THE INFLUENCE OF TIME ON MECHANICAL WEATHERING RATES, MODES, AND MECHANISMS

Fracture mechanics theory (e.g. Charles, 1958) and rigorously observed experimental data (e.g. Atkinson and Meredith, 1987) establish that individual crack growth rates increase with time as the crack lengthens. In nature, however, mechanical weathering encompasses the full suite of cracks within any given rock.

Here, to provide some of the first field documentation of natural, long-term, overall rock cracking evolution in different climates, we present measurements of all visible cracks >2 cm long on 2163 granitoid, carbonate, and volcanic boulders deposited atop fan, terrace, and moraine surfaces aged ~0 to ~150 ka.

We find that both surficial crack intensity and the length of long, individual cracks increase relatively rapidly within the first 5-10 ka in all rock types, then cracking stabilizes. Representative individual crack growth rates range from 0.1-35.6 mm/ka with the fastest rates occurring initially. These individual rates exhibit rock type dependence but little climate dependence. However, the highest crack intensity (sum crack length per rock surface area) in granitoid rocks was observed in the hottest, driest site.

After accounting for the inherent biases in these type of data (e.g. once a crack propagates through the boulder, the crack is no longer measurable) we explore multiple internal and external phenomena that could explain these results. For example, both rock stress fields and elastic properties will change not only with increasing porosity as individual cracks grow in abundance and length, but also with changing climates. The time-dependent mechanical weathering rates that we observe have important implications for understanding rock’s evolving susceptibility to chemical weathering and erosion.