Near-surface thermal profiles in alpine bedrock: Implications for the frost weathering of rock

The rates of many processes that control rock breakdown into transportable particles are dependent upon rock temperature. In particular, frost-crackin g depends largely upon the time spent within a range of subzero temperature s I call the frost cracking window. I present simple analytic solutions, de tailed 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 hi stories result in predicted vertical profiles of frost-cracking intensity t hat depend upon the location of the frost-cracking window within the range of surface temperatures. In some cases, the expected frost cracking intensi ty decreases monotonically with depth, while in others it displays a distin ct maximum at depth. I use hourly temperatures measured over the I-yr inter val 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 ther mal diffusivity of 1.7 mm(2) s(-1). These temperature histories suggest tha t frost cracking at this site should decrease monotonically with depth into the rock. I also use this time series of temperatures to calibrate a numer ical 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 expec ted near-surface temperatures at other sites by running the model with diff erent annual mean temperatures, latitudes, and slopes. The resulting simula tions suggest that at lower mean annual temperatures, as are found in highe r latitude and altitude sites, the frost cracking maximum should both ampli fy and deepen into the subsurface.