New Zealand Journal of Geology and Geophysics abstracts
1995 Ruapehu lahars in relation to the late Holocene lahars of
Whangaehu River, New Zealand
SHANE J. CRONIN1
K. A. HODGSON2
V. E. NEALL1
A. S. PALMER1
J. A. LECOINTRE1
1Department of Soil Science
Massey University
Private Bag 11 222
Palmerston North, New Zealand
2Institute of Geological & Nuclear Sciences
Private Bag 2000
Taupo, New Zealand
Abstract Observations of active flows, their deposits, and
the effects of lahars generated during the 1995 eruptions of Ruapehu are
compared to those of the last 2000 years. The 1995 lahars were generated by
similar mechanisms, and had similar volumes and flow rheologies, to those of
the last 135 years. However, the large number of lahars in the 1995 sequence
and the eventual emptying of the entire Crater Lake on Mt Ruapehu by lahars is
distinctive in the context of the historic record. The 1953 and 1975 lahars,
although of smaller total volume than the largest 1995 lahar, had higher peak
discharges and consequently greater effects on life and property within
57 km from the source. This implies that failure of a lake dam is the most
efficient mechanism for generating a fast lahar with high peak discharge, and
that eruptions through Crater Lake are more efficient at generating a lahar
with high peak discharge when the lake is full.
The scant 1995 lahar deposits preserved indicate that the geological record of
lahars is likely to be incomplete and biased to events of greatest magnitude.
Given this limitation, lahars which formed deposits of the prehistoric (>135
years old) Onetapu Formation were of larger magnitude in all respects, and
often more concentrated with sediment, than those of the last 140 years
(including 1995). In order to generate such events, catastrophic total emptying
of Crater Lake is required. Alternatively, some of the events may have been the
result of small flank collapses on the eastern slopes of Ruapehu. These larger
events are less frequent than small eruption-triggered lahars on Ruapehu, but
they may be preceded by little or no warning. Future lahar hazard mitigation
strategies in the Whangaehu River should begin with improving the detection and
early warning of such flows.
Keywords lahars; Ruapehu volcano; Holocene; historic lahars;
Onetapu Formation; volcanic hazards; debris flows; hyperconcentrated
streamflows
New Zealand Journal of Geology and Geophysics, 1997, Vol. 40: 507-520
0028-8306/97/4004-0507 $7.00/0 (c) The Royal Society of New Zealand
1997
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