GNGTS 2013 - Atti del 32° Convegno Nazionale

years. The last event was in 1538 AD, after about 3.0 ka of quiescence, and formed the Mt. Nuovo tuff cone (Di Vito et al. , 1987; Piochi et al. , 2005a). Sixty-four of these eruptions were phreato-magmatic to magmatic explosive events, and 76% of these eruptions occurred from vents active in the central-eastern sector of the caldera (Mormone et al. , 2011). Solfatara geological setting. The Solfatara volcano, about 2 km east-northeast of Pozzuoli, is a tuff cone (180 m above sea level) characterized by a sub rectangular (0.5x0.6 km) crater, shaped by NW-SE and SW-NE trending faults along which the vegetation lacks. The volcano generated a low-magnitude explosive eruption that deposited a tephra over a small area (<1 km 2 ), named Solfatara Tephra, during phreatomagmatic and subordinate magmatic explosions (Di Vito et al. , 1999). This tephra overlies the Monte Olibano and Accademia lavas, both younger than Agnano-Monte Spina Tephra (4.1 ka), and underlies the Astroni Tephras (3.8 ka), from which it is separated by a thin paleosoil containing many charcoal fragments. The Solfatara Tephra comprises a phreato-magmatic coarse breccias overlain by a sequence of stratified, dune-bedded deposits composed of accretionary lapilli-bearing ash surge layers, alternating with thin, well sorted, rounded pumiceous lapilli beds pyroclastic pumiceous fallout beds. The breccia contains large blocks of green tuff, altered lavas and dark scoriaceous bombs engulfed in a hydrothermally altered matrix. The scoriae of the basal breccia are porphyritic containing crystals of sanidine, plagioclase, clinopyroxene, biotite and Fe-Ti oxides, in order of decreasing abundance. Rare crystals of leucite converted to analcime are also present. The late erupted pumice fragments are alkali-trachytic in composition, crystal-poor to subaphyric pumice (upper sequence), and contain rare crystals of plagioclase. A thin massive fallout layer, grey to yellowish in color, consisting of fine-to-coarse ash with scattered pumice clasts and interbedded pumice beds represents the distal counterpart of the Solfatara Tephra. It is a deposit dispersed towards the north-east with a minimum measured thickness of 5 cm at Verdolino, at about 7 km from vent. The crater of the Solfatara has been the site of an intense hydrothermal activity since Greek times. It is the most impressive manifestation of the present hydrothermal activity of the caldera, which includes both focused vents, with a maximum temperature of about 160°C (Bocca Grande fumarole), and large areas of hot steaming ground. The average molar composition of the fluids is H 2 O about 82 %, CO 2 17.5%, H 2 S 0.13% and minor amounts of N 2 , H 2 , CH 4 and CO. Systematic measurements of the gas fluxes from the soil evidenced up to 1500 tonnes/day of CO 2 emission (Chiodini et al. , 2011) through the main fault system, coinciding with temperature up to 95°C (Granieri et al. , 2010); the degassing area is enlarging since the first analytical campaign. The isotopic compositions of H 2 O, CO 2 and He suggest the involvement of magmatic gases in the feeding system of the fumaroles. Subsequently the original magmatic gases are condensed by an aquifer system as suggested by the absence of the soluble acid gases SO 2 , HCl and HF, typical of the high-temperature volcanic gas emissions. Boiling of this heated aquifer(s) generates the Solfatara fumaroles. Based on geochemical data, the hydrothermal system at the Solfatara crater consists of a heat source, possibly represented by a relatively shallow (few kilometres deep) magma batch, a geothermal system located above the magma, and the shallow hydrothermal system. During the last 24 years of monitoring of the geochemical composition of the fumaroles the ratio of the concentration of CO 2 /H 2 O showed three clear peaks in 1985, 1990 and 1995 which was followed a few months later, a lifting of the ground. According with Caliro et al. (2007) these peaks reflecting the composition of the fumaroles rich component of magmatic, probably due to episodes of degassing of the magma in depth during periods of lifting the soil. Other physical and numerical simulations have shown that periods of intense degassing of fluids rich in CO 2 can explain other relevant characteristics of the crisis of 1984, 1990 and 1995, such as ground deformation and gravity anomalies (Todesco et al. , 2004; Todesco and Berrino, 2005). After 2000, the ratio of the concentration of CO 2 /H 2 O fumaroles showed no peaks but a slow upward trend still underway. This different behaviour of the composition of fumarolic reflect a change in the style of degassing at depth. If this growing trend is the ascending portion 246 GNGTS 2013 S essione 1.3