Assessing lava flow evolution from post-eruption field data using Herschel-Bulkley rheology

We present a method to estimate the rheology of a lava flow and reconstruct variations of eruption parameters from measurements of flow dimensions together with petrological analysis of samples from the solidified flow. The model assumes a Herschel-Bulkley lava rheology, which we demonstrate is a reasonable approximation from new analyses of samples and rheological measurements from past eruptions of Etna volcano, as well as published data for other lava compositions and crystal contents. We present a simplified 2-D model for the flow of a Herschel-Bulkley fluid over an inclined plane with a constant flux from a vent, which we validate with analogue experiments scaled to typical conditions of lava flows. Further analogue experiments with two fluids of different rheologies demonstrate the dominance of the rheology at the flow front in controlling the advance rate of the flow. The Herschel-Bulkley flow model was applied to a lava flow of the 2002 eruption on Etna volcano. Flow velocity and effusion rate were calculated from the post-eruption flow dimensions and rheology estimates along the length of the flow were based on the crystal content and glass compositions of samples. The results compare well with the observations during the eruption, with a mean flow rate of 19.5m³/s, which is in the range of the 15-20m³/s calculated by previous authors.