Sedimentary records of 
changes between granular-fluid based and fully dilute pyroclastic density 
currents.Michael J. Branney (University of Leicester) and Peter Kokelaar 
(Liverpool University).Pyroclastic density currents have been modelled 
variously as grainflows, laminar semi-fluidised dispersions, and 
low-concentration turbulent suspensions.  However, they are inherently 
inhomogeneous, polydisperse, spatially and temporally variable, and exhibit 
a wide range of particle concentrations.  Simultaneously operating 
clast-support mechanisms (aerodynamic lift, traction, saltation, 
hindered-sedimentation, fluidization, granular interactions, clast 
buoyancy and interlocking) each have associated particle segregation 
effects.  The relative importance of each support and segregation mechanism 
varies with clast type, clast-concentration, shear rate, and deposition rate 
(e.g. effects of granular temperature may decrease with height though a density
stratified current).A major challenge is to use deposit characteristics 
(ignimbrite lithofacies) to infer flow conditions, so that vertical 
successions of lithofacies in the field can be used to constrain temporal 
changes in currents, and lateral variations can be used to constrain how 
flow properties vary spatially within a current.  Deposit lithofacies are 
created within a flow-boundary zone, which spans the basal part of the 
current and the upper part of the deposit.  Downcurrent and lateral 
variations in deposit character, e.g. between stratified and massive 
lithofacies, indicate that the nature of the current flow-boundary 
zone varies with location.  We explore what may control such 
variations in sustained currents.