Tsunamis from Impacts
Tsunamis from impact events have the potential to produce the largest
tsunamis ever seen. The best documented tsunami deposits from an impact are
the K/T deposits from the 170 km diameter Chixculub crater. Based on the size
of the rocks that it moved, the K/T impact event on the Yucatan peninsula
produced a wave that was 50 to 100 meters high at the present location of the
Brazos river in Texas[1]. It is not widely recognized that these sediments were
deposited below wave base and that there are no known preserved K/T tsunami
deposits from above sea level.
There are at least 2 sets of Holocene age megatsunami deposits in both eastern
and western Australia (> 4 total). On the east coast, near Jervis Bay, the
deposits have claimed maximum run-ups of 130 meters[2]. On the west coast,
near Perth, the tsunami deposits are chevron dunes that were deposited as
much as 150 meters above sea level and at least 4 km inland[3]. The chevron
dunes are not oriented in the same direction as the prevailing winds. In some
cases, the chevron dunes are in locations where there is no beach. In each of
those locations, there is no known seismic source or volcanic explosion that
could have produced these megatsunami deposits. The alternatives are either a
large landslide or a large bolide impact.
To date, no landslide source has been identified for any of the megatsunami
deposits in either eastern or western Australia. Potential impact crater sources
have been identified for one set of megatsunami deposits in both eastern and
western Australia. In eastern Australia, the potential tsunami source is the 24
km diameter Mahuika crater candidate located on the western edge of the New
Zealand shelf at about 48°S in about 300 meters of water[4, 5]. The crater
candidate is in the right location to produce the largest tsunami run-up in eastern
Australia within Jervis Bay, as is observed. In western Australia, the potential
tsunami source is the 29 km diameter wide Burckle crater at 31°S, 61°E just
south of the crest of the southwest Indian ridge[6]. The latitude of the crater is
identical to the latitude of Perth, as predicted for the source of one set of
megatsunami deposits in western Australia[3].
Based on their crater size, the energies needed to make the Mahuika and
Burckle impact craters are > 400,000 and > 2,000,000 megatons, respectively[7].
For comparison, the Krakatau volcanic explosion had an energy of between 100
and 150 megatons[8]. The tsunami from the Krakatau volcanic explosion moved
10 ton coral blocks at least a km inland. The maximum runups from the Krakatua
tsunami were at least 40 meters above sea level and at least 4 to 7 km inland.
Thus, although these 2 crater candidates are 1000s of km away from their
respective tsunamis deposits in Austalia, the observed runups are quite
plausible.
The bolides needed to make the Mahuika and Burckle craters are just over 1 km
in diameter. Based on published charts of impact rates, one might expect a 1 km
bolide to hit the Earth about every 100,000 to 1 million years[9]. However, the
charts implicitly assume that all known large impact craters represent all of the
large impacts that have occurred. In fact, the best assessment is that we have
discovered between one quarter and one-tenth of all impact craters with a best
guess of about one-sixth of all impact craters[10, 11]. In addition, the published
charts of impact rates assume that the long term impact rate is stable and does
not fluctuate over time. This is contradicted by time series analysis of impact
rates that find a 32 to 36 million year periodicity in impact rates[12, 13]. Based
on these periodicities, we are presently in or just ending a time of a 2 to 3 times
enhancement of the present impact rate over the long term rate. Thus, it is quite
plausible that there have been 2 (or more) impacts of >1 km bolides during
Holocene time.
Research Needed
The idea that the megatsunami deposits on the Australian coast have been
produced by cosmogenic impacts is very controversial. Several types of
research would help to address the issue. On land, the tsunami deposits need
to be thoroughly mapped and traced into lowland areas like marshes and lakes.
In marshes and lakes, the upper contact of a cosmogenic tsunami deposit may
still contain samples of the impact ejecta layer. As the proposed source craters
are quite distal from Australia, the ejecta layers will be extremely thin and spotty.
Thus, tens of cores need to be examined. By mapping the thickness of the
tsunami layer in detail, directional and runup information can be refined.
In the ocean basins, the areas around the proposed craters need to be mapped
with multibeam bathymetry and single channel seismic reflection profiling. The
rims of the craters should exhibit prominent resurge gullies and redeposited
ejecta blankets. The very centers of the craters should exhibit prominent
magnetic anomalies from the impact melt body and central uplifts buried by the
resurgent wave.
References:
1.
Bourgeois, J., et al., A tsunami deposit at the Cretaceous-Tertiary boundary
in Texas. Science, 1988. 241: p. 567-570.
2.
Bryant, E., Tsunami: The Underrated Hazard. 2000, Cambridge, UK:
Cambridge University Press. 425.
3.
Kelletat, D. and A. Scheffers, Chevron-shaped accumulations along the
coastlines of Australia as potential tsunami evidences. Science of Tsunami
Hazards, 2003. 21: p. 174-188.
4.
Abbott, D.H., et al., Did a bolide impact cause catastrophic tsunamis in
Australia and New Zealand? Abstracts with Programs, Geological Society of
America, 2003. 35: p. 168.
5.
Bryant, E.A., G. Walsh, and D. Abbott, Cosmogenic mega-tsunami in the
Australia region: Are they supported by Aboriginal and Maori Legends? in Myth
and Geology. in press.
6.
Abbott, D.H. Burckle Abyssal Impact Crater: Did this Impact Produce a
Global Deluge and Indian Ocean Megatsunamis? in Atlantis 2005. 2005. Milos.
Greece.
7.
Collins, G.S., H.J. Melosh, and R. Marcus, Earth Impact Effects Program: A
Web-based Computer Program for Calculating the Regional Environmental
Consequences of a Meteoroid Impact on Earth. Meteoritics and Planetary
Science, in press.
8.
Simkin, T. and R.S. Fiske, Krakatau 1883; the volcanic eruption and its
effects. 1983, Washington, D.C.: Smithsonian Institution Press. 464.
9.
Gehrels, T.H., Hazards Due to Comets and Asteroids. 1994, Tucson,
Arizona: University of Arizona Press. 1300.
10. Shoemaker, E.M., R.S. Wolfe, and C.S. Shoemaker, Asteroid and comet
flux in the neighborhood of earth, in Global Catastrophes in Earth History: An
Interdisciplinary Conference on Impacts, Volcanism, and Mass Mortality, V.L.
Sharpton and P.D. Ward, Editors. 1990, Geological Society of America: Boulder,
CO. p. 155-170.
11. Shoemaker, E.M., Asteroid and comet bombardment of the Earth. Annual
Reviews of Earth and Planetary Science, 1983. 11: p. 461-494.
12. Rampino, M.R. and R.B. Stothers, Geological rhythms and cometary
impacts. Science, 1984. 226(4681): p. 1427-1431.
13. Rampino, M.R., Impact crises, mass extinctions, and galactic dynamics:
The case for a unified theory, in Large Meteorite Impacts and Planetary Evolution
II, B.O. Dressler and V.L. Sharpton, Editors. 1999, Geological Society of
America: Boulder, CO. p. 241-248.