Deposits from explosive submarine eruptions have been found in the deep sea, 1–4 km below the surface, with both flow and fall deposits extending several km’s over the seafloor. A model of a turbulent fountain suggests that after rising 10–20 m above the vent, the erupting particle-laden mixture entrains and mixes with sufficient seawater that it becomes denser than seawater. The momentum of the resulting negatively buoyant fountain is only sufficient to carry the material 50–200 m above the seafloor and much of the solid material then collapses to the seafloor; this will not produce the far-reaching fall deposits observed on the seabed. However, new laboratory experiments show that particle sedimentation at the top of the fountain enables some of the hot, buoyant water in the fountain to separate from the collapsing flow and continue rising as a buoyant plume until it forms a radially spreading intrusion higher in the water column. With eruption rates of 10$$^6$$ 6 –10$$^7$$ 7 $$\hbox {kg s}^{-1}$$ kg s - 1 , we estimate that this warm water may rise a few 100’s m above the fountain. Some of the finer grained pyroclasts can be carried upwards by this flow and as they spread out in the radial intrusion, they gradually sediment to form a fall deposit which may extend 1000’s m from the source. Meanwhile, material collapsing from the dense fountain forms aqueous pyroclastic flows which may also spread 1000’s m from the vent forming a flow deposit on the seabed. Quantification of the controls on the concurrent fall and flow deposits, and comparison with field observations, including from the 2012 eruption of Havre Volcano in the South Pacific, open the way to new understanding of submarine eruptions.

Abstract

<div>Deposits from explosive submarine eruptions have been found in several deep-sea locations, with both flow and fall deposits of small clasts, 1-3mm, extending 1000’s m over the seafloor. Here we propose that after mixing with seawater, the erupting fragmented material typically forms a negatively buoyant fountain. To explore their dynamics, we present a simple numerical model to describe the evolution of the eruption column and series of laboratory experiments of turbulent particle-laden fountains rising through a stratified water column.  Our experiments show that at the top of the fountain, some of the erupted material collapses to the seafloor to form a pyroclastic flow. However, some of the buoyant water in the fountain may separate from the top of the fountain, to form a buoyant plume which can carry particles higher into the water column. Eventually this mixture will be arrested by the ambient stratification and intrudes into the water column. Subsequently, the particles settle from this intrusion to form a fall-type deposit. Quantification of the controls on the concurrent fall and flow deposits, and comparison with field observations, including from the 2012 eruption of Havre Volcano in the South Pacific, open the way to new understanding of submarine eruptions.</div>

We present new experiments of particle-laden turbulent fountains in a uniform horizontal crossflow, $u_a$ , with momentum flux, $M_0$ , and buoyancy flux, $B_0$ . We use the ratio, $P$ , of the crossflow speed to the characteristic fountain speed, $M_0^{-1/4}|B_0|^{1/2}$ , and the ratio $U$ , of the Stokes fall speed of the particles, $v_s$ , to the characteristic fountain speed, to characterise the dynamics of a particle fountain in a crossflow. We find that the dynamics of these particle fountains can be categorised into three distinct regimes. In regime I when the fall speed of the particles is small in comparison with the characteristic fountain speed ( $U\ll 1$ ), the particles remain well-coupled to the fountain fluid and the flow essentially behaves as a single-phase fountain in a crossflow. In the transitional regime II ( $0.1< U<1$ ), when the fall speed of particles is comparable to the characteristic fountain speed, we observe some particles separating from the fountain fluid during the descent of the flow which leaves some fluid neutrally buoyant. As $U>1$ (regime III), we observe particles separating from the fountain as it rises from the source. We measure the average dispersal distance of the particles and the speed of the descending particles as a function of $U$ and $P$ and compare these results with models of a single-phase fountain in a crossflow. We build a regime diagram to describe the effect of $U$ and $P$ on the flow dynamics and consider our work in the context of deep-submarine volcanic eruptions.