• Entrainment threshold;
  • pumice;
  • settling tube analysis;
  • settling velocity;
  • Shields criterion;
  • volcaniclastic sedimentation

Abstract Large-scale explosive eruptions from silicic caldera volcanoes can generate huge volumes of pyroclastic material in terrestrial and marine environments. On land, erosion, remobilization and redeposition of this debris is predominantly carried out by running water in the form of precipitation run-off. Conversely, in the submarine realm, both primary emplacement and subsequent remobilization are influenced by the presence of water as a transporting medium. Despite this, and the number of studies devoted to volcaniclastic sedimentation, relatively little attention has been paid to the hydrodynamic behaviour of the particles themselves, which ought to underpin any assessment of transport or depositional process. This is crucial, as many volcanic particles exhibit variable density: according to composition and as functions of differing degrees of vesiculation and the extent to which pore space is filled by water and/or gaseous phases during transport and deposition. Investigation of the physical and hydrodynamic properties of Taupo 1800a pumice, with reference to sedimentary facies developed during the eruption aftermath, shows that, although buoyant when dry, when sufficiently waterlogged, cool pumice clasts will sink and behave more like quartzo-feldspathic material. Saturation is apparently achieved by a combination of rapid capillary flooding of large interconnected vesicles and slower diffusional air–water exchange in smaller pores. Low saturated pumice densities result in lower settling velocities and easier entrainment by tractional currents than those for equivalent-sized quartzo-feldspathic or crystal/lithic particles. Fine-grained pumice is conversely harder to entrain because of the frictional interlocking of angular particles. These unusual properties of temporary buoyancy, variable saturation, low density and size-dependent cohesion complicate interpretations of the depositional setting and energy of pumiceous sediments and give rise to several unique facies. These findings have implications not only for the analysis of remobilized pyroclastic facies in terrestrial and marine environments, but also for primary depositional processes during subaqueous explosive volcanism.