ARE THERMAL STRESSES IN ROCKS EXPOSED TO THE SUN SUFFICIENT TO BREAK THEM? YES (Invited Presentation)

Seminal experiments by D. Griggs (1936) showed that rock tablets subjected to large variations in temperature did not show signs of break down. Decades later, however, the idea that solar exposure could significantly contribute to the mechanical of weathering of rocks has resurfaced because of compelling recent field observations and laboratory experiments. We launched a collaborative study to examine the potential for solar-driven thermal cycling to progressively breakdown rocks on the surface of the Earth and other planets. This study integrates modern instrumental and numerical approaches to monitor the surface temperatures, strains, and microfracture activity in exposed boulders in the field, and to shed light on the thermo-mechanical response of boulders to diurnal solar exposure. Herein, we focus on the modeling work, which addresses 1) the time-varying thermal regime of rocks exposed to diurnal variations in solar radiation as dictated by latitude, and time of the year, and as influenced by the surface emissivity and thermal properties of the rock and soil, and size and shape of the rock, and 2) the corresponding time-varying stress and strain fields in the rocks. We idealize rock as a homogeneous and isotropic material, but use representative linear elastic properties, and realistic rock shape and orientation. Model results suggest that thermal tensile stresses due to recurrent solar exposure are large, and ample to fracture rocks larger than ~0.1 m especially over geologic time (>100-10⁴years). Because these stresses decrease with size for rocks smaller than about 1 m-dia., we expect solar exposure to be effective in breaking down boulders and cobbles, while having little impact on gravel size and smaller clasts.

The field and modeling components of this study complement each other well. Numerical results help understand the evolution of thermal stresses, their relation to the direction of solar radiation, and their strong non-linear dependence on the size of the rocks. Our approach, combining modeling with field measurements, holds considerable promise for advancing research on physical weathering, with diverse potential implications including the deterioration of man-made monuments and sculptures, and breakdown of surface rocks and bedrock on our planet and others.