The influence of solar-induced thermal stresses on the mechanical weathering of rocks in humid mid-latitudes

The role of solar-induced thermal stresses in the mechanical breakdown of rock in humid-temperate climates has remained relatively unexplored. In contrast, numerous studies have demonstrated that cracks in rocks found in more arid mid-latitude locations exhibit preferred northeast orientations that are interpreted to be a consequence of insolation-related cracking. Here we hypothesize that similar insolation-related mechanisms may be efficacious in humid temperate climates, possibly in conjunction with other mechanical weathering processes. To test this hypothesis, we collected rock and crack data from a total of 310 rocks at a forested field site in North Carolina (99 rocks, 266 cracks) and at forested and unforested field sites in Pennsylvania (211 rocks, 664 cracks) in the eastern United States. We find that overall, measured cracks exhibit statistically preferred strike orientations (47 degrees +/- 16), as well as dip angles (52 degrees +/- 24 degrees), that are similar in most respects to comparable datasets from mid-latitude deserts. There is less variance in strike orientations for larger cracks suggesting that cracks with certain orientations are preferentially propagated through time. We propose that diurnally repeating geometries of solar-related stresses result in propagation of those cracks whose orientations are favorably oriented with respect to those stresses. We hypothesize that the result is an oriented rock heterogeneity that acts as a zone of weakness much like bedding or foliation that can, in turn, be exploited by other weathering processes. Observed crack orientations vary somewhat by location, consistent with this hypothesis given the different latitude and solar exposure of the field sites. Crack densities vary between field sites and are generally higher on north-facing boulder-faces and in forested sites, suggesting that moisture-availability also plays a role in dictating cracking rates. These data provide evidence that solar-induced thermal stresses facilitate mechanical weathering in environments where other processes are also likely at play. Copyright (c) 2015 John Wiley & Sons, Ltd.