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Rock fall in high alpine regions
The destabilisation of rock faces is not a process that happens overnight but is instead the result of a long process that may last many thousands of years. Depending on the rock structure (e.g. fractures, stratification, hardness) and the topography, erosion processes act slower or faster. In high alpine regions, glaciers and permafrost also play a key role. Various factors may lead to fractures forming and opening up, increasingly destabilising the rock:
These factors have an impact over very different time periods. Over thousands of years, seasonal shifts in temperature weaken the rock. By contrast, heavy rainfall only becomes a decisive factor when there are already open crevices into which water can accumulate and thereby exert pressure on the rocks. Ice pressure in fractures plays a key role in high alpine regions. Such pressures occur when water from precipitation or pore spaces in the rock permeates into crevices and freezes there. The resulting ice wedges expand and fragment the rock.
During the ice ages, the glaciers formed steep mountainsides. When glaciers retreat, steep rock faces may be exposed and lose a crucial support.
Figure: Time frames for the preparation of a rock fall (R. Kenner, SLF)
The role of permafrost
Permafrost refers to soil or rock substrate with a temperature below 0°C for many years. In the Alps, permafrost in rock faces is found in areas above 2500 metres. On south-facing rock walls, permafrost is found above 3000 metres. In terms of rock fall, permafrost has two different roles:
Therefore, permafrost may accelerate the fragmentation of rock faces yet at the same time stabilise these weak areas.
The role of global warming
Global warming accelerates various processes:
Rock fall Parpaner Weisshorn, 29. 11. 2016 (Photo Tom Meissen)