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

Abstract

The study of medium-large magnitude and intermediate-high intensity (VEI = 4/5; Column Height 15-20 km) eruptions fed by poorly evolved magmas is one of the main topics in volcanology. In this framework, the 472 CE (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. Pyroclast textures record a variable degree of outgassing efficiency and lateral textural stratification of magma in the conduit, related to differential magma ascent rates and resulting in variable eruption intensity. Repetitive patterns of magma densification, achieved by progressive outgassing and crystallization, led to stages of decreasing ascent velocity resulting in the end of the eruptive pulse (during the oscillatory Phase I) or anticipating PDC emplacement (at the end of Phase I and during the pulsatory Phase II). Further decompression of the outgassed magma induced intense clinopyroxene microlite crystallization, that markedly increased magma viscosity and promoted the restoration of sustained columns during the eruption climaxes (L3 and L8). Magma densification patterns and consequent unsteadiness in magma discharge at the surface may derive from small heterogeneities in the initial volatile budget and/or represent a natural evolution of low viscosity magmas, as those feeding Pollena eruption, where efficient crystallization in the conduit can deeply and abruptly modify magma rheology, outgassing ability, conduit flow and, ultimately, eruptive style.