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Effect of bubble dynamics in driving explosivity of peralkaline trachytic/phonolitic magmas

Lorenzo Cappelli1,Gianmarco Buono2, Lucia Pappalardo2, Karen Fontijn1

  • Affiliations:  1Department of Geosciences, Environment and Society, Université libre de Bruxelles, Bruxelles, Belgium; 2Istituto Nazionale di Geofisica e Vulcanologia, Italy 

  • Presentation type: Poster

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

  • Poster Board Number: 217

  • Programme No: 3.6.21

  • Theme 3 > Session 6

Abstract

Explosivity of rising magma is influenced by conditions established at the storage level, but is within the magmatic conduit that magma's fate is determined. Peralkaline magmas (agpaitic index > 1) exhibit high water saturation levels, providing sufficient fuel to drive explosivity. At the same time, their low viscosities---resulting from alkali-induced silica depolymerisation---impede fragmentation conditions compared to high-silica calc-alkaline magmas. Yet, peralkaline magmas can still produce violent eruptions, as evidenced by numerous trachytic and phonolitic documented events. The Rungwe Pumice eruption (Tanzania) serves as a striking example of unexpected eruptive behaviour. This Plinian VEI 5 eruption was generated by a crystal-poor, microlite-free phonolitic/trachytic magma stored at high temperatures and low water concentrations, indicative of shallow storage depths. Under these conditions, likely milder eruption intensity is expected. However, detailed 2D and 3D textural analyses of pumice ash clasts, reveal delayed homogeneous bubble nucleation (ΔP~50MPa) occurring abruptly at shallow depths (Pn~17 MPa) due to fast ascent rates (1-2 MPa). The rapid nucleation and growth of bubbles left insufficient time to form a highly vesicular foam (f < 75%), while low magma permeability hindered efficient outgassing. This maintained a strong coupling between magma and gases, and, combined with a sudden rheological shift likely triggered by volatile loss and a temperature drop, ultimately led to fragmentation and the explosive nature of the eruption. The Rungwe Pumice eruption highlights the critical role of conduit dynamics in shaping the behaviour of peralkaline magmas, which can unexpectedly deviate from predictions based solely on their composition and storage conditions.