Late Quaternary evolution of rock cracking rates and morphology in Owens Valley, California

Mechanical weathering is a key component of critical zone architecture, landscape evolution, rock erosion resistance, fluid infiltration, source-to-sink sedimentology, and interpretation of cosmogenic dating. Yet weathering rates have rarely been studied and are poorly understood. Numerical modeling and experimental observations suggest that the cracking that embodies mechanical weathering is likely subcritical in nature, and that cracking rates vary as a function of climate, stresses, lithology, and pore water availability and chemistry (Eppes & Keanini, 2017). Further, fracture mechanics theory (Griffith, 1921) and controlled subcritical crack growth experiments show that, under the same stress magnitude, longer cracks propagate faster, and that cracks in humid environments propagate more quickly than those in drier conditions (Nara et al., 2012). In contrast, as any given crack propagates, it may encounter rock heterogeneities that slow or arrest its growth (Anderson, 2017). Few, if any, studies document these types of possible changes in cracking rates through time in a natural setting.

Here we employ a chronosequence approach to document cracking rates over geologic time scales (10⁰ - 10⁵ years), measuring the length and density of all cracks >2cm in length on over 1000 granitic surface boulders on dated alluvial fans and moraines of the Lone Pine and Mono Lake areas of the Eastern Sierra Nevada (averaged ¹⁰Be exposure ages from meter-scale boulders: 148 ka, 117 ka, 46 ka, 33 ka, 21 ka, 18 ka, and 6 ka, plus modern deposits) (D'Arcy et al., 2015; Rood et al., 2011). Preliminary calculations of cracking rates, considering both crack lengths and crack density over the exposure age of the boulders, show that mechanical weathering rates peak in late Pleistocene for Lone Pine boulders, and in late Holocene in the cooler, wetter Mono Lake area. These apparent rates are slower over longer time scales at both sites, although some decrease in calculated cracking rates is likely the result of through-going cracks splitting rocks. Nevertheless, the data suggest that rocks are more susceptible to fracture upon first exposure. This quantification of rates - and changing rates - of mechanical weathering will improve our understanding of Earth surface processes and their evolution through time and changing climates.