Geochemical monitoring of the volcanic unrest and the eruption in La Palma island (Canary Islands, Spain): the 2017-2021 dataset and first results

In 2017, La Palma Island entered a state of volcanic unrest, with nine pre-eruptive seismic swarms detected by the seismic monitoring network of the Instituto Geográfico Nacional (IGN) up to 2021, most of the events occurring at depths of 20-35 km. During this period, the IGN geochemical network detected significant changes in deep gas emissions. On 11 September 2021, the last and most energetic pre-eruptive unrest began with more than 1500 earthquakes located at 10-15 km depth and migrating upwards, accompanied by ground deformation with up to ~20 cm vertical inflation detected by the IGN deformation network (GNSS, Insar, tiltmeters). On 19 September 2021 at 15:08 UTC, a volcanic eruption began on the western flank of Cumbre Vieja in El Paso village. This was the first eruption in the island after 50 years of quiescence.

The 2021 eruption has lasted almost three months, ending on 13-14 December 2021 (last activity at the time of this writing). It began as a SE-NW fissural eruption and rapidly evolved to construct a main volcanic edifice up to ~200 m high, with several craters partially overlapping in a SE-NW direction, and later it constructed a secondary cone with a horseshoe shape open to the NE. The eruptive activity has been both strombolian and effusive, sometimes alternating and many times simultaneous with different behaviour at different emission points, a typical situation being strong degassing and strombolian jet and ash emission from an upper crater simultaneous to emission of fluid lavas and lava lake formation and periodical overflowing from a lower crater. Significant volcanic plumes have reached up to 8500 masl (typical value of 3000-3500 masl), and a large set of successive basanitic lava flows has been emitted to the west, developing a volcanic lava-fan covering ~12 km² (~3000 buildings) and reaching the sea at several points along the western coast of La Palma.

During the eruption, the IGN geochemical monitoring network included four stations measuring diffuse radon/thoron in soil, one station measuring diffuse CO₂ flux in soil, an infrared thermal camera coupled with a visual camera and six water sampling points, regularly sampled for water composition, dissolved radon content, total and isotopic composition of dissolved gas (5 points) and free gas (1 point). Physical-chemical parameters (pH, Eh, T, EC, alkalinity) were also regularly measured in situ at these points. In this work we present the obtained dataset and first results. Changes in dissolved gas, mainly H₂ and He, were recorded before and during the eruption. In two radon/thoron stations, abrupt increases in both gaseous species related to the eruptive process were also detected. Changes in dissolved radon in water were also observed at some of the sampling points. Finally, the analysis of the thermal image set can be used to monitor the surface volcanic activity in correlation with visual images and geophysical signals (volcanic tremor).