QUATERNARY GEOLOGY & GEOMORPHOLOGY DIVISION J. HOOVER MACKIN AWARD: CHRONOSEQUENCES OF MECHANICAL WEATHERING: THE INFLUENCE OF ROCK TYPE ON TIME-DEPENDENT CRACKING IN THE MOJAVE DESERT, CALIFORNIA

Mechanical weathering (rock cracking) is a fundamental soil- and sediment-forming process that is scantly quantified in natural settings. Geomorphologists commonly attribute differential landscape response (e.g. erosion) to variations in lithology (e.g. Young and Hilley, 2018) yet these rates are rarely linked to specific material properties (e.g. Montgomery, 2004), which themselves may change over time. Basic fracture mechanics theory and experiments (e.g. Atkinson and Meredith, 1987) show that when cracks propagate slowly at stresses below the rock strength, cracking rates depend on rock type-dependent material properties and climate. To our knowledge, however, no prior study has documented how/if short-term experiments and theory reflect cracking behavior over geologic time. Thus, this field study begins to test the application of decades of cracking experiments and theoretical models in the context of mechanical weathering.

Here we document rock cracking on ~900 granitoid, carbonate, and volcanic boulders on alluvial fans of the semiarid western Providence Mountains, CA, dated ~1, 5, 10, 30, and 70 ka (McDonald et al., 2003) plus modern deposits (assumed 0 ka). We measure length, intensity, orientation, and morphology of all cracks longer than 2 cm; and document rock properties including grain size, fabric, and rock shape. Preliminary results suggest that the three rock types all exhibit high initial cracking rates that decrease over time, but that the rates vary with rock type. These observations corroborate basic fracture mechanics theory. For example, the ‘Felicity’ and ‘Kaiser’ theories predict that cyclical stresses of the same magnitude cause significant initial cracking that decelerates or stops over time as growing cracks accommodate the subsequent stress cycles. Additionally, our observations suggest a progressive rock strengthening process, e.g. case hardening by mineral precipitation. We also acknowledge a possible “survivor’s bias” effect whereby rocks remaining on older deposits are disproportionately resistant to cracking. The quantitative cracking rates presented here have implications for long-term erosion and sediment production rate research that generally assume mechanical weathering rates tie only to climate and rock type, and not to time itself.

This work was partially supported by NSF/GSA Graduate Student Geoscience Grant # 13131-21, which is funded by NSF Award # 1949901.