Page 87 - Caldera Volcanism Analysis, Modelling and Response

62 Roberto Sulpizio and Pierfrancesco Dellino

layer. Massive deposits result from highly concentrated mixtures, pyroclastic flows,
in which particle–particle interactions dominate the pyroclast motion.
Recent works (e.g. Burgissier and Bergantz, 2002; Branney and Kokelaar, 2002;
Sulpizio et al., 2007) have demonstrated how these two categories are just the end
members of a continuous spectrum of PDCs, whose sedimentological character-
istics mainly depend on the interplay of particle concentration, shear rate and
depositional rate over time and space. Hereafter, we will discard the terms
pyroclastic flow and pyroclastic surge as deposit characteristics. We will refer instead
to the different typologies of PDCs using the terms diluted or concentrated, with
sedimentological information supplied by lithofacies codes and architecture.

2.3. Steady vs. unsteady currents

A given PDC can experience variations of physical parameters (velocity,
concentration, mass flux, etc.) with time at a fixed location. A current can be
defined as steady with respect to a given parameter if that parameter does not vary
with time at a fixed location. If the parameter fluctuates with time, the current is
unsteady (Kneller and Branney, 1995). This means that we observe the current in an
Eulerian reference frame and we can define the current as time variant or time
invariant.
There are three main types of unsteadiness: waxing is when a given parameter at
a fixed location increases with time, waning is when a given parameter at a fixed
location decreases with time, and quasi-steady is when a parameter fluctuates only
slightly around some average value (Branney and Kokelaar, 2002).
It is important to note that the concept of steadiness and unsteadiness applies
to parameters that characterise the whole current. In comparison to behaviour of
particles within the PDC as a whole, particles at the point of observation can
exhibit contrasting behaviour. For example, even for waxing and steady currents,
a particle can experience a decrease in the considered parameter. If we consider
the velocity, a particle can decelerate and settle down the current thickness in
waxing, waning and steady flows, depending on the support mechanisms that act
within the flow and their spatial distribution. This means that using a Lagrangian
reference frame we can describe the variation of parameters for a single particle
within the PDC.

2.4. Sedimentation vs. deposition

The terms sedimentation and deposition are sometimes confused and improperly
used in describing the processes that influence PDC behaviour. Therefore, it is
important to clarify their relationships here. The term sedimentation describes all
processes that accompany the motion of a particle throughout the current thickness
down to the flow-boundary zone. The mass flux of particles that settle across
the upper part of the flow-boundary zone define the R s . This implies that only
hypotheses on the sedimentation processes can be made based on deposit
characteristics. The term deposition describes all processes that affect a particle in
the flow-boundary zone until the definitive rest in the deposit. The mass flux of