Rheology and flow of crystal-bearing lavas: Insights from analogue gravity currents

Abstract

The rheology of suspensions of cubic crystals in viscous liquids was investigated with a series of experiments, consisting of the release of a fixed volume of fluid inside a horizontal channel. A Herschel–Bulkley rheology was assumed and the consistency K and the shear rate exponent n of this constitutive equation were calculated using the evolution of the flow front; the yield strength was calculated using the final shape of the flow. A solid fraction by volume of ϕ=0–0.67 and a liquid viscosity range of 1–370 Pa s were used in the experiments. Results show an increase in K when crystal content increases. The mixtures start to show a shear thinning behaviour at ϕ~0.3 with n values going from approximately 1 (Newtonian behaviour) to 0.5 at ϕ=0.6. Yield strength was detected at the same ϕ as the beginning of shear thinning behaviour and increases with a power-law relationship with crystal content. Suspensions with bimodal size distribution of crystals show a dramatic decrease of the apparent viscosity compared to unimodal suspensions, especially at the higher total crystal concentrations. The results were applied to theoretical 2-D flows on a slope, showing large variations in velocity profiles for the same crystallinity depending on the rheology assumed. A case study of a 2002 lava flow from Etna volcano demonstrates that measured lava flow speeds are similar to speeds calculated from 2-D theory with rheologies of lava based on laboratory experiments and measured lava crystal content. The results illustrate that the dynamics of lava flows depend on the crystal size distribution in addition to the total crystal concentration.