Page 93 - Caldera Volcanism Analysis, Modelling and Response

68 Roberto Sulpizio and Pierfrancesco Dellino

with respect to the matrix (e.g. normal grading of lithic clasts and reverse grading of
pumice).
Another mechanism that can support clasts and promote segregation in fines-rich
concentrated PDCs is particle cohesion. Cohesion is a force characteristic of moist clay
that aggregates and imparts a yield strength (or matrix strength) to the particulate. The
amount of clay controls the strength, defined as the ability of a given material to absorb
a finite amount of stress without deformation. Cohesion is often a characteristic of
volcaniclastic flows, especially of those generated by erosion or failure of weathered,
soil-rich and/or hydrothermally altered volcanic terrains. Cohesion effects in PDCs
are usually negligible, although some ignimbrite characteristics have been interpreted
in terms of current segregation induced by current yield strength (Wright and Walker,
1981; Freundt and Schmincke, 1986; Carey, 1991). However, PDCs are unlikely to
contain moist clay aggregates because they are usually hot and do not contain large
amounts of very fine particles (o2 mm). An exception could be represented by low-
temperature, fines-rich phreatomagmatic PDCs, in which the presence of water could
induce sticky effects among particles, promoting agglutination and subsequently
increasing the yield strength of the particulate.

3.3. Support due to particle interactions
The granular flow theory (Campbell, 1990; Iverson and Vallance, 2001)isimportant
in understanding some important sedimentological characteristics of PDCs. In a rapid,
gravity-driven, shearing mass of sediment, the repulsion force originated by particle–
particle collisions causes their motion in each direction, irrespective of the mean
shear flow direction of the whole PDC. Clast vibration can be considered as an
analogue for thermal motion of molecules in the kinetic theory of gases, and is
known as granular temperature (Savage, 1983, 1984; Iverson, 1997). Similar to the
thermodynamic temperature, the granular temperature generates pressure and governs
the transfer of mass and momentum. However, the granular temperature cannot auto-
sustain because particle collisions are inelastic and mechanical energy dissipates as
thermodynamic heat. This implies that the granular temperature is maintained by
conversion of kinetic energy to mechanical energy during the motion of the granular
mass. The granular temperature varies as function of the square of the shear rate of the
granular mass (Campbell and Brennen, 1985) and then is influenced by the slope
angle or by the shear stress exerted by the more diluted, upper part of the flow.
The pressure associated with the granular temperature is called dispersive
pressure (Bagnold, 1954), and causes inflation of the whole granular mass. At high
granular temperatures, the dispersive pressure can maintain a granular mass in a
liquefied state, while the inflation of the granular mass promotes segregation
processes due to kinetic sieving. Kinetic sieving promotes the migration of small
particles toward the base of the current because smaller particles can easily fall in the
intergranular voids among larger particles (Figure 5). This induces an apparent
migration of the coarser clasts towards the top of the flow-boundary zone, a process
also promoted by the kinematic squeezing undergone by larger clasts because of
their more frequent collisions when compared to smaller clasts (Savage and Lun,
1988; Sohn and Chough, 1993; Le Roux, 2003; Figure 5).