How do subcritical cracking rates and styles influence rock erosion? A test case from the Blue Ridge Mountains of Virginia.

There is a disconnect between measured rates of rock erosion and regolith production and our understanding of the factors and processes that drive them. Here we examine the mechanical weathering (cracking) characteristics of natural, bare bedrock outcrops characterized by 10Be derived erosion rates that vary from 2 to 40 m/my in the Blue Ridge Mountains, VA. Observed erosion rate variance generally correlates with rock type; we seek to characterize and quantify to what extent the mechanical weathering properties of the different rock types drive erosion rates. We assert that subcritical cracking constitutes the primary mechanism by which the outcrops increase their porosity and subsequently weather and erode. We therefore hypothesize that rock parameters that control rates and styles of subcritical cracking set the outcrop erosion rates. For each outcrop, we measured crack characteristics along transects: for every crack >2 cm length, we measured its length, width, orientation, and weathering characteristics (rounded vs sharp edges); and we measured the thickness of all `steps' (spallation remnants) encountered in the transects. For most outcrops, we collected surface samples in order to characterize their mineralogy and microcracking characteristics through thin section analysis. For each rock type, we collected samples for which we measured fracture toughness, as well as the subcritical crack growth index under different moisture conditions. Preliminary analysis of the field crack data indicates that each rock type (granite, sandstone, quartzite) is characterized by unique macro- and micro-scale crack characteristics consistent with known generic subcritical cracking parameters for those rocks. Crack density and length correlate with erosion rates in faster eroding rock types, but not slowly eroding ones. Overall, we hope these data will help to shed light on the driving and limiting factors for the mechanical production of porosity in rock at and near Earth's surface.