FIELD STUDY, VOLCANIC ASH PETROLOGY, AND COMPILATION OF EVENT CHRONOLOGY TO UNDERSTAND THE HOT ASH CLOUD AND MECHANISM OF MOUNT SEMERU ERUPTIONS ON DESEMBER 2020, 2021, AND 2022

• Main Author: Benita Nareswari, Ratika
• Format: Theses
• Language: Indonesia
• Subjects: Geologi, hidrologi & meteorologi
• Online Access: https://digilib.itb.ac.id/gdl/view/86894
• Institution: Institut Teknologi Bandung

Semero is the most active stratovolcano in East Java and poses significant hazards due to its proximity to populated areas. Since 1967,the activity has been dominated by Vulcanian eroptions and the growth and destroction of lava domes or coulees, with the primary hazard being "hot ash clouds" moving south and southeast. From 2020 to 2022, Semero produced hot ash clouds traveling the farthest distances since 2002, specifically on December 1, 2020, December 4, 2021, and December 4, 2022. Although the travel distances were similar, it is believed that the 2021 event was triggered by summit material collapse due to surface processes induced by rain, while the 2020 and 2022 events were associated with magmatic eroptions. This study aims to examine and compare the events of December 2020-2022 and characterize their deposits in the context of pyroclastic density currents (PDCs) to understand the eroption mechanisms. The research was conducted in the southeastern sector of Semero, 8-16 km from the summit. Field studies were carried out to obtain information on the distribution, facies, and components of the deposits, reflecting the deposition mechanisms. Meanwhile, the petrological study of the PDC matrix ash including component analysis, grain shape, and microphenocryst-microlite texture of feldspar, providing insights into material origin, magma fragmentation, and conduit processes. Additionally, the chronology of events and morphological changes around the crater were compiled to support the interpretation. The study shows consistent PDC deposits from 2020-2022 characterized as block­and-ash flows, predominantly granular flows, with relatively warm temperatures and moist conditions. Block fragments were dominated by gray lava with plagioclase and pyroxene phenocrysts. Breadcrost blocks were also found. The matrix comprised lapilli and brownish-gray ash. Component analysis of the ash matrix from the 2020-2022 events showed similar compositions, consisting of nonjuvenile oxidized lithic lava, crystal fragments of plagioclase, pyroxene, and Fe-Ti oxides, and fresh crystal-rich juvenile lava with low to moderate vesicularity. Juvenile fractions were classified based on color gradients from black to light brown, indicating fresh magmatic material ejected during eruptions. Grain shape analysis of the juvenile components revealed glassy/microcrystalline dense forms, predominantly blocky with a few irregular shapes. This reflects magma with long crystallization times, relatively low gas content, and experienced brittle fragmentation near the surface. The groundmass of juvenile grains consisted of brown to transparent glass with microlites of feldspar, pyroxene, and Fe-Ti oxides. Textural analysis showed that microlite formation conditions were dominated by crystal growth over nucleation, with basic habit of prismatic crystal and overgrowth leading to swallowtail and skeletal forms. These characteristics reflect slow initial magma ascent followed by rapid decompression near the surface. The eruption chronology indicated small-scale lava avalanches and pyroclastic flows before the main events, and the appearance of new lava tongues after the events. Together with the deposit and petrology studies, this shows that the PDCs ofSemeru in 2020, 2021, and 2022 were caused by similar eruption mechanisms of active lava coulee collapse due to increased extrusion rates, with magma already present at shallow levels. All three eruptions originated from the same magmatic system and were controlled by similar conduit processes, resulting in consistent eruptions. Variations in microlite texture can reflect heterogeneous conditions in the conduit during magma ascent.