Earth's Surface Transformed by Three Massive Asteroid Impacts 3.2 Billion Years Ago

http://info.anu.edu.au/mac/Media/Med...805glikson.asp

Earth's surface transformed by massive asteroids
Australian National University media release
August 5, 2005

A cluster of at least three asteroids between 20 and 50 kilometres
across colliding with Earth over 3.2 billion years ago caused a massive
change in the structure and composition of the earth's surface,
according to new research by ANU earth scientists.

According to Dr Andrew Glikson and Mr John Vickers from the Department
of Earth and Marine Sciences at ANU, the impact of these asteroids
triggered major earthquakes, faulting, volcanic eruption and
deep-seated
magmatic activity and interrupted the evolution of parts of the Earth's
crust.

The research extends the original discovery of extraterrestrial impact
deposits, discovered in South Africa by two US scientists, D.R. Lowe
and
G.R. Byerly, identifying their effects in the Pilbara region in Western
Australia.

"Our findings are further evidence that the seismic aftershocks of
these
massive impacts resulted in the abrupt termination of an over 300
million years-long evolutionary stage dominated by basaltic volcanic
activity and protracted accretion of granitic plutons," Dr Glikson
said.

The identification of impact ejecta - materials ejected by the hitting
asteroid - is based on unique minerals and chemical and isotopic
compositions indicative of extraterrestrial origin, including iridium
anomalies.

The impact ejecta from the Barberton region in the eastern Transvaal
indicate the formation of impact craters several hundred kilometres in
diameter in oceanic regions of the earth, analogous to the lunar maria
basins (large dark impressions on the surface of the moon). The seismic
effects of the impacts included vertical block movements, exposure of
deep-seated granites and onset of continental conditions on parts of
the
earth surface.

In the Pilbara, the formation of fault escarpments and fault troughs is
represented by collapse of blocks up to 250-metres wide and 150-metres
high, buried canyons and a major volcanic episode 3240 million years
ago.

"The precise coincidence of the faulting and igneous activity with the
impact deposits, coupled with the sharp break between basaltic crust
and
continental formations, throws a new light on the role of asteroid
impacts in terrestrial evolution," Dr Glikson said.

Preliminary indications suggest that at about the same time the Moon
was
also affected by asteroid impacts and by resurgent volcanic activity.

Dr Glikson and Mr Vickers will continue to investigate the extent and
effects of large asteroid impacts by studying early terrains in other
parts of the world, including India and Canada.

Further Information

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