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Understanding the mechanisms driving large-scale basic Plinian eruptions from textural and rheological studies on the Pozzolane Nere eruption (Colli Albani, Italy)
Abstract
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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In many volcanic studies, estimates must be made of the changes that magma and its associated gases experience when traveling through an eruptive conduit to the surface. Exsolution of magmatic gas, acceleration, changes in pressure and temperature, depth of fragmentation, and final exit velocities affect such features as lava fountain heights, the ability of a volcanic column to convect or collapse, and the degree to which water can enter the conduit during eruptive activity. Most of these quantities cannot be easily estimated without some sort of numerical model. This report presents a model that calculates flow properties (pressure, vesicularity, and some 35 other parameters) as a function of vertical position within a volcanic conduit during a steady-state eruption. The model idealizes the magma-gas mixture as a single homogeneous fluid and calculates gas exsolution under the assumption of equilibrium conditions. These are the same assumptions on which classic conduit models (e.g., Wilson and Head, 1981) have been based. They are most appropriate when applied to eruptions of rapidly ascending magma (basaltic lava-fountain eruptions, and Plinian or sub-Plinian eruptions of intermediate or silicic magmas) that contains abundant nucleation sites (microlites, for example) for bubble growth. The numerical parts of the program were written in Fortran 90 and can be compiled on any platform (DOS, Unix, Macintosh etc.) that has a Fortran 90 compiler.
In response to rapid decompression, porous magma may fragment explosively. This occurs when the melt can no longer withstand forces exerted upon it due to the overpressure in included bubbles. This occurs at a critical pressure difference between the bubbles and the surrounding magma. In this study we have investigated this pressure threshold necessary for the fragmentation of magma. Here we present the first comprehensive, high temperature experimental quantification of the fragmentation threshold of volcanic rocks varying widely in porosity, permeability, crystallinity, and chemical composition. We exposed samples to increasing pressure differentials in a high temperature shock tube apparatus until fragmentation was initiated. Experimentally, we define the fragmentation threshold as the minimum pressure differential that leads to complete fragmentation of the pressurized porous rock sample. Our results show that the fragmentation threshold is strongly dependent on porosity; high porosity samples fragment at lower pressure differentials than low porosity samples. The fragmentation threshold is inversely proportional to the porosity. Of the other factors, permeability likely affects the fragmentation threshold at high porosity values, whereas chemical composition, crystallinity and bubble size distribution appear to have minor effects. The relationship for fragmentation threshold presented here can be used to predict the minimum pressure differential necessary for the initiation or cessation of the explosive fragmentation of porous magma.
Our ability to forecast volcanic hazards at active volcanoes stems from our knowledge of parameters affecting eruption dynamics. Persistent activity displayed by basaltic volcanic systems, such as Etna and Stromboli, is governed mainly by the textural evolution and rheological properties of eruptive products. Here, we investigate the high temperature (1131 °C < T < 1187 °C) rheological behavior and textural evolution of remelted natural basaltic magmas from Stromboli and Etna volcanic systems upon cooling and crystallization at atmospheric conditions. The pure liquid and subliquidus iso-thermal viscosities are investigated using a concentric cylinder apparatus by varying strain rate applied to the investigated materials. Detailed textural analysis has been performed in order to evaluate the effect of crystal distribution on the rheolog-ical evolution of magmas. At subliquidus conditions, the mineralogical assemblage is dominated by the presence of spinel and plagioclase with the occurrence of stable clinopyroxene only at the lowest temperatures (1157 and 1131 °C for the Stromboli and the Etna samples, respectively). The overall crystal fraction (/) varies between 0.06 and 0.27. In the high T regime the viscous response to applied stress and strain rate is strongly affected by the presence of even small amounts of crystals. Large discrepancies between measured and predicted viscosities obtained using the Einstein–Roscoe (ER) equation are also found at low crystal fractions. We find here that, at the investigated conditions, the physical effect caused by the presence of elongated crystals is orders of magnitude higher than that predicted by existing models which only account for spherical particles. It also appears that a weak shear thinning behavior occurs at the lowest temperatures investigated. Crystal shape and, with it, the strain-rate dependence of the rheological properties appear to play primary roles in influencing the transport properties of these basaltic magmas.