Abstract
Pyroclastic density currents (PDCs) are a life-threatening volcanic hazard. Our understanding and hazard assessments of these flows rely on interpretations of their deposits. The occurrence of stratified layers, cross-stratification, and bedforms in these deposits has been assumed as indicative of dilute, turbulent, supercritical flows causing traction-dominated deposition. Here we show, through analogue experiments, that a variety of bedforms can be produced by denser, aerated, granular currents, including backset bedforms that are formed in waning flows by an upstream-propagating granular bore. We are able to, for the first time, define phase fields for the formation of bedforms in PDC deposits. We examine how our findings impact the understanding of bedform features in outcrop, using the example of the Pozzolane Rosse ignimbrite of the Colli Albani volcano, Italy, and thus highlight that interpretations of the formative mechanisms of these features observed in the field must be reconsidered.
Highlights
Pyroclastic density currents (PDCs) are a life-threatening volcanic hazard
The existing widespread interpretation of backset features in PDC deposits is that they are a product of upper flow regime/Froude supercritical flow within dilute PDCs31–35,64, or that relatively steep backset bedforms are a record of the formation and propagation of Froude jumps, where flow transforms from Froude supercritical (>1) to Froude subcritical, similar to fluvial chute-and-pool structures[20,31,35,37,39,64,65,66] (Fig. 1a/e, f)
Planar beds are deposited at Froude number (Fr) 3–5, shallow backset bedforms at Fr 2–3 and steep backset bedforms at Fr 0.59–2
Summary
Pyroclastic density currents (PDCs) are a life-threatening volcanic hazard. Our understanding and hazard assessments of these flows rely on interpretations of their deposits. Bedform-related sedimentary structures in PDC deposits include backset features (i.e., upstream-dipping beds) formed by stoss-side aggradation, similar to chute-and-pool structures and antidunes found in fluvial systems (Fig. 1a, f and Fig. 1b, d), which are generally thought to be formed under supercritical flow conditions[16,19,29,30]. Work on such structures in PDC deposits interpreted them as the result of supercritical flows[31,32,33,34]. Since regressive has been commonly used to describe stossaggrading features in PDC deposits, linking this to flow conditions, rather than temperature and moisture content[21,35,36,37]
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