Evidence of explosive-effusive volcanic eruption transitions from a fossilized magma conduit on the flank of the Nevados de Chillán volcano

Abstract

Volcanic activity is often characterised as either effusive or explosive, but both the 2008 Chaitén eruption and the 2011 Puyehue-Cordón Caulle eruption exhibited periods of hybrid eruptive style, indicating that both eruption styles can occur simultaneously. The underlying processes that control eruptive styles within a single eruptive event are still poorly understood. As such, more insight is needed to determine factors that govern both a particular style of eruptive activity and the changes between different style within the same event. Data to aid this understanding is necessary for accurately determining the potential risks related to future eruptions at any volcano. In this work, we investigate the question of how effusive and explosive eruptive phases interact and evolve during the ascent of magma through a single conduit? To provide insight into these processes, we present new data from a fossilized conduit on the South Eastern flank of the Nevados de Chillán volcano which is active and hazardous Andean volcanic complex that presents a diverse range of magma compositions and long history of eruptions. The 2m wide shallow conduit records textural evidence indicative of diverse transitions in magma behaviour. We combine field and laboratory methods focused on the integration of field-scale and micro-scale observations, using three-dimensional outcrop reconstruction from photogrammetry, optical microscopy and Scanning Electron Microscopy (SEM) for textural analysis, Fourier Transform Infrared spectroscopy (FTIR) to map total water content in different areas of the conduit and Helium Pycnometer and image analysis to quantify porosity and particle size distribution. The conduit presents xxx distinct zones which were defined based on their textures and morphologies. At the contact between zone I and II , we identify a 0.5 m thick zone of sub-vertically intercalated bands of yellowish pyroclastic material with darker densified lava. f two main domains , one pyroclastic and the other of densified lava. The intercalated bands form undulating but linear structures with individual band thicknesses ranging from millimetres to decimetres. Devitrification textures, high glass content margins and alteration zones are present in the inter-band contacts. Spot data from SEM analysis shows little chemical composition difference within and between the different bands, indicating a common magma origin. Detailed textural analysis indicates the occurrence of clast morphologies with globules and rounded particles welded the crystal surfaces, indicative of sintering and densification.