Lithofacies and juvenile clast variations at Ranu Grati Maar, East Java: Insights into eruptive transitions in monogenetic volcanoes

Maar-diatremes are valuable archives for understanding phreatomagmatic eruption dynamics and associated hazards. Their stratigraphic records often preserve complex eruptive styles and insights into magmatic conditions prior to eruption. In this study, we integrate detailed lithofacies analysis with petrographic and geochemical investigations of pyroclastic deposits from Ranu Grati Maar, East Java, Indonesia, to reconstruct eruptive transitions and pre-eruptive magmatic conditions. A ∼ 15 m thick tephra succession exposed along the southern and eastern crater rims reveals four distinct stratigraphic units (Units 1–4), differentiated by variations in grain size, componentry, and depositional features. These units record oscillations between phreatomagmatic and magmatic eruptive styles during the eruption. Three juvenile pyroclasts, each scoriaceous (black, gray, and golden), are identified based on textural and color differences. Petrographic and geochemical data indicate that these scoria types share similar bulk compositions and mineral assemblages but display distinct textural variations, including microlite content (∼16.3 % in black scoria vs. ∼9.1–9.6 % in gray and golden scoria), microlite number density, and vesicularity (∼55.5 % in black scoria vs. ∼69.7–71.5 % in gray and golden scoria). These differences suggest that black scoria resided longer in cooler conduit margins, allowing for enhanced microlite crystallization and degassing, whereas gray and golden scoria experienced more rapid ascent and sustained vesiculation in the hotter central conduit zone. Despite their textural diversity, the juvenile clasts from Ranu Grati show minimal geochemical differentiation, indicating that they likely originated from a single magma batch, with heterogeneity driven by dynamic conduit processes rather than complex magmatic evolution. This study demonstrates that eruptive transitions and oscillatory eruption styles in monogenetic systems such as Ranu Grati Maar can be governed by shallow-level magmatic processes (including bubble growth, cooling, and crystallization) without requiring external triggers such as new magma input, aquifer fluctuations, or long-term chamber evolution. These highlight the importance of textural monitoring of pyroclasts for interpreting eruptive conditions and improving hazard assessments, particularly in monogenetic volcanic fields where short-lived eruptions can exhibit abrupt changes in explosivity.