A study on the influence of internal structures on the shape of pyroclastic particles by X-ray microtomography investigations

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.