Vesicle evolution during rhyolitic Plinian eruptions

Jong Gil Park ¹, Helge M. Gonnermann¹

• Affiliations: 1. Department of Earth, Environmental, and Planetary Sciences, Rice University, Houston, TX, USA

• Presentation type: Poster

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

• Poster Board Number: 221

• Programme No: 3.2.44

• Theme 3 > Session 2

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Abstract

Plinian eruptions of rhyolitic magma are characterized by magma fragmentation, producing pyroclasts with bubble number densities (BNDs) of 10^15±1 m^-3 at discharge rates of 10⁶ - 10⁹ kg/s. What magma ascent pathways, that is conduit geometries, give rise to Plinian eruptions, and what are the resulting magma decompression rates? Our study addresses this by modeling bubble nucleation and growth under a large number of randomly chosen decompression paths between the chamber and surface, generated using the Monte Carlo method. We use a conduit model to interpret the simulated results, which allows us to derive conduit size and discharge rate and account for permeable outgassing after fragmentation. For each given decompression scenario, the model predicts whether fragmentation occurs and at what depth. Bubble number density, vesicularity, and residual water content of pyroclasts, as well as the magma discharge rate and associated conduit geometry, are all model predictions, as opposed to prior chosen parameters. We find that multiple bubble nucleation events, ranging from three to five, occur during rhyolitic Plinian eruptions, producing a wide range of size distributions. These degassing processes are explained by volcanic conduits, which narrow as they approach the surface. A bubble number density of 10^15±1 m^-3 results from a decompression rate of 1-2 order of magnitude in MPa/s at fragmentation. Additionally, the vesicularity and water content are 0.7-0.9 and 1.0 wt.%, respectively, aligning with observational data.