New Zealand Journal of Geology & Geophysics abstracts
Onetapu Formation: the last 2000 yr of laharic activity at Ruapehu
volcano, New Zealand
Katy A. Hodgson
Western Heights High School
Old Quarry Rd
Rotorua, New Zealand
Jérôme A.
Lecointre*
Vincent E. Neall
Soil and Earth Sciences
Institute of Natural Resources
Massey University
Private Bag 11222
Palmerston North 4442, New Zealand
*Corresponding author: J.A.Lecointre@massey.ac.nz
Abstract The Onetapu Formation represents a
sequence of at
least 17 volcanic diamictons that were emplaced over the last c. 2000
14C
yr by lahars in the Whangaehu River catchment, covering part of the
eastern flank of Ruapehu volcano and its adjacent ring plain.
Interbedded andesitic tephras from the Tufa Trig Formation help in
reconstructing the chronology of volcanogenic events that led to the
formation of the Crater Lake basin and its subsequent evolution.
Onetapu Formation contains deposits laid down by lahars spanning
several orders of magnitude. Channel deposits left by the largest
lahars are bouldery to distances reaching c. 50 km
downstream from the source region. Much of the coarsest fraction of the
medium to smaller lahar deposits (i.e., volume
<5 × 107
m3) was emplaced
close to the source on the Whangaehu Fan, where the river debouches
onto the ring plain. Mainly thin, fine grained, pebbly overbank
debris-flow deposits are preserved downstream of the fan, with
increasing proportions of sandy hyperconcentrated-flow deposits. The
largest Onetapu lahars are inferred to have been generated by partial
collapse of the wall of Crater Lake as a result of powerful explosive
eruptions. In contrast, smaller lahars were the consequence of
phreatic/phreatomagmatic activity within Crater Lake, expelling lake
waters onto the flanks of the volcano.
Keywords Onetapu Formation; lahar; Crater Lake;
Ruapehu;
volcanic hazards
G04010; Online publication date 17 April 2007; Received 27
February
2004; accepted 8 March 2007
New Zealand Journal of Geology & Geophysics, 2007, Vol. 50:
81-99
0028-8306/07/5002-0081 © The Royal Society of New Zealand 2007
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