Study of degassing processes in active and quiescent volcanic areas

Abstract

Study of degassing proceedings in active and quiescent volcanic areas

Quattrocchi F., Procesi M., Sciarra A., Voltattorni N., Cinti D., Galli G., Pizzino L.

Istituto Nazionale di Geofisica e Vulcanologia, Roma 1 section, Via di Vigna Murata n° 605, Rome (Italy)

Active and quiescent volcanic areas release high amounts of gases (i.e. CO2, Rn, He, H2S, CH4) to the atmosphere from both active craters, as plumes and fumaroles, and the flanks of volcanic edifices as diffusive soil emanations. In the frame of “Diffuse Degassing Structures (DDS)” risk assessment project (funded by the Italian Civil Protection Department), many areas have been investigated in order to estimate a quantitative minimum total budget of the DDS and discriminate the different transport processes. Almost 1000 samples have been collected (fluxes measurements and punctual concentration analyses) and around 60 groundwater sites analysed. All the investigated areas are characterised by the presence of tectonic lineaments that strongly affect the degassing proceedings. In particular, one of these areas (the Tor Caldara-Lavinio area) belongs to the Quaternary Alban Hills volcanic complex that spans around 1500 km2, South-East of Rome. The Alban Hills region undergoes to frequent seismic swarms, typical for recent volcanism, concentrated along a NW-SE belt intersecting the Western sector of the volcanic structure (in correspondence of the belt where degassing is concentrated). Both CO2 flux and Rn measurements were performed in June, August (soon after a M=5.0 earthquake occurred in the area) and December 2005. Despite we found higher 222Rn values in soils after the earthquake than before, suggesting an enhanced local stress signal throughout the DDS as a whole, possibly due to the vicinity of the activated seismogenic structure, the results highlights an unmodified shape and location of the 222Rn anomalies inside the DDS before and after the earthquake. This evidence excludes both that the activated seismogenic segment has affected in some way the DDS degassing patterns and new fracture field changes. The shape and location of the 222Rn anomalies inside the DDS are strictly inversely correlated with the CO2 flux areal peaks and they are more linked to the sealing power of the clays strata generated by the leaching of the outcropping sedimentary Pleistocene rocks by secondary mineralization due to the continuous and the huge ongoing and past CO2 flux. The CO2 could be carrier for 222Rn since a certain threshold, over which dilution and sealing processes of host-rocks are prevailing for the overall 222Rn transport at surface. Herewith we distinguish the contribution of organic, diffusive and advective CO2 flux. Hints of convection, added to the advective and diffusive gas transport mechanisms have been envisages on selected points, where continuous monitoring station of 222Rn/CO2 flux could be strategic, for seismic, volcanic and natural gas hazard surveillance.