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Quantifying "boiling-over" versus discrete eruption column collapse to predict the timing and intensity of pyroclastic density currents

Johan T. Gilchrist 1, A. Mark Jellinek2

Abstract

Field and eruption modelling studies show that explosive eruptions in collapsing fountain regimes produce deadly pyroclastic density currents (PDCs) that can originate from relatively high column collapses and are linked to discrete, dilute and relatively thin PDC flow deposit layers, sometimes interbedded with fallout layers. Collapsing eruption columns can also emerge in or evolve to a continuously collapsing "boiling-over" regime linked to massive and relatively thick ignimbrite layers. We use scaled analog experiments on multiphase fountains to show that the period between fountain collapses compared to the time for ground-hugging gravity currents (GCs) to flow out of the impact zone forms a Collapse Frequency number (Cf) that predicts the timing and intensity of PDCs. For relatively strong fountains with Cf < 1, collapse occurs less frequently via periodic sedimentation waves to produce discrete GCs. As fountains become weaker with Cf ≥ 1, collapses occur more frequently such that collapsing sedimentation waves merge in the impact zone and provide a continuous mass flux to GCs. We suggest that Cf quantitatively defines a continuous eruption source parameter space between discrete and continuous eruption column collapse regimes, can be used to predict PDC timing and intensity during eruptions, or be inferred from deposits to constrain eruption source parameters post-eruption. Future work should investigate how column collapse processes, distinct from sedimentation waves, influence the effective value of Cf.