Page 91 - Caldera Volcanism Analysis, Modelling and Response

66 Roberto Sulpizio and Pierfrancesco Dellino

larger clasts (traction carpet; Sohn, 1997; Dellino et al., 2004; Figure 3b). Elongated
clasts usually exhibit a preferential orientation of long axis perpendicular to the flow
motion.
The proposed division of three partially overlapping layers is useful for a
comprehensive description of main support mechanisms that act within a fully
turbulent PDC. However, it is an oversimplification of reality because a particle can
experience different support mechanisms during its down-current movement, or
the support mechanisms can change according to current unsteadiness. Regardless,
the progressive density stratification of turbulent PDCs with time increases the
importance of particle-interaction processes, and may limit or suppress the effects of
turbulence in the lower part of the current.

3.2. Fluid-escape regime and matrix support
Gas fluidisation has long been considered one of the main particle support
mechanisms in PDCs. It is frequently invoked to explain both the mobility of
concentrated PDCs and some characteristics of their deposits, such as gradation,
sorting and the presence of elutriation pipes.
Several fluidisation mechanisms have been hypothesised to act in a moving
PDC, and exhaustive reviews of these mechanisms can be easily found in the
literature (Sparks, 1976, 1978; Wilson, 1980; Carey, 1991; Branney and Kokelaar,
2002). The escaping fluid supplies the clast support, but only for particles with
terminal velocity comparable or lower than the force exerted by upward fluid
movement (e.g. Roche et al., 2004). The particles with lower terminal velocity are
elutriated, whereas those with higher terminal velocity should sink towards the base
(Figure 4). Sinking processes occur only if the density of the particle is greater than
the fluid density in which it is immersed. Since PDCs are usually density stratified,
the larger clasts can be supported by the lower part of the current (a process also
known as matrix support). PDCs can only be partially fluidised due to the wide
grain-size spectra present in typical volcanic eruptions.
Fluid-escape processes and matrix support can be important mechanisms in
highly concentrated PDCs. In these cases, the sedimentation and deposition of
particles induces fluid expulsion from the flow-boundary zone, and the upward
motion of escaping fluid delays the settling of particles toward the deposition
zone. At the same time, part of the fluid remains entrapped in the flow-boundary
zone. The amount of this fluid depends mainly on the porosity of particulate
and on rate of new gas released by breakage or degassing of juvenile material
(Figure 4). This type of fluidisation is probably the most effective and most
common in PDCs that contain a broad range of particles, and does not require the
occurrence of complicated external mechanisms that continuously induce
fluidisation. The pressure drop with height induced by fluidisation has been
demonstrated to be directly proportional to the escaping gas velocity and inversely
proportional to the porosity of the particulate through the gas flows (Ergun
equation; Roche et al., 2004). This implies that fines-rich grain sizes (low porosity)
and/or large amounts of gas content/generation (increasing pore fluid pressure
and/or escaping gas velocity) within a PDC are favourable conditions for