The genesis, transport and deposition of volcanic ash in the context of Vulcanian activity

Key processes and parameters controlling the unsteady dynamics of low-to-mid intensity, high- frequency and cyclic ‘Vulcanian’ eruptions are to date only partially understood. Vulcanian eruptions are classically decribed as low-to-mid intensity and transitory phenomena involving only moderate amounts of tephra materials and resulting from the sudden pressure releases of an over-pressurized magma conduit. This classical definition, still widely used in the literature for the description of these eruptions, does not efficiently represent and entirely capture the natural complexity associated to these cyclic and transitory phenomena, which actually represents the typical and more frequent eruptive style of many active volcanoes around the world. The presented work aims to characterize and better constrain different aspects that are involved in the natural life-cycle of volcanic ash in the context of the mid-intensity, unsteady and cyclic Vulcanian activity. In the recent years, volcanic ash has progressively claimed an increasing attention by the volcanological community, due to its high informative potential upon the dynamics of explosive eruptions. The study of tephra fallout products (i.e. ash, lapilli and ballistics projectiles) under a depositional and morpho- textural perspective has traditionally represented a valid approach in order to study the physical processes controlling explosive volcanism, and still now it is widely employed to intepret the erupive dynamics of ongoing explosive eruptions. The morphological and textural characterization of volcanic ash and ballistic products represents the basic data to intepret the compositional variability often encountered in the analysis of tephra products. The morpho-textural approach to the study of fallout products represent also a valuable analytical instrument for the understanding of the complex interplay between the different processes involving magma prior, during and after its fragmentation in the conduit, also in the case of the ash-dominated, Vulcanian-like eruptions. . In fact, the mutual interaction of these processes is interpreted as the main factor which actively controls the dynamics of cyclic and unsteady eruptions. For example, inside the conduit, different mechanisms of primary magma fragmentation and pre-fragmented material transport (e.g. fine ash elutriation) can superimpose, and the morpho-textural characterizatics of volcanic ash can provide important insights for discriminating the role of all these processes in affecting the style and the intensity of an eruption. On the other hand, the dispersal of high quantities of volcanic ash resulting from this high-frequency eruptive activity can represent a significant factor of risk for human activities (e.g. air transport) and infrastructures localized in the close proximity of active volcanoes (e.g. Casadeval 1994). Ash aggregation has proved to represent a key process able to largely control the atmospheric dispersal of volcanic ash and modify its ground deposition both in proximal and in distal areas. However, still now, the important effects of the aggregation process appear as not properly considered inside the numerical models aimed at forecasting the atmospheric dispersal of volcanic plumes. In fact, even the most recent numerical schemes lacks of an acceptable parametrization of the ash aggregation process and suffer of an inappropriate calibration of the forecasting results through affordable field data.