Rheology and dynamics of lava flows

Abstract

This thesis presents the results of laboratory experiments, theoretical analysis, field work and petrological analysis conducted to study the rheology and dynamics of lava flows. 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. Results show an increase in K when crystal content increases. The mixtures start to show a shear thinning behaviour at 4>-0.3 with n values going from approximately 1 (Newtonian behaviour) to 0.5 at 4>=0.6. Yield strength was detected at the same 4> as the beginning of shear thinning behaviour and increases with a power-law relationship with crystal content. Suspensions with bimodal crystal sizes show a dramatic decrease of the apparent viscosity compared to unimodal suspensions, especially at the higher total crystal concentrations. Further experiments, with the same types of fluids (with approximately Herschel- Bulkley rheology) in a sloping channel with a constant flux rate were approximated with a simplified 2-D model. The results show an excellent agreement between the theory and the experiments. Additionally, experiments with two fluids, where a low viscosity fluid (syrup) is injected in a fixed volume of more viscous material (syrup plus crystals) suggest that the front advance and height is controlled mainly by the rheology of the fixed volume fluid. The simplified 2-D theory was applied to real lava flows. Three dynamical regimes were considered: A Newtonian viscous regime, a yield strength-dominated regime, and a crust-dominated regime. The resulting equations were applied to published data from eruptions of 10 lava flows with a range of compositions and conditions. Comparisons of the fits of the models to the data reveal that short-lived, high effusion rate eruptions typical of Hawaiian lava flows, are dominated by the internal viscosity of the lava, whereas low effusion rate or long lived eruptions are dominated by the yield strength in the growing crust. Finally, eruptions with very high initial crystal contents are dominated by the internal yield strength. The developed rheological model and 2-D theory were applied to the 2002 lava flow deposits at the NE flank of Etna volcano to reconstruct the variations in flow rate and advance of the flow front. Analyses of samples were used to estimate crystal content and liquid viscosity variations with distance. Yield strength variations were estimated from levee width measurements. The modelled flow rate and advance of the flow front compare well with the measured data, with a mean flow rate of 19.5 m3/s. The method developed in this thesis provides the prospect of using measurements of flow dimensions together with analysis of samples from the deposits to estimate the rheology of the flow and reconstruct variations of eruption parameters.EThOS - Electronic Theses Online ServiceGBUnited Kingdo