Page 85 - Caldera Volcanism Analysis, Modelling and Response

60 Roberto Sulpizio and Pierfrancesco Dellino

Regardless of whether they are concentrated or diluted suspensions of gas and
particles, PDCs consist of two essential and intergradational counterparts: an
underflow and a phoenix plume (e.g., Fisher, 1966; Dade and Huppert, 1996;
Baer et al., 1997; Branney and Kokelaar, 2002). The underflow is denser than the
atmosphere and flows in direct contact with the ground. It usually comprises a basal
part dominated by particle–particle interaction overlaid by a turbulent part
dominated by traction processes (also known as ash-cloud surge; e.g. Cas and
Wright, 1987). The phoenix (or coignimbrite plume) is less dense than atmosphere
and lofts convectively (Dobran et al., 1993; Sparks et al., 1997a). Mass partitioning
between the underflow and phoenix changes continuously during motion,
particularly when: (1) a slope change induces sedimentation (concave local
curvature; Denlinger and Iverson, 2001; Macias et al., 1998; Saucedo et al., 2004);
or (2) mixing with air is enhanced (i.e. an accelerating moving mixture, convex
local curvature; Branney and Kokelaar, 2002), or (3) where a change in the substrate
(e.g. topographic jumps, surface roughness, standing water) affects the current
(Fisher, 1990; Carey et al., 1996; Gurioli et al., 2002). The rate and behaviour of
mass partitioning between the underflow and the phoenix plume influence the
runout distance of a current as well as mass flux (e.g. Bursik and Woods, 1996;
Branney and Kokelaar, 2002), density and grain size of pyroclasts (e.g. Taddeucci
and Palladino, 2002) and rate of air entrainment (e.g. Huppert et al., 1986; Woods
and Bursik, 1994; Nield and Woods, 2003).
The complex interplay between sedimentation and depositional mechanisms,
coupled with the difficulty of direct observation of the internal flow organisation,
make the study of PDCs a great challenge for volcanologists. The resulting
literature is extensive, complex and sometimes contradictory. In this work, we will
review and reinterpret some of the most crucial points in the physics of PDCs over
topography. We will also illustrate some examples of PDC behaviour inferred from
their deposits.

2. Key Concepts
2.1. The flow-boundary zone approach

PDC deposits record processes and physical conditions in a generically defined
flow-boundary zone that includes the lowermost part of the current, the flow-
deposit boundary and the uppermost part of the deposit (Branney and Kokelaar,
2002; Figure 2). From a sedimentological point of view, the flow-boundary zone
can be considered that (lower) part of the current where particle–particle
interaction dominates the transport mechanisms and promotes deposition.
It is important to note that the concept of flow-boundary zone (Branney and
Kokelaar, 2002) neither contains limitations on its thickness nor takes into account
the behaviour of the overriding flow. The concept of flow-boundary zone has been
used in describing deposition of both fully turbulent and granular flow dominated
small-scale PDCs (Sulpizio et al., 2007), with the thickness of the flow-boundary
zone ranging between several centimetres and meters. The extreme case of a