Magnall, Nathan and James, Michael and Tuffen, Hugh and Vye-Brown, Charlotte (2018) Commonalities in the emplacement of cooling-limited lavas : insights from the 2011-2012 Cordón Caulle rhyolitic eruption. PhD thesis, Lancaster University.
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Abstract
Lava flows pose a risk to infrastructure and communities near many volcanoes. Although the emplacement processes of low-viscosity basaltic lava flows are relatively well studied, the infrequency of silicic eruptions has limited our understanding of the emplacement of high-viscosity rhyolitic lavas. The 2011-2012 eruption of Puyehue-Cordón Caulle in southern Chile provided a unique opportunity to make scientific observations of an active rhyolitic lava flow. The thesis utilises a multiscale approach to draw comparisons between the emplacement of mafic and silicic lavas by building on the established understanding of basaltic flows. The thesis demonstrates the similarities and differences in the emplacement of cooling-limited lavas at contrasting ends of the compositional spectrum. Observations and quantitative models of lava flow lengthening demonstrate that advance of the Cordón Caulle lava flow was controlled in its latter phases by a cooled surface crust, which suggests similarities to the rheological control of cooling-limited basaltic lava flows. Cessation in flow advance was followed by breakout formation, the first such observations in a rhyolitic lava flow. Breakout formation was triggered by a pressure build-up at the flow margins, due to continued supply of lava along preferential thermal pathways, as well as late stage vesiculation of the flow core. The breakouts developed a morphological range classified into domed, petaloid, rubbly and cleft-split, which reflect the results of advance and inflation processes. Many silicic lavas show pumice diapirs and crease structures at their surface, but such features were rarely observed at Cordón Caulle. The inferred strong surface crust of the Cordón Caulle lava flow, as well as higher viscosity than other rhyolite lava flows, may have locally impeded upwelling to the lava surface, so favouring breakout formation. Insights gained from Cordón Caulle will aid the interpretation of ancient silicic lava flows, and help anticipate the hazards posed by future eruptions of rhyolitic lava.