Abstract
Basaltic fissure eruptions may evolve rapidly and unpredictably complicating hazard management. Localization of an elongate fissure to one or more focused vents may take days to months, and depends on fluid dynamic processes, such as thermally-driven viscous fingering, in the sub-volcanic plumbing system. However, fluid dynamics in a dyke geometry are poorly understood. We perform scaled analogue experiments to investigate convective magma exchange flow within a dyke-like conduit, and discover flow regimes ranging from chaotic mingling to stable, well-organized exchange, over the parameter space relevant for natural eruptions. Experiments are scaled via the Grashof number Gr, which is a Reynolds number for buoyancy-driven exchange flows. We propose that chaotic exchange at high Gr hinders thermally-driven localization by suppressing viscous fingering, whereas flow organization at low Gr enhances localization. Consequently, progressive decrease in Gr through increasing magma viscosity or decreasing dyke width pushes a fissure eruption towards a tipping point that results in rapid localization. Our findings indicate that current conceptual models for magma flow in a dyke require revision to account for this convective tipping point, and provide a quantitative framework for understanding the evolution of fissure eruptions.
Highlights
The fluid dynamics of magma ascent in the sub-volcanic plumbing system play a key role in determining the eruptive style of a volcano (Houghton et al, 2004)
The instabilities grow and organize into diverse flow patterns that depend on the viscosity of the downwelling fluid; typical sequences of this start-up flow are shown in Fig. S1, but are not the focus of this study
Across all Gr, the observed wavelengths (Videos S1–4) of both developing instabilities and developed flow patterns are longer than λc, indicating that surface tension does not play a role in determining the development of the fluid dynamic phenomena, or their observed variation with Gr
Summary
The fluid dynamics of magma ascent in the sub-volcanic plumbing system play a key role in determining the eruptive style of a volcano (Houghton et al, 2004). We investigate how convective flow patterns interact with thermo-rheological effects to enhance or hinder flow localization. Multiple lines of evidence indicate that basaltic conduits commonly host density-driven convection, in which a lower-density upwelling magma exchanges with a higher-density downwelling magma (Fig. 1): (1) Excess SO2 flux at persistently-active basaltic volcanoes indicates that magma ascends to the shallow conduit, outgasses, descends without being erupted (Allard et al, 1994; Burton et al, 2007; Kazahaya et al, 1994; Oppenheimer et al, 2004; Palma et al, 2008; Stevenson and Blake, 1998; Witham, 2011). (2) Zonation in erupted crystals indicates that magma may repeatedly ascend and descend in the conduit (i.e. convect) before being recycled into magma at depth (Francalanci et al, 2012; Landi et al, 2008). Data for the 1986-90 eruption of Izu-Oshima (Kazahaya et al, 1994), indicate a mean magma exchange flux of ∼3.4 m3 s−1, but a net eruption flux of only ∼0.2 m3 s−1. (2) Zonation in erupted crystals indicates that magma may repeatedly ascend and descend in the conduit (i.e. convect) before being recycled into magma at depth (Francalanci et al, 2012; Landi et al, 2008). (3) Lava has been directly observed to drain back down a fissure system during an eruption (e.g. 1974 eruption of Kılauea; Wilson et al, 1995). (4) Concurrent upward and downward magma flow has been inferred from studies of flow textures in exposed vent deposits and dyke feeder systems at a range of depths (Geshi and Neri, 2014; Jones et al, 2017; Wadsworth et al, 2015)
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