Pedro A. Hernández^1,2, Einat Lev³, German Padilla^1,2, Janine Birnbaum³, Gladys Melian^1,2, María Asensio-Ramos¹, Eleazar Padrón^1,2, Luca D'Auria^1,2 and Nemesio M. Pérez^1,2

¹Instituto Volcanológico de Canarias (INVOLCAN), 38400 Puerto de la Cruz, Tenerife,

Canary Islands, Spain

²Instituto Tecnológico y de Energías Renovables (ITER), 38600 Granadilla de Abona, Tenerife, Canary Islands, Spain

³Lamont-Doherty Earth Observatory, Columbia University, New York City, NY, United States.

Abstract: Quantifying water vapor (H₂O) emissions is critical for understanding volcanic energy budgets and volatile cycling, yet it remains challenging. We use ground-based thermal imagery to quantify an exceptional H₂O mass flux during the 2021 Tajogaite eruption (La Palma), totalling 597.9 ± 24 Mt. This emission represents one of the largest tropospheric water injections recorded, with peak rates exceeding 150 Mt d^-1. Our results reveal a significant decoupling between H₂O and SO₂ fluxes, characterized by an anomalous H₂O/SO₂ mass ratio of 373.7. The integration of seismic tomography and volcanic tremor analysis suggests that these emissions were driven by the mobilization of a shallow (<3 km) hydrothermal reservoir, characterized by an anomalously high Vp/Vs ratio (>2.0). The sustained H₂O discharge was governed by a hybrid fluid-dynamic engine, where volcanic tremor, reflecting melt-gas-rock interactions, signaled the continuous thermal vaporization of external water even as magmatic gas overpressure declined. These findings redefine the role of crustal fluids in sustained, high-magnitude basaltic eruptions and highlight the necessity of accounting for non-magmatic water sources in volcanic energy and climate models.

Keywords: Volcanic water vapor flux; Thermal infrared remote sensing; Latent heat anomalies; Hydrothermal fluid dynamics; Tajogaite eruption; Co-eruptive seismicity

Author Profile (first or corresponding author):

Nemesio M. Pérez, INVOLCAN’s Scientist-in-charge, mainly engaged in geochemical research applied to volcano monitoring, surface geothermal exploration, groundwater studies and environmental issues, etc.

E-mail: nperez@iter.es