Titolo della tesi: Environmental and health risk assessment of volcanic ash in densely populated areas
Explosive volcanic eruptions are among the most powerful natural phenomena on Earth, capable of injecting immense quantities of volcanic ash into the atmosphere. Volcanic ash, the finest fraction of pyroclastic material, can be transported over hundreds to thousands of kilometres, producing wide-ranging effects on the environment, the climate, infrastructures, and human health. At the core of these impacts lie the physical and chemical characteristics of the particles—such as size, shape, componentry, composition, and surface reactivity—which jointly determine how ash is dispersed, deposited, and interacts with the environment and living organisms. Understanding these properties is therefore essential for performing realistic hazard assessments.
This thesis investigates the physicochemical properties of volcanic ash and their role in controlling both atmospheric dispersal and environmental and health impacts.
The research combines petrological, morphological, and spectroscopic techniques and is structured around three complementary case studies that represent the full spectrum of explosive volcanism from large-magnitude to low-intensity eruptions: (i) fine ash from a large-magnitude super-eruption, the VEI-7 Campanian Ignimbrite of Campi Flegrei (southern Italy), which dispersed ash over an area of approximately 3 million km². The ~39.8 ka Campanian Ignimbrite is the largest known volcanic event in Europe over the past 200 ka, contributing to significant environmental and climatic changes and possibly influencing early human populations. Understanding the dispersion and impact of such catastrophic events is fundamental for predicting the potential consequences of future large-magnitude eruptions. This study quantifies how particle shape and density jointly control terminal velocity and, consequently, long-range ash dispersal; (ii) ash from recent paroxysmal activity at a persistently active basaltic volcano, Mount Etna (Italy), to link syn- and shallow pre-eruptive processes to ash physicochemical characteristics, with particular emphasis on Fe speciation; and (iii) a comparative multi-volcano dataset, encompassing a wide range of compositions, to investigate grain size, Fe host-phase distribution, and reactivity-relevant Fe speciation.
The results provide new quantitative constraints on how aerodynamic properties determine the fate of volcanic ash. The study of the Campanian Ignimbrite deposits demonstrates that spherical models strongly overestimate particle settling velocities, whereas measured, non-spherical shapes and true densities yield significantly lower terminal velocities. This implies that the actual dispersal and atmospheric residence time of volcanic ash are greater than previously estimated. The analysis quantified the difference in terminal velocity between glass and mineral phases, as well as the relative influence of shape and density on their aerodynamic behaviour. These findings highlight the need to include realistic shape and density data in dispersal models, particularly for estimating the spread of the respirable fraction (< 4 µm).
The investigation of ash from recent Mount Etna paroxysms reveals the close connection between eruption dynamics, magma fragmentation, and the distribution of Fe-bearing phases, which ultimately control Fe speciation. The results indicate a mixed Fe²⁺–Fe³⁺ speciation with surface Fe³⁺ enrichment on juvenile glassy particles, a feature that may enhance surface reactivity, leading to potential health implications. Moreover, the unexpected occurrence of high content of crystalline silica as lithic in the fine fractions highlights an additional health concern and the need to pay attention to not a primary mineral phase. These discoveries are particularly significant given the proximity of basaltic volcanoes to densely populated areas, where frequent ash emissions may represent a chronic exposure risk.
Finally, the comparative study of volcanic ash from multiple active volcanoes worldwide (Eyjafjallajökull, Sakurajima, Kelud, Fuego, and Etna) shows that grain size and Fe-bearing phases distributions are controlled by eruption style and transport distance, influencing both dispersion and exposure potential. The variability observed in Fe-phase distribution, speciation, and oxidation state among samples may affect the accessibility of reactive Fe and, consequently, the potential ash reactivity.
Overall, this thesis provides a comprehensive framework that investigates all the key factors underlying the hazard potential of volcanic ash. Although reactivity and toxicological studies are needed to fully assess the biological effects of ash exposure, this research establishes the basis for improving ash dispersal models and exposure assessments for both local and global volcanic hazards. By bridging disciplines — from volcanology and petrology to environmental geochemistry — this dissertation contributes to a more integrated understanding of volcanic ash behaviour and its potential impact on ecosystems and human health.