HOW DO POSTCALDERA SYSTEMS BEHAVE? FIELD AND PETROLOGICAL RECONSTRUCTIONS OF MT. IJEN’S INTRACALDERA SCORIA CONES

The Ijen Caldera Complex (ICC) in East Java, Indonesia, is one of SE Asia’s most complex and least understood active caldera systems. The ICC has two active eruptive centers along its crater rim (CR), as well as more than a dozen intracaldera (IC) features spreading across its crater floor. Kawah Ijen, the one of two CR stratovolcanoes, has fumaroles which emit high quantities of CO₂, sulfur, and other toxic metals into the atmosphere. It also has the world’s most acidic lake, whose most recent eruption was a phreatomagmatic event in 1817. The system still shows signs of activity, and there are worries about how a future eruption might impact a community with little collective memory of the last event. West of Kawa Ijen lies the other CR peak, Mt. Raung, which produces regular Strombolian eruptions. To the north of these stratovolcanoes are many IC scoria cones, formed after the initial caldera-forming eruption. Mt. Anyar is one such IC scoria cone, and it exhibits some of the best exposure of lava flows and pyroclastic fall deposits in the caldera. In this study, we aim to characterize the geological history of Anyar in an attempt to better understand the post caldera evolution of the ICC. Using rock samples gathered from Mt. Anyar in June of 2024, we have begun a petrographic, microanalytical, and modeling study of Anyar’s eruptive products. Mt. Anyar has stratigraphic evidence of past effusive and mafic activity, as evidenced by both its crystal cargo and field observations. Different layers of pumice airfall, scoria bombs, and lava fragments show varying patterns in vesicle density and shape factors, indicating variable eruptive behaviors over the life of this scoria cone. We combine these petrographic insights with both qualitative (SEM-EDS) and quantitative (EPMA) chemical analyses of mineral and glass phase chemistry. When modeled alongside whole rock geochemical records using the rhyolite-MELTS software, this study provides the first glimpse of Anyar’s storage conditions and volcanological evolution through time. This provides us with a glimpse into the varied hazard potential of this active system.