Linking magma texture, rheology and eruptive style during the 472 AD Pollena Subplinian eruption (Somma-Vesuvius)

摘要

The study of medium-large magnitude and intermediate-high intensity eruptions (VEI 4/5: Column Height 15-20 km) fed by poorly evolved magmas is one of the main topics in volcanology. In this framework, the 472 AD (Pollena) eruption from Somma-Vesuvius (Italy) represents a key case study, as it is one of the subplinian eruptions used to constrain the reference scenario adopted by the Italian Department of Civil Protection in case of renewal activity at Somma-Vesuvius. The Pollena eruption experienced a complex dynamics, with abrupt shifts in eruptive style. This study focused on the fall products (L1-L8) of the magmatic eruptive phases (Phases I and II), which preceded the onset of the final phreatomagmatic phase (Phase III). Phase I was characterized by unsteady magma discharge resulting in an oscillating convective column, whereas Phase II involved pulsating activity with alternation of sustained and collapsing columns. To evaluate the role of textural variability in controlling magma rheology (and therefore variations in magma discharge), a detailed textural analysis of the juvenile products has been performed.