See also fig. The flow deposits that overlie the welded succession are about 18m thick and show a drastic increase of lithics content. Single beds are generally more than one meter thick and are massive or show the grading of the coarser lithic and pumice clasts. The uppermost part of the sequence has a thickness of about 13 m and is made by a succession of layers forming dunes and cross stratification, alternated with massive beds. The characteristics of the PGT, such as the large average size of the clasts, the abundance of lithic clasts and the presence of ballistic blocks suggest a very close source area to Ventotene Island.
Bomb sags indicate a vent location near the western coast of the island.
The first products of the PGT eruption overlay a paleosoil. They are lithic-rich fall layers and thin ash flow layers indicating an initial phase of the eruption characterised by small phreatic explosions, alternated with brief phreatomagmatic phases. The paroxysmal phase, which develops with increasing violence, is characterized by the formation of a sustained plinian column and the emission of pumice clasts. The pumice layer overlaying the phreatomagmatic deposits, testifies this shift of activity after the opening of the conduit and the precursory phase. The increase of clasts size toward the top of the deposit is attributed to increasing discharge rate and column height with time.