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Modeling explosion dynamics during phreatic eruptions at Campi Flegrei

Nils Mekelburger 1,2, Tomaso Esposti Ongaro1, Mattia de\' Michieli Vitturi1

  • Affiliations: 1Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Pisa, Pisa, Italy 2Dipartimento di Fisica, Università di Pisa, Pisa, Italy

  • Presentation type: Poster

  • Presentation time: Thursday 16:30 - 18:30, Room Poster Hall

  • Poster Board Number: 77

  • Programme No: 3.14.11

  • Theme 3 > Session 14

Abstract

Despite their relatively small volume (compared to magmatic volcanic eruptions), phreatic eruptions have proven to be hazardous, due to their unpredictability and significant proximal impact in terms of ballistic ejection and generation of pyroclastic density currents (Ontake, 2014, Whakaari, 2019).  In densely populated active calderas, hosting large hydrothermal systems, like Campi Flegrei (Italy), this is especially critical. Therefore numerical modeling is needed to prepare for possible eruptive scenarios during volcanic unrests. We use a three-dimensional multiphase flow model to simulate phreatic explosions. The model describes the sudden decompression of a high-pressure, high-temperature particle-gas mixture (after fragmentation) and the simultaneous ejection of coarse lithic blocks. After the decompression phase, the expanded mixture forms an eruptive cloud and, eventually, pyroclastic density currents. We present a preliminary calibration and benchmark study comparing results of 3D numerical simulations and laboratory experiments of a shock tube with compressed gas and ash samples, with different grain sizes and with initial pressures. In detail, the rarefaction speed and the particle ejection velocity are compared showing good agreement. The influence of the specific energy (as a function of the initial gas content, overpressure and temperature) is investigated to provide an integral parameter characterizing explosion scenarios. Application to phreatic eruption scenarios at Campi Flegrei show that the area affected by ballistics and pyroclastic flows is strongly controlled by the initial specific energy, while changes in temperature and geometry had only second-order impacts. This is aligned with the approximately adiabatic nature of these explosions.