Page 95 - Caldera Volcanism Analysis, Modelling and Response

70 Roberto Sulpizio and Pierfrancesco Dellino

although modified grainflows are more common due to the abundance of gas in
pyroclastic mixtures.
In PDCs, dominated by grain interaction, large clasts can bounce downcurrent,
driven by both gravity and their momentum, causing a particular type of clast
saltation called debris-fall (Sohn and Chough, 1993). The debris-fall process, also
known as overpassing, requires slopes very close to or steeper than the repose angle
of falling clasts (Nemec, 1990; Branney and Kokelaar, 2002). The larger clasts can
overpass the smaller ones because: (i) the larger clasts migrate toward the top of the
flow-boundary zone due to collisional forces (kinematic squeezing), where they
experience greater shear intensities; (ii) in debris-fall regimes, the larger clasts have
greater momentum; and (iii) the larger clasts that move along an interface are less
influenced by surface roughness than smaller clasts. In turn, small clasts can easily
be stopped by surface irregularities and/or can fill intergranular voids when they
move over coarse-grained deposits (Branney and Kokelaar, 2002). Laboratory
experiments have demonstrated that for this type of segregation, clast size is more
important than clast density (Drahun and Bridgewater, 1983).

4. Depositional Processes in PDCs

Depositional mechanisms affect all particles in the flow-boundary zone until
they come to a complete stop to form the deposit. Deposition accompanies
transport, and transport may be affected by deposition (Branney and Kokelaar,
2002). Deposition can occur grain by grain or en masse, when shear velocity fall
below the resistance force in large parts of the flow-boundary zone. Deposition can
be a steady, discontinuous or stepwise process. Steady deposition (Branney and
Kokelaar, 1992) is a continuous phenomenon in which the thickness of a deposit
increases at constant rate with time (Figure 6). Stepwise deposition (Branney and
Kokelaar, 1992) defines periods of steady or sharp increase in deposit thickness
alternating with periods of non-deposition (Figure 6). Alternating periods of steady
or stepwise deposition with erosion of deposits characterises discontinuous
deposition, resulting in fluctuation of deposit thickness with time (Figure 6).
Here, we review the main ‘‘historical models’’ and discuss some new ideas about
depositional mechanisms in PDCs.

4.1. En masse freezing vs. progressive aggradation

Historically, two main models of deposition have been proposed for PDCs: en masse
freezing (Sparks, 1976; Wright and Walker, 1981; Carey, 1991) and progressive
aggradation (Fisher, 1966; Branney and Kokelaar, 1992). In en masse freezing, the
flow comes to an abrupt halt over its entire depth, whereas in the progressive
aggradation model, the deposit builds progressively due to the continuous supply of
material from the current through the flow-boundary layer. The debate about the
two models has been vigorous in the past, although it was recognised early that
some internal shear occurs even in avalanche-type flows and that progressive