Page 112 - Caldera Volcanism Analysis, Modelling and Response

Pyroclastic Density Currents 87

Figure 17 Examples of lithofacies associations in PDC deposits of Pollena (left) and Avellino
(right) eruptions of Somma-Vesuvius (Italy).

Many deposits from diluted PDCs show a chaotic alternation of coarse-grained
lenses (lapilli) and coarse to fine ash (Figure 18). This lithofacies architecture has
been interpreted as the result of deposition by two different portions of the various
pulses that constitute the entire PDC (Sulpizio et al., 2007). In this model, the
coarse-grained lenses represent the basal part of the pulse, dominated by grain
interaction. The lenses can exhibit inverse grading and imbrication of larger clasts.
The stratified ash represents the tail of the pulse, less concentrated and richer in
fine-grained material. In this case, the thickness of each pulse is only a few
centimetres, and the turbulence that dominates the upper part of the current can, in
some cases, penetrate the deposit and truncate the sedimentary structures (Figure 7f).
The extreme case of interaction between deposition processes and turbulence is
shown in Figure 7h and i, in which the current is fully turbulent and the low
concentration of material does not allow the development of any granular flow-
dominated layer. In these cases, deposition occurs grain by grain with development
of dune-bedding and internal cross-stratification (lithofacies xsA; Figure 18e). Fluid
drag can induce traction on the material previously deposited and cause imbrication
of clasts in the upper part of the deposit (lithofacies lensL (i) ; Figure 18e).

6. Conclusive Remarks and Future Perspectives

Research on PDC behaviour is one of the main topics in present day
volcanology, and involves disciplines ranging from sedimentology to geophysics to
laboratory experiments to numerical simulations. The vast interest is justified by the
complex nature of these currents and by their very dangerous nature that threaten
millions of people around the world. In recent years, significant goals have been
reached in understanding the physics of PDCs. Perhaps the most important