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

Abstract

Volcanic clasts in many pyroclastic density current deposits are notably more round than their counterparts in corresponding fall deposits. This increase in roundness and sphericity reflects different degrees of comminution, abrasion and breakup during transport. We performed experimental measurements to determine an empirical relationship between particle shape and mass loss caused by particle-particle interactions. We consider, as examples, pumice from four volcanoes: Medicine Lake, California; Lassen, California; Taupo, New Zealand; Mount St Helens, Washington. We find that average sample roundness reaches a maximum value once particles lose between 15% and 60% of their mass. The most texturally homogeneous clasts (Taupo) become the most round. Crystal-rich pumice abrades more slowly than crystal-free pumice of similar density. Abrasion rates also decrease with time as particles become less angular. We compare our experimental measurements with the shapes of clasts in one of the May 18, 1980 pyroclastic density current units at Mount St Helens, deposited 4-8 km from the vent. The measured roundness of these clasts is close to the experimentally determined maximum value. For a much smaller deposit from the 1915 Lassen eruption, clast roundness is closer to the value for pumice in fall deposits and suggests that only a few volume percent of material was removed from large clasts. In neither field deposit do we see a significant change in roundness with increasing distance from the vent. We suggest that this trend is recorded because much of the rounding and ash production occur in proximal regions where the density currents are the most energetic. As a result, all clasts that are deposited have experienced similar amounts of comminution in the proximal region, and similar amounts of abrasion as they settle through the dense, nearbed region prior to final deposition.

Construction of volcanic records from marine sediment cores: A review and case study (Montserrat, West Indies)

“…The aspect ratio/area index relates to both the elongation of the clasts and their regularity. The value of this index is generally higher (more elongate and irregular) for the fallout deposits, as clasts in these deposits have been less subject to processes such as comminution, saltation and attrition relative to the other volcaniclastic deposits, which experience high energy, erosive, transport (e.g., Manga et al, 2011). Similarly, the perimeter/area index equates closely with the angularity of the clasts, and is again generally higher for tephra fallout clasts due to the same transport-related processes.…”

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

“…Rounding of pumice in PDC deposits reflects abrasion and production of comminuted ash during transport (e.g., Manga et al, 2011). Increasing the amount of comminution-ash in a PDC can result in an increase internal pore pressure, which has been shown to increase flow mobility and carrying capacity resulting in longer runout distances than PDC flows with less ash (e.g., Dufek and Manga, 2008;Roche, 2012).…”

“…Increasing the amount of comminution-ash in a PDC can result in an increase internal pore pressure, which has been shown to increase flow mobility and carrying capacity resulting in longer runout distances than PDC flows with less ash (e.g., Dufek and Manga, 2008;Roche, 2012). We used a 2-D image analysis technique described in Manga et al (2011) where the roundness value of a pumice is calculated using (A is the cross sectional area and P is the perimeter of the pumice clast). For simplification only the outcrop data for Unit IV is presented although the results are consistent for all units analyzed (Dawson et al, 2011).…”

A tuff cone erupted under frozen-bed ice (northern Victoria Land, Antarctica): linking glaciovolcanic and cosmogenic nuclide data for ice sheet reconstructions

“…Some support for this suggestion comes from the presence of abraded juvenile lapilli and the high content of fine ash in the lapilli tuffs. Lapilli abrasion can occur during transport in pyroclastic density currents and much of the abrasion takes place relatively In shallow water, eruptions will be explosive throughout and lack pillow lava proximally, but a few kilometres of travel is required (Houghton and Smith 1993;Manga et al 2011). The Harrow Peaks deposits are demonstrably proximal and lateral transport was minimal (few hundred metres at most), and only minor abrasion would be expected.…”