The role of frost cracking in local denudation of steep Alpine rockwalls over millennia (Eiger, Switzerland)

Abstract

Abstract. Denudation of steep rockwalls is driven by rock fall processes of various sizes and magnitudes. Rockwalls are sensitive to temperature changes mainly because thermo-cryogenic processes weaken bedrock through fracturing, which can precondition the occurrence of rock fall. However, it is still unclear how the fracturing of rock together with cryogenic processes impacts the denudation processes operating on steep rockwalls. In this study, we link data on long-term rockwall denudation rates at the Eiger (Central Swiss Alps) with the local bedrock fabric and the reconstructed temperature conditions at these sites, which depend on the insolation pattern. We then estimate the probability of bedrock for failure through the employment of a theoretical frost cracking model. The results show that the denudation rates are low in the upper part of the NW rockwall, but they are high both in the lower part of the NW rockwall and on the SE face, despite similar bedrock fabric conditions. The frost cracking model predicts a large difference in cracking intensity from ice segregation where the inferred efficiency is low in the upper part of the NW rockwall but relatively large on the lower section of the NW wall and on the SE rock face of the Eiger. We explain this pattern by the differences in insolation and temperature conditions at these sites. Throughout the last millennium, temperatures in bedrock have been very similar to the present. These data thus suggest the occurrence of large contrasts in microclimate between the NW and SE walls of the Eiger, conditioned by differences in insolation. We use these contrasts to explain the relatively low denudation rates in the upper part of the NW rockwall and the rapid denudation in the SW face and in the lower part of the NW rock face where frost cracking is more efficient.

Topographic signatures of progressive glacial landscape transformation

“…This was predicted by numerical modelling (e.g. Anderson et al, 2013;Egholm et al, 2015;Hales & Roering, 2007), and shown by cosmogenic nuclide (Mair et al, 2020) and monitoring studies (Hartmeyer et al, 2020). Thus, the few metres of periglacial erosion predicted in our experiments might strongly underestimate real-world headwall retreat, at least in temperate mountain ranges.…”

Rarefied particle motions on hillslopes – Part 4: Philosophy

“…As a point of reference, Benjamin et al (2020) provide an assessment of efforts to observe and measure rockfall events contributing to cliff erosion and thus to downslope delivery of particles. The frequency and magnitude of these events may vary widely, from the chronic activity of small rockfall events to large infrequent events, depending on the geological and environmental factors that influence the mechanisms of weathering and failure (Luckman, 2013;Strunden et al, 2015;Mair et al, 2020). The frequency of occurrence of rockfall volume typically varies as an approximate inverse power function of volume, where the specific relation depends on the spatial coverage and temporal duration of the data set (Benjamin et al, 2020).…”

Rarefied particle motions on hillslopes: 4. Philosophy

“…As a point of reference, Benjamin et al (2020) provide an assessment of efforts to observe and measure rockfall events contributing to cliff erosion and thus to downslope delivery of particles. The frequency and magnitude of these events may vary widely, from the chronic activity of small rockfall events to large infrequent events, depending on the geological and environmental factors that influence the mechanisms of weathering and failure (Luckman, 2013;Strunden et al, 2015;Mair et al, 2020). The frequency of occurrence of rockfall volume typically varies as an approximate inverse power function of volume, where the specific relation depends on the spatial coverage and temporal duration of the data set (Benjamin et al, 2020).…”