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A thin, fines-poor, and lithic- and crystal-rich layer\ud locally present, from proximal to distal areas, at the base of the\ud pyroclastic density current deposits of the Campanian\ud Ignimbrite eruption (39 ka), sourced from Campi Flegrei\ud (Italy), is interpreted as a ground layer (GL). It has an average\ud lithic and crystal content of 57 and 25 wt%, respectively. The\ud GL rests on a paleosol or an early fall deposit and is capped by\ud a stratified ash deposit. It is bounded by erosion surfaces and\ud its thickness does not change systematically from the source.\ud The occurrences of GL in places separated from the source by\ud a stretch of sea and the angular to sub-angular shape of the\ud lithic clasts are consistent with deposition from a dilute current.\ud The presence of the GL up to 971 m above sea level and\ud beyond 1400–1500-m-high mountain ridges, allows estimation\ud of the thickness of the PDC as having been at least\ud 1.5 km. To investigate the parameters influencing the transport\ud and emplacement mechanisms that produced the GL, we use\ud field and sedimentological data (grain size and componentry),\ud from samples collected between 30 and 70 km from the\ud source. The progressive decrease in maximum lithic clast\ud and median grain-size with distance from the vent and increasing\ud altitude corresponds to lateral and vertical grading of lithic\ud clasts within the current. The coarser clasts moved toward the\ud basal part of the current, settling in more proximal areas and at\ud low altitude. Farther from the vent (45–50 km), the lower\ud part of the pyroclastic current impacted against the reliefs\ud surrounding the Campanian Plain, causing the emplacement\ud of coarser and more poorly sorted deposits on slopes oriented\ud toward the source

Large ignimbrites are the product of devastating explosive eruptions that have repeatedly impacted climate and life on global scale. The assemblage of vertical and lateral lithofacies variations within an ignimbrite sheet, its internal architecture, may help to determine how the parental pyroclastic current evolves in time and space. The 39 ka Campanian Ignimbrite eruption, vented from Campi Flegrei caldera, laid down a thick ignimbrite over an area of thousands of km2. A detailed reconstruction of the vertical and lateral variation of the seven lithofacies recognised in the ignimbrite medial sequence constrains the behaviour of this event. The pyroclastic current flowed over a wide area around Campi Flegrei without depositing (bypass zone), and inundated a huge area during most of the paroxysmal, waxing phase, emplacing a mainly incipiently- to strongly- welded ignimbrite. Following this waxing phase, the leading edge of the current retreated back towards the source as the current waned, impacting a progressively smaller area and leaving an unconsolidated ash and lapilli deposit, later lithified. Our study illustrates how large pyroclastic currents can evolve in time and space and the importance of both internal (eruptive and transport mechanisms) and external (topography, surficial water and rain) factors in governing their behaviour.

A twenty years lasting geo-volcanological survey allowed us to reconstruct the eruptive history of the city of Naples which is part of the active Campi Flegrei (Phlegraean Fields) volcanic field. Naples hills are mainly modelled by volcanic and volcanotectonic processes, partly masked by historical floods that buried the lower part of the city as well as the main roman buildings. The ancient core of these hills is made by coalescent tuff cones and a lava dome older than 78 ka. Remnants of this ancient activity are draped by a thick succession of pyroclastic deposits comprising large, caldera-forming ignimbrites (Campanian Ignimbrite, 39 ka and Neapolitan Yellow Tuff, 15 ka), phlegraean tephra and monogenetic vents (e.g. Trentaremi and Nisida volcanoes). The western area of Naples collapsed during both the ignimbrite episodes. Caldera faults bound Camaldoli hill, San Martino hill, Capodimonte hill and Posillipo hill. Stratigraphical and geochronological data show that the volcanic activity in the city of Naples has lasted at least 78 ka and probably longer on the basis of the undated ancient tuffs at the base of the neapolitan succession. This persistent volcanic activity indicates that this urban sector of the Campi Flegrei volcanic field, could be considered as a likely eruption location for the next event