| Summary: | Rock avalanching is a very hazardous process in mountain areas, that generates a high risk in inhabited valleys. Present glacier shrinkage and permafrost degradation in steep rockwalls could increase the frequency and magnitude of rock avalanching in the context of the current climate change. In high glaciated mountain, rock avalanches interact with glaciers in different ways. Oversteepening of rock slopes by glacial erosion, and effects of glacier thinning and retreat on debuttressing of rockwalls and on permafrost prepare rock avalanche triggering. Channelling of rock avalanche by moraines and glacial valleys, incorporation of ice, and irregularities on the glacier surface modify the rock-avalanche mobility. High elevated scars resulting from rock avalanches favour the formation of small glaciers. Here, we describe how rock avalanche deposits modify glacier dynamics, and how complex could moraine complexes result from rock avalanches onto glaciers, basing our arguments on case studies mainly located in three of the larger massifs of the Western Alps: Mont Blanc, Vanoise and Ecrins massifs. Large debris accumulation deposited by a rock avalanche onto the glacier surface is a driving factor of the glacier dynamics because it can strongly reduce its ablation, especially when the glacier becomes debris-covered. Modification of the glacier mass balance by a dense debris cover allows: (i) the rate of retreat or advance of its front to be different than this of a clean-type glacier, with accelerated advance during positive mass balance periods, and slow down retreat during negative ones; this is illustrated for instance by the control of 1920 and 1997 rock avalanches on the dynamics of Brenva Glacier; (ii) advances to last longer or to reach further downvalley, as shown by the case of Miage Glacier both during the Neoglacial and the 20th Century; and (iii) small glaciers to survive at a lower elevation than the regional ELA (e.g. Grande Casse or Drus Glaciers). In a secondary way, the supplementary mass of debris can ...
|
|---|