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The interplay among clast size, vesicularity, postfragmentation expansion, and clast breakage: An example from the 1.8 ka Taupo eruption

By
S.J. Mitchell
S.J. Mitchell
Department of Geology & Geophysics, University of Hawai‘i at Mānoa, Honolulu, Hawaii 96822, USA
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S. Biass
S. Biass
Department of Geology & Geophysics, University of Hawai‘i at Mānoa, Honolulu, Hawaii 96822, USA
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B.F. Houghton
B.F. Houghton
Department of Geology & Geophysics, University of Hawai‘i at Mānoa, Honolulu, Hawaii 96822, USA
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A. Anderson
A. Anderson
Hawaii Institute of Geophysics and Planetology, University of Hawai‘i at Mānoa, Honolulu, Hawaii 96822, USA
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E. Bonny
E. Bonny
Hawaii Institute of Geophysics and Planetology, University of Hawai‘i at Mānoa, Honolulu, Hawaii 96822, USA
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B.H. Walker
B.H. Walker
Department of Geology & Geophysics, University of Hawai‘i at Mānoa, Honolulu, Hawaii 96822, USA
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B.G. Mintz
B.G. Mintz
Department of Geology & Geophysics, University of Hawai‘i at Mānoa, Honolulu, Hawaii 96822, USA
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N.R. Turner
N.R. Turner
Department of Geology & Geophysics, University of Hawai‘i at Mānoa, Honolulu, Hawaii 96822, USA
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D. Frank
D. Frank
Hawaii Institute of Geophysics and Planetology, University of Hawai‘i at Mānoa, Honolulu, Hawaii 96822, USA
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R.J. Carey
R.J. Carey
Centre for Ore Deposit and Earth Sciences, School of Earth Science, University of Tasmania, Hobart, Tasmania 7001, Australia
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M.D. Rosenberg
M.D. Rosenberg
Wairakei Research Centre, GNS Science, Taupo 3384, New Zealand
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Publication history
24 January 201819 September 2018

ABSTRACT

Field 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.

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Contents

GSA Special Papers

Field Volcanology: A Tribute to the Distinguished Career of Don Swanson

Geological Society of America
Volume
538
ISBN electronic:
9780813795386

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