Sustained High-Magnitude H2O Flux: Quantifying Exceptional Water Vapor Emission and Shallow Fluid-System Dynamics at the 2021 La Palma Eruption

The 2021 Tajogaite eruption on La Palma, Canary Islands, was a prolonged event characterized by high intensity and significant emission of volcanic gases. Water vapor (H2O), the most abundant volcanic volatile, is often significantly under-measured due to challenges associated with plume measurement and atmospheric entrainment. This study applies and validates a novel methodology using a portable thermal infrared (TIR) camera combined with a mass and energy conservation model to quantify the H2O mass flux throughout the 85-day eruption. We estimate the total H2O released at 597.9 ± 24 Mt, classifying Tajogaite as one of the highest sustained high-flux tropospheric degassing events recorded globally. An exceptional peak rate of 156 Mt/d was observed on September 22, 2021, shortly after the eruption onset. The temporal evolution of the H2O flux shows a strong correlation with long-period (1–5 Hz) seismic tremors, suggesting a direct link between shallow magmatic/fluid processes and gas release dynamics. We calculate an H2O/CO2 mass ratio of 21.3, which is consistent with the high CO2 signature of the island's intra-plate alkaline magmatism (Burton et al., 2023). However, the resulting H2O/SO2 ratio (373.7) is significantly higher than previous estimates and global basaltic analogues (e.g., Miyakejima approx 10), underscoring the dominance of a shallow, hydrothermal-driven H2O component, which decoupled from the exponentially decaying SO2 flux in the final stages of the eruption.