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

Abstract

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