Subaqueous Flows of Pyroclastic Debris and Their Deposits

In this chapter, we consider the intriguing and provocative topic of what happens when pyroclastic density currents enter the sea or large lakes from subaerial vents, or are erupted from subaqueous vents. Does dynamic interaction with the ambient water mass occur? Do the flows mix in ambient water, and transform from hot, gas-supported pyroclastic (mass) flows into water-supported mass flows (e.g. syn-eruptive debris flows and megaturbidity currents)? Can we recognise differences in the characteristics of the deposits of subaqueous mass-flow density currents of pyroclastic debris, and subaerial pyroclastic flow deposits? What do we call the deposits? Initially, it is important to consider the terminology that has developed historically to describe such subaqueous deposits. By analogy with subaerial pyroclastic flows, use of the term “pyroclastic flow” should be restricted to demonstrably hot, gas-rich mass flows of pyroclastic debris. Using this definition, very few examples of subaqueous pyroclastic deposits have been described with both evidence for hot emplacement and of having been wholly submerged. From the results of more recent experimental and numerical modelling studies, together with observations of some contemporary eruptions, we conclude that, when pyroclastic flows enter water, they are generally disrupted explosively and/or ingest water and transform into water-supported mass flows, although some higher mass and volume flux flows may instantaneously displace the shoreline seawards and deposit near shore ignimbrite. Based on a multitude of likely scenarios in which pyroclastic flows may enter subaqueous environments (either through eruption from subaerial vents and flow of pyroclastic flows into a body of water or through eruption from subaqueous vents), we then provide an in-depth review and assessment of examples described in the literature. There is no evidence to suggest that welding in wholly subaqueous environments is common, although, we highlight several situations where this potentially could occur.