The Phlegraean Volcanic District is probably one of the most complex volcanic structures in the world, due to caldera collapse and repeated volcanic activity within a limited area. The centre is situated upon a Pliocene-Quaternary extensional domain with normal faults which trend NE-SW and NW-SE from the margin of a thrust belt (Apennine belt). The geological and deformational history of the Campi Flegrei complex has been subdivided into three periods, based upon the a chaotic ignimbrite (Campanian Ignimbrite), which marks the end of the first period in 37 ka, a yellow tuff unite (Neapolitan Yellow Tuff), which marks the end of the second period in12 ka, and eruptions in the third period which dates from 12 ka to the present.
Period I: The beginning of this period has not yet been well defined, though rocks which are older than the Campanian Ignimbrite can be seen within the cliffs of Mt.Procida, the hill of Cuma and the northern border of the Quarto and Soccavo plains. An approximate age for the Cuma lava domes of 37 ka was calculated by Cassignol and Gillot ( 1982) and an age of >42 ka was deduced from the pyroclastic deposits of Tuff at Torre Franco (Alessio et. al., 1973). The oldest dated exposures of 60 ka using 40Ar/39Ar isotopes can be seen on the slopes which border the northern edge of the Quarto plain. This period ended with the eruption of the Campanian Ignimbrite (37 ka) which covered approximately 30,000 km2 with 150 km3 of magma with a composition which ranges from trachyte to phonolitic-trachyte; this event is suggested to have been the biggest event within the Mediterranean area over the past 200 ka (Barberi et. al., 1978), and it has also been suggested that the epicentre might have migrated during the course of the eruption (Civetta et. al., 1997).
Period II: Prior to the eruption of the Neapolitan Yellow Tuff there has been identified within limited outcrops an explosive hydromagmatic phase. This event can be calculated to have occurred between 37 and 18-14 ka as it has been moulded by the Würmian level surface with its epicentre being located within the Campi Flegrei caldera depression. The most important event during this period was the Neapolitan Yellow Tuff eruption which, it has been suggested, was of phreatoplinian to phreatomagmatic in its nature. It was the second largest eruption in the Campanian area, covering an area of approximately 1,000 km2 with 40 km3 of magma with a composition which ranges from alkali-trachyte to latite. From its stratigraphical, compositional etc. characteristics is has been suggested that the caldera began to collapse during the eruption (Wohletz et. al., 1995) though the rim of this caldera is poorly exposed as it was disturbed during later eruptions.
Period III: This period covers the last 12 ka. Di Vito et. al., 1998 has defined three epochs of activity between 12-9.5 ka, 8.6-8.2 ka and 4.8-3.8 ka with the last eruption occurred in 1538 AD They were also able to recognise 75 volcanic units, 61 of which had vents located within the Campi Flegrei caldera. The first epoch started soon after the Neapolitan Yellow Tuff caldera collapse and lasted approximately 2.5 ka, during which period there were 34 explosive eruptions with a mean frequency of one eruption every 70 years. The eruptions of this epoch were explosive, varying from magmatic to phreatomagmatic depending upon the position of the vents with respect to the coastline. The caldera floor at this time was on average 60m lower than at present with the major part of caldera depression being invaded by the sea, only the northern sector remaining constantly above sea level. The second epoch started with the eruption of the Fondi di Baia in the western sector at approximately 8.6 ka. Within this epoch there were 6 low magnitude explosive eruptions with a mean frequency of one every 65 years; all the eruptions with the exclusion of the first occurred along the north eastern sector of the Neapolitan Yellow Tuff caldera. After this epoch there was a period of quiescence which lasted approximately 3.5 ka during which the sea level rose once more and partly eroded the volcanic features within this area. During the third epoch there were 16 explosive and 4 effusive eruptions with a mean frequency of one every 50 years. The majority of the eruptive vents were located within the north eastern sector of the Neapolitan Yellow Tuff caldera; only two occurred within the north western sector in the area where the later eruption of Monte Nuovo took place.
Sites to see
Within this area there are sites of special interest; these include the Solfatara fumarolic field, Monte Nuovo caldera, Astroni caldera and the evident bradyseism.
The Solfatara crater has been a site of intense hydrothermal activity since Grecian times and is the most impressive manifestation of the current hydrothermal activity of the Campi Flegrei caldera. The manifestation includes both focused vents with a maximum temperature of approximately 160°C, and large areas of hot steaming ground. The average molar composition of the fluids is approximately 82% H2O, 17.5%CO2, 13% H2SO4 with minor amounts of N2, H2, CH4 and CO. The isotopic composition suggests an aquifer system, also evidenced by the absence of soluble acid gases typical of high temperature volcanic gas. At present the solfatara is the main object of the geochemical surveillance of the Campi Flegrei. In particular both the chemical and fluid compositions and the CO2 fluxes from the soil within the crater are monitored.
The most obvious manifestations of bradyseism occur along the coastline where the Roman coastline can be noted to lie approximately 10m below the current sea level. One of the areas where this phenomenon is most easily seen and recorded is in the town of Pozzuoli where within the main square there is the temple of Serapeo which was constructed between the I and II century AC The site had to be restored at the beginning of the III century AC as sea water had invaded its floor. The subsidence continued until the X century AC when uplift started and continued until the eruption of Monte Nuovo in 1538. Capocci, 1835, reported an uplift of 5 to 8 m within a few days before the eruption of Monte Nuovo. This phenomenon was noted several times up to 1992 since when no great movement has occurred greater than a few centimetres.
The calderas of Monte Nuovo and Astroni are well preserved manifestations of the last period of eruptive activity.