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Harsh Climate Scoured Early Earth, Study Says

Mason Inman
for National Geographic News
June 17, 2008
 
Earth's early atmosphere may have been highly corrosive to rocks, gradually dissolving away all but the toughest of minerals, a new study suggests.

The findings could explain a gap in Earth's geologic record that has puzzled scientists.

"It's possible that [the new study] answers the riddle," said Takayuki Ushikubo, a geologist at the University of Wisconsin, who led the study published online in the journal Earth and Planetary Science Letters.

When Earth formed about 4.5 billion years ago, it was a fiery, inhospitable mass of rocks and gases.

(Related: Prehistoric Time Line)

Scientists believed this era—known as the Hadean, or "hellish," era, for its molten seas of magma—lasted until the planet was as much as 500 million years old.

However the new findings and other recent evidence bolsters the idea that Earth actually cooled quite quickly and became more like the planet we're used to within just a couple of hundred million years.

Ancient Record

Over the last three to four billion years, Earth's tectonic plates have continually shifted around, and their minerals have been melted down and recycled—erasing much of the early geologic history of our planet.

But tiny shards of very early rock have survived in the Jack Hills of western Australia. Some of these microscopic crystals, known as zircons, date to as much as 4.4 billion years ago.

"The Jack Hills zircons are the oldest objects from Earth and the only geochemical evidence we have of the earliest Earth history," Ushikubo said.

Ushikubo and his colleagues performed chemical analysis on the zircons to measure trace amounts of lithium.

The element is scant in zircons formed on the ocean floor from magma but abundant in zircons formed in Earth's continental crust.

The Jack Hills zircons contain high levels of lithium, the researchers found, suggesting that the tiny crystals formed from continental crust—even though some of these rocks formed when our planet was less than 300 million years old.

Further analysis showed that the zircons were low in heavier forms of lithium atoms. Such low ratios occur only in heavily weathered rocks, said study co-author John Valley, a geochemist also at the University of Wisconsin.

The new study supports prior research suggesting the Jack Hills zircons formed from other rocks that had originally formed at relatively low temperatures and in the presence of liquid water.

"Once you have liquid water on the surface [of the Earth], then you'll have precipitation," which can weather the rocks, Valley said.

The team's findings suggest that very early in its history, Earth had crust that was being eroded by a harsh climate.

Mixed Reviews

Bruce Watson, a geochemist at Rensselaer Polytechnic Institute in Troy, New York, who was not involved in the study, said its findings were "important."

"They definitely bolster the enticing picture of an early Earth with continental crust and possibly water."

But other researchers are more skeptical.

Thorsten Geisler-Wierwille of the University of Münster in Germany, said: "I think that most of the [changes in] lithium is indeed due to weathering, but from recent times."

He believes radioactivity from uranium in the zircons could have damaged them over eons, allowing lithium to move in or out of the rocks after they formed.
 

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