New Zealand Journal of Geology and Geophysics abstracts

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Pyroclastic stratigraphy and eruption dynamics of the 21.9 ka Okareka and 17.6 ka Rerewhakaaitu eruption episodes from Tarawera Volcano, Okataina Volcanic Centre, New Zealand

Miles Darragh*
Jim Cole^†

Department of Geological Sciences
University of Canterbury
Private Bag 4800
Christchurch, New Zealand

Ian Nairn

45 Summit Road
RD5, Rotorua
and
GNS Science, Wairakei Research Centre
Private Bag 2000
Taupo, New Zealand

Phil Shane

Geology Department
The University of Auckland
Private Bag 92019
Auckland, New Zealand

^*Present address: Sunrise Dam Gold Mine, Anglogold Ashanti Australia Ltd, Australia.

^†Author for correspondence.

Abstract  The 21.9 ka Okareka and 17.6 ka Rerewhakaaitu rhyolite eruption episodes began the construction of Tarawera Volcano in the Okataina Volcanic Centre, Taupo Volcanic Zone. Examination of the proximal and medial stratigraphy of these moderate-size (c. 5 km³ magma) but poorly exposed pyroclastic deposits has increased understanding of their eruption and dispersal processes. The Okareka Tephra consists of at least nine units (A–I), with unit A basaltic scoria at the base, overlain by the rhyolitic units B–I. Unit C is the largest individual plinian fall deposit (c. 0.4 km³), dispersed from an eruption column that reached c. 19 km height in the presence of strong crosswinds. The other pyroclastic units record a variety of phreatomagmatic, sub-plinian, and small ignimbrite eruptions, which were followed by extrusion of voluminous lava flows. The Rerewhakaaitu Tephra consists of 15 rhyolitic fall units A–N. An initial short plinian phase dispersed lapilli-fall unit A, mostly to ENE, from columns c. 15 km in height. Units B–D have high ash contents, indicating phreatomagmatic eruptions with varying magma/water ratios, and were widely dispersed, with lobes to the northeast and southeast. Units E–J were deposited from 20–25 km high plinian eruption columns into strong crosswinds that dispersed tephra to the southeast. The E–J package contains the largest tephra volume of the episode (1.18 km³) and is thought to dominate the deposits widely dispersed in Pacific Ocean sediments to the east of New Zealand. Rerewhakaaitu pyroclastic deposits are interbedded with, and underlie, voluminous lavas.

Keywords  eruption episode; Tarawera; rhyolite; tephra; stratigraphy; dispersal; grain sizes; components; column dynamics; duration

G05038; Received 3 August 2005; accepted 13 June 2006; Online publication date 26 July 2006
New Zealand Journal of Geology & Geophysics, 2006, Vol. 49: 309–328
0028–8306/06/4903–0309  © The Royal Society of New Zealand 2006

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