Coarse-tail vertical and lateral grading in pyroclastic flow deposits of the Latera Volcanic Complex (Vulsini, central Italy): origin and implications for flow dynamics

Coarse-tail vertical and lateral grading are common features of pyroclastic flow deposits of the Latera Volcanic Complex (Vulsini Volcanoes, central Italy), and are described for four representative flow units. Lithic clasts show normal vertical and lateral grading in all four units. Pumice clasts show reverse vertical and lateral grading within three flow units; normal vertical and lateral grading of scoria clasts has been observed in one flow unit. The origin of vertical and lateral grading of lithic and pumice clasts is related to mechanisms operating during the transport process within the high particle concentration basal avalanche of pyroclastic flows. Vertical grading results from the balance between gravitational and dispersive forces, and is transferred to a lateral grading by vertical velocity gradients within a nonturbulent flow zone of pyroclastic flows. The pyroclastic flows are modeled as Bingham-type fluids, a framework that explains some of the basic deposit features even though it is a highly simplified treatment of the flows. Plug flow zones in the flows were a relatively minor part of their thicknesses. Evidence for the flows having had high densities, probably within a factor of two of the final deposit density, is presented. Finally, their origin by “single pulse” or “progressive formation” is discussed.