• radon;
  • thoron;
  • precursory signal;
  • geochemical monitoring;
  • submarine eruption;
  • El Hierro


[1] Soil gas Radon ( 222Rn) and Thoron ( 220Rn) activities have been continuously measured during the period of the recent volcanic unrest that occurred at El Hierro, Canary Islands, at two different monitoring stations, namely HIE02 and HIE03. Significant increases in soil 222Rn activity and 222Rn/220Rn ratio from the soil were observed at both stations prior to the 2011-2012 submarine eruption off the coast of El Hierro, showing the highest increases before the eruption onset and before the occurrence of the strongest seismic event (M = 4.6). Statistical analysis showed that the long-term temporal trend of filtered radon data matched closely that of seismic energy release during the volcanic unrest. The observed increases of 222Rn are related to the rock fracturing processes (seismic activity) and the magmatic CO2 outflow increase, as observed in HIE03 station.