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Dynamics and deposits of a two-layer model for pyroclastic density currents in magmatic and phreatomagmatic eruptions

Hiroyuki A. Shimizu1, Takehiro Koyaguchi2

  • Affiliations: 1Sabo & Landslide Technical Center, Tokyo, Japan; 2Emeritus Professor of the University of Tokyo, Tokyo, Japan

  • Presentation type: Poster

  • Presentation time: Friday 16:30 - 18:00, Room Poster Hall

  • Poster Board Number: 177

  • Programme No: 3.9.13

  • Theme 3 > Session 9

Abstract

One of the most important challenges in volcanology is a unified understanding of the dynamical features of pyroclastic density currents (PDCs) and the characteristics of PDC deposits for various eruption styles such as magmatic and phreatomagmatic eruptions. We have developed a dynamical model for large-scale PDCs in magmatic and phreatomagmatic eruptions to explain both the dynamics and deposits of strongly stratified PDCs. In the model, the two-layer depth-averaged model consisting of upper dilute and lower dense currents (Shimizu et al. 2019) is combined with a thermodynamical model for magmatic and phreatomagmatic eruptions where the heat balance and phase changes associated with mixing of magma (pyroclasts and volcanic gas), external water, and air are considered (Koyaguchi and Woods 1996). The numerical results show that the run-out distance of upper dilute currents significantly increases with the increasing amount of external water due to the delay of lift-off. For a relatively small proportion of external water, the lower dense current tends to be absent due to the dilution of particle concentrations in the dilute current, resulting in the direct formation of the deposits from the dilute current in the entire area. These results capture the diverse features of natural PDC deposits of phreatomagmatic eruptions. In the presentation, we also discuss the topographic effects on the dynamics and deposits of large-scale PDCs by applying a two-dimensional (2D) depth-averaged model to the lower dense current. The semi-2D two-layer model successfully reproduces the valley-fill distribution of distal deposits.