Phlegrean Fields ('Campi Flegrei') translates from the Italian as “fiery fields”. It refers to an area to the west of Naples where there is a steep geothermal gradient, with a magma chamber lying beneath the surface; evidence suggests that the magma is actually much closer to the surface here than that beneath Vesuvius. The main volcano is Solfatara (read below), though this is only one of approximately 40 identified in the area. The type of activity found at Solfatara is found in many parts of the world and is commonly referred to by this name.
No lava is produced or any pyroclastic material ejected here, but there is the emission of large quantities of steam and gas. Some sulphur dioxide is produced too, which causes the precipitation of sulphur crystals around the gas-vents (so-called fumaroles). Percolating groundwater quickly boils at a shallow depth, being returned to the surface as steam; this mixes at the surface with superficial deposits to create bubbling mud-pools. There are also a number of hot springs on the nearby island of Ischia.
At Solfatara itself, which appears as a shallow crater, the surface obviously covers a void space filled with hot gases – a large heavy object dropped on the surface causes the ground to shake. The whole area is also subject to a phenomenon known as “Bradyseism”, which can be thought of as slow earthquakes, where the ground can rise or fall by more than a metre over a period of several months. The area has mostly sunk, with Roman remains in Pozzuoli now well below sea-level.
Nearby is also the small cone of Monte Nuovo: this came into existence in 1538, when a major earthquake struck Pozzuoli, providing a line of weakness for magma to rise to the surface. The crater is only 150m above the surrounding land surface; it has not erupted since its formation.
The Vulcano Solfatara – the mythcal entrance to the Ancient Romans’ Hell – is an inactive volcano, part of the volcanic region known as the Phlegrean Fields. It is famous for its steaming jets of sulphurous vapour at temperatures of 160° C, the emission of carbon dioxide and small volcanoes of boiling mud. Sulphur was extracted here in the past, and until 1800 this was a spa renowned for its saunas in which natural mineral water and mud were used. The Vulcano Solfatara is the epicenter of the cyclic rising and lowering of the ground level in the Phlegrean Fields – a phenomenon known as ‘bradyseism’.
The Bocca Grande is the largest fumarole – ‘Grande Fumarola’ – in the Vulcano Solfatara. In the past an Observatory dedicated to Friedlander (a German scientist) was situated here, and today this is the site of a volcanic research station for the “Vesuvian Observatory” network and other scientific bodies; traces of rare red arsenic sulphur crystal called ‘Realgar’ were also found here.
The Stufe – Stufe Antiche – ('ovens'), one known in the past as Purgatory and the other as Hell, were used in ancient times as natural saunas and for the inhalation of sulphureous vapours which were considered beneficial for respiratory illnesses; rocks with sulphur and alum crystals can be found near here too.
The Fangaia is fed by innumerable small fumaroles, spa water and natural mud – which boils at a temperature of 140° C – and it was considered an excellent remedy for rheumatism; a rare heat resistant micro-organism, Sulpholobus solfataricus, has its rare habitat here.
The Old Well – Pozzo dell’Acqua Minerale – famous since the Middle Ages, was the source or mineral water which was said to have a lot of powers for the cure of sterility and ulcers.
The mine – Cava di Pietra Trachite – testifies to the old mining activity of alum and lime that peaked in the Middle Ages and lasted until the fifties.