Conference Paper

Understanding the mechanisms driving large-scale basic Plinian eruptions from textural and rheological studies on the Pozzolane Nere eruption (Colli Albani, Italy)

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The Pozzolane Nere formation (PNR) represents one of the largest explosive events in the history of the Colli Albani volcano (407 ka, Vulcano Laziale phase). The PNR is characterized by a basal scoria fallout deposit showing an east-trending axis of dispersion overlain by a widespread low aspect ratio ignimbrite, estimated at 30km3 as bulk volume. PNR magmas are very undersaturated, tephri-phonolitic in composition and represent the basic end-member of the spectrum of explosive caldera-forming eruptions. Despite extensive studies on the deposits of the volcano, the mechanisms governing the explosive activity of these magmas are still poorly known. In order to understand the role of the internal properties of magmas in influencing and guiding the dynamic of magma ascent and eruption we carried out a combined investigation of structural features of the deposits (grain-size, maximum clast size, composition), textural, physical and chemical characteristics of the juvenile material (Vesicle Size Distributions, Crystal Size Distribution, density, viscosity) and eruption parameters (ejected volume, column height and discharge rate). Textural and minero-chemical investigations of the samples have been combined with their rheological characterization. Low T viscosity measurements (690°C<T<800°C) were performed by micropenetration technique, while the high T viscosities of fully molten (1250°C<T<1569°C) and partially crystallized specimens (1100°C <T<1225°C) were measured at 1 bar in air with a concentric cylinder viscometer. Above the liquidus temperature, viscosity ranges from 10^1.04 to 10^3.64 Pa s (HT) and 10^12.15 to 10^9.23 Pa s (LT) in good agreement with the viscosity model by Giordano et al. [2008]. In the subliquidus region, isothermal crystallization experiments allowed to quantify the role of crystals on the rheology of PNR magmas. Ten isothermal crystallization experiments were performed in the temperature range T=1193-1240°C with a constant strain rate at 0.1 s-1. The final viscosities ranged between 10^2.82 and 10^3.14 Pa s. The increase of apparent viscosity together with the onset of strain rate-and strain-dependent behavior could play a critical role during PNR degassing history, influencing the elevated explosivity of these very undersatured magmas.. Vesicle Number Densities (VNDs) are higher than those observed in literature for basic explosive eruptions and are more comparable to VNDs pertaining to explosive eruptions of evolved composition. We implemented the original version of CONFORT, the Fortran version of CONFLOW [Mastin and Ghiorso, 2000], inserting the most recent viscosity formulations [Giordano et al, 2008; Costa et al, 2009; Vona et al, 2011; Di Genova et al, 2013]. As far as PNR eruption is concerned, we demonstrated through numerical simulations that, at very fast decompression rates, despite the physical and chemical conditions (e.g. mafic composition, low viscosities and porosities) the condition necessary for fragmentation can be achieved. We postulated that water-magma interaction triggered the eruption, generating the fast decompression rates that allowed it to initiate and to progress into a purely magmatic stage. Changes in vent/conduit geometry are hypothesized to be responsible for the transition fallout-ignimbrite. We suggest opening of fractures and caldera collapse as responsible for an increase in mass discharge rates and a decrease in ascent velocities, which produced a final column collapse leading to the generation of great intensity ignimbrite, such as that relating to the PNR eruption. Giordano D, Russell JK, Dingwell DB (2008) Viscosity of magmatic liquids: A model. Earth A model for the rheology of particle-bearing suspensions and partially molten rocks. Geochemistry Geophys Geosystems.

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