A dearth of direct field observations limits our understanding of individual mechanical weathering processes and how they interact. In particular, the specific contributions of solar-induced thermal stresses to mechanical weathering are poorly characterized. Here, we present an 11 mo data set of cracking, using acoustic emissions (AEs), combined with measurements of rock temperature, strain and other environmental conditions, all recorded continuously for a granite boulder resting on the ground in open sun. We also present stresses derived from a numerical model of the temperature and stress fields in the boulder, idealized as a uniform elastic sphere experiencing simple solar temperature forcing. The thermal model is validated using this study’s data.
Most observed cracking coincides with the timing (∼midday and sunset) of calculated maximum, insolation-driven, tensile thermal stresses. We also observe that most cracking occurs when storms, or other weather events, strongly perturb the rock surface temperature field at these times. We hypothesize that these weather-actuated thermal perturbations result in a complex thermal stress distribution that is superimposed on the background stresses arising from simple diurnal forcing; these additive stresses ultimately trigger measurable cracking. Measured locations of observed cracking and surface strain support this hypothesis in that they generally match model-predicted locations of maximum solar-induced ensile stresses. Also, recorded rock surface strain scales with diurnal temperature cycling and records progressive, cumulative extension (dilation), consistent with ongoing, thermal stress-driven subcritical crack growth in the boulder.
Our results therefore suggest that (1) insolation-related thermal stresses by themselves are of sufficient magnitude to facilitate incremental subcritical crack growth that can subsequently be exploited by other chemical and physical processes and (2) simple insolation can impart an elevated tensile stress field that makes rock more susceptible to cracking triggered by added stress from other weathering mechanisms. Our observed cracking activity does not correlate simply with environmental conditions, including temperature extremes or the often-cited 2 °C/min thermal shock threshold. We propose that this lack of correlation is due to both the ever-varying ambient stress levels in any rock at Earth’s surface, as well as to the fact that ongoing subcritical crack growth itself will influence a rock’s stress field and strength. Because similar thermal cycling is universally experienced by subaerially exposed rock, this study elucidates specific mechanisms by which solar-induced thermal stresses may influence virtually all weathering processes.
- Received 11 September 2015.
- Revision received 5 February 2016.
- Accepted 1 March 2016.
- © Geological Society of America