Soil H2 degassing studies: a useful geochemical tool for monitoring Cumbre Vieja volcano, La Palma, Canary Islands

La Palma Island (708 km²), situated in the northwest of the Canarian Archipelago, stands as one of the youngest (~2.0 My) islands. A new volcanic eruption took place at the Cumbre Vieja volcanic system, located in the southwest flank of the island, on September 19, 2021. Cumbre Vieja is renowned as the most active basaltic volcano in the Canaries. The eruptive event, which lasted for 85 days, featured various volcanic activities, including lava effusion, strombolian activity, lava fountaining, ash venting, and gas jetting, and concluded on December 13, 2021.

Regular surface geochemical studies have been conducted focusing on hydrogen (H₂) emissions along Cumbre Vieja. H₂, being one of the most abundant trace species in volcano-hydrothermal systems, plays a pivotal role in numerous redox reactions occurring in the hydrothermal reservoir gas. This comprehensive study of H₂ emissions has been ongoing since 2001, encompassing continuous monitoring of soil gas samples collected at a depth of approximately 40 cm across 600 sites during each survey. H₂ concentrations have been meticulously analyzed using a micro-gas chromatograph (Agilent 490 microGC).

Spatial distribution maps have been generated using sequential Gaussian simulation (sGs) techniques to quantify the diffuse H₂ emissions from the study area. The time series data of the diffuse H₂ emissions indicate significant increases before and during the occurrence of seismic swarms observed between 2017 and 2021. Furthermore, during the eruptive phase, substantial spikes in the diffuse H₂ emissions were observed, closely correlating with the volcanic tremor escalation. These fluctuations in diffuse H₂ emissions were observed preceding the peak of diffuse CO₂ emissions, aligning with the anticipated behavior of these gases. Over the last two years following the eruption, the values have reverted to levels like those observed during periods of volcanic calm, reinstating the stability in the diffuse H₂ emissions.

The absence of visible volcanic gas emissions before the eruption, such as fumaroles or hot springs, on the surface of Cumbre Vieja underscores the importance of such studies in serving as a critical tool for continuous volcanic surveillance and monitoring purposes. This update represents ongoing efforts to comprehensively study and understand the behavior of hydrogen emissions within the volcanic system, providing essential insights into volcanic activity and potential precursor signals for enhanced monitoring and risk assessment.