MECHANICAL ROCK BREAKDOWN DUE TO DIURNAL SOLAR EXPOSURE: RESOLVING A PARADOX BETWEEN OBSERVATIONS AND THEORY (Invited Presentation)

A growing number of studies suggest that cyclic thermal stressing drives considerable mechanical weathering of rocks at the Earth’s surface. The evidence is observational and instrumental: preferred crack orientation in boulders^1,2, temperature and micro-crack activity in boulders³, and thermally induced rock deformation and opening of exfoliation fractures⁴. This evidence appears paradoxical, however, in view of the classic experimental work by Griggs⁵. He exposed small rock specimens to a multitude of temperature oscillations, larger in both amplitude and frequency than those in nature, but saw no crack growth. This led to the conclusion that stresses induced by diurnal solar exposure are insufficient to induce even slow crack growth.

We revisit Griggs’ experiment, using a thermo-elastic simulation to assess the stress state in his specimens, showing that high tensile stresses are significant and do reach the strength of some granites, but only within a very thin surface layer. These high, local stresses are unlikely to induce propagation of cracks beyond a depth of 2-3 mm, which contrasts with the >10 mm cracks observed by Griggs that did not grow. Hence, his finding has little bearing on the breakdown of rock due to diurnal solar irradiation under natural settings.

To gain further insight into thermal stresses in boulders and complement numerical studies, we developed an analytical solution for sinusoidal, spherically symmetric surface heating/cooling of rock spheres, identifying the effect of relevant physical properties and length scales on tensile stresses. Three characteristic boulder sizes emerge from this solution with boundaries defined naturally in terms of diurnal skin depth (~0.15 m). The small domain, <0.1 m, is immune to splitting; breakdown is driven solely by intergranular stresses. The large domain, >10 m, ranges up to bedrock landforms; it is characterized by thermal stresses mostly within one to two skin depths below the surface. The medium size boulders show significant complexity in the time-varying stress field.

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3. Eppes MC et al. 2016. GSA Bull.128(9), 1315–1338
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5. Griggs D 1936. J. Geology, 44(7), 783–796