Page 105 - Caldera Volcanism Analysis, Modelling and Response

80 Roberto Sulpizio and Pierfrancesco Dellino

Figure 11 Variation of lithofacies of PDC deposits from Upper Pollara eruption (Salina Island,
Aeolian Archipelago, Italy) in response to changes in topography. (a) Morphological pro¢le
(red line) from the Pollara depression (vent area) to the Malfa terrace.The black lines indicate
approximately the topography before the eruption; (b) massive, coarse-grained deposits on the
inner slopes of Pollara depression; (c) di¡usely strati¢ed deposit on the outer slopes of Pollara
depression; (d) massive, ¢ne-grained, lenticular deposits in the Malfa terrace area.The white
lines indicate di¡erent eruptive units, while the dashed lines indicate di¡erent depositional
units within a single eruptive unit.

4.3.3. Vertical barrier
The occurrence in which a PDC encounters a vertical barrier is the extreme case of
a flow encountering an increase in slope. The effects of a vertical topographic
barrier can vary in function of: (i) physical properties of the PDC (e.g. density
stratification, height); (ii) height of the obstacle; (iii) angle of incidence; and
(iv) topography of the surroundings.
Physical properties are important because they define how a current behaves
as a fluid or as a granular mass of solid particles. If the current behaves as a fluid
(e.g. fully diluted PDCs) we can imagine different mechanisms of interaction with
a vertical obstacle, with effects on flow propagation and deposition that depend
mainly on height of the flow (h f ), height of the obstacle (h b ) and Froude number
(Rottman et al., 1985; Branney and Kokelaar, 2002; Figure 12a).
In the case of stratified currents, only the part of the flow above a certain critical
height, known as the dividing streamline (Sheppard, 1956; Snyder et al., 1985;
Valentine, 1987; Figure 12b), is able to overcome the obstacle. In this case, the
denser levels of the current are blocked or diverted, depending on the incidence
angle with respect to the obstacle. The diverted current can flow along the obstacle
and pass through saddles or valleys (Figure 12b).
The higher, less-dense levels of the current are less constrained by topography
and can pass over the obstacle. The separation of the basal, concentrated flow and
the upper less dense one is known as flow-stripping (Piper and Normark, 1983).
The stripped upper part of PDCs caused the most devastation and fatalities in the
1902 eruption of Mt. Pele `e(Fisher and Heiken, 1982), in 1991 at Unzen volcano
(Yamamoto et al., 1993) and in 1997 at Montserrat (Loughlin et al., 2002).