A review of the supercritical state of eruption‐fed volcanic density currents in subaerial and subaqueous settings

ABSTRACT Subaerial Pyroclastic Density Currents (PDCs) and Subaqueous Eruption‐Fed Density Currents (SEFDCs) produced during volcanic eruptions can present major hazards to surrounding communities and ecosystems. The bedforms deposited by these volcanic density currents can provide insights into the nature of transport and depositional processes, which can aid in hazard prediction. Most existing models for the deposits of volcanic density currents have been developed from turbidite systems using numerous outcrop, seafloor and experimental observations. However, little work has been done to study the nature of deposition and bedform development under supercritical conditions in PDCs or SEFDCs. Here, we review our current knowledge of the nature of supercritical flow in subaerial PDCs and SEFDCs, as well as the character of backstepping bedforms that develop in these systems. We aim to address the following questions: (1) what are the effects of the ambient fluid (i.e. water vs. air) on flow dynamics in volcanic density currents, (2) how does the role of the ambient fluid affect deposition, (3) what are the alternative mechanisms for the formation of backstepping bedding in volcanic density currents, (4) what are the criteria for differentiating between deposits in the subaerial and subaqueous realm and (5) what are the criteria for differentiating between backstepping bedding formed under supercritical flow conditions and backstepping bedding formed under alternate mechanisms. We show that SEFDC deposits generally conform with established models for deposits of supercritical flow in subaqueous systems, as Froude‐supercritical conditions are favoured in the subaqueous realm due to the dynamic viscosity and density of the ambient fluid. We further show that the internal character and flow dynamics of subaerial PDCs are much more variable and do not promote the development of large, laterally continuous structures. We compare and contrast these findings and suggest future directions for research.