Sediment waves and the gravitational stability of volcanic jets

Gilchrist, J. T., & Jellinek, A. M. (2021). Sediment waves and the gravitational stability of volcanic jets. Bulletin of Volcanology, 83(10), 1-59.

My former Ph.D. supervisor Dr. Mark Jellinek and I have built on the methods of Carazzo and Jellinek (2012) and Jessop et al. (2016) to investigate the effects of intermediate sized pumice "lapilli" particles (Lherm and Jellinek 2019) on the dynamics of relatively large "Plinian" eruptions, such as the eruption of Mt. Vesuvius in AD 79 in which the collapse of the eruption column generated devastating pyroclastic density currents (PDCs) that destroyed the cities of Herculaneum and Pompeii. We focus on the transition of Plinian eruption columns from a gravitationally stable Buoyant Plume regime to a gravitationally unstable Total Collapse regime through a Partial Collapse regime. Our work follows the pioneering work on the Partial Collapse (or "transitional") regime of Plinian eruption columns by Neri et al. (1992), Kaminski and Jaupart (2001), Neri et al. (2002) and Di Muro et al. (2004). Going one step further in experimental complexity than previous studies, our sand-laden saltwater jets are injected with a constant flow rate and particle volume fraction (steady source parameters) into a tank with a linear density gradient that has saltwater at the base changing continuously to fresh water at the top. This allows us to investigate how multiple cloud layers (modeling volcanic ash clouds) can spread from the jet column (modeling the eruption column) and the top of ground-hugging gravity currents (modeling PDCs), both of which have implications for ash spreading and sedimentation in the atmosphere, which present hazards for aviation and local communities.

Excitingly, our work led to the discovery that multiphase jet columns with steady source parameters collapse periodically as annular-shaped sedimentation waves (sediment waves) that drive the formation of multiply-layered clouds and ground-hugging gravity currents spreading axisymmetrically from the volcanic vent. A serendipitous discovery is that sediment waves emplace deposits with axisymmetric and regularly-spaced terraces, which might be analogs to those found around shallow submarine calderas such as Santorini, Greece (Hooft et al. 2019), Sumisu, Izu-Bonin arc (Tani et al. 2008) and Macauley caldera, Kermadec arc (Rotella et al. 2013). Crucially, our work shows that Plinian and the largest explosive "Ultraplinian" eruptions predominantly occur in the Partial Collapse and Total Collapse regimes where sediment wave dynamics are a key process linking eruption source parameters, column height fluctuations, distinct ash cloud structures and deposit architectures, each of which can be diagnostic of the eruption regime. Our peer-reviewed journal article "Sediment waves and the gravitational stability of volcanic jets" published in the Bulletin of Volcanology describes this work in more detail and the supplemental videos of our experiments are shown below.