Breaking down walls: conduit wall evolution during explosive activity at an andesitic stratovolcano

The sub-Plinian AD1655 Burrell eruption of Taranaki Mounga, Aotearoa/New Zealand, underwent multiple changes in eruption style from effusive dome growth to explosivity with a buoyant plume followed by total plume collapse. This contrasts the effusive-only eruptions that both preceded and followed it. Understanding drivers of effusive to explosive transitions in eruptive style is important for hazard mitigation at any stratovolcano. Textural analysis of pumice and lithic clasts from fall and pyroclastic density current deposits of the Burrell eruption is used to present a model for the development and evolution of a shallow conduit system across a multi-phase eruption. We show how the composition and textures of the conduit margins changed through time. During the initial dome-forming phase there was progressive deformation of remnant linings forming the conduit wall. As the eruption progressed the conduit was locally lined with variably vesicular juvenile lithologies, although zones of brecciated and altered linings remained. Both vesicular and dense juvenile linings from earlier eruption phases formed the lining of subsequent phases, highlighting how shallow conduit wall processes are a balance between accumulation and erosion/fragmentation. Our findings show that to build a complete picture of conduit conditions and the drivers of changes in eruption style, the textures of both pumices and lithics must be investigated within pyroclastic deposits.