Audrey Michaud-Dubuy receives the CCR Cat Nat 2020 thesis prize

Summary of her thesis:

Volcanic plumes produced by explosive eruptions represent a major hazard in areas near volcanoes. Physical models developed over the last forty years have aimed to better understand these eruptions and to quantify the associated hazards. Tests of the robustness of these predictive models must be based on accurate and detailed field data on past eruptions of active volcanoes. In this thesis, we propose to revisit the Plinian eruptive history of Mount Pelée in Martinique (Lesser Antilles) over the last twenty-four thousand years. Our results, combining fieldwork and carbon-14 dating, enable us to establish a new chronology of past eruptions, in agreement with observations made on a core sample taken from the seabed. We then reconstruct the dynamic evolution of the newly-discovered eruptions at Bellefontaine (13,516 cal BP), Balisier (14,072 cal BP), Carbet (18,711 cal BP) and Étoile (21,450 cal BP), whose main interest lies in their unusual southward dispersal axis, which encompasses areas considered safe on current hazard maps. The strong similarities observed between all the documented Plinian eruptions of Mount Pelée allow us to draw up a portrait of the eruptive scenario most likely to occur in the future.

Since this scenario could include collapse of the eruptive column and the production of pyroclastic density flows, we are modifying a 1D physical model of the volcanic plume in order to improve its predictions. We first study the impact of the size distribution of volcanic fragments on the transition from a stable Plinian column to a collapsing fountain. The effect of wind is then taken into account using novel laboratory experiments to simulate turbulent jets forming in an environment subject to wind. We propose a new theoretical model, validated by experiments, which reconciles data from several major historical Plinian eruptions. We then study the dispersion of volcanic ash during the Bellefontaine and Balisier eruptions using a 2D physical model to understand the origin of their preferential direction towards the south, and therefore towards Fort-de-France, the capital of Martinique. Our results make it possible to identify specific atmospheric contexts during which the path of the subtropical jet stream is modified, producing northerly winds over Martinique that can disperse volcanic ash over the most densely populated areas. This integrated approach, combining field studies, numerical simulations and laboratory experiments, has enabled us to draw up a new volcanic hazard map for Martinique, taking account for the first time of past Plinian eruptions on Mount Pelée over the past 24,000 years, as well as the monthly variability of atmospheric winds.

More information on the CCR website: https://www.ccr.fr/prix-ccr-cat-nat