Settling behaviour of concentrated dispersions and some volcanological applications

Abstract

Experiments have been carried out on the settling of non-aggregating, poorly sorted aqueous dispersions laced with lowdensity particles in order to gain insight into the sedimentation behaviour of volcaniclastic flows which contain pumice. Two poorly sorted dispersions of silicon carbide particles with continuous size distributions were studied; plastic beads were added as a light component. Experiments were carried out at solids concentrations between 10 and 60 vol%. Dispersions of low concentration generated normally graded deposits in which particles of different densities were in approximate hydraulic equivalence. Above a critical concentration (45 to 60% depending on the grain-size distribution) particle segregation was suppressed, and the dispersion settled to form a virtually ungraded bed. Hindered settling at intermediate concentrations generated normally graded deposits with elutriation pipes, “floating” rafts of plastic, and capping layers of fines elutriated by escaping pore fluid. Buoyancy forces resulted in strong hydraulic inequivalence between light and dense components in the deposits from intermediate concentrations. Two mechanisms of elutriation-pipe growth were observed. At intermediate concentrations, pipes grew progressively upwards in the deposit as the latter aggraded, whereas at high concentrations they nucleated and grew uniformly throughout the whole dispersion. Elutriation pipes form readily during sedimentation of some poorly sorted aqueous dispersions; the presence of elutriation pipes cannot be used alone to distinguish between deposits of epiclastic and pyroclastic flows. Reverse grading of pumice, pumice rafts, elutriation pipes, and layers of elutriated fines should characterise deposits from pumice-rich submarine mass flows. Despite differences in the fluid phase, the experiments also have implications for fluidization mechanisms in pyroclastic flows. Hindered settling of a concentrated, poorly sorted dispersion is a type of self-fluidization in which the sedimentation of large particles causes a sufficiently strong vertical fluid flux to fluidize or elutriate smaller ones. Hindered settling is intrinsic to all pyroclastic flows as they spread out and deflate across the landscape, and may reinforce other gas sources as an effective fluidization mechanism during emplacement of type 2 and type 3 ignimbrites and of coignimbrite lag breccias.