Abstract Pyroclastic density currents are inhomogeneous mixtures of volcanic particles and gas that flow according to their density relative to the surrounding fluid (generally the atmosphere) and due to Earth's gravity. They can originate by fountain-like collapse of parts of an eruption column following explosive disintegration of magma and rock in a volcanic conduit, or from laterally inclined blasts, or from hot avalanches derived from lava domes. They can transport large volumes of hot debris rapidly for many kilometres across the ground and they constitute a lethal and destructive volcanic hazard. Ground-hugging pyroclastic density currents produce a buoyant counterpart, known as a phoenix cloud or co-ignimbrite ash plume, which can carry ash and aerosols into the stratosphere and so cause significant climatic perturbation. Most processes within pyroclastic density currents are impossible to observe and so are commonly inferred from the associated deposits.

Abstract In this chapter we consider the initiation and transport behaviour of pyroclastic density currents that deposit ignimbrites. We deal with a wide range of phenomena and assess the limitations in present understanding. Some limitations considered in this chapter and the next lie in the possible differences between pyroclastic currents, which are gas-particle systems, and aqueous analogue experiments from which some understanding has been gleaned. Air has a substantially lower viscosity and density than (liquid) water, is far more compressible and shows far greater thermal expansion. Therefore flow rheologies and processes, like particle settling and sorting in pyroclastic currents, are likely to differ quantitatively from those in aqueous currents, and there may also be some more fundamental differences in behaviour, such as in fluidization, the development and propagation of shock waves and thermal effects, and in the agglomeration (clustering) behaviour of fine ash particles.

Abstract In this chapter, we consider the various mechanisms of clast support, and the associated clast-segregation effects, that are relevant to pyroclastic density currents.

Abstract In this chapter we present ways of conceptualizing ignimbrite deposition. We explore possible types of deposition and what processes may influence them.

Abstract This chapter presents an approach for ignimbrite description and interpretation. It draws on field, granulometric and fabric data from published descriptions of ignimbrites. To describe ignimbrites, we adopt a non-genetic lithofacies scheme (Table 5.1). This avoids possible connotations of ‘ideal’ sequences or of specific emplacement models (as in the previous schemes of 'Layers 1, 2a, 2b', 'ignimbrite types 1-3', ‘ground layer’ and ‘basal layer’). We describe some of the more common lithofacies in ignimbrites. Our list is not intended to be prescriptive, and it is to be expected that workers will in the future modify or subdivide our groupings. We then show how the lithofacies might be interpreted in terms of flow-boundary zone processes. Understanding is far from complete, and in some cases we give possible alternative interpretations that require testing (also see summary on Table 7.1, p. 120). Consideration of lithofacies that record sedimentary reworking (e.g. by wind or water) is beyond the scope of this work.

Abstract This chapter explores how ignimbrite architectures and lithofacies associations can be used to infer how flow-boundary zones of pyroclastic density currents vary with time, downcurrent, laterally and with topography. The approach hopefully will be developed further so deposits can be used more precisely to constrain the dynamics of pyroclastic density currents.

Abstract The vast extent of many ignimbrites shows that eruptions have occurred on almost unimaginable scales, well beyond any modern human experience. Evidence is emerging that plumes derived from large pyroclastic currents have impacted climate and biota on a global scale, whilst certain types of ignimbrites (e.g. extensive rheomorphic ignimbrites) indicate particularly awesome styles of eruption and emplacement that are regionally devastating and which we do not fully comprehend. Such unimaginable eruptions are bound to occur again. If we are to interpret such catastrophic events correctly, and possibly even anticipate the impact of future occurrences, it is essential that ignimbrite sheet architectures are studied further in order to understand the mechanisms, rates and durations of the fundamental processes. Of particular importance in risk mitigation will be the understanding of early stages of such devastating eruptions. The new approaches and descriptive schemes presented in this Memoir are intended to stimulate and facilitate such future work.

Abstract accumulative . A downstream increase in a current parameter, such as velocity, concentration or capacity (see p. 2). for example, accumulative velocity is downstream spatial acceleration. Term coined by Kneller & Branney (1995).

Abstract Pyoclastic density currents are awesome volcanic phenomena that can wreak destruction on a regional scale and can impact global climate. They deposit ignimbrites, which include vast impact lansdscape-modifying sheets with volumes exceeding 1000 km 3 .This book takes stock of our understanding of pyroclastic density currents and presents a new conceptual framework for investigating how ignimbrites are deposited. It integrates the results of field-based studies, laboratory experiments and numerical modelling, including work on clastic sedimentologym and industrial particle transport. Topics covered include the behaviour or particulate currents, mechanisms of clast support and segregation, interpreting ignimbrite lithofacies and architectures, and future research directions. The new approach focuses on processes and conditions within the lower flow-boundary zone of currents. Superb diagrams explain many new concepts, while the 95 photographs make an explanatiry atlas of deposit types. This is essential reading for workers investigating volcanic hazards, and for anyone wishing to interpret modern or ancient ignimbrites, as well as other catastrophically emplaced sediments. “Given the depth of scholarship that they have brought to the subject, the power of their arguments, and the degree of synthesis with other fields, this would seemto qualify as a seminal work… I think that this will be the paper on the topic that others will have to contend with for many years to come.” Marcus Bursik, State University of New York