The occurrence and origin of prominent massive, pumice-rich ignimbrite lobes within the Late Pleistocene Abrigo Ignimbrite, Tenerife, Canary Islands

Dynamics of pyroclastic density currents: Conditions that promote substrate erosion and self-channelization — Mount St Helens, Washington (USA)

“…In Unit IV, the increase in lensP thickness and length, increase in lensP average grain size and juxtaposition of lensP with lithic-rich mLT with distance from source suggests the pumice did not segregate and decouple from a more lithic rich basal region of the flow, as interpreted in Calder et al (2000) and Pittari et al, (2005). Instead, in this case it is more likely that the lensP effectively segregated a proportion of the pumice into concentrated lenses, which traveled at the level of neutral buoyancy until the current's density decreased to less than that of the pumice concentrations, resulting in en masse deposition of the lensP (e.g., Druitt, 1995).…”

“…LensP that are asymmetric with pronounced downstream terminations and diffuse tails are interpreted to represent exposure in the direction of flow (e.g., Ad-2b, Figure 7b; Pittari et al, 2005); lensP that are more symmetric and diffuse in two directions, or are discontinuous lenses that grade vertically and laterally into additional pumice lenses or mLT (Figure 9c -base of Unit I exposure) are interpreted to represent exposure perpendicular to flow. The lensP within Units I and II are more symmetric and diffuse laterally at locations AD-1 ( Figure 5) and AD-3 ( Figure 9c), suggesting deposition occurred into or oblique to the outcrops.…”

“…Therefore it is unlikely that they resulted from segregation of low density material that completely decouple from the lithic-rich portion of the current (e.g., Calder et al, 2000;Pittari et al, 2005). Density segregation and the formation of 'pumice rafts' apparently occurred within the flow as the low density pumice rose and accumulated at a level of neutral buoyancy in the flow (e.g., Druitt, 1995).…”

“…The lensed nature of the lensP suggests they are deposited en masse (e.g., frozen) upon entering the depositional region of the current (e.g, Pittari et al, 2005). …”

Rounding of pumice clasts during transport: field measurements and laboratory studies

“…Rounding of pumice clasts is often interpreted as evidence for transport in pyroclastic density currents (e.g., Wilson and Hildreth 1998), hence clast shape may preserve information about the transport processes within currents. Particle roundness is expected to increase with transport distance in density currents, and significant rounding of clasts is often found in distal deposits (e.g., Calder et al 2000) and in pumice lobes (e.g., Pittari et al 2005). In some deposits there is enough variability in clast roundness and shape to distinguish among genetic processes and elucidate transport dynamics (Dellino and Volpe 1996).…”