Understanding Basaltic Plinian Eruptions

Abstract

Plinian eruptions are the explosive endmember of basaltic volcanism, ejecting several km3 of tephra, with significant impacts for local communities and climate. The low viscosity of basaltic magma should prevent magma fragmentation during ascent, unless subjected to exceptionally high strain rates. However, the discovery of several Plinian deposits has challenged this model. Understanding the conditions that may promote a basaltic Plinian eruption is crucial for hazard assessments. To address this question, I have investigated three basaltic Plinian eruptions: the Fontana Lapilli and Masaya Triple Layer eruptions of Las Sierras-Masaya volcanic system, and the 122 BC Etna eruption, combining field studies, analytical techniques and a numerical conduit model. I first constrained pre- and syn-eruptive conditions from analysis of natural samples. Using X-ray microcomputed tomography, I have provided the first constraints on the 3D pore network geometry which controls magma permeability and degassing during a basaltic Plinian eruption. I incorporated these data into an existing numerical conduit model to simulate eruption dynamics and explore the sensitivity of modelled eruptive behaviour to different input parameters. The results of this integrated approach find that the initial pre-eruptive conditions strongly influence the eruptive style at basaltic volcanoes. Basaltic Plinian eruptions are driven by a common set of physico-chemical conditions, such as moderate-low pre-eruptive storage temperatures, low volatile concentrations and high crystal contents. Moderate-low pre-eruptive storage temperatures can drive rapid syn-eruptive crystallisation within the conduit, leading to brittle magma fragmentation. High magma ascent rates maintain gas-melt coupling, even at high magma permeabilities, restricting outgassing. Cooling, crystal-rich basaltic magma bodies within the shallow crust should be monitored carefully, due to their potential to erupt with Plinian magnitude. By using an integrated approach that combines field studies, analytical techniques and numerical modelling, the initial conditions which may promote a basaltic Plinian eruption can be identified and constrained. These data can be used to inform monitoring strategies at basaltic volcanoes, as precursory signals may indicate a transition to a highly explosive volcanic system.

Date of Award	1 Aug 2022
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Mike Burton (Supervisor) & Margherita Polacci (Supervisor)