EXPLORING LINKAGES BETWEEN SUBCRITICAL CRACK GROWTH AND ROCK TYPE DEPENDENT EROSION RESISTANCE, A CASE STUDY FROM THE BLUE RIDGE, VA

The ridge-forming rocks of the Valley and Ridge and Blue Ridge physiographic provinces in the eastern United States have long been cited as exemplars of erosion resistant rock types. Few if any studies, however, have explored the specific processes behind the lithology-erosion susceptibility relationship. New data derived from Shenandoah National Park in central Virginia suggest a correlation between 10Be-derived bedrock outcrop erosion rates (Hancock, unpublished) and mechanical weathering rates for different rock types and outcrops (this study). Building on these results, we hypothesize that surface and near surface rock fracture is a rate-limiting process that influences observed erosion rates, and further, that variables which influence subcritical crack growth provide a primary control on each outcrop’s susceptibility to erosion.

Weathering-related stresses that typically arise in surface or near surface bedrock are relatively low and unlikely to exceed the tensile strength of most rock types. Thus mechanical weathering in these rocks likely occurs most commonly through subcritical crack growth, an idea that has been largely overlooked (Eppes et al., 2016). Here, we explored this hypothesis by comparing and contrasting known fracture mechanics properties for the Shenandoah study rock types: sandstone, quartzite and granitoids. From the exact outcrops for which 10Be samples were collected, we also gathered in-situ crack data (size, density, degree of rounding of crack edges, spall thickness), as well as samples for SEM analysis, thermal conductivity and uniaxial compression tests. Preliminary results support causal relationships between rock properties that influence subcritical crack growth and cracking patterns. For example, observed high crack density in slowly eroding quartzite outcrops is consistent with measures of a low subcritical crack growth index but high fracture toughness for that rock type and with the complex microstructure of the skolithos-bearing Antietam quartzite of Shenandoah. Ultimately we believe that conceptualizing erosion resistance in terms of subcritical crack growth could provide a unifying mechanistic paradigm for other previously observed relationships between erosion and rock properties like joint spacing, quartz content and compressive strength.