Early Earth Wasn't So Hellish

http://www.universetoday.com/am/publish/anu_hadean_earth.html?18112005

New ANU research is set to radically overturn the conventional wisdom that early
Earth was a hellish planet barren of continents.

An international research team led by Professor Mark Harrison of the Research
School of Earth Sciences analysed unique 4 to 4.35 billion-year-old minerals
from outback Australia and found evidence that a fringe theory detailing the
development of continents during the first 500 million years of Earth history -
the Hadean ("hellish") Eon - is likely to be correct.

The research, published in the latest edition of Science, follows on from
results by Professor Harrison and his colleagues published earlier this year
that confirmed that our planet was also likely to have had oceans during most of
the Hadean.

"A new picture of early Earth is emerging," Professor Harrison said. "We have
evidence that the Earth's early surface supported water - the key ingredient in
making our planet habitable. We have evidence that this water interacted with
continent-forming magmas throughout the Hadean.

"And now we have evidence that massive amounts of continental crust were
produced almost immediately upon Earth formation. The Hadean Earth may have
looked much like it does today rather than our imagined view of a desiccated
world devoid of continents."

Professor Harrison and his team gathered their evidence from zircons, the oldest
known minerals on Earth, called zircons. These ancient grains, typically about
the width of a human hair, are found only in the Murchison region of Western
Australia. The team analysed the isotopic properties of the element hafnium in
about 100 tiny zircons that are as old as 4.35 billion years.

Conventionally, it has been believed that the Earth's continents developed
slowly over a long period of time beginning about 4 billion years ago - or 500
million years after the planet formed.

However, hafnium isotope variations produced by the radioactive decay of an
isotope of lutetium indicate many of these ancient zircons formed in a
continental setting within about 100 million years of Earth formation.

"The evidence points to almost immediate development of continent followed by
its rapid recycling back into the mantle via a process akin to modern plate
tectonics," according to Professor Harrison.

The isotopic imprint left on the mantle by early melting shows up again in
younger zircons - providing evidence that they have tapped the same source. This
suggests that the amount of mantle processed to make continent must have been
enormous.

"The results are consistent with the Earth hosting a similar mass of continental
crust as the present day at 4.5-4.4 billion years.

"This is a radical departure from conventional wisdom regarding the Hadean
Earth," said Professor Harrison.

"But these ancient zircons represent the only geological record we have for that
period of Earth history and thus the stories they tell take precedence over
myths that arose in the absence of observational evidence."

"The simplest explanation of all the evidence is that essentially from its
formation, the planet fell into a dynamic regime that has persisted to the
present day."

Original Source: ANU News Release
http://info.anu.edu.au/mac/Media/Media_Releases/_2005/_November/_181105harrisonc
ontinents.asp



Alan

http://www.veloceraptor.free-online.co.uk/enigma.html

http://veloceraptor.blogspot.com/
.

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