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Investigating Canada's Deadliest Volcanic Eruption and Mitigating Future Hazards

Abstract : Monogenetic volcanoes are the most common volcanic landforms on Earth and usually form isolated small-volume volcanic centres with a wide range of eruptive styles and products. Here, I focus on the case of Tseax volcano (Wil Ksi Baxhl Mihl) in north-western British Columbia, Canada's deadliest volcanic eruption; its ~ 1700 CE eruption killed up to 2,000 people of the Nisga'a First Nation. Tseax is composed by two imbricated volcanic edifices (an outer breached spatter rampart and an inner 70 m high tephra cone) and 4 far-travelled, valley-filling lava flows (2 pahoehoe and 2 'a'a) for a total volume of 0.5 km³ submerging the former Nisga'a villages. All the erupted products are Fe-, Ti-rich, basanite-to-trachybasalts and their geochemical homogeneity suggests the eruption of a single magma batch that was produced by low partial melting of a cpx-poor wehrlite at 52 - 66 km depth. The magma was stalled for > 10³ days in the upper crust and cooled down to 1094 - 1087 °C prior to eruption. The eruption lasted between 1 to 4 months and was divided in two main periods. The first period occurred in a typical Hawaiian-style with lava fountaining, spatter activity and the eruption of long pahoehoe flows. Almost half of the total lava volume was erupted in the first days of the eruption with fluxes > 800 m³/s. The lava may have engulfed the Nisga'a villages in a few tens of hours and thus be one of the cause for the fatalities. A "vog" produced when the lava entered the Nass River may have been also responsible for the Nisga'a deaths. The second period of activity was characterized by low intensity Strombolian explosions with the building of the tephra cone and eruption of the shorter 'a'a lava flows.In high speed channelised lava flows, standing waves are often interpreted as hydraulic jumps, indicating supercritical conditions. Using open channel hydraulic theory for supercritical flows, the geometry of the standing waves to constrain eruption flux and viscosity. I propose that investigating standing waves during ongoing eruption is a powerful tool to help for lava flow modelling and hazard mitigation.
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Submitted on : Tuesday, June 1, 2021 - 12:22:14 PM
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Yannick Le Moigne. Investigating Canada's Deadliest Volcanic Eruption and Mitigating Future Hazards. Earth Sciences. Université Clermont Auvergne; Simon Fraser university (Burnaby, Canada), 2020. English. ⟨NNT : 2020CLFAC067⟩. ⟨tel-03244605⟩



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