Skip to content

Unravelling Transient Phreatomagmatism in the 1982-83 Galunggung Eruption through Grain Size and Textural Analysis of Pyroclasts

Tabegra Disando1, Karen Fontijn1, Mirzam Abdurrachman2

  • Affiliations: 1Department of Geosciences, Environment and Society, Université libre de Bruxelles, Bruxelles, Belgium; 2Department of Geological Engineering, Institut Teknologi Bandung (ITB), West Java, Indonesia 

  • Presentation type: Poster

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

  • Poster Board Number: 176

  • Programme No: 3.3.13

  • Theme 3 > Session 3

Abstract

The 1982-83 Galunggung eruption was a complex eruption involving both magmatic and hydromagmatic phases. The eruption began with the destruction of the 1918 dome (P1a) involving fresh basaltic magma (P1b), followed by a paroxysmal phreatomagmatic phase (P2) with the same magma composition, and ended after a strombolian phase involving more primitive magma (P3). In this study, we characterise the pyroclasts of the different eruption phases based on their grain size distribution, components, morphometry, and textures. Based on the stratigraphic profile, pyroclastic layers show a compositional change from matrix-rich density currents in P1 to clast-rich fall-dominated deposits in P2 and P3. The abundance of lithics and accreted ash-rich layers in the P2 strata indicates conduit erosion, water interaction, and more effective fragmentation during this transient phase. Subangular to subrounded ash particle shapes in P2 illustrate brittle and ductile fragmentation, possibly with variable excess water ratios relative to the basaltic magma. Vesicularity in P2 is lower (ca. 26%) than in both P1 and P3 (ca. 50%). Phenocryst crystallinity in P2 is higher than in the other phases, around 57%. P2 magma is therefore thought to represent the deeper levels of the magma reservoir feeding P1b. With >10% MgO, P3 indicates a new supply of primitive magma. Microlite number densities in both P2 and P3 range from 106.6 to 107.2 mm⁻³,  consistent with decompression rates expected for strombolian-style eruption mechanisms.