Fissures and fountains: magma dynamics in basaltic conduits

Abstract

Basaltic volcanoes are responsible for the bulk of the planet’s magma output. Their eruptions are often spectacular and can have serious impacts on a local scale (through lava flows) and regional scale (through emission of ash and toxic gases). However, hazard planning and mitigation is challenging because these eruptions can be highly variable both temporally, lasting from hours to months, and spatially, localizing from a long, linear fissure to a discrete vent. In this thesis, I will explore the role that magma transport and eruption dynamics play in controlling the evolution of basaltic fissure eruptions in time and space. Firstly, I present results from a detailed field investigation of the proximal deposits from episode 1 of the 1969 fissure eruption of Mauna Ulu, Kīlauea, Hawai‘i. Exceptional preservation of the deposits allows for the reconstruction of vent-proximal lava drainage patterns and the assessment of the role that drainage played in controlling vent localization. Secondly, I provide the first in-depth textural classification of low Hawaiian fountaining (< 100 m) products. Using spatter from the Mauna Ulu fissure eruption, I describe how the magma matured and produced late-stage spatter during the waning stages of the eruption. Furthermore, a new type of spatter is identified and interpreted to form by the disruption of lava flows above tectonic ground cracks; presenting a potential for misinterpretation of eruptive vents. Thirdly, through a series of scaled analogue experiments, I explore how the simultaneous drainage of dense, degassed, viscous magma and upwelling of less dense, less viscous magma can influence flow processes in the volcanic conduit. This convective system is characterized via the dimensionless Grashof number (Gr), which is a ratio of viscous to buoyancy forces. At low Gr (≲ 0.1), efficient laminar flow is observed with narrow, well-defined fingers of upwelling fluid separated by broad regions of downwelling fluid. As Gr increases (to ~ 100), the flow becomes increasingly chaotic and exchange becomes inefficient – no stable fingers or regions of coherent flow are established. Furthermore, the intrinsic properties of the fissure system (Gr) are related to the extrinsic properties (Reynolds number, Re – a ratio of inertial to viscous forces) to give the empirical relationship: Re = 0.0401Gr^(0.767). Together, these field and lab data suggest that “top-down” lava drainage processes and “bottom-up” convective processes in basaltic fissure eruptions conspire together to control their localization and longevity. This new knowledge advances our understanding of how magma dynamics can influence key, hazard-relevant eruption parameters, such as: mass eruption rate, dynamics of proximal lava flow emplacement, lava fountain vigour, and eruption evolution. This work highlights the importance of understanding the central role that magma dynamics have in shaping volcanic eruptions in order to develop more robust conceptual and physical models of eruptive behaviour.