Experiments on mixing in pyroclastic density currents generated from short-lived volcanic explosions

During short lived volcanic eruptions, dilute, turbulent pyroclastic density currents are often observed to spread laterally from a collapsing fountain. These flows entrain and heat air while also sedimenting particles. Both processes lead to a reduction in the bulk density and since these flows often become vertically stratified, the upper part of the flow may then exhibit a reversal in buoyancy and lift off. The relative importance of entrainment and sedimentation in controlling the lift-off and the associated run-out distance of short-lived flows is not well-understood. We report a series of novel analogue laboratory experiments in which a suspension of dense particles and salt powder is released into a flume filled with CO2-laden water. A strong circulation develops in the head of the current: as current fluid reaches the front of the flow, it rises and mixes with ambient fluid which is displaced upwards over the advancing head. As the salt powder in the current mixes with the ambient fluid, small CO2 bubbles are released, decreasing the bulk density below the ambient and the mixture then rises off the current. As it advances, progressively more of the material in the flow circulates through the head, becomes buoyant and rises from the flow. Within a distance of order 9-12 times the initial size of the flow, all the original fluid has cycled through the head of the flow, mixed with ambient and lifted off. This suggests that dilute turbulent pyroclastic density currents produced by short-lived explosions, of initial length-scale L, will only propagate distances of order 9-12L. Currents with larger particles sediment more of their particles before the flow has fully mixed with the ambient, and this leads to a reduction in the mass which lifts off from the flow.