New Zealand Journal of Geology and Geophysics abstracts
Proximal stratigraphy and event sequence of the c. 5600 cal.
yr BP Whakatane rhyolite eruption episode from Haroharo volcano,
Okataina Volcanic Centre, New Zealand
Tetsuo Kobayashi
Department of Earth Sciences
Kagoshima University
Kagoshima 890, Japan
Ian Nairn*
Lake Okareka
Rotorua, RD5, New Zealand
i.a.nairn@xtra.co.nz
Vicki Smith
Phil Shane
Geology Department
University of Auckland
P.O. Box 90 129
Auckland, New Zealand
*Corresponding author.
Abstract The c. 5600 cal. yr BP Whakatane eruption
episode consisted of a sequence of intracaldera rhyolite eruptions from
at least five vents spread over 11km of the Haroharo linear vent zone
within Okataina Volcanic Centre. Initial vent-opening eruptions from
the Haroharo vent produced coarse lithic clast "blast beds" and
pyroclastic density currents (surges). These were immediately followed
by eruption of very mobile pumiceous pyroclastic surges from the
Makatiti vent 6 km to the southwest. Major plinian eruptions from the
Makatiti vent then dispersed Whakatane Tephra pumice fall deposits
(bulk volume c. 6 km3) across the northeastern North Island
while smaller explosive eruptions produced pyroclastic flows and falls
from the Haroharo-Rotokohu vents and at the Pararoa vent on the caldera
rim 11 km northeast from Makatiti. The pyroclastic eruptions at all
vents were followed by the extrusion of lava flows and domes; extruded
lava volumes ranged from 0.03 km3 for the Pararoa dome to
7.5 km3 for the Makatiti-Tapahoro lava flows and domes.
Minor
variations in whole rock and glass chemistry show that the three main
vent areas each tapped a slightly different high-silica rhyolite magma.
About 10 km3 of M-type magma was erupted from the
Makatiti-Tapahoro vents; c. 1.3 km3 of H-type magma from the
Haroharo-Rotokohu vents, and 0.04 km3 of P-type magma from
the Pararoa vent. There are no significant weathering or erosional
breaks within the Whakatane eruptive sequence, which suggests that all
Whakatane eruptions occurred within a short time interval. However,
extrusion of the Haroharo dome within the Makatiti pyroclastic eruption
sequence suggests a duration of c. 2 yr for the main pyroclastic
eruption phase. Emplacement of the following voluminous (7.5 km3)
lavas from the Makatiti-Tapahoro vents would have occurred over
>10 yr at the c. 10–20 m3/s inferred extrusion rates.
Keywords stratigraphy; Haroharo volcano;
high-silica rhyolite magmas; multiple vents; pyroclastic eruptions;
lava extrusions; episode duration
G04032; Received 4 August 2004; accepted 28 April 2005; Online
publication date 22 August 2005
New Zealand Journal of Geology & Geophysics, 2005, Vol. 48:
471–490
0028–8306/05/4803–0471© The Royal Society of New Zealand 2005
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