Page 110 - Caldera Volcanism Analysis, Modelling and Response

Pyroclastic Density Currents 85

Figure 15 Examples of PDC deposits originated by aggradation of di¡erent pulses developed
within the same current. Inverse grading of lithic blocks indicate a £ow-boundary zone
dominated by granular £ow regime. (a, b) Massive deposits of Pollena eruption (Somma-Vesuvius,
Italy). Inverse grading of large lithic blocks indicates the aggradation of three main pulses;
(c) block and ash £ow deposits at Nevado deToluca (Mexico).The white solid lines indicate
di¡erent eruptive units, while dashed lines indicate depositional units within the same current;
(d) stacked, massive PDC deposits with inverse grading of large lithic blocks atTungurahua
volcano (Ecuador); (e, f ) sedimentological scheme that illustrates the development of di¡erent
pulses within a current and their stepwise aggradation.

A more convincing interpretation of this lithofacies architecture is offered by
stepwise aggradation of different pulses developed within a PDC (see Section 4.2;
Sulpizio et al., 2007). This model explains the repetitive occurrence of lithofacies
mLA (il) and mLB (il) (Figure 15) as the progressive deposition of different pulses
developed within a single PDC (Figure 15e). These pulses are dominated by grain
interaction (granular flow regime), with the processes of kinetic sieving and kinematic
squeezing causing an increased concentration of larger blocks toward the top of each
single pulse. In this model, each pulse stops en masse when the driving force falls