On the mobility of pyroclastic currents in light of deposit thickness and clast size trends

Pyroclastic currents (PCs) represent the most hazardous manifestations of explosive volcanic activity. The evaluation of theirmobility, as related to the eruptive source parameters and the interactions with the surroundings along their path, is crucial for hazard assessment at active volcanoes in order to define the areas exposed to their impacts. Here we report on two case studies from the Latera Volcanic Complex (Vulsini Volcanic District, central Italy) that may approach the two end-members of the classical spectrumof PC dynamics in terms of particle concentration and dominant flowregime(i.e., dilute, turbulent vs. concentrated, laminar PCs). For the two examples, we document specific (and contrasting) variation trends of deposit thickness and maximum lithic and pumice clast sizes with distance from vent, which provide a further characterization in terms of forced vs. inertial flow behavior. In the first case, thickness and maximumlithic and pumice sizes showa mild, linear decreasewith distance, implying a sort of balance between forced and inertial regimes. Flowpressure andmass progressively drop via deposition, along with flow competence. For given source conditions (i.e., mass eruption rate, fountaining height, erupting mixture, etc.) and slope angle of topography, the PC runout depends primarily on the waning density contrast between the flow and the surrounding atmosphere (density current), ending with the loft of the current. The second case, representative of moderate- to large-volume, dense PCs derived by continuous feeding with high mass discharge rate (i.e., from collapsing Plinian columns or caldera-forming events), shows an increase of deposit thickness with distance, which is accompanied by opposite lithic and pumice clast sizedistance trends (i.e., decreasing lithic sizes and increasing pumice sizes downcurrent), compatible with high particle concentration and forced, non-turbulent, granular flow regime. By analogy with the mobility of dry debris flows, we suggest that the sliding component of transport, which prevails in proximal to intermediate settings, may account for the prevailing tendency of the PC to transport than to deposit, which leaves behind relatively thin sheetswith coarse lithics, while the dominant spreading component downcurrent would result in increasing accumulation and the stacking of coarse pumice-rich flow lobes inmore distal settings. The spreading component of transport would become more and more important with increasing material supply at PC source, resulting in additional runout. These findings cast newlight on the first-order controlling factors of flowmobility for the two PC end-members, laying the groundwork for determining the most relevant PC hazard parameters, i.e. the maximum runout and the flow properties upon impact (dynamic pressure, burial potential, temperature).