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Pyroclastic density currents (PDCs) are density-driven flows of gas, ash and rock generated during explosive volcanic eruptions, or by the collapse of volcanic lava domes. They can reach temperatures of >500 °C and are highly mobile, with the ability to travel at speeds of >200 m/s and to scale obstacles hundreds of metres high. PDCs are a devastating volcanic hazard and have killed >90 000 people since 1600 AD1; understanding their behaviour is an important aspect of volcanic disaster risk reduction. Because of their extreme nature, PDCs are difficult to observe and quantify in real time. Volcanologists therefore use field studies and modelling techniques to investigate their dynamics.As PDCs travel they progressively deposit ignimbrite, a poorly sorted volcanic rock typically rich in pumice and ash. Ignimbrites commonly display a variety of bedforms and stratigraphic architecture; such architecture can be interpreted to unpick the behaviour of the original PDC, and of the evolution of the eruption that created it. However, there is considerable uncertainty involved in ignimbrite bedform interpretation, due to the potential for complexity (such as cryptic bypass, erosion, and hybrid processes) and also due to fundamental gaps in our knowledge of the physical properties of PDCs.Our work uses analogue modelling of gas-fluidised, dense, granular currents in a laboratory flume together with examples from the field to explore what the dynamics of experimental PDCs (and their resultant bedforms) can tell us about our interpretations of real-world volcanic stratigraphy. The examples presented examine a range of bedforms and processes, including the impact of fluidisation and grainsize on PDC mobility and bedform morphology, the influence of moisture on cohesivity and bedform preservation, and the implications of grading in volcanic stratigraphy. The significant challenges in quantifying the sedimentation of ignimbrites are discussed. This work seeks to improve field interpretations of volcanic bedforms and contribute to the development of volcano risk models. 1: Auker et al. (2013) J Appl. Volcanol. https://doi.org/10.1186/2191-5040-2-2 |
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