Particle size-density relationships in pyroclastic deposits: inferences for emplacement processes

In moving pyroclastic systems, particles are sorted as a function of their sizes, densities and shapes. The analysis of the distribution of these characteristics in particle populations of pyroclastic deposits is a major tool in evaluating properties and regimes of parent transport systems. This paper reports a study of size-density relationships of component subpopulations from selected Quaternary pyroclastic deposits of the Latera Volcanic Complex (Vulsini, Italy), by means of integrated field and laboratory investigations, including grain-size and component analyses, density measurements and SEM crystal morphoscopy. Three kinds of particle size-density relationships characterize the studied deposits, as follows: (1) a significant correlation of decreasing sizes with increasing component densities exists for direct suspension sedimentation products, including Plinian fall and pyroclastic surge deposits; (2) a wide data dispersion with poor correlation normally occurs for the main zones of deposits from (high-concentration) pyroclastic flows; and (3) component subpopulations occur in a narrow size range irrespective of density in granular systems dominated by grain collisions, such as traction zones beneath (low-concentration) pyroclastic surges and sheared basal layers of pyroclastic flows. The regression coefficients of size-density relationships appear as a comparative measure of particle concentration in the parent systems, consistent with that inferred from the degree of sorting and abrasion of crystal subpopulations from sequential fragmentation/transport analysis of grain-size data, and quantitative morphoscopy. The crystal component becomes less sorted and more abraded as the fit to the size-density relationship decreases from Plinian fall to pyroclastic surge to pyroclastic flow, consistent with increasing particle concentration in the transporting current.