The Astroni volcano (about 3700 y.b.p.) belongs to the last cycle of Campi Flegrei volcanic activity. Its morphology is associated with an ash ring; its effusive and explosive products chemically range in composition from K-trachytes to alkali-trachytes. The eruptive sequence of Astroni has been reconstructed like that: - ascending, along fractures, of viscous magma with emplacement of lava dome (Caprara); - Through an efficient water-magma ratio interaction, an explosive "surge cloud" formed the ash ring and wide speeded the pyroclastic products on 50 kmq area. At the end of this phase a coarse pumice flow was emplaced; - the last activity occurred inside the ash ring with formation of scoria rampart (Toppo dell'Imperatrice) and final scoriaceous lava flow (Rotondella). During these three phases about 0,6-0,7 kmc of magma (D.R.E.) was erupted. A detailed field survey has identified the pyroclastic facies relative to second phase of explosive activity; the products of these facies were studied in detail by grain size analyses (70 samples). The grain size distributions of the fine facies (massive, planar) indicate the particles were transported inside a "surge cloud" characterized by turbulent flow.