Channeled lava dynamics: implications for Hawaiian lava studies

Lava from large volcanic eruptions on Hawai'i often self-organizes into localized channels, which can rapidly transport lava over large distances. The dynamics of lava transport through these channels (and particularly the dynamics of crust growth and a changing internal rheology) determine lava cooling rates; thus their dynamics form a crucial part of the large-scale problem of lava flow emplacement and extent. We present recent experimental, theoretical and numerical analysis which sheds light on the complex interplay between rheology and solidification in channelized lavas, and discuss how these general physical insights may be applied to field studies of Hawaiian lava flows in particular. The experimental work used slurries of polyethylene glycol and kaolin as an analogue for lava, which flowed with a constant flux down an inclined channel under water. The water temperatures were varied to give three sets of complementary experiments: isothermal, cooling and solidifying flows. Isothermal flows, in which the slurries and water have the same temperature, are used as a base case for the study. Surface velocities, measured by tracking particles dropped onto the surface of the flow, are compared with numerical solutions for the velocity field. A root-finding scheme using the numerical results allows us to infer flow depth, viscosity and yield strength from a known flow rate and the measured surface velocities, and we show that the existence of unyielded regions and the variation of velocity profiles with flow rate is consistent with a Bingham rheological model. We also discuss how these methods can be used to determine yield strengths in lava flows in the field from measurements of surface velocities. Cooling flows, in which the water temperature is between the initial and solidification temperatures of the slurry, are used to quantify the effects of the viscoplastic rheology on thermally-driven convection within the flow. These experiments showed that thermal convection occurs in organized rolls aligned with the shear flow, but that unyielded regions are not broken up by the convective overturning. Solidifying flows, where the water temperature which was below the solidification temperatures of the slurry, were used to quantify the effects of the viscoplastic rheology on crust formation. In these experiments a quasi-stable width of crust formed on the surface of the flow. The degree of surface crust coverage and the transition between crust cover regimes is quantified in terms of a dimensionless parameter which characterizes the relative importance of the strain and crust growth rates. We show that this parameter accurately predicts the crust distribution for both viscous and viscoplastic flows in the mobile crust regime, as well as the transition between tubes and mobile crust flows. Finally we examine the implications of these results for both field studies and lava flow models (e.g. FLOWGO), and highlight some areas where field studies could help to bridge the gap between lab and field, and improve our physical understanding of flow dynamics.