Geochemical and Morphometric Characterization of Mt. Ronggo (Scoria Cone), East Java, Indonesia: Insights into Monogenetic Volcanism and Edifice Instability

Abstrak

Mt. Ronggo is a Quaternary monogenetic scoria cone on the western flank of the Mt. Semeru volcanic complex, East Java. This study integrates whole-rock geochemistry (XRF), field stratigraphy, and morphometric reconstruction to delineate its eruptive evolution and structural instability. Geochemical analysis reveals predominantly basaltic products (SiO₂ ≈ 46–52 wt%) with significant compositional overlap with regional calc-alkaline trends of the Tengger-Bromo, Semeru, and Lamongan systems, suggesting the vent taps a shared or similar transcrustal mush reservoir. The field stratigraphic record captures an eruptive transition, evidenced by a pyroclastic density current (PDC) deposit and preserved charcoal fragments suitable for future radiocarbon dating. Morphometric modeling of the original cone (V ≈ 126.6 × 10⁶ m³) identifies a 120° sector collapse involving 20.4 × 10⁶ m³ of material. Mass balance calculations show the Western Hill remnant accounts for only 23.7% of the failed mass. This volume discrepancy indicates slope failure was governed by a pre-existing NW-SE structural weak zone that compromised the western flank, determining the collapse azimuth and facilitating distal dispersal or burial of the remaining 76.3% of material. These results highlight the critical role of structural heritage in controlling lateral instability of monogenetic volcanoes in the Java Volcanic Arc.

Kata kunci

• east java
• monogenetic volcano
• scoria cone
• sector collapse