Page 104 - Caldera Volcanism Analysis, Modelling and Response

Pyroclastic Density Currents 79

Because momentum decreases due to deceleration on a slope also reduce
turbulence generation, a fully turbulent, homogeneous flow may develop density
stratification. Depending on the balance achieved between driving and resisting
forces, the fate of the basal, more concentrated part of the flow, may be different
than that of its diluted upper part. Increasing concentration of particles at the base
of the flow may result in enhanced deposition and loss of mass from the current,
with feedback effects on the momentum equation (Legros and Kelfoun, 2000):

ndn 2 ðdm=mÞ
M f ¼ þ v (12)
dx dx

where m is the initial mass. The deposited material may either rest on the slope or
start to flow backward, depending whether the Coulomb equilibrium is exceeded
or not (neglecting cohesion):

mg sin y4½ðmg cos yÞ dŠtgf (13)

where y is the slope angle, d the pore fluid pressure and f the internal friction angle
of the material.
Because the rate of material delivered to the flow-boundary zone depends on
the rate of supply (R s ), the upper part of the current loses mass at a rate proportional
to dm/dt. At the same time, the flow decelerates on a slope at a rate proportional to
dv/dt. Therefore, the upper part of the flow may stop on a slope if R s drives the
bulk density of the flow below that of the surrounding atmosphere in a time less
than that necessary for the flow to overcome the length of the slope. If the time is
not enough, the upper part of the flow can overcome the slope to form a new flow
with different physical characteristics than the previous one, driven by residual
kinetic energy and potential energy transformation. An example of this behaviour is
represented by PDCs originated during the Upper Pollara eruption (13 ka;
Calanchi et al., 1993; De Rosa et al., 2002) at Salina Island (southern Italy; Sulpizio
et al., 2008b; Figure 11). The eruption was characterised by several Vulcanian
explosions that generated small-volume PDCs, which propagated in the Pollara
depression, overcame the rim and finally stopped beyond the break in slope
between the outer slopes and the Malfa terrace (Figure 11a). On the inner slopes of
the depression, coarse-grained, poorly sorted, massive deposits characterise the
entire stratigraphic succession (Figure 11b), indicative of rapid deposition from a
flow-boundary zone with a high concentration of material. The inner slopes were
not great enough to stop the whole flow, which passed over the depression rim and
flowed along the outer slopes of the depression. In these areas, the deposits
comprise lapilli and ash, with diffuse stratification, indicative of a flow-boundary
zone affected by traction processes (Figure 11c). The deposits are, as a whole, finer
grained than those on the inner slopes. The more distal deposits include fine and
coarse ash with lenses of lapilli (Figure 11d), indicative of a flow-boundary zone
dominated by granular interaction. The deposits thicken close to the break in slope
zone and disappear within 1 km beyond this point (Figure 11a).