Transition from Magmatic to Phreatomagmatic Eruptions in Young Ciremai Volcano, Indonesia: Insights from Stratigraphy, Componentry, and Textural Analysis of Tephra Deposits

Vulcanian eruptions, characterized by intermediate magma compositions, pose significant hazards due to their potential for both magmatic and phreatomagmatic fragmentation. The Young Ciremai volcano located in Indonesia has undergone recent phreatic-phreatomagmatic eruptions (from 1698 to 1951), with previous eruptions likely exhibiting both magmatic and phreatomagmatic fragmentations. In order to reconstruct the eruptive histories and elucidate the fragmentation mechanism, we integrate stratigraphic analysis, grain size distribution, componentry, bulk XRD analysis of fine ash, and petrographic analysis, which encompassed the morphometry, vesicularity, and crystallinity of ash particles. The results indicate a complex eruption history characterized by changing fragmentation mechanisms. Magmatic fragmentation correlates with the Vulcanian eruption style, which is characterized by diverse grain size distributions and higher vesicle number densities. The interactions between magma and water drive phreatomagmatic fragmentation, characterized by predominant finer ash exhibiting blocky textures and lower vesicle number densities. The eruptive evolution is described into three distinct stages: Stage 1 indicates magmatic fragmentation resulting from conduit plugging, Stage 2 signifies phreatomagmatic activity originating from the interaction with shallow groundwater, and Stage 3 emphasizes phreatic-hydrothermal activity with continuous recent solfatara manifestation. The results highlight the potential hazards of alternating magmatic and phreatomagmatic eruptions, including pyroclastic density currents and sudden phreatic explosions.