Rounding of pumice clasts during transport: field measurements and laboratory studies |
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Authors: | Michael Manga Ameeta Patel Josef Dufek |
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Institution: | (1) Department of Earth and Planetary Science, University of California, Berkeley, 307 McCone Hall, Berkeley, CA 94720-4767, USA;(2) School of Earth and Planetary Sciences, Georgia Tech, Atlanta, GA, USA |
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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, near-bed region prior to final deposition. |
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