Gas‐Pyroclast motions in volcanic conduits during strombolian eruptions, in light of shock tube experiments

In a Strombolian volcanic eruption, bursting of a pressurized gas pocket accelerates a mixture of gas and pyroclasts along a conduit and out of a vent. While mixture ejection at the vent is the subject of direct geophysical measurements, and a key to eruption understanding, the dynamics of how the mixture moves in the conduit are not observable and only partly understood. Here, we use analog, transparent shock tube experiments to study the dynamics of gas and particles under fast gas decompression in a vertical tube. Maximum particle exit velocity increases linearly with increasing energy (pressure times volume) of the pressurized gas and, subordinately, with decreasing particle size and depth in the tube. Particles, initially at rest, are at first accelerated and dispersed in the conduit by the expanding gas. When the gas decelerates or even reverses its motion due to pressure changes in the tube, the particles, moving under their inertia, are then decelerated by the gas drag. Deceleration lasts longer for lower initial gas energy and for deeper particle starting position. Experiments and eruptions share two key vent ejection features: (1) particles exit the vent already decelerating, and (2) the exit velocity of the particles decays over time following the same nonlinear law. Friction with slower or even backflowing gas likely causes pyroclast deceleration in volcanic conduits during Strombolian explosions. Pyroclast deceleration, in turn, affects their exit velocity at the vent, as well as estimates of the explosion source depth based on temporal changes in exit velocity.