Geological Society, London, Memoirs
Geological Society, London, Memoirs; 2003; v. 27;
p. 7-21;
DOI: 10.1144/GSL.MEM.2003.027.01.02
© 2003 Geological Society of London
Chapter 2 The origin, nature and behavior of pyroclastic density currents
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.
Origin and development of pyroclastic density currents
Eruption styles
Pyroclastic density currents originate in different ways and from various sources (Fig. 2.1). They may be short-lived (highly unsteady) phenomena (Fig. 2.1 A, D and E) or relatively long-lived (sustained unsteady to quasi-steady; Fig. 2.IB and C). In currents that initiate explosively, the clast concentration of the erupting dispersion relates to the eruption style (e.g. high or low pyroclastic fountaining), which is a function of (1) the magma rheology and mass flux, (2) the volatile abundances, species and exsolution rates, (3) the size, abundance and timing of formation of vesicles and cracks at the time of fragmentation, and (4) the size and abundance of accidental
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