Effects of flow density on the dynamics of dilute pyroclastic density currents

Dilute turbulent pyroclastic density currents may have densities as large as 10–100 kg m−3. Laboratory experiments using dense gases show that for such currents, the speed of the head of the current depends on the density difference between the current and the surrounding air in a complex way. Here, a model of a dilute pyroclastic density current generated by a short-lived volcanic eruption is developed to account for this effect. The model predicts that the head of the current travels more slowly than in existing models, leading to shorter run-out distances and smaller dynamic flow pressures. The effects are most significant for dense pyroclastic density currents such as those produced by dome collapse events. We apply the new model to the eruptions of Mount St. Helens 1980, Taupo 1800 years ago and Soufrière Hills 1997.