Soil gas Rn monitoring at Cumbre Vieja prior and during the 2021 eruption, La Palma, Canary Islands

Cumbre Vieja volcano (La Palma, Canary Islands, Spain) suffered a volcanic eruption that started on September 19 and finished on December 13, 2021. The eruption is considered the longest volcanic event since data are available on the island: it finished after 85 days and 8 hours of duration and 1,219 hectares of lava flows. La Palma Island is the fifth in extension (706 km2) and the second in elevation (2,423 m a.s.l.) of the Canarian archipelago. Cumbre Vieja volcano, where the volcanic activity has taken place exclusively in the last 123 ka, forms the sand outhern part of the island. In 2017, two remarkable seismic swarms interrupted a seismic silence of 46 years in Cumbre Vieja volcano with earthquakes located beneath Cumbre Vieja volcano at depths ranging between 14 and 28 km with a maximum magnitude of 2.7. Five additional seismic swarms were registered in 2020 and four in 2021. The eruption started ~1 week after the start of the last seismic swarm.

As part of the INVOLCAN volcano monitoring program of Cumbre Vieja, soil gas radon (²²²Rn) and thoron (²²⁰Rn) is being monitored at five sites in Cumbre Vieja using SARAD RTM2010-2 RTM 1688-2 portable radon monitors. ²²²Rn and ²²⁰Rn are two radioactive isotopes of radon with a half-life of 3.8 days and 54.4 seconds, respectively. Both isotopes can diffuse easily trough the soil and can be detected at very low concentrations, but their migration in large scales, ten to hundreds of meters, is supported by advection (pressure changes) and is related to the existence of a carrier gas source (geothermal fluids or fluids linked to magmatic and metamorphic phenomena), and to the existence of preferential routes for degassing (deep faults). Previous results on the monitoring of soil Rn in the Canary Islands with volcano monitoring purposes are promising (Padilla et al, 2013).     

The most remarkable result of the Rn monitoring network of Cumbre Vieja was observed in LPA01 station, located at the north-east of Cumbre Vieja. Since mid-March 2021, soil ²²²Rn activity experienced a sustained until reaching maximum values of ~1.0E+4 ²²²Rn Bq/m³ days before the eruption onset. During the eruptive period, soil ²²²Rn activity showed a gradual decreasing trend. The increase of magmatic-gas pressure due to magma movement towards the surface and the transport of anomalous ²²²Rn originated from hydrofracturing of rock, from direct magma degassing or from both, is the most plausible explanation for the increases in radon activity before the eruption onset observed at LPA01. As soil gas radon activity increased prior to the eruption onset, this monitoring technique can be efficiently used as an initial warning sign of the pressurization of magma beneath La Palma Island.

Padilla, G. D., et al. (2013), Geochem. Geophys. Geosyst., 14, 432–447, doi:10.1029/2012GC004375.