The Solfatara Volcano

The Solfatara volcano is a hydrothermally altered tuff-cone built between 4.6 and 3.7 ka BP. It represents the most active zone of the Campi Flegrei, a caldera related to the Campanian Ignimbrite (35 ka BP) and Neapolitan Yellow Tuff (12 ka BP) eruptions.

Two bradyseismic crises, centred at Pozzuoli, were detected in the caldera in recent times: in 1969-1972 (maximum uplift of 1.7 m) and in 1982-1984 when the area was affected by a maximum uplift of 1.8 m and by several thousands earthquakes (0<depth<4 km, and Md < 4). More recently, two minor sudden ground uplifts and seismic swarms were recorded in 1989-90 and in 1994.

Fumarolic activity occurs in different sectors of Campi Flegrei but concentrates at Solfatara. Two fumaroles called Bocca Grande (BG) and Bocca Nuova (BN) have the highest temperatures (145 to 165°C), while temperatures of other fumarolic vents are close to 100°C.

Fumarolic effluents have similar chemistry, with H2O as the main component, followed by CO₂ and H₂S. The principal elements of the hydrothermal system of Solfatara are: (1) A heat source which is made up of a relatively shallow (a few kilometers deep) magma chamber; (2) one or several aquifers located over the magma chamber - the degassing magma supplies fluids and heat to the overlying aquifer(s) which dissipate the heat through boiling and condensation; and (3) an intensely fractured zone, situated above the uppermost aquifer and occupied by a pure vapour phase ('superheated' vapour zone), which is produced through boiling of the underlying aquifer(s).

Gas equilibria in the CO₂-CO-CH₄-H₂O-H₂ system indicate that temperature and PH₂O conditions in the 'superheated' vapor zone varied from 240°C and 30 bar during the 1982-84 crisis (Fig. 44) , to 210 to 220°C and 3-7 bar at present.

Solfatara, the most active volcanic area of Campi Flegrei, is one of the most famous tourist sites of Italy which and is also intensely urbanized. Four different kinds of natural hazards can be distinguished:

• Gas release during quiescent periods from the entire volcanic cone (about 1,500 tons/day of CO₂);
• Magmato-tectonic seismic activity related to unrest episodes;
• Hydrothermal explosions;
• Phreatomagmatic eruptions.

At present, the Osservatorio Vesuviano performs the volcanic surveillance of the area. Their activity includes periodic sampling of fumarolic gases, periodic soil CO₂ flux measurements, continuous monitoring of soil CO₂ fluxes, and geophysical monitoring (seismicity, gravity and deformation).

Fig. 44 Observed variations of H₂O/CO₂ and S/C ratios of the Solfatara di Pozzuoli fumarolic gases since 1980; ground deformation and seismic activity are shown. Osservatorio Vesuviano Napoli (OVNI)
(Click on image to enlarge).

created by geopublic