The Campi Flegrei, with Ischia and Procida, represent a complex volcanic system consisting of a series of nineteen craters concentrated in an area of about 65 km² and arranged according to an east-west alignment. These craters are so close together that in several cases they overlap; it is thus that an older crater has been partially destroyed by the formation of a more recent one.
The volcanic activity of Campi Flegrei predates that of Vesuvius and the area seems to have remained in a state of quiescence during historical times with the exception of the eruption of the Solfatara volcano in 1198 (named Solfatara because of sulfurean gasses), and that of Mount Nuovo, which occurred in 1538.
The eruptive mechanisms of the volcanic activity of the Phlegraean Fields are of various types with a clear prevalence of the explosive ones, and in particular deriving from the water-magma interaction, compared to the effusive ones.
Effusive eruptions are characterized by low explosiveness and by the emission of lava flows that flow along the flanks of the volcanic building. If the lava cools without being able to flow, it can give rise to accumulations of circular shape called lava domes.
Explosive eruptions can present various eruptive “styles”: if they form a high eruptive column that expands upwards with a typical pine-shaped ash cloud, they are defined as “plinian” taking their name from Pliny the Elder who died during the eruption of Vesuvius in 79 AD and by Pliny the Younger who described it. In these eruptions the magma is fragmented before reaching the surface and then thrown upwards in the form of bombs, slag, pumice, lapilli and ashes called “pyroclastic products”.
In explosive eruptions there are different ways of depositing the erupted magma. A type of deposition of pyroclastic products occurs by “falling”: from the cloud that expands from the top of the eruptive column generated by the volcano, the ashes and incandescent lapilli fall to the ground by gravity. A second type of deposition is by pyroclastic flow which is made up of a “mixture” of magma fragments (ash, lapilli, and slag) and gas that flows rapidly on the ground away from the eruptive center. Such a phenomenon can be highly destructive: just think of the destruction of the city of Herculaneum which occurred during the eruption of Vesuvius in 79 AD.
In other cases, however, the activity of the volcano can be of the “Strombolian” type, with intermittent explosions and the launch of incandescent slag and lapilli that alternate with lava flows. This activity is typical, for example, of the Stromboli volcano which is in a state of perennial volcanic activity.
Generally explosive eruptions are the phreatomagmatic ones in which there is direct contact between magma and water: the water is “impacted” by the incandescent magma rapidly rising through the conduit. It can be deep water, of underground aquifers or surface water, of crater lakes (eg Lake Averno), or marine, in which case the volcanic activity takes place under water.
The examination of the stratigraphic sequences, together with the absolute dating, allowed the subdivision of the Phlegraean volcanic activity into four cycles, listed below.
1 Cycle – The volcanic products belonging to this cycle have an age prior to 35,000 years. The following volcanic formations can be ascribed to this cycle: Vivara, Punta Serra, Torre Murata, Fiumicello which emerge above all in Procida, formation of the S. Martino rock, Monte Grillo (Monte di Procida) and lava domes of S. Martino, formation of the lava domes and breaches of Punta Ottimo (Procida), Cuma and Marmolite (Quarto). In addition to this cycle also belong the tuffs of Torre Franco and a level of falling pumice found, mostly, at the base of the Campana Ignimbrite.
2 Cycle – The volcanic deposits belonging to this cycle have an age between 35,000 and 30,000 years. The placement of the Campana Ignimbrite and the formation of the Piperno-Breccia Museum are to be ascribed to this time interval. Some authors consider the two deposits the consequence of a single eruption while others maintain that they are distinct even if there is an evident stratigraphic continuity between the two formations.
3 Cycle – The White Tufas (emerging in Soccavo), the Ancient Tufas (emerging in the urban area of Naples), the formation of the Volcano of Solchiaro (Procida), and the formation of the Torregaveta volcano belong to this cycle. These deposits are probably between 18,000 and 15,000 years old. This cycle also includes the placing in place of the Neapolitan Yellow Tufo having an age of about 13,000 years.
4 Cycle – The absolute age of the formations belonging to this last cycle starts at 10,000 years to reach the historical eruption of Monte Nuovo in 1538. The analysis of the geo-chronological data shows a state of quiescence of the volcanic activity in the time span between 30,000 and 20,000 years; which arose after the eruption, and therefore after the placing in place, of the Ignimbrite Campana. According to some authors, this major event that generated deposits that were then widely exploited as building stone (piperno, tuff in s.s.) gave rise to the formation of the Phlegraean caldera whose edges are still well preserved today. The aforementioned area is bordered to the N by P.ta Marmolite at the northern limit of the Piana di Quarto, to the NE by the Camaldoli hill, to the SE by the Posillipo hill and to the W by the Monte di Procida. Of this eruption, of enormous size both for the volume of erupted magma (estimated in the order of 80 Km³) and for the extension of the areas covered by these deposits (about 7,000 Km²), there are not many outcrops within the caldera area. .
The other major eruption of the Phlegraean area is that of the formation of the Neapolitan Yellow Tufo, whose eruption (50 km³ of magma over an area of 350 km²) is placed at the peak of an intense eruptive activity preceding this major event. There is evidence of a sinking of the central part of the Phlegraean Fields along fracture lines that will then constitute the easy way up for the magmas that have characterized the last intense 10,000 years of volcanic activity. The eruptions of Gauro (M. S. Angelo and M. Barbaro), Archiaverno, Punta dell’Epitaffio, Monteruscello, Capo Miseno, Porto Miseno, Nisida, Banco di Nisida (submerged), La Pietra are attributable to this period; these are all yellow tuff volcanoes.