The origin, nature and behaviour 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.