Stratigraphy and physical parameters of the Plinian phase of the Campanian Ignimbrite eruption

The Campanian Ignimbrite eruption (39 ka) was the most powerful eruptive event of the Campi Flegrei caldera (southern Italy). This event coincided with the onset of a cold climatic phase and the Paleolithic transition from Neanderthals to modern humans. The eruption started with a sustained column that emplaced a widespread pyroclastic fall deposit covering an area >4000 km2 within the 15 cm isopach. For the first time, we present the complete longitudinal variation from the coarse and 10-m-thick proximal (down to 15 km) sequence, through the well-stratified pumice lapilli deposit in medial areas (30–80 km), to the distal tephra (hundreds to thousands of kilometers distant). The Plinian pumice fall deposit shows a strong lateral heterogeneity due to variations in stratification, grading, and abundance of components. All Campanian Ignimbrite fall layers display Plinian dispersal. Variation of grain size with stratigraphic height suggests that the convective plume was far from steady state. Initially, the plume dispersed 1.3 km3 of tephra toward the ENE (N75°E) from a height of 29 km (layer A). A gradual increase in intensity resulted in inverse grading of layer B. Column height increased from 26 to 37 km at a vertical velocity of 3.6 m/s. It had a main dispersal axis to the east (N97°E) and a secondary lobe to the southeast (N137°E). During this phase, a maximum volume of 1.73 km3 of tephra was emplaced. Accessory lithics concentrated in layer C are possibly due to vent clearing after partial blockage of the vent. The 33-km-high column dispersed ejecta to the east (N95°E), with only 0.2 km3 of tephra erupted during this phase. During the successive gradual decline in eruption intensity (column height decreased from 38 to 32 km at a velocity of 4.2 m/s), a normally graded layer (D) with a volume of 1 km3 accumulated to the east (N95°E). The sustained column phase ended with a pulsating and partially collapsing column that reached 23 km in height and dispersed 1.1 km3 of a stratified and lithic-rich succession (layer E) to the southeast (N112°E). If we include the distal co-Plinian deposit, a total volume of 7.8 km3 (dense rock equivalent [DRE]) of magma was released (containing 0.09 km3 of accessory lithics). Tephra mass for the single layers is on the order of 1011 kg, and the total mass is ~2 × 1012 kg (2 × 1013 kg including the co-Plinian ash). Mass discharge rates ranged from 0.9 to 6.7 × 108 kg/s. The calculated magnitude of the sustained column phase is 6.3. The duration of the Plinian phase of this eruption, based on the ratio of two parameters, erupted mass divided by discharge rate, is estimated to have been ~20 h (including co-Plinian ash). This study shows that Plinian deposits are not always homogeneous and, as for pyroclastic current deposits, can show an articulate architecture. Only the complete reconstruction of vertical and lateral variations in components, stratification, and grading might provide insights into the temporal and spatial evolution of the sustained plume