Evidence for increase in crack damage in rocks with duration of exposure at Earth’s surface.

Fractures in rock are ubiquitous; from cold dry planetary bodies to the hottest, wettest climates on Earth, and from km-scale tectonic fractures deep in Earth’s crust to microcracks in surficial rocks. Yet, many of these fractures propagate progressively over geologic timescales, making their development complex and enigmatic. Therefore, to measure how fractures have developed in rocks exposed at Earth’s surface over millennia, and how this consequently changes rock physical properties, we collected ten ~25 cm diameter granitic boulders from two sites in the Eastern Sierra, California, USA. The boulders were deposited on the surface of alluvial terraces and fans during geologically instantaneous glacial and alluvial events at different times since about 148ka BP, then the depositional surfaces were subsequently abandoned. The chronosequences of geomorphic surfaces provide a natural laboratory in which rocks of consistent lithology have been exposed to similar environmental conditions for different lengths of time, allowing us to compare rock property evolution on the order of 0 to 10⁵ years of environmental exposure; an approach that allows us to better understand and characterize mechanical weathering processes, especially long-term changes in rock fracturing. Note that fresh (time-zero) rocks in this study are represented by boulders found within active channels, and that the measured changes in rocks with longer exposure times are interpreted by comparison with the fresh rocks. Focusing only on similarly sized boulders removes any ambiguities in tectonic and exhumation history that might arise in outcrop samples, thus ensuring that rocks from each site have experienced similar stress conditions; namely those restricted to the environment.

We performed laboratory measurements on 10 granitic boulders (four from Lundy Canyon, with exposure ages of ~0 to ~148 ka; six from Shepherd Creek, with exposure ages of ~0 to ~117 ka) to quantify how rock physical properties changed as a function of environmental exposure age. We measured key parameters commonly used as proxies for crack damage, including porosity, compressional wave velocity (Vp), and shear wave velocity (Vs). We hypothesize that changes in crack damage are likely to affect rock mechanical properties, so we also measured tensile strength, uniaxial compressive strength (UCS), and Young’s modulus (E). We find that all measured parameters evolve as a function of exposure age, with systematic increases in porosity, and systematic decreases in Vp, Vs, tensile strength, UCS, and E. For example, porosity increases from 0.5 – 1.0 % in the fresh rock to 2.6 – 3.2 % in the oldest rocks. We interpret these changes as reflecting progressive subcritical crack growth that arises due to ubiquitous, but relatively low magnitude, environmental stresses continuously acting on the boulders, as opposed to differences inherited before their erosion from bedrock.

Apart from demonstrating the importance of environmentally driven cracking in rock weathering, these observations of progressive crack damage accumulation also have significant implications for the interpretation of any measurements made on rocks exposed at Earth’s surface, even if the age of exposure is relatively short compared to the age of the geologic deposit itself.