Title of the talk: Microstructural controls on thermally-induced crack damage surrounding magma intrusions
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 I discuss results from a series of 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 were placed inside a tube furnace and subjected to temperature cycling up to 1000°C. 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 it is possible to determine key processes related to differential thermal cracking. 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, it can be concluded 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, potentially priming a volcano for later eruption, and be linked to the development of permeable fluid-flow paths in geothermal settings.
Short bio: John is an Associate Professor at Pontificia Universidad Catolica de Chile, where he combines structural geology techniques as well as numerical modelling and rock deformation experiments to understand processes related to volcanotectonics, rock fracture, geothermal reservoir characterization and subsurface mine stability. He is currently the Vice President of the IAVCEI commission on Volcanic and Igneous Plumbing Systems and an editor at the diamond open access journal Volcanica.
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