Capponi, Antonio (2016) The consequences of gas slug ascent in a stratified magma on Strombolian eruption dynamics. PhD thesis, UNSPECIFIED.
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Abstract
Strombolian volcanic activity, one of the most common on Earth, results from the bursting of large gas pockets (slugs) following ascent through relatively low-viscosity magma within the volcanic conduit. However, this paradigm was forged when the complex rheology of the magma at Stromboli, the model-type volcano for this activity, was still poorly constrained. Textural and petrological evidence has recently suggested the presence of viscous, degassed magma layers in the upper portion of the conduit at Stromboli. This layer acts as a plug, through which slugs burst, controlling the eruptive dynamics. To date, little has been done to integrate this scenario into current models of volcanic eruptions and interpretation of geophysical signals. This study investigates slug ascent through a rheologically stratified magma column using analogue laboratory experiments, numerical modelling and 3D computational fluid dynamic simulations. The results illustrate (1) the range of slug flow configurations that develop in a rheologically stratified column, (2) the relevance of such configurations to Strombolian-type volcanoes, and (3) the key parameters controlling the transition in flow configurations. Each identified configuration encompasses processes affecting slug expansion and burst: for example, dynamic narrowing and widening of the conduit, instabilities along the falling liquid film and slug break-up. These complexities lead to variations in eruption magnitude, style and consequent geophysical signals. The similarity between laboratory infrasonic waveforms, whose amplitudes are strongly dependant on the flow configuration in which the slugs burst, and measured infrasonic signals from Stromboli suggests that the slug burst through a plug represents a viable first-order mechanism for the generation of volcano-infrasonic signals. Furthermore, the presence of a plug seems to be a pre-requisite for the generation of eruptive pulses observed in single explosions at Stromboli, and the interaction between an ascending slug and the liquids promotes magma mingling, therefore affecting the properties of the ejecta.