The Evolution of a Basaltic Fissure Eruption

Abstract

Effusive basaltic eruptions, the most common form of volcanism on Earth, produce lava flows that may destroy local infrastructure and emit toxic gas and particles that may adversely impact public health on a regional scale. These eruptions typically begin as long, linear fissures producing a “curtain of fire”, which gradually localizes into separate, isolated vents along the fissure line. Predicting the eruptive style and its evolution for basaltic volcanoes is therefore a key goal in volcanology and requires an understanding of the multiphase flow processes within the sub-volcanic plumbing system. This PhD project aimed to characterise the evolving organisation of gas-driven flow patterns within basaltic feeder dyke systems, with a particular focus on the effects of volcanic outgassing via discrete vents along fissures. Field observations of both active and historic basaltic fissures were synthesised with laboratory experiments using two different analogue apparatuses to examine the relationships between conduit geometry, gas content and spatial organization, and eruption dynamics.

The novel experimental dyke kit “LAVA” was used to perform scaled analogue experiments reproducing bubbly flows in a 3 x 2 x 0.03 m glass-walled slot, allowing us to examine gas-driven flow patterns in the 2D conduit geometry of dykes that feed most basaltic eruptions, whereas previous experimental studies had usually adopted a cylindrical conduit. The investigation revealed that large decoupled bubbles in such 2D fissure conduits can achieve higher ascent velocities than those in 1D (cylindrically confined) or 3D (unconfined) scenarios. It also showed that surface vent distribution along a fissure has minimal impact on subsurface flow patterns in the absence of co-current liquid flux, but the spacing between gas pathways within the fissure affects bubble ascent dynamics and vent arrangement at the surface, thus likely playing a role in fissure localization. Shifting the focus to eruption dynamics, a new method for determining the vertical scale in eruption videos was then developed using the motion of ballistic clasts near the zenith. This method allows crowd-sourced footage of basaltic fissure eruptions to be leveraged for scientific analysis and inference of subsurface processes driving eruption dynamics and evolution. Subsequently, an analogue fountain kit was designed and built to explore how the relative proportions of gas and magma fluxes, and the presence of lava ponding at the surface, affect eruptive style at separate vents along a basaltic fissure. The ensuing fountain experiments demonstrated that the gas volume fraction plays a crucial role in determining lava fountain heights, with a lesser impact from magma flux, while also revealing that this influence is significantly moderated by lava ponding at the fissure surface. The thesis finally discusses the influence of topography on lava re-entrainment and ponding dynamics during a fissure eruption, thereby shaping eruption dynamic. Overall, this research advances our understanding of basaltic fissure eruptions by examining both the “bottom-up” effects of subsurface gas dynamics and the interlinked “top-down” effects of surface processes like topography, spatter cone formation, and lava drainage and ponding on eruption dynamics and evolution.