LEVERAGING MODERN WEATHER DATA TO MODEL THE DRIVERS OF ROCK MECHANICAL WEATHERING IN EASTERN CALIFORNIA

A long-held belief in geology is that climate impacts rock weathering (e.g., Saunders and Fookes, 1970). For mechanical weathering (cracking), the environment provides a source of stress (e.g., thermal expansion/contraction) and defines the conditions in which chemophysical cracking processes progress, with higher temperature and/or moisture leading to faster cracking. Daily cyclical stresses can be a significant driver of rock cracking, but geologists are limited by our datasets, defining “climate” as mean annual temperature and precipitation. Is this sufficient for understanding mechanical weathering?

Here we posit that daily conditions and high-magnitude weather events must be considered to understand the impacts of climate on rock cracking. We follow Eppes and Keanini (2017) to implement Paris’ law (Paris and Erdogan, 1963), modeling single-crack growth as a function of intergranular stresses imposed by diurnal temperature changes. Values are derived from weather station measurements in three field sites in Eastern California. The sites represent a traditional “climosequence” of data, reflecting a stepped increase in aridity and temperature from north to south.

For modeling validation, we first present field cracking data showing that significant crack growth occurs during the first ~5 – 6 kyr. Then, we model crack growth using averaged conditions from the full weather datasets, calculating stress from average daily temperature flux and subcritical crack growth index from average daily relative humidity. Modeling results in negligible (<0.001 mm) crack growth after 5 kyr of daily cycles.

By contrast, modeling the variable daily weather values results in cracking that is strongly dependent on days of unusually high diurnal temperature flux, with 46% (hottest site) and 99% (intermediate site) of crack growth occurring on the highest single stress day. At the coolest site, daily temperature flux is lower, and cracking occurs incrementally on multiple high-flux days.

These results suggest that estimating unusual weather days is essential for understanding long-term rock cracking, and diurnal temperature flux may be a more important variable than mean annual temperature. We caution that the typical “climate” comparison employed by geologists may be insufficient for predicting the true drivers of mechanical weathering at Earth’s surface.