Characterizing magma fragmentation and its relationship with eruptive styles of Somma-Vesuvius volcano (Naples, Italy)

tAmong the active volcanoes worldwide, Somma-Vesuvius is one with the highest volcanic risk as thesurrounding areas are highly populated. Somma-Vesuvius is quiescent since 1944, but geological andhistorical records reveal a frequent violent explosive activity in the last 4000 years, representing a severerisk for the ∼700,000 inhabitants currently living in the area having a high probability for being impactedby pyroclastic density currents (“red zone”) and more than one million people who can be potentiallyaffected by tephra fallout. This study aims to analyze the distribution and grain-size of tephra falloutdeposits from several Somma-Vesuvius eruptions of different styles, ranging from Violent Strombolianto sub-Plinian and Plinian, for characterizing the associated magmatic fragmentation through the assess-ment of their total grain-size distribution (TGSD). Chronologically, we focus on the Avellino (4365 BP) andPompeii (A.D. 79) Plinian eruptions, the Pollena (A.D. 472) sub-Plinian eruption, and the 1906 and 1944Violent Strombolian eruptions. The related TGSDs were estimated by means of the Voronoi tessellationmethod, which requires a suitable number of local grain-size distributions and estimation of the areaof minimum tephra loading (zero-line contour). However, field-derived TGSDs can be biased towardsthe coarse and fine grain-size populations due to the typical paucity of available field outcrops of falloutdeposits. To encompass this issue, we performed a sensitivity study on the assumption behind TGSDreconstructions and described TGSDs through analytical grain-size distributions that best fit the fieldTGSDs. Our main objective is a more robust estimation of the TGSDs associated with the different erup-tive styles, which is crucial, together with the other eruption source parameters, for robustly predictingtephra loading and airborne ash dispersal of future eruptions at Somma-Vesuvius.