Page 111 - Caldera Volcanism Analysis, Modelling and Response

86 Roberto Sulpizio and Pierfrancesco Dellino

Figure 16 (a, b) Alternating massive deposits with inverse grading of lithic blocks and ¢ne
ash (Pollena eruption, Somma-Vesuvius); (c -- e) sedimentological model that illustrates the
observed alternation of pictures (a) and (b).

below the resistance force due to friction and grain interlocking. Since different pulses
form within the PDC, the deposition occurs stepwise (Figures 6 and 7).
Figure 16 shows another good example of stepwise aggradation of granular
pulses. In this case, the lithofacies mLA (il) have interbeds of massive ash (lithofacies
mA; Figure 16a and b). When each single pulse approaches the change in slope
between the upper and lower slopes of the volcano, the sharp decrease in velocity
causes loss of some finer materials due to convective lofting (Figure 16c). The
lofting fine material is successively entrapped by the following granular pulse
(Figure 16d). This occurrence preserves the succession of lithofacies mLA (il) -mA
(Figure 16e). Further support for this mechanism is the rapid disappearing of
lithofacies mA a few hundred meters down valley, where the succession includes
only lithofacies mLA (Figure 15b).
Another common occurrence in PDC deposits is the alternation of massive and
stratified lithofacies (Figure 17). This type of lithofacies architecture generally
records different events of deposition, with alternation of diluted PDCs, dominated
by tractive processes, and concentrated PDCs, dominated by fluid-escape or
granular flow regimes. However, this does not necessarily imply different eruptive
mechanisms because traction-dominated deposits can be generated at the front of
highly concentrated PDCs (e.g. Branney and Kokelaar, 2002), which travel in
advance of granular flow- or fluid escape-dominated pulses from the main body and
tail of the PDC.