The 2021 Cumbre Vieja eruption: an overview of the geochemical monitoring program

Cumbre Vieja (220 km²) is the most active volcano in the Canary Islands. It has been the location of 8 of the 17 historical eruptions in the archipelago during the last 600 years. The establishment of a geochemical monitoring program by our research group for the volcanic surveillance of Cumbre Vieja started in 1997. This program was mainly focused on diffuse degassing monitoring because of the absence of visible volcanic degassing manifestations (fumaroles, plumes, etc.) as well as other obvious geothermal features at Cumbre Vieja up to the 2021 eruption which started on September 19, ended on December 13 and lasted 85 days.

The INVOLCAN’s soil degassing monitoring at Cumbre Vieja is carried out by means of a geochemical instrumental permanent network (soil CO₂ efflux, soil gas ²²²Rn and soil C isotope ratio) and regular geochemical surveys covering the entire area of Cumbre Vieja (diffuse CO₂, He and H₂ emissions). Several soil degassing anomalies have been observed and some of them years before the 2021 eruption, which illustrates the importance of diffuse degassing monitoring for volcanic surveillance. The single visible manifestation of volcanic degassing at La Palma is a cold CO2-rich site at Taburiente volcano. Regular helium-3 emission monitoring of this observation site has been carried out since 1991 in collaboration with Tokyo Univ., and provided a clear early warning signal of the 2021 Cumbre Vieja eruption. Because of the registration of seismic swarms, and to strengthen the INVOLCAN geochemical monitoring program of Cumbre Vieja volcano,  regular sampling of groundwater for chemical and isotopic analysis started in October 2017. The results of this hydrogeochemical monitoring also showed significant changes related to the recent volcanic unrest of Cumbre Vieja.

Since the 2021 eruption onset, INVOLCAN performed daily observations of SO₂ emissions using a miniDOAS in traverse mode, on terrestrial (car), sea (ship) and air (helicopter) mobile position recording relatively high SO₂ emissions (> 50.000 t/d). Static scanners and satellite instruments were used also to monitoring the SO₂ emission released by this eruption; a task lead by the volcano research group of Manchester University. Additional plume geochemical monitoring was carried out using OP-FTIR spectrometers and UAV, helicopter and ground-base MultiGas units to characterize the chemical composition of the plume degassing in collaboration with scientists from Manchester Univ., Palermo Univ., UCL, INGV, IPGP and Azores Univ.  Carbon isotope analysis of the CO₂ gas plume was also undertaken in collaboration with New Mexico Univ. Analysis of pristine ash leachates has been also performed in collaboration with Durham Univ. and Tokyo Institute of Technology since it is often used to estimate the composition of the gas phase during volcanic eruptions and provides important information on the eruption processes was also performed.

The results of these geochemical observations during the inter-eruptive, pre-eruptive, eruptive and post-erupive phases have been tremendously useful to understand the recent magmatic reactivation of Cumbre Vieja volcano.