PLUMBING THE PRE- AND POST-CALDERA: FIELD AND PETROLOGICAL RECONSTRUCTIONS OF MT. IJEN'S VOLCANIC HISTORY

Caldera-forming eruptions represent the most dangerous volcanic events in our planet’s history. Recent advances in our understanding of the architecture of caldera plumbing systems highlight that even post-caldera edifices maintain long-lived crystalline mushes capable of trapping recharging liquids, allowing for postcaldera resurgence. However, in much of Indonesia, one of the most volcanically active nations on Earth, many calderas lack detailed petrological models of magma storage and behavior. This knowledge gap inhibits our ability to estimate the past and present magmatic activity of such systems, presenting a substantial threat to millions of people.

One of the systems most in need of petrological attention is the Ijen Caldera Complex (ICC) in East Java, Indonesia. Kawah Ijen, the active stratovolcano within ICC, has one of the world’s most acidic lakes, and fumaroles emitting globally significant quantities of CO₂, native sulfur, and toxic metals into the surrounding environment. Despite being mostly quiescent today, Kawah Ijen experienced a large phreatomagmatic eruption as recently as 1817. Along the southern rim of caldera, a chain of inactive stratovolcanoes (Ringgih, Jampit, and Suket) are aligned E-W between Kawah Ijen and the Strombolian-eruption-producing volcano Mt. Raung. North of these stratocones, there are over a dozen postcaldera scoria cones on ICC’s caldera floor, including Mt. Anwar, Wurung, Ilalang, and Papak. These scoria cones have emitted numerous lava flows fall deposits, and pyroclastic surges, shaping the topography of the modern caldera floor. The magmatic drivers of these diverse volcanic features are poorly understood.

To address such uncertainty, we conducted a field campaign in June of 2024 to holistically study the deposits, mineralogy, and petrogenesis of Ijen’s postcaldera products. Here, we present the preliminary results of our field campaign, including field relationships between units, petrography, and phase (mineral and glass) chemistry. These results provide the first petrological picture of Ijen’s postcaldera resurgence, as well as highlighting the complexity of Ijen’s plumbing system.