Transient volcanic ash plumes: Morpho-dynamical evolution and source properties

Transient volcanic plumes, typically generated by Strombolian and Vulcanian eruptions, are time-dependent features characterized by rise and development time scales similar to the eruption duration. Their morphological and dynamical properties are thus strongly related to the source conditions and evolution over time, i.e. (ejection duration, spatial spreading, ejection angle, time interval between pulses). In this study, the shape evolution and dynamics of initial transient volcanic plumes development, as well as their relation with discharge history, have been investigated using high-speed and high-resolution visible-light and thermal infrared videos. Physical parameterization of the plumes has been performed by defining their front velocity, volume and apparent surface temperature. Optical flow computer vision tool and fractal dimension analysis were applied for the first time in order to extract plume velocity field and shape complexity evolution over time, respectively. The source conditions were characterized both qualitatively, in terms of number, location, duration, and frequency of individual ejection pulses, and quantitatively, in terms of time-resolved ash eruption rate and a newly-defined instability factor. The newly proposed, image-based method I developed to retrieve discharge rate provides results that are comparable with previous methods but with more than one order of magnitude increase in time resolution. Results show that the connection between source properties and the dynamical and morphological features of transient plumes holds true for every one of our study cases, which encompass a variety of eruption styles and plume heights and shapes. In particular, plume front velocity, temperature decay, and plume complexity, as measured by fractal dimension, all follow complex evolutions which are intimately linked with the discharge history at the vent. Of the different factors that characterize vent discharge, lateral shifts in the ejection (from, e.g., vent shifts or changes in vent geometry or angle of the ejection) and temporal fluctuations, including the tempo and intensity of ejection pulses and other changes in the discharge rate, exert the strongest controls on plume evolution. These lateral and temporal changes at the vent can be combined in a general source instability factor that, by controlling the formation of the vortexes at the base of the plume, eventually determines the modes of air entrainment and the overall evolution of the plume. The connection between source instability and plume dynamics that I quantified in this study brings new understandings on the formation and initial development of unsteady volcanic plumes. Settings of new characterization tools such as fractal analysis and time-dependent discharge rate show promising results and potential for new monitoring resources.