Magma ascent dynamics and transport process of the Mt. Semeru lava dome that collapsed on 4th December 2021 (East Java, Indonesia)

This study aimed to discuss the magma ascent dynamics and transport process of the Mt. Semeru lava dome that collapsed on 4th December 2021 by coupling the field data with analysis of crystal textures (phenocrysts and microlites) and whole-rock geochemistry of clasts embedded in the 2021 pyroclastic density currents (PDCs) of the channel facies. We identify four clast types within the channel PDC facies, namely vesicular-black and porphyritic (V-BP), dense-grey and porphyritic (D-GP), dense-grey and highly porphyritic (D-GhP), and vesicular-reddish and weakly porphyritic (V-RwP). The V-BP, D-GP, and D-GhP clasts are free of olivine, having identical phenocryst size distribution (avg. slope − 3.0) and whole-rock composition of high Zr/Y basaltic andesites (∼56.5 wt% SiO₂; Magma A). Meanwhile, the V-RwP clast includes some olivine, has a characteristically steeper slope of the phenocryst crystal size distribution (CSD) (−3.8), and is classified as low Zr/Y basalt (∼51 wt% SiO₂; Magma B). This evidence suggests the occurrence of at least two magma reservoirs beneath Semeru. Judging from Semeru's historical record, dome formation via the extrusion of Magma A took place between 2004 and 2012, while Magma B was extruded before dome formation (between 1941 and 1994) and produced lava flows and V-RwP clasts; the latter was carried by the 2021 PDCs and thus classified as accidental lithics. The V-BP yielded the steepest CSD slope of plagioclase microlites (−50.8), followed by D-GP (−43.7) and D-GhP (−24.3), suggesting that the type A magma ascended and quenched at different rates. The less porphyritic and more gaseous magma quickly reaches the surface, resulting in V-BP with relatively small microlite size and steeper CSD. While the more porphyritic and gas-poor magma ascends slowly, resulting in slower cooling and thus facilitating the microlites to attain a larger size with gentler microlite CSD. The increasing SO₂ emissions and the presence of harmonic tremors before the collapse event suggest that the dome was destabilized by the ascending magma, forming initially hot PDCs. However, heavy rainfalls (∼15 mm/h) successfully turned the PDCs into wet and cold (<100 °C) conditions, as evidenced by the prevalence of accretionary lapilli in the overbank facies and unsinged trees and wooden and plastic materials.