Texture and strength changes of buried surface-hoar layers with implications for dry snow-slab avalanche release

Buried layers of surface hear are the failure plane for many slab avalanche s, including fatal human-triggered avalanches in various mountain regions. These layers may persist as weak layers in the snow cover for weeks or mont hs. It is therefore essential for operational avalanche forecasters to moni tor the evolution of persistent weak layers, such as buried surface hear. T raditional grain-shape observations of isolated grains with a magnifier and crystal screen do not show bonding that is decisive for strength. In this study we used in situ microphotography and observations of texture to compl ement strength measurements from shear frame tests. Buried layers of surfac e hear consist of crystals most of which extend from the layer below to the layer above, and may exhibit a columnar or truss-like structure. Observati ons and measurements show that texture and crystal size change little over periods of up to several months during which the snowpack remains dry. Unde r these conditions, layer thickness decreases while density and strength in crease. Based on field measurements, we argue that the increase in strength is primarily due to penetration of the surface-boar crystals into the adja cent layers, especially at the bottom of the buried surface-hear layer, whe re bonding is critical. The weak bonding at the bottom implies that shear f ailure occurs at the lower interface rather than within the weak layer. On slopes: we find that surface-hear crystals that were initially surface norm al are tilted downslope faster than predicted by published shear strain rat es for settled snow, indicating that shear strain is concentrated in these layers. The characteristic texture of buried surface boar (columnar or trus s-like) permits collapsing at the time of fracture. The gravitational energ y released by the displacement of the slab may contribute to the extensive fracture propagation associated with buried surface-hear layers.