Structurally controlled diffuse 222Rn degassing along the NW volcanic rift-zone of Tenerife, Canary Islands

Three major volcanic rift-zones trending NW-SE, NE-SW and N-S are identified at Tenerife island. Most of the volcanic activity have occurred along these three major structures. The most recent eruption, Chinyero (1909), occurred along the NW rift-zone. The aim of this research is to evaluate the relationship between structural characteristics of the study area with diffuse ²²²Rn degassing rates. Nearly fifty cinder cones have been analysed in detail to infer the eruptive fissures trend. The strike values of the 26 eruptive fissures identified ranged from 95 to 176° E, showing a mean value of 116° E. These results suggest that most of the eruptions took place along preferential pathways of WNW-ESE trended fissures and that these eruptions were effectively true fissure type eruptions. Parallel to the structural study a soil gas ²²²Rn survey of 424 sampling sites was carried out along the NW rift-zone from July 17 to August 6, 2003. Soil gas ²²²Rn was analyzed in situ by means of a portable radiation monitor Pylon AB-5. The highest observed soil gas ²²²Rn was 136 pCiL^-1. Statistical-graphical analysis showed two overlapping populations. Soil gas ²²²Rn concentration background geometric mean was 23 pCiL^-1 and represents 98% of the total data. Most of the study area showed background levels of soil ²²²Rn concentration, while peak levels were aligned along the NW rift-zone. The good spatial correlation of the soil gas ²²²Rn concentration with the WNW-ESE trend of the rift-zone suggest that the ²²²Rn diffuse emission is structurally controlled in the study area.