Page 106 - Caldera Volcanism Analysis, Modelling and Response

Pyroclastic Density Currents 81

Figure 12 Scheme of possible interactions among a £uid-like current and obstacles with
di¡erent heights. (a) Diagram of Froude number vs. ratio between obstacle (h b ) and £ow
heights (h f ) (after Branney and Kokelaar, 2002; modi¢ed); (b) interaction between a
density-strati¢ed £ow with ridges. Only the part above the dividing streamline is able to pass
over the obstacle. If diverted, the basal part can pass over the obstacle through saddles or at the
end of the barrier (afterValentine,1987; modi¢ed).

An example of interaction between a radial, expanding PDC with a
vertical barrier has been recently recognised in the Avellino eruption of Somma-
Vesuvius (Sulpizio et al., 2008a; Figure 13). This eruption was characterised
by extensive generation of PDCs during the final phreatomagmatic phase,
whose deposits show large lithofacies variability from proximal to distal sites. In
particular, massive, accretionary lapilli-bearing beds crop out on eastern and
southern slopes of Somma-Vesuvius volcano (Figure 13), in a stratigraphic position
that corresponds to the dune-bedded, thick PDC deposits of the western
and north-western sectors. This dramatic change in lithofacies was caused by
the presence of the morphologic obstacle of the Mount Somma caldera wall,
which blocked the basal, coarse-grained part of the currents and caused the
stripping of the upper, finer grained, more diluted flows that gently settled beyond
the obstacle.