Dynamics of Volcanic Jets from Short-Duration Eruptions: Mixing and Rise

Abstract:

Mixing controls volcanic plume behavior. Our understanding of mixing therefore places a critical limit on our capacity to manage volcanic plume hazards. Researchers have investigated mixing for two situations: volcanic plumes with rise times that are long or short relative to the eruption duration. The plumes generated in these two situations are then modeled as thermals or maintained plumes, respectively. The case in the middle, when plume rise times are comparable to the eruption durations, is not often considered. This third class is appropriate to discrete eruptions, such as Strombolian- or Vulcanian-style eruptions, which last a few seconds or tens of seconds and generate relatively small plumes. Here we use numerical experiments to show that mixing occurs differently in this third eruption class because of dynamic features that are distinct from the long- and instantaneous-duration end members. In this presentation we focus on the effect of eruption duration on mixing in the inertia-driven or jet-like phase of ascending volcanic plumes. Our simulation results reveal that air entrainment rates increase after the eruption ends, resulting in enhanced overall mixing in jets from shorter-duration eruptions. Models developed for the other two mixing situations do not predict these results. New models are required for volcanic plumes with rise times comparable to their durations in order to reliably estimate their behavior for hazard management purposes.