Microstructural Controls on Thermally-Induced Crack Damage in Rocks

Crack damage induced in rocks by thermal stresses is an important process in both geology and geomorphology. Geologically, it reduces rock strength and contributes to enhanced fluid storage and flow, while, geomorphologically, it can provide a crucial contribution to mechanical weathering and, hence, landform evolution. Crack damage due to thermal stresses can be induced in rocks during heating, under all-round compression; during cooling, under all-round tension; and is commonly also enhanced by temperature cycling. Whilst there remains a paucity of data relating to cyclic thermal stressing in rocks, previous studies have demonstrated that, for some rocks the great majority of thermal cracking is generated during heating, while for other rocks most of the cracking is generated during cooling. Here, we report results from thermal stressing experiments on three rocks with different mineralogical compositions and microstructures, that were conducted under identical experimental conditions using the same experimental apparatus. During all the experiment, the output of acoustic emissions (AEs) was used as a proxy for crack damage evolution. As observed in previous studies, we found that most thermal cracking was induced during heating in some rocks, while most was induced during cooling in others. Specifically, we observed that thermal cracking during heating was dominant in coarse-grained, quartz-rich rocks, while cracking during cooling was dominant in finer-grained, quartz-poor rocks. Since all the experiments were conducted under an identical protocol, we conclude that the difference in behaviour is a product of the differences in rock composition. We explain our observations qualitatively via models of (1) internal self-constraint due to thermal expansion anisotropy, and (2) angular inclusions embedded in an essentially homogeneous matrix. Finally, we propose a strategy for further finite element modelling that can help to provide a more quantitative explanation of our experimental observations.