PHYSICAL WEATHERING BY INSOLATION: COMPARISON OF YEAR LONG, PER-MINUTE OBSERVATIONS OF TEMPERATURE, STRAIN AND CRACKING IN GRANITE BOULDERS IN A TEMPERATE PASTURE AND SEMIARID DESERT, NORTH CAROLINA AND NEW MEXICO

The efficacy of diurnal, solar-related thermal-mechanical processes in cracking rocks cannot be fully understood until links between natural environmental conditions and cracking are directly observed. Here we compare and contrast two, year-long datasets of strain, temperature, moisture and cracking that were collected for granite boulders sitting in exposed sites in North Carolina and New Mexico. A full suite of environmental data including insolation, rain, and soil moisture were also collected. Acoustic emissions were monitored continuously and all other data were collected by the minute.

Cracking events, as measured by acoustic emissions, were more common in the New Mexico boulder (~110,000 events in 1 year) than in the North Carolina boulder (~35,000 events in 11 months). The majority of events occurred during winter months in NC and in summer in NM. In both localities, the majority of events occurred during late afternoon and evening hours. The largest clusters of cracking events commonly occur during times of rapid change in the temperature gradient across the rock surface that at times includes a corresponding increase in strain. These conditions typically arise during sudden changes in weather such as increasing cloud-cover or wind suggesting that both insolation-related temperature as well as more complex weather phenomena that affect rock thermal properties play a key role in rock cracking. These data are providing key inputs to numerical models by B. Hallet and P. Mackenzie (University of Washington) that seek to calculate the stresses produced by diurnal thermal conditions. Models must reconcile 1) rock surface temperatures that we measure in the field, 2) stresses that might exist during clusters of microcrack events and 3) stresses that would produce the NE striking/SE dipping preferential orientations of macrocracks observed in 1027 rocks in the Mojave desert (Eppes et al., 2010). Our overall combined field- instrumentation- and modeling-based study provides a unique new insight into the long-studied idea that thermal stresses caused by the sun can crack rocks.