Mechanical weathering and rock erosion by climate‐dependent subcritical cracking

Abstract

This work constructs a fracture mechanics framework for conceptualizing mechanical rock breakdown and consequent regolith production and erosion on the surface of Earth and other terrestrial bodies. Here our analysis of fracture mechanics literature explicitly establishes for the first time that all mechanical weathering in most rock types likely progresses by climate‐dependent subcritical cracking under virtually all Earth surface and near‐surface environmental conditions. We substantiate and quantify this finding through development of physically based subcritical cracking and rock erosion models founded in well‐vetted fracture mechanics and mechanical weathering, theory, and observation. The models show that subcritical cracking can culminate in significant rock fracture and erosion under commonly experienced environmental stress magnitudes that are significantly lower than rock critical strength. Our calculations also indicate that climate strongly influences subcritical cracking—and thus rock weathering rates—irrespective of the source of the stress (e.g., freezing, thermal cycling, and unloading). The climate dependence of subcritical cracking rates is due to the chemophysical processes acting to break bonds at crack tips experiencing these low stresses. We find that for any stress or combination of stresses lower than a rock's critical strength, linear increases in humidity lead to exponential acceleration of subcritical cracking and associated rock erosion. Our modeling also shows that these rates are sensitive to numerous other environment, rock, and mineral properties that are currently not well characterized. We propose that confining pressure from overlying soil or rock may serve to suppress subcritical cracking in near‐surface environments. These results are applicable to all weathering processes.

Unraveling the Mechanics of Thermal Stress Weathering: Rate‐Effects, Size‐Effects, and Scaling Laws

“…Molaro et al () carried out fully three‐dimensional finite element calculations of macroscopic thermal stress development in various sized rocks on the Moon ( δ ∼0.8 m), and reported values of the maximum surface principal stresses of ∼2 MPa, ∼5 MPa, and ∼9 MPa for D =0.3 m, D =0.5 m, and D =0.7 m. Remarkably, these stress values are fairly consistent with predictions (respectively ∼1.5 MPa, ∼4 MPa, and ∼8 MPa for the same set of material properties) of Equation despite its simplicity and lack of fitting parameters. These stress values are generally smaller than fracture toughness of typical rocks (Asadi, Taeibi‐Rahni, Akbarzadeh, Javadi, & Ahmadi, ; El Mir, Ramesh, & Delbo, ; Eppes & Keanini, ; Gui, Bui, Kodikara, Zhang, & Zhao, ) and would lead to slow steady fatigue process.…”

Controls on the geotechnical response of sedimentary rocks to weathering

“…The observed reductions in intact rock strength are likely to result from a combination of factors, in addition to those that are evident in the observed surficial changes in the rock, such as slaking, cracking and grain loss. This can occur via stress corrosion cracking where molecular bonds are strained and stretched at crack tips by a chemically active environmental agent, such as water (Atkinson, 1984;Eppes and Keanini, 2017;Voigtländer et al, 2018). Small amplitude stress as a result of environmental processes such as insolation or wetting and drying can therefore drive microcrack growth (Eppes and Keanini, 2017).…”

“…Slopes can be destabilised rapidly in response to sudden and short-lived changes in stress conditions that trigger failure, such as those resulting from strong earthquake ground shaking or heavy rainfall (Iverson, 2000;Keefer et al, 1987). Rock slope instability can also develop over longer (10 0 -10 3 years) timescales in response to incremental and cumulative reductions in rock mass strength driven by micro-fracture development and/or weathering processes that reduce the cohesional strength of rocks (Collins and Stock, 2016;Eppes and Keanini, 2017;Gunzburger et al, 2005).…”

Mineral Weathering and Bedrock Weakening: Modeling Microscale Bedrock Damage Under Biotite Weathering

“…For example, damage differs from the volumetric concentration of microcracks, which, unlike the damage variable used in our model, is not necessarily linearly related to changes in rock stiffness (e.g., Budiansky & O'connell, ; Germanovich et al, ). It is also distinct from mechanical weathering, a term that encompasses all processes that drive near‐surface rock fracturing, such as freeze‐thaw cycling, solar heating, crystal growth, root expansion, and erosional unloading (e.g., Eppes & Keanini, ; McFadden et al, ). In this study, we use the term damage rather than mechanical weathering because in our model; damage quantifies the loss of elastic deformation energy; and, by proxy, the loss of stiffness (section ).…”