Microstructural controls on the physical and mechanical properties of edifice‐forming andesites at Volcán de Colima, Mexico

Abstract

The reliable assessment of volcanic unrest must rest on an understanding of the rocks that form the edifice. It is their microstructure that dictates their physical properties and mechanical behavior and thus the response of the edifice to stress perturbations during unrest. We evaluate the interplay between microstructure and rock properties for a suite of edifice-forming rocks from Volcán de Colima (Mexico). Microstructural analyses expose (1) a pervasive, isotropic microcrack network, (2) a high, subspherical vesicle density, and (3) a wide vesicle size distribution. This complex microstructure severely impacts their physical and mechanical properties. In detail, porosities are high and range from 8 to 29%. As a consequence, elastic wave velocities, Youngs moduli, and uniaxial compressive strengths are low, and permeabilities are high. All of the rock properties demonstrate a wide range. For example, strength decreases by a factor of 8 and permeability increases by 4 orders of magnitude over the porosity range. Below a porosity of 11-14%, the permeability-porosity trend follows a power law with a much higher exponent. Microstructurally, this represents a critical vesicle content that efficiently connects the microcrack population and permits a much more direct path through the sample, rather than restricting flow to long and tortuous microcracks. Values of tortuosity inferred from the Kozeny-Carman permeability model support this hypothesis. However, we find that the complex microstructure precludes a complete description of their mechanical behavior through micromechanical modeling. We urge that the findings of this study be considered in volcanic hazard assessments at andesitic stratovolcanoes.

Blowing Off Steam: Tuffisite Formation As a Regulator for Lava Dome Eruptions

“…Taking an aspect ratio of 2, then to achieve the effective viscosities expected, the crystal volume fraction would have to be ∼0.59. This is close to the maximum packing values for those aspect ratios, and corresponds well to the observed crystal fraction in these samples, and previous studies of Colima lavas (e.g., Lavallée et al, 2012;Kendrick et al, 2013;Heap et al, 2014b).…”

Geomechanical rock properties of a basaltic volcano

“…Only recently have laboratory studies begun to systematically investigate the microstructural, physical, and mechanical properties of volcanic rocks, a material infamously known for its formation under disequilibrium conditions, thereby rich in heterogeneities at all scales. These include experiments on a range of material under relevant volcanic conditions such as thermal stressing (Vinciguerra et al, 2005;Kendrick et al, 2013a;Heap et al, 2014a), cyclic inflation-deflation cycles by intrusions (Heap et al, 2009(Heap et al, , 2010Kendrick et al, 2013a), fragmentation (Spieler et al, 2004;Kueppers et al, 2006;Scheu et al, 2008), and flow or fracture at high temperatures and/or pressures (Balme et al, 2004;Rocchi et al, 2004;Smith et al, 2005;Lavallée et al, 2007Lavallée et al, , 2008Benson et al, 2008;Cordonnier et al, 2009;Loaiza et al, 2012;Kendrick et al, 2013b).…”

From magma ascent to ash generation: investigating volcanic conduit processes by integrating experiments, numerical modeling, and observations

“…Textural measurements on samples Textural data provide information on magma pressure and thermal histories in conduits and plumes. For example, vesicle number densities (VNDs; i.e., the number of vesicles per unit bulk volume) provide estimates of magma decompression rates [Toramaru 2006, Shea et al 2011, Wright et al 2012b] and eruption intensities Cashman 2011, Alfano et al 2012], while crystal size distributions (CSDs) and vesicle size distributions (VSDs) (i.e., the number of crystals or vesicles in each size class per unit bulk volume) provide information on crystallization and differentiation in magma reservoirs and conduits [Fornaciai et al 2009, Shea et al 2009, Brugger and Hammer 2010, Arzilli and Carroll 2013, and on magma vesiculation [Bai et al 2008, Gurioli et al 2008, Colò et al 2010, Shea et al 2010, Carey et al 2012, permeability [Mueller et al 2008, Bouvet de Maissoneuve et al 2009, Polacci et al 2014, Heap et al 2014, Kendrick et al 2016, Colombier et al 2017, and degassing/outgassing [Burton et al 2007, Degruyter et al 2010a, 2012. Furthermore, VSDs, CSDs, melt chemistry and volatile content feed into magma rheology estimates [Mader et al 2013, Vona et al 2013.…”