|
Significance of current heterogeneity and pyroclast diversity
Ignimbrite-forming pyroclastic density currents have been modelled as grainflows (Denlinger 1987; Dobran et al. 1993; Straub 1994), semi-fluidized beds (Sparks 1976; Wilson 1980, 1984), low-concentration turbulent suspensions (Valentine 1987; Dade & Huppert 1996) and laminar plug flows (Wright & Walker 1981; Battaglia 1993). However, although simplification is necessary in modelling, assumption of just one clast-support mechanism or of notional bulk current properties is not appropriate for the consideration of sedimentation from a pyroclastic density current. This is because pyroclastic density currents are inherently heterogeneous and involve a range of clast-support mechanisms (Table 3.1). The heterogeneity that significantly affects clast transport and sedimentation can result from: (1) source variability (e.g. fluctuations in velocity, concentration and rheology) with time and with location, according to the nature of any unsteady or inhomogeneous vent discharge; (2) segregation within the current (Fig. 3.1), for example due to development of density stratification and stratification of support mechanisms (e.g. see Fig. 3.3) or of clusters and concentrations with a different rheology to that of the surrounding current (e.g. pumice rafts); (3) contrasting behaviour and properties of upper and lower flow-boundary zones (e.g. involving sedimentation, elutriation, entrainment and turbulence); (4) unsteady and non-uniform air ingestion and expansion; (5) effects of topography; and (6) sedimentation and/or erosion-induced changes in properties of the current and substrate with time or location.
Clasts in a current are supported by
...
This 250-word extract was created in the absence of an abstract.
