More than Two Decades of Geochemical Surveillance on El Hierro, an Oceanic Volcanic Island in the Canary Islands

El Hierro, covering an area of 278 km² is one of the eight islands that make up the Canary Islands archipelago. This oceanic island emerged approximately 1.2 million years ago and is among the most volcanically active in the region. Its most recent volcanic activity was a submarine eruption 2 km off its southern coast, lasting from October 12, 2011, to March 5, 2012. This event was significant as it marked the first eruption in the Canary Islands to be closely monitored. Since 1998, diffuse CO₂ emissions across the island have been systematically measured using the accumulation chamber technique. These measurements are taken at 601 sites regularly distributed to cover the island’s surface. During periods of volcanic unrest, such as the 2011-2012 eruption, the frequency of these surveys increases. The island’s CO₂ emission rates have varied over time, with the most notable increases occurring during pre-eruptive and eruptive phases (Melián et al., 2014). In the last survey, performed in the summer period of 2024, soil CO₂ efflux ranged from levels below detection (<0.5 g·m^-1·d^-1) to a maximum of 44.0 g·m^-1·d^-1, with an average value of 3.0 g·m^-1·d^-1. The diffuse CO₂ degassing rate was estimated in 699 ± 32 t·d^-1. This value is slightly higher than the average background emission (412 t·d^-1) but remains within the background range of 181-930  t·d^-1as determined during the quiescent period from 1998 to 2010. The diffuse degassing studies carried out at El Hierro, have demonstrated that, at those volcanoes without visible volcano degassing, geochemical programs for volcano surveillance should be focused on diffuse degassing monitoring even if only low soil CO₂ efflux measurements are recorded (Pérez et al., 2012). The regular monitoring of diffuse CO₂ emissions has proven to be a valuable tool for detecting early signs of volcanic unrest, especially on islands like El Hierro, where visible gas emissions are not present.

 

Melián et al., (2014), doi:10.1002/2014JB011013.

Pérez et al., (2012), doi:10.1029/2012GL052410.