First gas and thermal measurements at the frequently erupting Gamalama volcano (Indonesia) reveal a hydrothermally dominated magmatic system

The first gas and thermal measurements at the summit of the Gamalama volcano indicate that the system is dominated by hydrothermal processes. This is highlighted by the prevalence of H2S over SO2 (H2S/SO2 = 2–8), a high CO2/SO2 ratio (76–201), and a low heat transfer (3.0 MW) to the surface. A relative variation in gas composition is observed along the degassing fracture zone, possibly due to partial S scrubbing. Despite this surface hydrothermal signature, the system exhibits high gas equilibrium temperatures (425–480 °C), indicating that fluids are not exclusively derived from a boiling hydrothermal aquifer, but also sourced by cooling and crystallizing basaltic magma at deep that continues to inject magmatic fluids into the system. This hydrothermally dominated activity on Gamalama possibly persisted over the last two decades, during which a high number of eruptive events were witnessed. The period coincides with the opening of large fractures at the summit that subsequently shifted the volcanic activity from the crater center to the peripheral fractures zones. These fractures that possibly developed in response to the regional geodynamics, have weakened the hydrothermal seal, allowing the pressure developed by the hydrothermal-magmatic system and promoted by the high annual rainfall, to rapidly exceeds the tensile strength of the seal leading to the numerous phreatic eruptions.