2020 KIRK BRYAN AWARD LECTURE: A NEW CONCEPTUAL FRAMEWORK FOR ALL MECHANICAL WEATHERING ON EARTH AND OTHER PLANETS – CLIMATE-DEPENDENT SUBCRITICAL CRACKING

The importance of mechanical weathering (defined as any rock fracture within ~10² m of Earth’s surface) to virtually all Earth systems cannot be overstated. Yet geoscientists have relied on incomplete conceptual models in studying this vital arm of the rock cycle. We have rotely portrayed mechanical weathering as a dissected cube governed by ‘mechanical weathering processes’ like freeze-thaw that are all, in fact, stress-loading processes that induce cracking. Only rarely evoked are the actual crack propagation processes themselves. In this work, we build a compendium of fracture mechanics research from rock physics and engineering fields and combine it with confidence drawn from ‘nose-to-crack’ field data from thousands of rocks and outcrops. We use this synergistic awareness to assert, explicitly for the first time, that all mechanical weathering proceeds dominantly by a particular type of cracking, subcritical cracking. In other words freezing, thermal cycling, topography-enhanced regional stresses, etc., all result in slow-steady crack growth like that occurring on your car windshield, not like that of your thrown vase (critical cracking). Importantly, subcritical cracking innately occurs via stress-enabled, crack-tip chemical reactions. Thus, we hypothesize that all natural rock cracking is dependent on both stress and – independent from stress - climate. We build a simple physical model that depicts how climate can affect mechanical weathering rates even when stress-loading is identical. Our numerical modeling – and subsequent field and laboratory data collected with colleagues - demonstrate that 1) both measured and modeled long-term rock cracking and erosion rates strongly correlate with rock material properties that define susceptibility to subcritical cracking; and 2) linear increases in moisture (modeled as atmospheric relative humidity and measured as vapor pressure) result in exponential increases in cracking rates. These results diverge from the long-held, implicit assumption of research on Earth and other planets that mechanical weathering hinges solely on factors that impact stresses. As such, we construct a new conceptual framework for mechanical weathering and highlight future research directions for those who seek a more physically accurate, comprehensive understanding of Earth’s weathering and climate systems.