New Zealand Journal of Geology and Geophysics abstracts
The c. AD 1315 syn-eruption and AD 1904 post-eruption breakout
floods
from Lake Tarawera, Haroharo caldera, North Island, New Zealand
K. A. Hodgson
8 Steep Street
RD5, Rotorua, New Zealand
I. A. Nairn
45 Summit Road
RD5, Rotorua, New Zealand
Abstract Breakout floods have resulted from
temporary blockage of the Lake Tarawera outlet during and after late
Holocene eruptions. The largest flood occurred near the end of the
c. AD 1315 Kaharoa rhyolite eruption episode of Tarawera
volcano after a fan of reworked volcaniclastic material had blocked the
existing outlet channel. The 41 km2 lake rose to
>30 m above its present elevation (298 m) before
overtopping and releasing c. 1.7 km3 of lake water
as the outlet was scoured down by >40 m. The resulting flood
excavated a 300 m wide, 3 km long spillway between the lake
and the c. 70 m high Tarawera Falls, and eroded a 1 km
wide valley below the falls. Flood deposits, including sand and gravel
bars with clast diameters locally exceeding 10 m, are traceable to
40 km downstream of the breach area. Peak discharge at the breach
area was c. 105 m3/s,
judging from breach geometry and lake hypsometry. Maximum clast sizes
at 5–10 km downstream from the breach indicate a peak discharge of
at least 104 m3/s in the main
river channel. Floodwater also traversed a 3 km wide interfluve to
cause massive erosion in an adjacent valley. A similar but smaller fan
dam that formed after the AD 1886 Tarawera eruption failed in
AD 1904, generating a much smaller flood of c. 700 m3/s.
Keywords breakout floods; caldera lake; fan dam;
volcanic eruptions; paleohydrology; Tarawera
G04007; Received 4 February 2004; accepted 22 December 2004; Online
publication date 22 August 2005
New Zealand Journal of Geology & Geophysics, 2005, Vol. 48:
491–506
0028–8306/05/4803–0491© The Royal Society of New Zealand 2005
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