Abstract
X-Ray computed microtomography is a non-destructive 3D imaging technique that can be used for the investigation of both the morphology and internal structures of a solid object. Thanks to its versatility, it is currently of common use in many research fields and applications, from medical science to geosciences. The latter includes volcanology, where this analytical technique is becoming increasingly popular, in particular for quantifying the shape as well as the internal structure of particles constituting tephra deposits. Particle morphology plays a major role in controlling the mobility of pyroclastic material in the atmosphere and particle-laden flows, while the internal structure (e.g. vesicles and crystal content) is of importance in constraining the processes that occurred in magmatic chambers or volcanic conduits.In this paper, we present results of X-Ray microtomography morphological and textural analyses on volcanic particles carried out to study how particle shape is influenced by internal structures. Particles were selected from tephra generated during explosive eruptions of different magnitudes and compositions. Results show that particle morphology is strongly influenced by internal structure, which is characterized by textural features like vesicularity, vesicle and solid structure distribution, vesicle inter-connectivity and distance between adjacent vesicles. These have been found to vary with magma composition, vesiculation and crystallization history. Furthermore, our results confirm that X-Ray microtomography is a powerful tool for investigating shape and internal structure of particles. It allows us to both characterize the particle shape by means of tridimensional shape parameters and relate them to their internal structures.
Highlights
During explosive eruptions, particles of variable size, shape and density are injected into the atmosphere and, depending on the eruptive size and style, can have an impact on human beings, infrastructure and activities from local up to global scale [Blong, 1984; Casadevall, 1994; Horwell and Baxter, 2006; Bonadonna et al, 2012; Wilson et al, 2012, 2014; Beckett et al, 2015]
3D shape descriptors quantified by means of μXCT analyses have been introduced and applied to predict the aerodynamic drag of volcanic particles [e.g. Dioguardi et al, 2017]
The use of μX-CT has allowed us to demonstrate that particle shape, which is here described by sphericity Φ3D and fractal dimension D3D, is strongly influenced by the internal structure of particles, here quantified by means of vesicularity, vesicle and solid structure distribution, vesicle inter-connectivity and distance between adjacent vesicles
Summary
Particles of variable size, shape and density are injected into the atmosphere and, depending on the eruptive size and style, can have an impact on human beings, infrastructure and activities from local up to global scale [Blong, 1984; Casadevall, 1994; Horwell and Baxter, 2006; Bonadonna et al, 2012; Wilson et al, 2012, 2014; Beckett et al, 2015]. Many studies have been carried out over the past few decades focusing on the dependency of the aerodynamic drag on particle shape, especially in the field of multiphase flow dynamics [Sneed and Folk, 1958; Wilson and Huang, 1979; Haider and Levenspiel, 1989; Swamee and Ojha, 1991; Ganser, 1993; Rodrigue et al, 1994; Chien, 1994; Taylor, 2002; Tran-Cong et al, 2004; Dellino et al, 2005; Pfeiffer et al, 2005; Loth, 2008; Hölzer and Sommerfeld, 2008; Mele et al, 2011; MELE ET AL. Dioguardi and Mele, 2015; Bagheri and Bonadonna, 2016; Dioguardi et al, 2017, 2018] In these studies, several shape-dependent drag laws have been proposed, which depend on one or more shape descriptors that are generally functions of 1D and 2D parameters. 3D shape descriptors quantified by means of μXCT analyses have been introduced and applied to predict the aerodynamic drag of volcanic particles [e.g. Dioguardi et al, 2017]
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