Solar-induced Thermal Stresses in Surface Rocks

There is growing interest in the potential role of recurrent solar exposure in the break down of rocks at the surface the Earth, Mars and the Moon. Considerable field evidence suggests solar exposure plays an important role in the mechanical breakdown of terrestrial boulders, but uncertainty persists regarding this mechanism and classic laboratory experiments have seriously cast doubt on this mechanism for decades. One of the factors limiting advances in this field has been the lack of solid evidence that the stresses induced by the sun approach the magnitude necessary to fracture rock. We present quantitative calculation of the stress field in boulders recurrently exposed to the sun, using realistic surface temperature boundary conditions guided by high resolution temperature measurements. A series of thermo-elastic finite element simulations of surface boulders subjected to solar heating was conducted to quantify the induced tensile stresses. The effect of three system parameters were studied: (1) size: boulders 12.5 cm, 25 cm, 35 cm, 50 cm and 100 cm in diameter were modeled; (2) burial ratio, which is a measure of how much the boulder extended into the underlying soil: varied in 3 steps from 0% to 50% of its volume; and (3) surface temperature: guided closely by multi-point surface measurements on a boulder in the Strzelecki Desert, South Australia (N 30o18', E 139o27'; elevation of 167 m), that define a characteristic one-week cycle applied to the model at a 15 minutes resolution. The key findings for the cooling phase include (1) tensile stresses near and parallel to the boulder surface reach high values approaching those required for crack initiation: around 0.85 MPa for 50 cm boulders, and 1.4 MPa for 100 cm boulders; 2) these tensile stresses increase with the boulder diameter and approach the magnitude estimated for an infinite half-space for large boulders, several meters in diameter; (3) boulders smaller than 25 cm in diameter experience significantly smaller stresses and much less likely to crack; and (4) the magnitude of surface stresses is nearly independent of the burial ratio. During heating phases, fully or largely exposed boulders can experience significant tensile stresses through a large portion of the boulder interior, and they can reach 50-75% of the near surface extremes. Though smaller than the surface extremes, crack initiation from the interior may result in splitting of (large) boulders. These and other numerical results and their implications for surface rock breakdown will be discussed.