Page 88 - Caldera Volcanism Analysis, Modelling and Response

Pyroclastic Density Currents 63

particles that pass from the flow-boundary zone to the deposit defines the R d . This
implies that the deposition processes can be inferred on the basis of deposit
characteristics.

3. Sedimentology: Main Particle Support
and Segregation Mechanisms in PDCs

PDCs are intrinsically heterogeneous media that comprise a fluid and a solid
phase. Several factors can influence the heterogeneity of PDCs and the transport of
solid particles, including: (i) variability at the source in time and space (e.g. velocity
fluctuation, concentration, etc.), due to non-homogeneous injections of pyroclastic
material into the atmosphere from the vent; (ii) segregation processes within the
current (e.g. due to development of density stratification); (iii) different
characteristics of the upper and lower flow-boundary zones, due to processes of
elutriation, entrapment and turbulence; (iv) ingestion of air; (v) influence of
topography; (vi) sedimentation and/or erosional processes that induce changes in
the physical properties of PDCs and substrate in time and space (Branney and
Kokelaar, 2002).
The fluid phase supports solid particles through different mechanisms, which
are usually: suspension due to fluid turbulence, intermittent saltation, traction
(sliding and rolling) and fluid support (fluidisation). Additional mechanisms of
particle support are particle–particle collisions and matrix support. The combina-
tion of different mechanisms acting on a particle changes in time and space, as it
experiences different concentrations and shear intensities during settling. Support
mechanisms influence the segregation processes and determine the settling velocity
of a particle.
Segregation processes induce density stratification within PDCs and determine
their depositional characteristics and subsequently, their lithofacies. Segregation
processes can occur within the whole PDC, at the flow boundary and/or within the
deposit (e.g. loading processes that induce sinking of large particles and/or
elutriation pipes).

3.1. Support due to fluid turbulence
Fluid turbulence acts as particle support mechanism through fluid drag and lift
forces (Figure 3a). If the concentration of particles in a PDC is lower than a few
volume percent, the particle interactions are negligible and turbulence dominates
the support mechanisms.
Clasts within a fully turbulent PDC segregate vertically. This implies that
clasts with different hydraulic properties occupy three partially overlapping levels
within the current, according to their main transport/support mechanism:
suspension, intermittent suspension and traction (Middleton and Southard, 1984;
Figure 3). Some dimensionless numbers and criterions describe the physical
conditions necessary to maintain suspension. In particular, the Rouse number