Page 96 - Caldera Volcanism Analysis, Modelling and Response

Pyroclastic Density Currents 71

Figure 6 Diagram showing the variation in thickness of deposit with time for steady, stepwise
and discontinuous deposition.

aggradation model is less consistent with small volume PDCs. Indeed, different
geological evidence supports both models. The existence of steep leve ´es and lobate
fronts in PDC deposits (e.g. Rowley et al., 1981; Miyabuchi, 1999; Iverson and
Vallance, 2001) has been interpreted as indicative of flows with Bingham rheology
and internal yield strength (Johnson, 1970; Sparks, 1976). However, the rheological
approach to granular mass-flow behaviour has been recently questioned (Iverson
and Vallance, 2001; Iverson and Denlinger, 2001; Iverson, 2003), and the
characteristics of these deposits have been attributed to the contrasting effects of
pore-fluid pressure, mixture agitation (granular temperature) and forces exerted
by grain interlocking at flow borders (Druitt, 1998; Iverson and Vallance, 2001;
Iverson and Denlinger, 2001).
On the other hand, several lines of evidence support the idea that extensive
ignimbrite sheets form by progressive aggradation. Their range as well as their
thickness and lateral grain-size variations are successfully predicted by models that
assume deposition by progressive aggradation from dilute turbulent suspension
(Bursik and Woods, 1996; Dade and Huppert, 1996; Freundt and Bursik, 1998).
Some ignimbrites show vertical variations in chemical composition (Branney and
Kokelaar, 1992, 1997) or in abundance, type or size of lithic clasts (Bryan et al.,
1998), which are thought to reflect temporal changes in the material supplied to the
flow-boundary zone of the flow.
Although the progressive aggradation model is satisfactory in describing large
ignimbrite sheet deposits, it is more difficult to apply to small-volume PDCs
because certain characteristics of their deposits (e.g. alignment of large blocks,
concentration of pumice and lithics at the base or at the top of the deposit) require
fluctuations in physical characteristics of the current (e.g. competence) that are
unlikely to occur at very short time scale.