CLIMATE, MECHANICAL WEATHERING, ROCK EROSION AND REGOLITH PRODUCTION: NEW INSIGHTS FROM FRACTURE MECHANICS

The mechanisms by which climate (moisture and temperature) can influence the mechanical breakdown of rock have been underappreciated and poorly understood. We posit that virtually all mechanical weathering at and near Earth’s surface is facilitated through subcritical cracking, whereby stresses lower than a rock’s critical strength result in progressive crack growth over geologic time. Furthermore, existing fracture mechanics experimental data demonstrates that the bond-breaking processes at crack tips that comprise most subcritical cracking in rock are dependent on environmental factors that influence chemical reactions such as moisture availability, temperature, and pore-water chemistry. Therefore, all mechanical weathering is climate-dependent, independent of the influence that climate has on stress-inducing processes like ice segregation or thermal cycling. To demonstrate these effects, we developed a simple physical model - whose assumptions are substantiated by field and lab observations - for subcritical cracking and consequent rock erosion and regolith production. We employed the model to evaluate how rates of rock erosion and/or regolith production - driven by subcritical cracking – may change as a function of humidity, temperature, and rock mechanical properties. We show that the magnitude of most known tectonic, topographic and environmental stresses acting near Earth’s surface can lead to subcritical cracking. We also find that linear increases in humidity lead to exponential increases in rock erosion via subcritical cracking. However, calculated erosion rates for any given combined stress and climate condition vary by several orders of magnitude for the existing known range of rock material properties considered herein. Further, as stresses approach critical levels, the influence of environmental factors decreases. In ongoing work we seek to quantify how rates of regolith production by subcritical cracking may vary as a function of soil depth. Overall, this work provides a new conceptual framework for considering the physical breakdown of rock on Earth and other planets.