Oldest Rocks on Earth Discovered?

September 25, 2008

An expanse of bedrock along Hudson Bay, Canada, may be a chunk of crust that formed not long after the solar system was born nearly five billion years ago, according to a new study.

The finding could push back the age of the most ancient remnant of stable crust on Earth by about 300 million years.

Previous research had dated rocks in northwestern Canada to 4.03 billion years ago, and tiny crystals of the mineral zircon in Western Australia are known to be upward of 4.38 billion years old.

It's not known whether the bedrock itself is also as old as the crust, a question that awaits further analysis, said study co-author Richard Carlson, a geochemist at the Carnegie Institution in Washington, D.C.

Regardless, the ancient date supports the once-controversial idea that Earth was cool enough to produce crust early in its history.

"Even though we expect the Earth formed [at] very hot [temperatures], at least Earth's surface had cooled down to temperatures … not dramatically hotter than today," Carlson said.

The finding disputes the idea that the first crust didn't form until a period of intense asteroid and comet bombardment ended about 3.8 billion years ago.

(Related: "World's Oldest Rocks Suggest Early Earth Was Habitable" [November 28, 2005].)

Dating Rocks

In 2007 Nicole Cates and Stephen Mojzsis of the University of Colorado at Boulder dated the Hudson Bay bedrock to about 3.8 billion years ago.

(See a prehistoric time line.)

Their team based their estimate on the decay of radioactive uranium found in tiny shards of zircon taken from the bedrock. The technique is widely used to date ancient rocks around the world.

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