A CLIMOSEQUENCE OF CHRONOSEQUENCES: EXPLORING THE TEMPORAL EVOLUTION OF MECHANICAL WEATHERING ACROSS VARYING CLIMATES, CALIFORNIA

Mechanical weathering – cracking – is a fundamental process which transforms bedrock and large boulders into mobile regolith, the foundation of sediment and soils. In turn, sediment size and distribution are thought to be largely predicated on the spacing of such fractures (e.g., Neely and DiBiase, 2020). Past studies document three dominant rock crack morphologies: 1) intergranular cracks, forming along grain boundaries and causing granular disaggregation; 2) spall-forming cracks, oriented parallel to the surface of the rock; and 3) through-going cracks, which can split the rock into two smaller rocks. Together, these crack styles can dislodge rock pieces ranging from mm to m in scale. However, little is documented regarding what controls the dominant morphology and spacing of cracking, and how morphologies may change through time and space.

Here we employ a chronosequence approach to document cracking rates over geologic time scales (10⁰ – 10⁵ years) in three localities with different climates. We measured the length, density, and morphology of all cracks longer than 2 cm on over 1000 granitic boulders on dated alluvial fans and moraines of the Lone Pine and Mono Lake areas of the Eastern Sierra Nevada and the western Providence Mountains in the Mojave Desert (surface ¹⁰Be and ¹⁴C ages: 148 ka, 117 ka, 70 ka, 46 ka, 33 ka, 30 ka, 21 ka, 18 ka, 10 ka, 6 ka, 4 ka, 1 ka, plus modern deposits)(D’Arcy et al., 2015; McDonald et al., 2003; Rood et al., 2011).We limit our study to granitic rocks in order to best compare the influence of time and climate on cracking rates and styles. We include other rock types (carbonates and volcanics) in the Mojave chronosequence.

Preliminary field observations suggest that overall cracking rates change through time. We also find that case hardening (through chemical mineral precipitation and lichen growth) appears to influence crack morphology in older rocks, diminishing the relative proportion of granular disaggregation compared to spallation and through-going cracks. By understanding how climatic and rock type variations effect crack growth, we can better understand how sediment is initially created, and how soils and landscapes evolve.