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The interplay among clast size, vesicularity, postfragmentation expansion, and clast breakage: An example from the 1.8 ka Taupo eruption
chapter
posted on 2023-05-24, 06:03 authored by Mitchell, SJ, Biass, S, Houghton, RF, Anderson, A, Bonny, E, Walker, BH, Mintz, BG, Turner, NR, Frank, D, Rebecca CareyRebecca Carey, Rosenberg, MDField studies of tephra-fall deposits traditionally use the density of juvenile pyroclasts to determine vesicularity of the host magma at the point of fragmentation. A range of pyroclast sizes between 16 and 32 mm has commonly been chosen for this purpose. Larger pyroclasts outside this range may undergo postfragmentation vesiculation due to slow cooling of the interior of the clasts, while smaller pyroclasts may be too small to represent accurately the distribution of the largest vesicles. The assumption of this method, of course, is that the 16-32 mm size range is representative of the fragmented magma. We explore, in detail, variations in density over a size range of 4-128 mm from Unit 2 pyroclasts of the 1.8 ka Taupo eruption and make inferences about the roles of postfragmentation vesiculation and secondary breakage of pyroclasts. We find (1) there is a clear threshold for onset of postfragmentation vesiculation at >32 mm, and (2) there are broken small pieces of the largest pyroclasts in the sample that artificially skew the density distribution for smaller size fractions. We constrain uncertainty associated with vesicularity measurements and offer best-practice recommendations in the hope of improving consistency of field sampling and laboratory processing of pyroclast populations for vesicularity studies.
Funding
Australian Research Council
History
Publication title
Field Volcanology: A Tribute to the Distinguished Career of Don SwansonVolume
538Editors
MP Poland, MO Garcia, VE Camp and A GrunderPagination
375-383ISBN
9780813795386Department/School
School of Natural SciencesPublisher
Geological Society of AmericaPlace of publication
United StatesExtent
21Rights statement
Copyright 2018 The Geological Society of AmericaRepository Status
- Restricted