NEW INSIGHTS INTO THE MECHANICAL WEATHERING OF ROCK IN HUMID-TEMPERATE CLIMATES

Despite the fact that mechanical weathering likely plays a critical role in Appalachian geomorphic processes ranging from fluvial incision to hillslope erosion and sediment production, there has been little study of mechanical weathering in humid-temperate climates. In particular, the specific environmental factors that limit or drive mechanical weathering in such settings is unknown. Here, we first present a summary of results from field and instrumentation studies in the eastern U.S. which suggest that insolation-related thermal stresses and moisture both heavily influence mechanical weathering in those settings. For example, both crack orientations (as measured on boulders found in late-Pleistocene to late-Holcene aged rock fields) as well as cracking rates and locations (as measured by acoustic emissions from a boulder sitting on the ground) are strongly correlated to rock surface temperature characteristics; which in turn can be linked to thermal stresses. Furthermore, we observe a strong relationship between moisture and crack density in observed boulders. Based on these data, we hypothesize that 1) omnipresent solar-related thermal stresses set a minimum pace for mechanical weathering in Appalachia and other humid settings; and that 2) in addition to the necessary role that moisture plays in weathering processes such as freezing, moisture strongly impacts the efficacy of all mechanical weathering processes, regardless of stress-loading mechanism, because it is inherently influential on crack propagation itself. To test these hypotheses, we developed a simple numerical model that calculates calculate time-, depth- and moisture- dependent crack evolution under solar-driven, cyclic, thermal stress loading. We find that calculated cracking rates are strongly dependent on rock thermal properties as well as moisture content of crack tips. These results represent a potential new framework for future study of mechanical weathering in Appalachia and elsewhere.