part of the snow cover is obvious from the ram profile, although the latter does not indicate the depth at which a slab would be expected to break away. Figure 7 illustrates an earlier stage in the hazard development shown in Figure 6. The basic depth hoar development in December provided insufficient support for the snowfalls of early January, resulting in a soft slab release two and one- half weeks earlier than the hard slab of Figure 6. Note that both slides released in the same way, by sliding on the thin, very fragile depth |
