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A new snow ablation form - avalanche mushroom - is deceribed from area of Giant Mts. Its rise, mature and collaps is conected with melting of dirt avalanche accumulations, i.e. it is short time form only in contrast to glacier tables.
Keywords: Cover forms of snow ablation, ground and mixed avalanche accumulations, avalanche mushroom, short time structures
1. INTRODUCTION
TROLL (1949) classified snow and ice ablation forms into two categories - "cover" forms ("Formen der bedeckten Ablation.) and "free" forms (.Formen der freien Ablation.). The formation of cover forms is conditioned by the presence of either:
1) fine mineral or organic material which gives rise concave forms called "Ablationshohlformen. (e.g. cryoconite holes), or
2) larger objects which shade the underlying snow and ice, creating convex forms called "Ablationsvollformen" (e.g. glacier tables, Schneekegeln).
As part of a programme of monitoring the influence of avalanche activity on the environment in the Giant Mountains, two types of cover ablation form were repeatedly observed in melting avalanche accumulations. These appear to be previously undescribed, and we name them "avalanche mushrooms. because of their origin and shape*.
2. DESCRIPTION
Avalanche mushrooms are analogous to glacier tables, sensu TROLL (1949), that typically occur on high mountain glaciers. Unlike them, avalanche mushrooms occur in avalanche fields. They are not formed of ice pillars covered by blocks of rock, but rather of snow (firn) pillars covered by turf, grass, branches or forest litter (Fig. 1.) mixed into snow during the descent of dirty avalanches, landslides or slushflows. Sometimes firn-only mushrooms occur in avalanche accumulations. In these cases, the protective cover is formed by a plate of harder snow or by an ice crust (Fig. 2.). Avalanche mushrooms reach a height from several centimetres to several decimetres (maximum about 1 m). The size depends on the size of the turf cover or snow plate, as well as on climatic conditions. The observed turf thickness is usually around 20 cm; length and width, up to 1 m.
2.1. Origin and period of occurance
The principle of origin of avalanche mushrooms is same as for glacier tables. A layer of turf protects the snow (firn) under it from solar radiation, like a block of rock on a glacier table shades the ice under it....