LAUSR

Abstract Mt. Tongariro is a multi-vent volcanic complex in the central North Island of New Zealand, constructed along the axis of an active graben. Its five largest-known explosive eruption episodes occurred 11,000 years ago during a ~200 year-long paroxysm. During these, stable Plinian and collapsing sub-Plinian columns erupted from multiple sequential and synchronous vents along the graben axis. We examined the spatially varying eruption dynamics during the 200-year paroxysm through the lens of pyroclastic textures. Each eruption episode produced micro-vesicular basaltic-andesitic juvenile lapilli with high vesicle number densities and high groundmass crystallinity. Textural characteristics indicate that pressurisation via second boiling drove eruptions and rhyolite-like melt viscosities promoted brittle failure. Highly variable and mingled textures within individual juvenile lapilli evidence diverse conditions developed across the conduit during the eruption. Foamy magma rose rapidly through the conduit core, driving explosive vigour, and dragged and sheared gas-poor crystallised magma from the conduit margins. Consistent with the northward-widening graben, the most stable Plinian eruptions occurred at northern vents, under highest extensional stresses and possibly widest most stable conduits. By contrast, collapsing, unstable eruption columns occurred at southern vents, where mainly degassed and crystalline magma erupted at lower ejection rates through narrower, unstable and shifting conduits. These data collectively show how spatial variations in structure and tectonics may strongly influence explosive eruption style.