Paraglacial rock-slope failure following deglaciation in western Norway

The paraglacial framework describes the geomorphological response to glaciation and deglaciation, whereby non-renewable, metastable, glacially-conditioned sediment sources are progressively released by a range of nonglacial processes. These include slope failures that directly modify the bedrock topography of mountain landscapes. This chapter synthesises recent research on the paraglacial evolution of western Norway’s mountain rock-slopes, and evaluates the importance of glaciation, deglaciation, and associated climatic and non-climatic processes. Following an introduction to the concept of paraglacial landscape change, current understanding of rock-slope responses to deglaciation are outlined, focussing on the spatial distribution, timing, duration and triggers for rock-slope failure (RSF). Preliminary analysis of an inventory of published ages for 49 prehistoric RSFs indicates that the great majority of activity occurred in the Late Weichselian / Early Holocene transition (~13-9 ka), within 2 ka of deglaciation. Subsequent RSFs were much smaller, though event frequency increased again at 8-7 ka and 5-4 ka BP. The majority of RSFs were not directly triggered by deglaciation (debuttressing) but were preconditioned for more than 1000 years after ice withdrawal, until slopes collapsed. It is proposed that the primary causes of failure within 2 ka of ice retreat were stress redistribution, subcritical fracture propagation, and possibly seismic activity. Earthquakes may have triggered renewed RSF in the Late Holocene, though it seems likely that permafrost degradation and water supply were locally important. Priority avenues for further research are briefly identified.