Near-surface Thermal Profiles in Alpine Bedrock: Implications for the Frost Weathering of Rock

Abstract

The rates of many processes that control rock breakdown into transportable particles are dependent upon rock temperature. In particular, frost-cracking depends largely upon the time spent within a range of subzero temperatures I call the frost cracking window. I present simple analytic solutions, detailed time series of thermal data from a field site, and a numerical model of subsurface temperatures that constrain the expected depth dependence of frost cracking. Analytic solutions using sinusoidal surface temperature histories result in predicted vertical profiles of frost-cracking intensity that depend upon the location of the frost-cracking window within the range of surface temperatures. In some cases, the expected frost cracking intensity decreases monotonically with depth, while in others it displays a distinct maximum at depth. I use hourly temperatures measured over the 1-yr interval April 1995 through March 1996, at 8 depths up to 42 cm into a granitic bedrock surface in the Laramie Range, Wyoming, to constrain an in situ thermal diffusivity of 1.7 mm² s^–1. These temperature histories suggest that frost cracking at this site should decrease monotonically with depth into the rock. I also use this time series of temperatures to calibrate a numerical thermal model in which the top boundary condition is set by a surface radiation balance. The data require both a low albedo of the rock surface (0.1), and a high atmospheric transmissivity (0.9). I then explore the expected near-surface temperatures at other sites by running the model with different annual mean temperatures, latitudes, and slopes. The resulting simulations suggest that at lower mean annual temperatures, as are found in higher latitude and altitude sites, the frost cracking maximum should both amplify and deepen into the subsurface.