Microstructural Controls on Thermal Crack Damage and the Presence of a Temperature‐Memory Effect During Cyclic Thermal Stressing of Rocks

We present results from a series of thermal stressing experiments that used three igneous rocks of different composition, grain size, and origin and contemporaneously recorded acoustic emissions (AEs) with changing temperature. Samples were subjected to both a single heating and cooling cycle and multiple heating/cooling cycles to different peak temperatures. The vast majority of thermal crack damage is generated during heating in the coarser‐grained (quartz rich) rock but during cooling in the two finer‐grained (quartz poor) rocks. Our AE results also demonstrate the presence of a temperature‐memory effect, analogous to the Kaiser stress‐memory effect observed during cyclic mechanical loading, but only in the coarser‐grained rock. We suggest that the total amount of crack damage induced during either heating or cooling is dependent on the mineral composition and, most importantly, the grain size and arrangement, as well as the maximum temperature to which the rock was exposed. We use our laboratory‐scale results to suggest ways in which crustal‐scale geophysical data may need to be reinterpreted to provide more accurate estimates of total, accumulated damage and the approach to macroscopic failure in crustal segments hosting magma chambers and geothermal reservoirs.