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.
Highlights
Alpine environments are characterized by high relief due to the interaction of tectonic uplift, climate and erosion (e.g. Schmidt and Montgomery, 1995; Whipple et al, 1999; Egholm et al, 2009)
On each rock sample (Fig. 2), two crackmeters were installed at the top (RD1) and at one side of the sample (RD2) to monitor rock deformation (RD) and rock-top temperature (RTT) at 1 min intervals
Cyclic thermal stresses can result in thermal fatigue that break down rock and are an important component of mechanical weathering (Fig. 10a)
Summary
Alpine environments are characterized by high relief due to the interaction of tectonic uplift, climate and erosion (e.g. Schmidt and Montgomery, 1995; Whipple et al, 1999; Egholm et al, 2009). Alpine environments are characterized by high relief due to the interaction of tectonic uplift, climate and erosion Tectonics result in the fracturing of rock (Molnar et al, 2007), which promotes erosion. Glaciers eroded deep alpine valleys (Harbor et al, 1988; Herman et al, 2015; Prasicek et al, 2018) and amplify fracturing by thermo-hydro-mechanical rock slope damage during glacial cycles (Grämiger et al, 2017; Grämiger et al, 2018, 2020) and internal stress changes following glacier retreat (Leith et al, 2014a, b). Glacial erosion preconditions and paraglacial processes including weathering prepare and trigger rock slope failures (McColl, 2012; McColl and Draebing, 2019), which are common and hazardous processes
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