Spatial distribution of alteration and strength in a lava dome: Implications for large-scale volcano stability modelling

Abstract

Volcanoes are unstable heterogeneous structures that can host hazardous mass movements. Hydrothermal alteration can create weak zones that promote instability. Volcanic instability can be assessed using large-scale numerical models, which require accurate and reliable physical and mechanical rock input parameters. Volcano stability models are often constructed using discrete zones that are assigned homogeneous parameters, rarely accounting for the heterogeneity of volcanoes. Given that the range and distribution of alteration and rock strength in volcanoes are likely highly variable, these factors should be carefully determined for accurate modelling. Here, we performed an integrated field and laboratory study. We examined a total of 544 variably altered andesites from seven sampling locations at La Soufrière de Guadeloupe (Eastern Caribbean). Based on a visual assessment, we assigned the rocks an alteration grade index, from 1 (least altered) to 5 (most altered), and measured the strength of rocks in the field using a point load tester. The alteration and strength distribution maps we provide highlight the extreme heterogeneity of a volcanic structure. We provide a method for direct on-site conversion from field to laboratory strength. We find that porosity and strength increase and decrease, respectively, as a function of increasing alteration. The most altered rocks were weak regardless of their porosity, suggesting that the alteration is the primary factor governing strength. We conclude that a volcano can be heterogeneous in terms of alteration and strength, between and within the discrete zones. Therefore, if possible, material property heterogeneity should be incorporated in future volcanic stability models.