Identification of rock and fracture kinematics in high alpine rockwalls under the influence of elevation

Abstract

Abstract. In alpine environments, tectonic processes, past glaciation and weathering processes fracture rock and prepare or trigger rockfalls, which are important processes of rock slope evolution and natural hazards. In this study, I quantify thermally and ice-induced rock and fracture kinematics and place these in the context of their role in producing rockfall and climate change. I conducted laboratory measurements on intact rock samples and installed temperature loggers and crackmeters at four rockwalls reaching from 2585 to 2935 m in elevation in the Hungerli Valley, Swiss Alps. My laboratory data show that thermal expansion followed three phases of rock kinematics, which resulted in a hysteresis effect. In the field, control crackmeters on intact rock reflected these temperature phases, and based on thermal expansion coefficients of these observed phases, I modelled thermal stress. Model results show that thermal stress magnitudes were predominantly below rock strengths. Crackmeters across fractures revealed fracture opening during cooling and reverse closing behaviour during warming on daily timescales. Elevation-dependent snow cover controlled the number of daily temperature changes and thermal stresses affecting both intact and fractured rock, while the magnitude is controlled by topographic factors influencing insolation. On a seasonal scale, slow ice-segregation-induced fracture opening can occur within lithology-dependent temperature regimes called frost cracking windows. Shear plane dipping controlled whether fractures opened or closed irreversibly with time due to thermally induced block crawling on an annual scale. Climate change will shorten snow duration and increase temperature extremes and will, therefore, affect the number and the magnitude of thermal changes and associated stresses. Earlier snowmelt in combination with temperature increase will shift the ice-induced kinematic processes to higher elevations. In conclusion, climate change will affect and change rock and fracture kinematics and, therefore, change rockfall patterns in alpine environments. Future work should quantify rockfall patterns and link these patterns to climatic drivers.

Temporal variability and site specificity of thermomechanical weathering in a temperate climate

“…Warmingcooling cycles producing stress of this magnitude are sufficient to cause subcritical fracture propagation by causing routine expansion and contraction of pre-existing fractures (i.e., thermal fatigue; Eppes and Keanini, 2017). Hysteretic crack expansion and contraction resulting from diurnal temperature changes has been observed to result in breakage by expanding discontinuities over time (Draebing, 2021;Racek et al, 2021). Collins and Stock (2016) measured the work performed upon discontinuities by hysteretic cycles which aligns with our finding that diurnal warming and cooling can exert a high magnitude of thermal stress.…”

“…We observed hysteresis between the mean hourly surface and fracture temperatures at all sites during the study period, indicating that both the rock surface and fractures experience routine cyclical heating and cooling year-round (Figure 7). Previous work suggests that these warming-cooling cycles result in a hysteretic temperature-deformation response throughout the day (Collins and Stock, 2016;Fiorucci et al, 2018;Draebing, 2021). The orientation of the hysteretic pattern remained relatively constant across seasons which reflects that the relationship between surface and fracture temperature is independent of seasonality in climatic conditions.…”

“…A more direct angle of insolation during the spring also increases radiative heat flux throughout the day. The effects of these factors have been documented elsewhere (e.g., Hall and André, 2001;Matsuoka, 2008;Gunzburger and Merrien-Soukatchoff, 2011;Vasile and Vespremeanu-Stroe, 2016;Kellerer-Pirklbauer, 2017;Draebing, 2021;Taye and Viles, 2021), although not in relation to weathering seasonality. Diurnal temperature cycles and minute-scale oscillations occurred more slowly in fractures and rates were consistent across seasons (Table 4; Figure 3).…”

Alpine Rockwall Erosion Patterns Follow Elevation-Dependent Climate Trajectories

“…That warming and retreat will intensify frost cracking at higher elevations and in proximity to retreating glaciers, respectively, whereas at lower elevations the warming will reduce frost cracking 32 . Although lower elevations will experience a reduction of periglacial and paraglacial processes, other climate-dependent weathering processes 19 such as chemical and thermal weathering [93][94][95] may replace them as the main drivers of rockwall erosion 96 . Over time, the areas of most intense periglacial and paraglacial processes and resulting erosion will shift to higher elevations, in locations where higher topography exists.…”

“…To model frost cracking and mean annual rock surface temperature (MARST), we measured rock surface temperature (RST) at five rockwalls along a topographic gradient from 2,580 to 3,148 m (Fig. 2) using Maxim iButton DS1922L temperature loggers installed in 0.1 m deep boreholes, similar to previous studies 32,80,93,102 . The loggers recorded RST at 3 h intervals (Supplementary Fig.…”