New Field Data Supporting the Role of Insolation in Physical Weathering

Despite the potential importance of mechanical weathering in landscape evolution of deserts and other environments, there is a general lack of documentation regarding rates, processes and driving environmental conditions associated with crack initiation and propagation in subaerial rocks. We collected new field data that provides support of the role of the sun, particularly directional insolation, in the physical breakdown of rock. We collected new orientation and size data for ~375 cracks in ~150 stones on Quaternary alluvial fan surfaces in Mojave and Australia deserts; and for ~300 cracks in ~125 boulders along transects on an open west-facing boulder field, and a treed southwest-facing boulder field in southwestern Virginia. We also examined two similar, large (~6x2.5x2m) meta-granite boulders located on south-facing hillslopes in the Mojave Desert and the southern Appalachians. For each boulder we measured the strike, dip, length, width and relative degree of weathering of all cracks on four different rock surfaces, as well as the spatial variability of surface temperature throughout a day. For the two boulders and the Mojave transect, cracks were defined as any visible void >1cm long. Finally, we collected ~8 months of temperature measurements every 3 minutes from the upper 2 cm of a marble corestone in the Mojave Desert. In general, the majority of all cracks measured exhibited non-random orientations. In both arid and temperate climates, east-facing surfaces exhibited higher crack densities than west-facing, and the Mojave boulder exhibited overall higher crack density (~66 cracks/m2) than the Appalachian Boulder (45cracks/m2). The relationship between rock size and in-situ cracking in desert pavements is difficult to interpret due to crack inheritance. Measured temperatures frequently meet or exceed the experimentally demonstrated temperature changes necessary to crack coarse crystalline marble. Overall, our data provide support for insolation-related cracking in surface rocks of deserts and elsewhere.