The influence of thermal stressing on the strength of andesites: implications for the stability of volcanic edifices during magma ascent

During magma ascent and eruption, the stability of volcano is challenged by heating and stressing, thereby affecting the physical state of the rocks. Here, we test the stability of active volcanic edifice through a study of the strength of volcanic rocks (present in the edifice of Volcán de Colima, Mexico) as well as after thermal stressing to 500°C. In this study, we chose andesitic rocks with a range of porosity (between 8 % and 30 %) representative of the material observed in the edifice. We characterized the modal composition, the initial microstructure, the density (air and water-saturated density), the porosity, ultrasonic velocity and dynamic elastic moduli of rocks prior to and after heat treatment. Thermal-stressing to 500°C (heated and cooled at 1°C/min) induced thermal cracks (seen as a reduction in elastic moduli and Vp, Vs), monitored via acoustic emissions, imaged via fluorescent light microscopic analysis and quantified (i.e. crack density, orientation and distribution anisotropy) using the crack density method and the semi-automated analysis software tool AMOCADO. We test the strength of the rocks before and after thermal stressing using uniaxial compressive experiments (under a strain rate of 10-5 s-1). Thermal stressing did not induce weakening of the andesite rocks studied. The strength of the samples does not vary more than expected from natural sample variability. Our work suggests that thermal stressing during periods of unrest increases the porosity and decreases the seismic velocities across the edifice. An increase permeability of the rocks may promote hydration of the rock and counteract cracking, causing an increase in seismic velocities. As for the stability of the edifice, thermal stressing does not necessarily, nor significantly, weakens the conduit wall rock and thus the strength of the edifice. We emphasize that interpretation of the stability of the volcanic edifice from seismic velocities, alone, needs caution; instead, it may require knowledge of the crack density when approaching catastrophic failure.