Abstract Recent structural analysis of the Grey’s Landing ignimbrite offers new insights into the emplacement of rheomorphic ignimbrites. We present several key localities, where volcanological and structural features reveal the emplacement history of a lavalike ignimbrite and the evolution of ductile deformation structures during and after deposition across complex topography. Excellent three-dimensional exposure allows us to interpret structural features of the Grey’s Landing ignimbrite in the context of diverse emplacement models for rheomorphic ignimbrites elsewhere and to consider field criteria to distinguish between lava-like ignimbrites and extensive silicic lavas .

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).