Rheological and morphological evolution of basaltic lava flows

Abstract

Over 500 million people live in proximity of an active volcano globally. Although lava flows rarely endanger human life, they often destroy critical infrastructure. Advancing our understanding of lava flow dynamics is therefore critical to developing accurate hazard assessment, with key socio-economic impacts for many communities. This work focuses on basaltic lava rheology, which exerts a first-order control on flow dynamics and is reflected in lava morphology. In particular, I address the following research questions: (1) How does the rheology of active flows evolve during emplacement; and (2) How can we use flow morphology to infer the rheology of inactive flows? The physical properties of a lava flow vary as it flows away from the source vent, cooling and crystallizing along the way. Temperature (T) controls crystal fraction (Φc) and residual liquid composition (X). The combination of these three factors (T, X, Φc) contributes to determining the main rheological properties of lava: viscosity (η) and yield strength (σ). Additionally, other external factors, such as underlying topography and magma effusion rate, also play important roles by modulating the applied stress (σ) and resulting strain rate (γ̇) experienced by a flow. Flow morphology reflects changes in rheological properties, eruptive parameters, and pre-existing topography, with the potential of revealing important clues as to the emplacement conditions of flows whose active stage was not observed. At Pacaya volcano (GT) I estimated the rheology of an active basaltic lava flow in the field through particle image velocimetry, and compared it with experimental rheological measurements (Soldati et al. 2016). The complementary field and laboratory data sets allowed me to isolate the individual contributions of cooling, crystallization, and changing ground slope to the nearly four-fold effective viscosity increase observed in the field over 550 m downflow distance. I concluded that in this case decreasing slope is the single most important factor, followed by crystallization. At Cima volcanic field (CA, U.S.A.) I used a combination of morphological (airborne photogrammetry-generated digital elevation model of the terrain), geochemical, and rheological data (the first ever for that composition) to reconstruct the eruption temperature, emplacement timescale, and emplacement history of the flow (Soldati et al. 2017). I inferred that lava emerged from the vent at a temperature of 1112°±60°C and reached its final length in about a week, through a multi-stage eruption process. Additionally, I used Monte Carlo simulations to propagate experimental uncertainties, and to select the rheological model (Bingham) that best describes the data. At Piton de La Fournaise (La Réunion, FR DOM), I addressed the longstanding question of how pre-existing topography controls lava flow system structure in volume limited flows (Soldati et al., accepted). I concluded that a steep slope results in a single, stable channel, whereas a gentle slope results in an unstable, braided channel. The findings of this study allow us to interpret and explain the observed flow structure on the basis of pre-existing volcano topography, and to forecast future flow structure. This allowed me to determine that rheology neither affects nor is affected by flow system configuration.