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A multivariate statistical technique (principal component analysis) has been used to investigate the granulometric features of 414 samples of pyroclastic rocks: 216 samples were collected from areas of recent and active volcanism in Central and Southern Italy; data for the remaining 198 samples were obtained from the literature and relate to products from Somma‐Vesuvio, Mount St Helens and the Japanese volcanic arc. The analysis offers an improved means of discriminating pyroclastic deposits using their grain‐size characteristics.

A computer code using sequential fragmentation/transport theory was used to deconvolute and characterize a large grain‐size data set taken from the AD 79 Vesuvio deposits. The results allow us to interpret transport and deposition processes. Four principal morphological classes of grain‐size spectra were recognized in the AD 79 deposits: 1 unimodal distributions with coarse modes and very good sorting; 2 polymodal distributions in which relative fractions of each subpopulation are considerably variable; 3 polymodal distributions, but with one mode greatly prevailing over the other ones; 4 flat spectra in which a large number of size classes show the same loading. Because different eruptive, transport and deposition conditions may have operated on pyroclasts which occur in the same bed, we have assigned grain‐size subpopulations, with different modes to specific mechanisms of particle movement and sedimentation depending on the size range of the particles and the textures of the beds. The fragmentation/transport processes considered here occur either within dilute flows (as fall, traction, saltation and suspension loads) or in high‐concentration flows (as a fluidized system or one with an extremely high sedimentation rate). Variation in strength and position of modes throughout the entire vertical section of AD 79 products illustrates changes in transport and deposition processes with time. Size spectra from Vesuvio quantitatively demonstrate contemporaneous deposition from fall and surge mechanisms as well as contributions from different levels of hydrovolcanic products. In contrast, vertical variations in size spectra within individual pyroclastic flow deposits suggest variation from high particle concentration near the base of the bed to more dilute depositional conditions towards the top. Lateral variations in size spectra for one marker horizon show how a local pyroclastic flow in a channel grades into a surge on the margins. This study supports the model of continuous modification in loadings of several discrete subpopulations during deposition from a single explosive cloud.