Pyroclastic density currents: state of the art and perspectives

Pyroclastic density currents (PDCs) are mixtures of twocomponents, namely solid particles and fluid (gas) phase. Theymacroscopically behave as dense,multiphase gravity currents (flowing pyroclastic mixtures of particles and gas) immersed in a less dense, almost isotropic fluid (the atmosphere). As for other natural phenomena, their study needs a multidisciplinary approach consisting of direct observations, analysis of the associated deposits, replication through laboratory experiments, and numerical simulations. This review deals with the description of the current state of the art of PDC physics, and combines analysis of data from various methodologies. All of the above-mentioned approaches have provided significant contributions to advancing the state of the art; in particular, laboratory experiments and numerical simulations deserve a special mention here for their tumultuous growth in recent years. A paragraph of the review is dedicated to the puzzling behaviour of large-scale ignimbrites,which are (fortunately) too rare to be directly observed; they cannot be easily reproduced through laboratory experiments, or investigated by means of numerical simulations. The final part is dedicated to a summary of the whole discussion, and to a comment on some perspectives for future developments of PDC studies.