About 14,000 years ago, the Campi Flegrei produced one of the two catastrophic eruptions that dominate its history. That event, the Neapolitan Yellow Tuff eruption, transformed the territory around present-day Naples, left the yellow stone that still defines much of the historic city, and was followed by collapse in the central part of the volcanic area.
A recent study discussed by Il Mattino dates the eruption to approximately 14,000 years ago, revising the previously used estimate of 15,000 years. The difference matters for readers: Italy's National Institute of Geophysics and Volcanology, INGV, still uses approximately 15,000 years in its general overview of Campi Flegrei, while the newer work reported in the interview points to about 14,000 years. Both figures describe the same event; the discrepancy reflects different studies and dating conventions rather than two separate eruptions.
How it changed the land around Naples
According to the INGV researcher interviewed by Il Mattino, the eruption deposited metres of ash across a wide area. INGV estimates that it affected approximately 1,000 square kilometres.
Around Naples, those deposits filled pre-existing depressions and older valleys. In particular, they filled depressed landforms between Soccavo and Fuorigrotta and between Arenella and Vomero, helping shape parts of present-day Naples. In other words, some of the ground Naples is built on today is, in effect, the levelled fill left by the Yellow Tuff eruption.
In the central Campi Flegrei, including Agnano, Pozzuoli and Bagnoli, the sequence was different: the eruption was followed by caldera collapse and the creation of depressed areas and plains. INGV describes the result as a smaller caldera, nested within the earlier volcanic system.
The scientific literature presents the Neapolitan Yellow Tuff eruption as the event responsible for caldera formation, in an interpretation involving downsag and trap-door subsidence. Downsag means broad downward bending of the ground above the eruption source; trap-door means collapse that is greater on one side, as if along a hinge. The precise extent of maximum collapse depends on the study and measurement convention used.
Why the deposit became yellow building stone
The eruption did not originally produce only yellow rock. The deposit included loose grey material known as pozzolana. Secondary mineralisation later transformed part of it into lithified yellow tuff, the Neapolitan Yellow Tuff.
The interviewed researcher describes that transformation as zeolitisation. The result was a yellow, durable tuff that was used extensively in Naples' historic buildings and underground cavities. When visitors see warm yellow masonry in historic Naples or enter older underground spaces cut into tuff, they are often looking directly at the solidified ash of this eruption.
That distinction is useful on site: grey, loose pozzolana and yellow, solid tuff can come from the same eruptive episode, with the yellow colour and hardness reflecting the later mineral transformation rather than a different volcano.
What came afterwards
INGV identifies the Neapolitan Yellow Tuff as one of the two catastrophic events dominating the region's eruptive history. It was not the end of activity: more than 70 eruptions occurred inside the caldera after the Neapolitan Yellow Tuff event.
The exact duration of the Yellow Tuff eruption itself remains difficult to estimate. The interviewed researcher describes it as geologically brief, but not as an instantaneous event.
The wider context also remains an area of scientific interpretation. In particular, the relationship between the earlier Campanian Ignimbrite eruption and the formation of the wider Campi Flegrei caldera is still debated, as is the exact volume of material erupted during the Yellow Tuff event.
