« Rethinking cracking: Combining field observations with fracture mechanics experiments and concepts to better understand what controls natural rock fracturing over geologic time

Traditional views in the context of surface processes have conceptualized natural rock fracture as a short-term rock strength-threshold dependent process. It is much more likely, however, that most rock fracture proceeds slowly and steadily – subcritically – induced by the very low stresses that are ever-present at and near Earth’s surface. As such, natural rock fracture is driven and limited by a much more complex array of variables than merely the largest stresses. These include climate – which impacts crack tip bond breaking rates separate from its influence on stresses – as well as internal feedbacks with cracking itself. In this talk, I will briefly introduce these ideas and then present new field data from the measurement of over 9000 cracks in 3 climate regimes and for 3 rock types. These data show that rock cracking rates are intimately tied to exposure age (and thus by inference erosion rate) of rock, whereby freshly exposed rock exhibits cracking rates on the order of 10s to 100s of meters per million years, slowing to <1 m per million years within about 10-15 ka. Preliminary data support our hypothesis that surface rocks can develop a ‘stress memory’ for very low stresses, and that time-dependent fracture deceleration is due to internal feedbacks between crack growth and the consequent increase in accommodation of stress through elastic strain. This work has important implications for how we understand feedbacks between weathering and erosion across all landscapes.