Energy facies : A global view of pyroclastic currents from vent to deposit

Abstract

Pyroclastic currents are described as gravity currents, and the classic conceptual model gives a first-order importance to the density of such currents. This directs quantitative models to assume specific flow structures (shallow water or equilib rium turbulent boundary layer), which may apply to restricted volcanic areas inde pendently of source dynamics or may correspond to source dynamics separate from topographic interaction. The recent introduction of two end-members of pyroclastic currents, inertial and forced, is further developed here, leading to a global conceptual model in which source dynamics and topographic interaction are both taken into account. The concept of energy facies is defined here as the ensemble of the first order indicators of pyroclastic currents (topological aspect ratio, competence ratio and emplacement temperature) that are proxies of the energy of such currents. Nine energy facies are introduced with general applicability and with the goal to globally characterize pyroclastic currents from vent to deposit.