Page 83 - Caldera Volcanism Analysis, Modelling and Response

58 Roberto Sulpizio and Pierfrancesco Dellino

pyroclastic density currents. The main mechanisms of particle support are critically
reviewed and their influence on depositional mechanisms is discussed. The depositional
behaviour of pyroclastic density currents is discussed in the light of new models that
consider the different typologies within a continuum spectrum that spans from very
dilute (fluid-dominated) to very concentrated (solid-dominated) flows. The combination
of progressive aggradation and en masse freezing models is proposed on the basis of a
model that takes into account the stepwise aggradation of discrete pulses that develop
within a single current and that stop en masse when resisting bulk forces exceed driving
ones. The influence of various types of morphological settings and obstacles on
depositional mechanisms and mobility of pyroclastic density currents is examined and
discussed in detail. Some examples of stepwise aggradation within pulsating PDCs
are discussed using analyses of lithofacies and lithofacies associations of deposits
emplaced under different flow-boundary conditions.

1. Introduction: What are Pyroclastic Density
Currents?

Pyroclastic density currents (PDCs) are among the most amazing, complex
and dangerous volcanic phenomena. They are moving mixtures of particles and gas
that flow across the ground, and originate in different ways and from various
sources, during explosive eruptions or gravity-driven collapse of domes. They may
be short-lived (highly unsteady) or relatively long-lived (sustained unsteady to
quasi-steady) phenomena, driven by both magmatic or phreatomagmatic melt
fragmentation (e.g., Cas and Wright, 1987; Carey, 1991; Branney and Kokelaar,
2002; Figure 1).
From a fluid dynamic point of view, PDCs macroscopically behave as dense
fluids (the pyroclastic mixture) immersed in a less dense, almost isothropic one (the
atmosphere). They mainly move under the effect of gravity (e.g., Burgissier and
Bergantz, 2002) and their mobility (distance travelled vs. difference in height from
source and deposit) is greatly controlled by mass and height of generation (potential
energy) and efficiency of conversion from potential to kinetic energy (i.e. loss of
momentum due to frictional processes both within the current and at current
edges). Because mass and frictional processes mainly relate to solid particles, the
particle concentration in a PDC is crucial in determining the physical parameters
of the moving flow (i.e. velocity, density, clast-support mechanism; Sparks, 1976;
Middleton and Neal, 1989; Bonnecaze et al., 1993; Hallworth and Huppert, 1998;
Branney and Kokelaar, 2002; Cao et al., 2003; Sulpizio et al., 2007). In currents
that initiate explosively, the clast concentration of the pyroclastic mixture relates to
the eruption style (collapse of a pyroclastic fountain or radial expansion of an over-
pressurised jet), the volatile content of the erupting mixture, the mechanical energy
released at fragmentation and the abundance of accidental fragments (Sparks et al.,
1997a; Dingwell, 1998; Alidibirov and Dingwell, 2000). In currents generated by
gravitational failures (e.g., dome collapses), the clast concentration in the mixture
relates to the amount of gas released during failure and by block breakage during
transport.