Thermal resilience of microcracked andesitic dome rocks

The strength of the rocks forming a lava dome informs on its structural stability, important for volcanic hazard assessments. Dome-forming rocks are persistently challenged by thermal stresses from recurring eruptive events that may reduce their strength and jeopardise the structural stability of the dome. Here, we present a series of experiments to better understand the impact of thermal stresses on the strength of an andesitic dome rock from Volcan de Colima (Mexico), a volcano that has witnessed some substantial dome collapses in recent years. Uniaxial compressive strength (UCS) was first tested at room temperature on as-collected samples and samples that had undergone either slow (heated and cooled at 1 degrees C/min) or shock (heated at 1 degrees C/min and shock-cooled in cold water) thermal stressing to target temperatures of 400-700 degrees C. Slow- and shock-cooling thermal stressing did not measurably alter sample strength, connected porosity, or permeability. UCS tests performed at high in-situ temperatures (400-700 degrees C), however, showed an increase in sample strength and stiffness. We interpret that the resistance of this rock to thermal stresses results from both the presence of abundant preexisting microcracks and the thermal stability of its mineral assemblage. Unchanged physical properties for the thermally stressed samples deformed at room temperature suggests that the preexisting microcracks close and reopen, respectively, as the rock expands and contracts during heating and cooling to accommodate the volumetric changes without further microcracking. The increase in strength and stiffness at high in-situ temperatures can be explained by the closure of microcracks due to thermal expansion. These observations suggest that the strength of microcracked dome rocks (1) may be slightly higher when hot (below the glass transition of the groundmass glass), although upscaled strength estimates highlight that dome strength will be largely unchanged by an increase in temperature, (2) may only be reduced following the first thermal stressing event, and (3) may not be further reduced by repeated thermal stressing events. Therefore, thermal perturbations, often observed at active domes, may not, as perhaps expected, repeatedly degrade the strength of individual blocks forming the lava dome and therefore may not jeopardise dome stability. (C) 2018 Elsevier B.V. All rights reserved.