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Deadly volcanic flows: understanding pyroclastic density currents

16 March 2016
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Geological Society Events
The Geological Society, Burlington House, London
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Volcanic flows include lava flows, lahars (mudflows) and pyroclastic density currents (PDCs). PDCs are flows of searing hot gas, ash and rocks that swoop down the sides of a volcano during the most violent eruptions. They can travel at speeds of up to 450 mph and can be as hot as 1000°C. Historically, they have been responsible for over 90,000 deaths, and so are the most deadly volcanic phenomenon.

In order to try and reduce the risk to people from these hazardous flows, we strive to understand how they behave. However, they are very difficult to observe. Even small flows are hidden in a cloud of ash. The largest types of these currents however, have not been observed at all, and no instrument exists that can be deployed (and survive) to record them. 

Ignimbrites, the deposits from these currents, can be studied to try and understand what the current might have been like. Volcanologists use sedimentological principles and a conceptual framework to interpret features seen in these deposits. We try to understand if the currents were dense, more like an avalanche, or dilute and mostly composed of hot gas and ash. We try to understand where the currents may have flowed and how.

Recent advances in field studies have helped unravel the internal architecture of ignimbrites, increasing our understanding of these hazardous flows. At the same time, computer models are becoming more powerful and have become an essential tool in hazard assessments.  Recent, rapid advances in analogue modelling are now allowing simulation of these currents and are able to generate deposits in the laboratory. 

In this lecture Rebecca Williams discussed these recent advances and whether we’re any closer to understanding these deadly volcanic flows.


Dr Rebecca Williams (University of Hull)

Dr Rebecca Williams is a Lecturer at the University of Hull. She gained a BSc in Geology from Royal Holloway, a MS in Volcanology from the University at Buffalo and a PhD in volcanology from the University of Leicester in 2010. Rebecca takes an interdisciplinary approach to research, integrating many techniques such as field work, geochemical analyses and computer modelling to understand complex problems ranging from the 3D architecture of volcaniclastics, to understanding magmatic processes, to evaluating the risk faced by communities from hazardous volcanic flows.

Most of Rebecca's work focuses on understanding catastrophic flows related to volcanism, such as pyroclastic density currents and lahars. Rebecca developed the technique of using a detailed chemical stratigraphy to unravel the emplacement history of ignimbrites. Using the Green Tuff Ignimbrite on Pantelleria, the work demonstrated how the pyroclastic density currents waxed and waned, gradually encroaching the landscape and scaling even the largest topographic barriers.

Rebecca is currently involved in developing techniques in the laboratory to simulate catastrophic flows through analogue modelling. The hazardous nature of these events mean that they are not easily studied in the field, and our interpretations of their deposits is often poorly constrained. It is hoped that analogue modelling will help bridge a gap between field studies, theoretical computer simulations and real life events.