Quantifying frost weathering induced rock damage in high alpine rockwalls
Abstract. Frost weathering is a key mechanism of rock failure in periglacial environments and landscape evolution. At high alpine rockwalls, freezing regimes are a combination of diurnal and sustained seasonal freeze-thaw regimes and both influence frost cracking processes. Recent studies have tested the effectiveness of freeze-thaw cycles by measuring weathering proxies for frost damage in low-strength and grain-supported pore space rocks, but detecting frost damage in low-porosity and crack-dominated alpine rocks is challenging due to small changes in these proxies that are close to the detection limit. Consequently, the assessment of frost weathering efficacy in alpine rocks may be flawed. In order to fully determine the effectiveness of both freezing regimes, freeze-thaw cycles and sustained freezing were simulated on low-porosity high-strength Dachstein limestone under temperature and moisture conditions that reflect those found in high alpine rockwalls. Frost-induced rock damage was uniquely quantified by combining X-ray computed micro-tomography (µCT), acoustic emission (AE) monitoring and frost cracking modelling. To differentiate between potential mechanisms of rock damage, thermal- and ice-induced stresses were simulated and compared with AE activity. µCT combined with AE data revealed frost damage on low-porosity alpine rocks with crack growth along pre-existing cracks with magnitudes dependent on the initial crack density. It was observed that diurnal freeze-thaw cycles have a higher frost cracking efficacy on alpine rocks compared to a seasonal sustained freezing regime. On north-facing high alpine rockfaces, the number of freeze-thaw cycles and the duration of sustained freezing conditions vary with elevation and seasonal climate. The experimental results establish a link between frost damage and elevation-dependent rockwall erosion rates, which has implications for hazard prediction in mountainous areas under a changing climate.
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