The fate of an extremely phenocryst-rich magma in producing small sub-Plinian plumes during the 17th and 30th April 2024 eruption of Mt. Ruang (North Sulawesi, Indonesia): The role of clast density

Abstract

After ∼22 years of dormancy, two vigorous sub-Plinian eruptions occurred at Mt. Ruang (North Sulawesi, Indonesia) on April 17th and 30th 2024, with an observed plume height of 9–12 and 19 km, respectively. Petrography and whole-rock XRF analysis reveal that Ruang pumices are extremely rich in phenocrysts (0.36–0.87 ${ф}_{\mathrm{PC}}$; plagioclase>amphibole>pyroxenes>olivine>oxides) and dense (1.31–2.33 g/cm³), with SiO₂ contents of 54.0–56.3 wt%. This suggests that the eruption was sourced by an extremely crystal-rich basaltic-andesite magma reservoir. The magma likely received substantial recharge from the deeper-hotter source as evidenced by the prevalence of amphibole megacryst, crystal clots, and disequilibrium phenocryst textures (i.e., reverse zoning, oscillatory zoning, and fine sieves). Ash from the 17th and 30th April 2024 eruptions are classified as very fine ash (median of 29 and 26 μm, respectively) and exhibit a blocky characteristic (0.75–0.99 convexity and 0.71–0.98 solidity). This evidence, coupled with the presence of stepped surface and hackle marks textures strongly suggests the involvement of external water during the eruption. The fact that Ruang pumices are characterized by high matrix-vesicle and feldspar microlite number density values (log ∼15 m⁻³) suggests that the magma decompression was rapid, reaching 0.05–27 MPa/s. However, despite this condition, the eruption was only able to generate relatively small sub-Plinian plumes. By consulting with the huge dataset from other previous studies, we found that plume height shows a negative correlation with phenocryst content and bulk density, with Ruang pumices having the highest phenocryst content and bulk density variations. This suggests that; besides magma decompression rates and magma-water interaction, the pre-eruptive crystallinity condition within the magma reservoir is essential for controlling the plume height and hence might be used as a proxy to predict the intensity of future eruptions.