Pontificia Universidad Católica de Chile Pontificia Universidad Católica de Chile

Microstructural controls on thermally-induced crack damage in intrusive rocks

Revista : CGCh XVI
Tipo de publicación : Conferencia No DCC


Crack damage due to thermal stresses can be induced in rocks during heating, under all-round compression or 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 discuss results from a series of novel experiments designed to capture acoustic emission (AE) (or micro-seismic) signals associated with the heating and cooling of three different igneous rocks (a granophyre from Iceland, a dacite from Santorini and a basalt from Iceland) over emplacement-relevant temperatures. Sample cores, 75 mm x 25 mm, were placed inside a tube furnace and subjected to temperature cycling up to 1000?C. The samples were held in purpose-built apparatus that was able to both contract and expand so as to maintain contact with the sample and act as a wave guide for propagating elastic waves induced by fracturing events. AE was monitored continuously along with temperature and all experiments were performed at ambient pressure conditions. By comparing the micro-seismicity with analysis of each rock’s microstructure we have been able to determine key processes related to differential thermal cracking. We observed that thermal cracking during heating was dominant in coarse-grained, quartz-rich rocks, while instead 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 and internal micro-structures. These results may be useful in determining natural seismic responses following shallow crustal magma intrusion and cooling and be linked to the development of permeable fluid-flow paths in geothermal settings.