Formation and Dispersal of Ash at Open Conduit Basaltic Volcanoes: Lessons From Etna

Matthew J Edwards,Julia Eychenne,Laura Pioli

doi:10.3389/feart.2021.709657

Abstract

Open conduit volcanoes are characterized by frequent, small scale explosive eruptions, which have a significant impact. Ash-forming explosions are impacting over larger areas with respect to effusive or poorly explosive events and, consequently, are more significant for hazard assessments. Quantifying the hazard associated with them requires understanding the processes and parameters controlling explosive style, and tephra dispersal and obtaining a comprehensive dataset to constrain syn-eruptive dynamics and particle transport in the volcanic plume. We present a study focused on Etna volcano (Italy), which, despite its continuous outgassing through the summit vents, has very frequent explosive eruptions dispersing ash along the southern Mediterranean area. The goal of this study is to obtain a statistically valid dataset on ash morphology and texture and investigate how various particle types distribute spatially in the tephra blanket. We chose a small scale, ash-forming eruption occurred in May 2016, sampled a few hours after tephra deposition. Analyses of grainsize distribution were coupled with further data on tephra texture and morphology, and numerical simulations. Several components were identified based either on purely textural or purely shape characteristics. Shape parameters related to the form of the grains (aspect ratio) are consistent across grainsizes and components. However, roughness parameters (solidity, convexity, concavity index) vary non-uniformly with particle size and componentry. Ash was formed through complex fragmentation of heterogenous magma, starting in the conduit, extending to the explosion jet, and resulting into a large variability of particle shapes, density and textures which distribute non-uniformly across grainsizes. This variability determines variable traveling potential within the volcanic plume and thus non uniform distribution in the deposit. Componentry variations along the dispersal axis suggest that density is the most effective parameter in controlling particle settling. However, extreme shapes, such as very elongated particles formed by surface tension instabilities in the jet, have the largest potential of being transported in the plume and can disperse downwind up to tens of km. Our results suggest that heterogeneities in textures and morphologies of particles are fundamental characteristics of tephra from frequently erupting volcanoes and should be accounted for plume dispersal modelling and hazard assessment.

Highlights

The impact of explosive eruptions on the environment depends on their magnitude, intensity, dispersal, frequency, and chemistry of the erupted magma (Deligne et al, 2017)
We briefly review the parameters used for shape analysis and the properties of the end-members ash components
The compositions of the glass and crystals in tephra from this eruption measured in a previous study (Edwards et al, 2018) indicate interaction and mobilisation of multiple magma batches during the entire May 2016 eruptive crisis. These findings suggest that all the sideromelane clasts represent the lower crystallinity, hotter magma which rose from a depth of about 1 km a.s.l. and erupted through a recently emplaced, cooler and more crystalline shallow magma

Summary

Introduction

The impact of explosive eruptions on the environment depends on their magnitude, intensity, dispersal, frequency, and chemistry of the erupted magma (Deligne et al, 2017). Explosive manifestations at mafic volcanoes are in general characterized by low to moderate explosivity (Taddeucci et al, 2015) but can be very frequent and last for several days to weeks, especially in the case of open vent systems They can emit large amounts of ash (Pioli et al, 2008; Andronico et al, 2009a), which is the finest fraction of tephra (grainsize smaller than 2 mm). Ash dispersal affects regional aviation and commercial networks due to the temporaneous disruption of air traffic, even at Strombolian scale (Scollo et al, 2013) For these reasons, small-scale, ash-forming eruptions at frequently erupting volcanoes represent a major hazard with highly relevant impacts. Quantifying the hazard related to these events requires forecasting ash cloud dispersal and sedimentation, which implies: i) understanding the mechanisms controlling the eruption onset and dynamics, and ii) determining the main eruption source parameters which are needed for modelling the eruption scenario (Marzocchi and Bebbington, 2012; Hayes et al, 2020), such as column height, the size distribution and morphological properties of tephra, which are key parameters in tephra transport modelling, in parallel with meteorological conditions (Mastin et al, 2009; De Micheli Vitturi et al, 2016)

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