Dilatancy, compaction, and failure mode in andesite: the transition from brittle faulting to shear-enhanced compaction in volcanic edifices

Andesite is an extrusive volcanic rock of intermediate composition (i.e., SiO2 varies between 52-63%). Andesitic volcanoes, typical of convergent plate margin settings, represent about 25% of volcanoes worldwide. However, our understanding of the physical and mechanical properties of andesites, important for volcanic hazard mitigation, remains sparse. We have therefore embarked on a systematic study on the mechanical properties of a suite of andesites collected from Volcán de Colima, one of the most active volcanoes on the Trans-Mexican volcanic belt, Mexico. Our andesite samples (ranging from 8 to 18% porosity) had high initial crack densities (as inferred from both a newly-devised sliding crack model and from more traditional stereological techniques), corroborated by low ultrasonic wave velocities (P-wave velocities were about 2.5 km/s for all samples). Bulk geochemical analysis showed that all samples were compositionally identical. Compressive strength experiments, performed at room temperature and under a constant strain rate of 10-5 s-1, were performed under a range of effective confining pressures (representative of those within a volcanic edifice). When rock is exposed to an applied differential stress, it can react in two different ways. The void space (a combination of cracks and pores) within the rock can either demonstrate net dilatation or net compaction. The resultant behaviour of the rock is governed by the competition between micromechanical processes, namely dilatational microcracking versus grain crushing and pore collapse. The potency of these competing processes is dependent on both the initial physical properties of the rock, such as porosity and grain size, and the conditions under which the rock deforms. In our experiments, all of the andesites displayed dilatancy and/or dilatant modes of failure, either axial splitting (restricted to the uniaxial experiments) or shear faulting at low effective confining pressures. Under uniaxial conditions, peak stresses varied from 18 MPa (for the samples of 18% porosity) and 84 MPa (for the samples of 8% porosity). However, as the effective confining pressure was increased (i.e., with increasing depth in the edifice), the failure stress increased and the deformation of the andesites gradually switched from net dilation (dilatancy) to net compaction (the higher porosity andesites passed through this transition at lower stresses). At these pressures the andesites failed via compaction and compactive shear bands formed in the samples. We flank our mechanical observations with comprehensive microstructural analysis. During the growth of andesitic volcanic edifice, a switch to compactive failure modes should therefore be expected within the porous andesitic rock as they are buried (>1 km) by subsequent eruptive products. This failure mode switch has important ramifications for volcano dynamics since the formation of compactive shear bands could influence the way fluids are transported within the edifice. We also suggest that, to fully understand deformation within an edifice, and hence its stability, we should be aware of the breath of the probable failure modes and the operative micromechanical processes.