A Confidence-Based Assessment Method for Distinguishing Pyroclastic Density Current Deposits From Other Volcaniclastic Units

James Cowlyn,Darren M Gravley,Shane J Cronin,Ben M Kennedy

doi:10.3389/feart.2020.581195

Abstract

Pyroclastic density currents (PDCs) are a destructive volcanic hazard. Quantifying the types, frequency and magnitudes of PDC events is essential for effective risk management, but since historical records at best extend a few hundred years this usually relies on identifying deposits in the geological record. However, small volume unconsolidated PDC deposits have low preservation potential and can be difficult to distinguish from other volcaniclastic units, especially in proximal locations. Consequently many small or poorly exposed deposits can be overlooked. Here, we introduce a structured field method for assessing volcaniclastic deposits of unknown origin with a particular focus on identifying deposits from concentrated PDCs (pyroclastic flows). The method differs from traditional identification schemes in that it does not uniquely attribute a deposit to a single depositional process, but instead assesses how confidently different volcaniclastic processes could explain the observed deposit features. Therefore, the underlying uncertainties in the assessment are explicitly addressed. The method allows consistent, rapid assessment of candidate pyroclastic flow deposits in the field, and the concept could easily be adapted for assessing other types of volcaniclastic deposit. The introduction of confidence levels in deposit interpretations should be useful for carrying though uncertainties into probabilistic assessments of volcanic hazards.

Highlights

A fundamental principle in volcanology is that a volcano’s past behavior provides insight into the kind of activity that can be expected in the future
The system differs from identification tables that typically list “ideal” deposit features, as these do not address the significant variability of natural systems or acknowledge the resulting interpretative uncertainty
The assessment scheme developed here provides a new method for rapidly and consistently assessing volcaniclastic deposits to determine if they could have been emplaced by a pyroclastic flow

Summary

Introduction

A fundamental principle in volcanology is that a volcano’s past behavior provides insight into the kind of activity that can be expected in the future. Quantifying the distributions, types, frequencies and magnitudes of previous hazards is an important component of volcanic hazard management (Nakada, 2000; Leonard et al, 2014). Since historical records only date back at most a few hundred years, this normally relies on identifying deposits in the geological record. It can be difficult to identify formative processes from small or poorly preserved deposits that lack distinguishing features, and absence of evidence is not always evidence of absence (White and Valentine, 2016). Properly assessing even small or poorly preserved deposits, and acknowledging any interpretative uncertainty, is essential for thoroughly understanding a volcano’s past activity

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Conclusion