Twenty four years of geochemical monitoring of the oceanic active volcanic island of El Hierro, Canary Islands

El Hierro (278 km²) is one of the eight islands of the Canary Islands archipelago. It is an oceanic island formed ~1.2 Ma ago and one of the most active from a volcanic point of view of all the Canary Islands. Its last historical eruption was submarine eruption that began on 12 October, 2011 until 5 March, 2012, off its southern coast, which was the first to be monitored in the Canary Islands. Since 1998, periodic diffuse CO₂ surveys have been carried out (except for the 2011-2012 seismic-volcanic unrest and eruptive periods, when the surveys frequency increases, Melián et al., 2014) by means of the accumulated chamber method measuring the diffuse CO₂ emission at 601 points distributed homogeneously throughout the emerged 278 km² of El Hierro. CO₂ emission rate measured in the entire island has been varying over time, registering highest values in the pre-eruptive and eruptive periods where a significant increase of this gas emission rate was registered. Soil CO₂ efflux values for the 2022 survey ranged between non-detectable (<0.5 g·m⁻²·d⁻¹) to 38.0 g·m⁻²·d⁻¹, with an average value of 2.6 g·m⁻²·d⁻¹. Diffuse CO₂ output from the studied area was estimated in 616 ± 26 t·d^-1 for El Hierro Island, a value above the background average of CO₂ emission estimated on 412 t d^-1, but within background range of 181 t·d^-1 (−1σ) and 930 t·d^-1 (+1σ) calculated at El Hierro volcano during the quiescence period 1998-2010 (Melián et al., 2014). The monitorization of the diffuse CO₂ emission has demonstrated that contributes to detect early warning signals of volcanic unrest, especially in areas where visible degassing is non-existent as in the El Hierro Island.

 

Melián et al. (2014), J. Geophys. Res. Solid Earth, 119, 6976–6991, DOI 10.1002/2014JB011013