Natural disasters: Explosive volcanic eruptions and gigantic landslides (Q1390029)

We discuss two natural disasters. In the first we show how to model massive, ground-hugging ash flows, known as pyroclastic flows by geologists, in terms of particle-driven, turbulent gravity currents. A framework for solving forward problems is set up so that, for different geometries, all the flow and deposit properties can be predicted given the initial conditions of the flow following a volcanic eruption. This is then used to discuss inverse problems, for which only the details of the deposit are provided and the initial conditions of the flow are to be calculated. This method is applied to analysing the eruption of Taupo, New Zealand, about 1800 years ago. The second study concerns the runout of massive landslides. A model employing the concepts of the flow of granular materials is presented in which the interior of the rockfall propagates uniformly above a thin shear layer of rocks through which all the rocks eventually fall, to leave the flow and add to its deposit. Quantitative predictions from this theoretical model agree well with observations from about 50 rockslides on Earth, the Moon and Mars.