The Lamongan Volcanic Field (LVF), East Java, Indonesia, has experienced numerous maar eruptions, producing varied properties and morphologies of ash particles. This study conducted textural, morphometric, and geochemical analyses of the juvenile particles to elucidate the factors governing their heterogeneous characteristics. Two distinct types of juvenile ash were identified: A (black and brown ash) and B (orange-brown ash), reflecting different fragmentation processes. The blocky to slightly elongate shapes of juvenile A across heterogenous basaltic compositions (resulting in variable textures, rheological properties, and/or cooling histories) highlight the phreatomagmatic process as the primary control of their shape. In contrast, the irregular-fluidal shapes of juvenile B particles indicate magmatic fragmentation of basaltic andesite magma. This study reveals that variable magma properties yield diverse ash components, yet fragmentation dynamics govern pyroclast shapes in the LVF maar complex. Our integrated approach emphasizes the importance of considering multiple variables when interpreting heterogeneous volcanic ash deposits.

Landslides are a common geomorphological hazard in tropical volcanic regions, particularly in areas with steep slopes and complex geological settings. Variations in soil properties on soil shear strength across different geological formations. This study investigated the physical and mechanical properties of soils developed from the Kebobutak, Semilir, and Nglanggeran Formations and evaluated their relationship with landslide susceptibility in the Watugede Sub-Watershed. Field surveys, geological observations, and soil sampling were conducted at landslide and non-landslide sites. Soil samples were collected from two depth intervals (0-30 cm and 30-60 cm) and analyzed for soil texture, maximum water content, plasticity index, clay sensitivity, cohesion, internal friction angle, and soil shear strength. A descriptive-comparative analysis was applied to assess differences among geological formations and slope conditions. The results indicate that landslide areas generally have higher moisture content and plasticity, accompanied by lower cohesion, internal friction angle, and shear strength than non-landslide areas. Soils derived from the Semilir Formation exhibited the greatest susceptibility to landslides due to their high water-retention capacity and relatively low shear resistance, whereas soils developed on the Nglanggeran Formation showed higher shear strength and greater slope stability. These findings demonstrate that geological formations play a significant role in controlling soil mechanical behavior and slope instability through their influence on soil physical and mechanical properties. The results provide a scientific basis for landslide susceptibility assessment, slope stability modeling, hazard mitigation strategies, and land-use planning in volcanic landscapes.

Mt. Guntur (West Java, Indonesia) has remained in surface repose since its last significant eruption in 1847 CE. Nevertheless, persistent volcano-tectonic seismicity and seismic tomographic anomalies indicate the possible presence of an active, melt-bearing magmatic system beneath the volcano. This study examines whole-rock geochemistry, high-resolution mineral chemistry, quantitative textural analysis (Crystal Size Distribution; CSD), and geothermobarometry of the youngest eruptive products to reconstruct the subvolcanic architecture. The results delineate a vertically extensive transcrustal plumbing system, consisting of a shallow, crystal-rich reservoir (1.0–3.0 kbar; approximately 3.5–11 km) and a deeper staging zone (approximately 3.0–3.5 kbar; 11–13 km, assuming an average crustal density of 2.7 g/cm³). This configuration spatially corresponds with seismic Low Velocity Zones (LVZs) and high-attenuation anomalies identified at 5–8 km depth in previous geophysical studies. Textural and micro-analytical data demonstrate a marked shift in magmatic dynamics over time. The mature GL-18 unit (>50 vol% crystallinity) records prolonged, closed-system fractional crystallization within a stable reservoir. It represents differentiated magma confined within a rigid crystal framework. In contrast, the younger basaltic units (GL-17, GL-11, GL-10) exhibit evidence of active disruption driven by successive mafic recharge events. These recharges largely replaced the resident interstitial melt, as indicated by primitive mineral compositions and distinct textural features that reflect pulsed thermal perturbations. The historical 1847 CE GL-17 eruption reached its final pre-eruptive equilibration at shallow depths (less than 1.5 kbar; approximately 1110–1125 °C), where the primitive magma was thermally buffered by cooler, partially crystallized portions of the resident reservoir. These findings classify Mt. Guntur as a system with Potentially Active Magmatic Storage (PAMS). Although persistent magmatic heat sustains regional geothermal fields, ongoing recharge dynamics suggest a continued potential for future volcanic unrest. As petrology alone cannot predict eruptions, a multidisciplinary monitoring framework that integrates continuous geophysical, deformation, and gas observations is necessary to mitigate risks in the Garut Basin.

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.