Pre- and syn-eruptive dynamics of coexisting andesite and basaltic-andesitic magmas driving the November 2024 eruption of Lewotobi Laki-laki, Indonesia

The November 2024 eruption of Lewotobi Laki-laki, Indonesia, was characterized by explosive activity, producing abundant pyroclastic deposits, including ballistic volcanic bombs. Main-phase deposits are predominantly juvenile, consisting of pumice, scoria, and banded pumice, with minor fresh to altered lithic fragments. Juvenile white pumice (Lwtb1) has an andesitic whole-rock composition with rhyolitic matrix glass, whereas gray-brown pumice or scoria (Lwtb2-P and Lwtb2-S), and black scoria (Lwtb3) are basaltic-andesite in whole-rock composition with dacitic matrix glass. The basaltic-andesite samples are characterized by relatively high Sr/Y and Cr and lower Zr concentrations, whereas the andesitic samples exhibit lower Sr/Y and Cr and higher Zr contents. Thermodynamic modeling using PELE demonstrates that both the andesite and basaltic-andesite compositions evolved from a common parental magma through progressive fractional crystallization. The basaltic-andesite magma shows mineral-scale compositional variations indicative of weak stratification, whereas the more evolved andesitic magma is highly crystalline (25–50 vol%) and and contains abundant crystal aggregates, consistent with storage within a crystal-rich mush. Pressure–temperature estimates overlap substantially, with the evolved andesitic magma stored at ~93–390 MPa and 1070–1125 °C and the basaltic-andesite magma at ~30–350 MPa and 1000–1135 °C, indicating a vertically extensive magmatic system. We infer that the andesitic magma represents the evolved, crystal-rich mushy portion of the system, extending from ~3.7 to 15.6 km depth, whereas the weakly stratified basaltic-andesite magma occupied a broader depth range of ~1.2–14 km. Disequilibrium plagioclase textures, including reverse, patchy, sieve, and resorbed zoning, indicate repeated magma recharge and remobilization of the mush prior to eruption. Disequilibrium plagioclase textures (e.g., reverse, patchy, sieve, and resorbed) in both magmas indicate repeated recharge, facilitating remobilization of the mush prior to eruption. Matrix vesicle number densities (10 13 to 10 14 m −3 ) of the juvenile products are consistent with explosive eruptions globally. Weakly stratified systems, such as that inferred for the basaltic andesite magma, may promote rapid shifts between effusive and explosive behavior in response to variations in recharge and volatile content. The presence of ballistic volcanic bombs further amplifies hazard due to their impact potential. These findings underscore the need for continuous monitoring of both geophysical signals and geochemical indicators to improve eruption forecasting and hazard mitigation at Lewotobi Laki-laki.