The Piperno Quarry in Masseria del Monte in Pianura

Geographically, the Phlegraean Fields covers an active volcanic area stretching from the Posillipo ridge northwards to Cuma. In effect, a distinction is made between this area, known as the Continental Phlegraean Fields, and the so-called Insular Phlegraean Fields - that is, the islands of Ischia, Procida and Vivara.

The products of the oldest volcanic activity (Paleo Campi Flegrae) are found on the islands of Procida and Vivara, in the Monte di Procida area and in the quarries bordering the Quarto Plain. The real volcanic activity of the Phlegraean Fields began with the great eruption of the Ignimbrite Campana (circa 39 kaB.P. - De Vivo et al., 2001).
 
This created a pyroclastic stream which spread over the Campania Plain as far as the carbonate spurs of the Monti Lattari and Picenti to the south and the Roccamorfina area to the north. There is no consensus on where the centre of the Ignimbrite Campana eruption was (I.C.).

Some writers associate it with the calderic collapse that caused a vast area corresponding to the present-day Phlegraean Fields to subside. Others see the Ignimbrite Campana as a product of a huge eruption of a fissural nature deriving from a series of fractures in the wider area of the plain.

Since the eruption of I.C., the Phlegraean Fields have seen only limited volcanic activity, the products being traceable within the city of Naples, along the Posillipo ridge, and above all on the Camaldoli Hill (Di Girolamo et al., 1984). These products consist of eject of variable welding, generically known as Old Tufo of the City of Naples. Products of the same era are to be found on the island of Procida.

Another important event in the volcanic activity of the Phlegraean Fields was the Yellow Neapolitan Tufo eruption, recently dated at 40 Ar/39 Ar, at about 15 ky B.P. (Deino et al., 2004). This formation, which has an incoherent facies (Pozzolana Auct) and a lithoid one, makes up the basic structure of the city of Naples. Connected with the Yellow Neapolitan Tufo eruption is the calderic collapse from which the sunken morphology of the Phlegraean Fields originates.

In the last 10 ky B.P., the interior of the Phlegraean caldera has seen various eruptions from menogenetic centres, episodes each consisting of a single eruption. In an area which is volcanically active, eruptions of an explosive nature are the main cause of rapid morphological modification of the appearance of the terrain.

In the Phlegraean Fields, the most recent of these modifications occurred following the eruption that in 1538 gave birth to the Monte Nuovo. They are still volcanically active places and are highly developed, densely populated urban centres such as Naples itself and the communes of Pozzuoli, last 5000 years, the explosive eruptions would emit products that would be distributed mostly like pyrcoclasic streams, whose trajectories would be greatly conditioned by the existing typographical barriers. The above photograph shows the map of where the volcanic risks lie, a simulation based on the positioning of deposits, and we see that that the areas of Naples within the Phlegraean caldera are in fact those potentially most exposed to risk.