The influence of effusion rate and rheology on lava flow dynamics and morphology: A case study from the 1971 and 1988–1990 eruptions at Villarrica and Lonquimay volcanoes, Southern Andes of Chile

Abstract

We analyzed two historical lava flows from the Southern Andes of Chile: The lava flows from the 1971 Villarrica volcano eruption and the 1988–1990 Lonquimay volcano eruption. The 1971 lava flow has a volume of 2.3×107m3, a maximum length of 16.5km and was emplaced in two days, with maximum effusion rates of ~800m3/s. The lava has a mean width of 150m and thicknesses that decrease from 10 to 12m at 5km from the vent to 5–8m at the flow front. The morphology is mainly ‘a‘ā. The 1988–1990 lava flow has a volume of 2.3×108m3, a maximum length of 10.2km and was emplaced in 330days, with peak effusion rates of ~80m3/s. The flow has a mean width of 600m and thicknesses that increase from 10 to 15m near the vent to >50m at the front. The morphology varies from ‘a‘ā in proximal sectors to blocky in the rest of the flow. We modelled the advance rate and thickness of these flows assuming two possible dynamical regimes: An internal rheology regime modelled as a Herschel-Bulkley (HB) fluid and a Yield Strength in the Crust (YSC) regime. We compared our results with the widely used Newtonian and Bingham rheologies. Our results indicate that the 1971 flow can be modelled either by the HB, Bingham or Newtonian rheologies using a single temperature, while the 1988–1990 flow was controlled by the YSC regime. Our analysis and comparison of models shows that care should be taken when modelling a lava flow, as different rheologies and assumptions can reach the same results in terms of advance rate and flow thickness. These examples suggest that the crustal strength should be taken into account in any model of lava flow advance.