On the dispersal of clasts from volcanic craters during small explosive eruptions

In small volcanic eruptions in which volcanic ash and gas erupts through a conical summit crater, the mixture may become so underpressured that a stationary shock wave forms in the crater. This leads to the eruption of a small, dense fountain which sheds ash flows around the vent. On passing through the shock, large clasts, with diameters in excess of 1–10 cm, separate from the main jet of fine ash and may be ejected from the crater rim with speeds as large as 300–400 m/s. Simple calculations of ballistic clast trajectories show that such high speed clasts may then be deposited several kilometres from the vent. The calculated dispersal distances are broadly consistent with field deposits. In summary our model suggests that, during small explosive eruptions, crater flow dynamics may lead to the dispersal of large clasts as ballistics and the transport of small clasts in dense ash flows.