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Abstract
The 15.7 cal. ka Rotorua eruptive episode involved two main phases of contrasting eruptive style, and tapped two separate but closely adjacent silicic magma batches with distinctive physical and chemical properties. The eruptive episode began when actively vesiculating rhyolite magma encountered shallow groundwater or a small body of standing water within the Okareka Embayment of the Okataina Volcanic Centre, and generated a locally dispersed deposit. The discharge rate then increased dramatically, driving off or excluding external water from the vent, and the eruption rapidly became dominantly magmatic, producing a widespread plinian fall deposit (2.6 km3) from northwest‐dispersed plumes. Plumes were buoyant throughout but magma discharge rate fluctuated significantly over short intervals of time, probably in response to variations in magma ascent rate and gas loss dynamics, causing instabilities in the eruption column. Maximum plume height (c. 20 km) was achieved early in the eruption and waned irregularly to c. 14 km in the later stages, associated with the progressive involvement of a second, largely degassed magma body. Phase 2 of the eruptive episode marked the transition from explosive to dominantly effusive activity, as the consequence of increased involvement of a separate batch of relatively gas‐poor magma (forming Middle and Trig 7693 rhyolite domes) in the Okareka Embayment. Eruptions associated with dome construction probably occurred irregularly over several years. Periodic vulcanian explosions and dome collapses produced locally dispersed pyroclastic density current and fall deposits, dispersed mostly to the south and east.
