Transition from stable column to partial collapse during the 79 cal CE P3 Plinian eruption of Mt. Pelee volcano (Lesser Antilles)

摘要

Explosive volcanic eruptions commonly form sustained Plinian columns that collapse at some stage producing dangerous pyroclastic density currents (PDC) on the ground. Numerical and laboratory models of volcanic plumes show that the conditions leading to total column collapse are strongly controlled by the amount of exsolved gas at the source and the mass eruption rate. However, column collapse is rarely total and the volcanic jet often separates in a dense collapsing part feeding PDC and a buoyant rising plume spreading volcanic gases and pyroclasts in the atmosphere. This transitional regime has been directly observed and/or inferred from the structure of the deposits for several past eruptions, but the number of cases for which the partial collapse regime is described in detail, including the 79 CE Vesuvius, and the 186 CE Taupo eruptions, remains too small to fully constrain physical models. Here, we present a detailed reconstruction of the time evolution of the P3 eruption at Mt. Pelee volcano (Martinique, Lesser Antilles) that underwent partial column collapse in order to discuss the mechanisms controlling the eruption dynamics and improve the volcanological database on transitional eruptions. The P3 eruptive succession consists of seven major phases that produced a total of 1 km 3 dense rock equivalent (DRE) of deposits (i.e., VEI 5 event), starting with a thick pumice fall deposit (0.1 km(3) DRE) overlain by alternating pyroclastic density current (PDC) (0.7 km(3) DRE) and pumice fall deposits (0.2 km(3) DRE). We use physical models together with field data on deposit dispersal, thickness, and grain-size distribution to reconstruct the dynamical evolution of the volcanic column. Our results show that the mass eruption rate (MER) increased from 1.2 x 10(8) to 1.7 x 10(8) kg s(-1) during the initial phase producing a 28 to 30 km-high Plinian plume. The MER later reached up to 25 x 10(8) kg s(-1) and the column entered the partial collapse regime characterized by the formation of a small (i.e., 12 to 17 km-high) ash plume and contemporaneous PDCs mainly channelized in three paleo-valleys. These estimates are used together with published data on magmatic water contents in glass inclusions to decipher the mechanisms leading to partial collapse. The P3 eruption column collapsed due to an increase in mass eruption rate and a decrease in gas content. A similar evolution was also inferred for the 1300 CE P1 and 280 CE P2 eruption deposits, revealing a systematic behavior in the recent Plinian eruptions of ML Pelee volcano. The comparison of model predictions of column collapse and field data reveals a good agreement for the P1, P2, P3 and Taupo eruptions, but not for the 79 CE Vesuvius eruption where thermal disequilibrium between gas and pyroclasts most likely strongly affected the column dynamics. (C) 2019 Elsevier B.V. All rights reserved.