Influences Driving and Limiting the Efficacy of Ice Segregation in Alpine Rocks

<title xmlns:mml="http://www.w3.org/1998/Math/MathML">Abstract</title><p xmlns:mml="http://www.w3.org/1998/Math/MathML" xml:lang="en">Rockwall erosion by rockfall is largely controlled by frost weathering in high alpine environments. As alpine rock types are characterized by crack‐dominated porosity and high rock strength, frost cracking observations from low strength and grain supported pore‐space rocks cannot be transferred. Here, we conducted laboratory experiments on Wetterstein limestone samples with different initial crack density and saturation to test their influence on frost cracking efficacy. We exposed rocks to real‐rockwall freezing conditions and monitored acoustic emissions as a proxy for cracking. To differentiate triggers of observed cracking, we modeled ice pressure and thermal stresses. Our results show initial full saturation is not a singular prerequisite for frost cracking. We also observe higher cracking rates in less‐fractured rock. Finally, we find that the temperature threshold for frost cracking in alpine rocks falls below −7°C. Thus, colder, north‐exposed rock faces in the Alps likely experience more frost cracking than southern‐facing counterparts.</p>

Plain Language Summary: Freezing results in the formation of ice that exerts stresses on fracture walls and draws in additional moisture to supply further growth and break down rocks, a process termed frost cracking. Frost cracking drives much erosion and rockfall in alpine environments. Here we test hypotheses from prior work about how frost cracking is impacted by saturation and rock properties. We exposed rock samples of different strength and saturation to identical freezing conditions in laboratory experiments. We monitored rock temperature and acoustic emissions (AE), assuming frost cracking produces the recorded AE hits. We find that initial full saturation is not required for frost cracking, as water transport is enhanced by fractures in alpine rocks. Furthermore, rock with initial higher short‐term strength showed more frost cracking because, we infer, of stiffness properties that make these rocks more brittle compared to lower strength rocks. Frost cracking occurred at a wide range of temperatures below freezing and was highest between −9 and −7°C. We thus conclude that frost cracking is most impacted by temperature and rock short‐term strength. In Alpine environments, this may result in more frost cracking and rockfall on colder north‐facing rockwalls than warmer southern exposures.</p>

Key Points: Initial saturation levels do not limit the efficacy of ice segregation in fractured alpine rocks. Rock initial crack density impacts rock stiffness and thermal properties and thus frost cracking efficacy. The “frost cracking window” temperature range is dependent on rock strength and crack‐controlled porosity in alpine rocks.

German Reserach Foundation

https://doi.org/10.6084/m9.figshare.23584686