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.

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Deposition of the 1.8 ka Taupo ignimbrite in the Hawke's Bay buried parts of the Ngaruroro and Mohaka River catchments beneath up to 40 m of loose pyroclastic debris. Re-establishment of the two river systems led to the remobilisation of the loose debris and followed similar patterns in both catchments. An initial period of laharic remobilisation and formation of lahar-deposit-dammed lakes was followed by two phases of fluvial remobilisation. During the first of these, streams were shallow and ephemeral to perennial, with sediment-laden flash-floods eroding headwards through valley-ponded primary pyroclastic and laharic deposits. With gradual re-establishment of vegetation, the sediment:water ratio decreased, and braided rivers with deeper, more stable channels became dominant.Although the general pattern of resedimentation was similar for both river systems, field evidence shows that the volumes of remobilised material, and the likely durations of resedimentation in the different catchments, differed. These variations reflect: (1) the different volumes of initial ignimbrite in the trunk valleys; (2) the different percentage of catchment areas covered by ignimbrite debris; and (3) the different river gradients and valley shapes. However, extremely high sediment yields in the Hawke's Bay region are estimated to have persisted for at least 4-17 yr after the eruption.

Almost 500 m of basaltic hyaloclastite tuffs, hyaloclastite lapillistones, and lithic breccias were drilled in the northern, southeastern, and southwestern flank of Gran Canaria, Canary Islands during Leg 157 (Hole 953C, total penetration 1159 meters below seafloor [mbsf], basal 293 m; Hole 954B, total penetration 446 mbsf, basal 38 m; and Hole 956B, total penetration 704 mbsf, basal 140 m). These deposits represent (1) mostly moderate to shallow water (<< ~500 m) eruptions, (2) transition to the emergence, and (3) the fully subaerial island shield stage. The volcaniclastic rocks are interlayered with minor thin layers of nannofossil ooze and clay.Volcanic clasts comprise blocky to vesicular, generally altered former sideromelane shards, tachylite crystallized basalt, round glassy shards, lapilli, and single crystals, chiefly titanaugite. Dominantly filled foraminifers, thick-walled shallow-water skeletal debris, and nannofossil ooze make up <5 vol% of the volcaniclastic rocks. Most of the basaltic volcaniclastic deposits are interpreted to have been deposited as debris flows resulting from (1) destabilization of hyaloclastites generated during voluminous moderate (<500 m?) to shallow-water explosive volcanic activity and temporarily accumulated prior to episodic failure and transfer to the deep basins, fragmentation of subaerial lava flows that entered the sea and collapse of lava deltas and by flank collapse. About 16 debris-flow units (lithologic Unit VII) in Hole 953C range in thickness from ~1 to 50 m. Most are composed of well-sorted massive lapillistone to coarse hyaloclastite tuff consisting of blocky, poorly vesicular shards, minor tachylite, and crystallized basalt. The top 5%-10% or so show laminar bedding to minor cross-bedding, the grain size rarely decreasing to fine sand (ash) size in the top beds. Basalt clasts up to 25 cm in diameter are common in the coarse-grained basal parts. Most particles in the stratigraphically highest deposits are vesicular to highly vesicular ash to lapilli-size clasts suggesting decreasing water depth. Coarse breccias at Hole 953C (lithologic Unit VI) consist of basalt clasts of diverse composition, angularity, and vesicularity, and some contain pillow rind fragments. Only the upper of three debrites at Site 956 (Cores 157-956B-43R through 45R) and underlying turbidites consist dominantly of highly vesicular formerly glassy ash to lapilli-sized clasts. Lithic-rich debris-flow deposits at Site 956 (Cores 157-956B-45R through 48R, and 49R through 57R) consist chiefly of poorly vesicular, angular tachylite, crystallized basalt, and minor formerly glassy shards set in ~30−50 vol% brown clay matrix.About 300 very thin turbidite beds, 1−40 cm thick, deposited prior to the first ignimbrite-related ash deposit at Hole 953C are composed of variable amounts of dominantly silt-to sand-sized tachylitic and lesser amounts of vesicular to blocky altered shards and minor biogenic debris. They are interpreted to represent chiefly the subaerial growth stage of the basaltic shield and to...