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“Historical Impact Craters” Guest: Dallas Abbott
DID A BOLIDE
IMPACT CAUSE CATASTROPHIC TSUNAMIS IN AUSTRALIA AND NEW
ZEALAND?
Dallas Abbott
and her colleagues have been
exploring evidence that some historical tsunami events in the
South Pacific resulted from bolide (asteroid or meteorite)
impacts, rather than earthquakes as previously postulated. In
her presentation, she will share some of her recent research
in Australia and New Zealand to test her theories.
She
and her co-authors made a presentation at the Geological
Society of America Annual Meeting in November 2003,
based on this Abstract:
ABBOTT, Dallas
H., MATZEN, Andrew, and PEKAR, Stephen F.
Lamont-Doherty Earth Observatory, Route 9W, Palisades, NY
10964,
dallas@ldeo.columbia.edu and BRYANT, Edward A., Faculty of
Science, University of Wollongong, Wollongong, Australia, 2522
Goff attributes coastal abandonment
in New Zealand at 1500 A.D. [1]
to an earthquake-induced tsunami event. However, the largest
historical earthquakes produced maximum tsunami runups of 40
to 60
meters. On Stewart Island, New Zealand, beach sand is present
~220
meters above sea level at Hellfire Hut and ~150 meters above
sea
level at Mason Bay. In eastern Australia, there are mega
tsunami
deposits with maximum run-ups of over 130 meters and a C-14
age of
~1500 A.D[2]. Megatsunami deposits occur on the eastern side
of Lord
Howe Island in the middle of the Tasman Sea, implying a source
crater
for the tsunami further east. We named this source crater
Mahuika
for the Maori god of fire. Mahuika crater is 20±2 km wide and
>153
meters deep. It is on the New Zealand continental shelf at
48.3 S,
166.4E. Several pieces of evidence point to Mahuika as the
source
crater for the 1500 A.D. event. The first is that the crater
lies on
a great circle path from Australia oriented at about a
45-degree
angle to the general trend of the eastern Australia coast.
Mega
tsunami deposits near Wollongong and at
Jervis Bay, Australia suggest
a tsunami wave oriented at this angle to the coast. The second
is the
sub bottom depth of the impact deposits. We have found impact
ejecta
in all of the dredges near the crater. Because marine
sediments are
deposited at a rate of about 1 cm per thousand years, this is
expected if the impact deposit is only 500 years old. We are
seeking
C-14 dates to confirm this. The third is the distribution of
tektites, which are found on the opposite side of the crater
from the
direction of impactor arrival[3]. Although we found impact
ejecta in
many samples, only some samples contained tektites. All
tektite-bearing samples are located SE of the crater, in the
opposite
direction from SE Australia, where the impact fireball was
seen by
the aborigines[2].
References: [1] Goff, J.R. and B.G.
McFadgen, Seismic driving
of nationwide changes in geomorphology and prehistoric
settlement-a
15th Century New Zealand example. Quaternary Science Reviews,
2002.
21: p. 2229-2236. [2] Bryant, E., Tsunami: The Underrated
Hazard.
2001, Cambridge, UK: Cambridge University Press. 320. [3]
McCall,
G.J.H., Tektites in the Geological Record. 2001, London:
Geological
Society of London. 256.
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