Acid rain may be reason for no 4-bln-year-old rocks

BEIJING, June 24 (Xinhuanet) -- The reason scientists can't find rocks more than 4 billion years old is they may not have been able to handle the weather -- stinging acid rains -- and an intensely warm surface.

The fate of all those rocks from the first 500 million years after Earth formed has been a longstanding question in geology. Scientists have advanced various explanations for the missing rocks, including destruction by barrages of meteorites and the possibility that the early Earth was a sea of red-hot magma in which no rocks could form.

Geologists from the University of Wisconsin-Madison examined zircon crystals, the oldest known materials on Earth, to shed light on the fate of rocks from the early Earth. Zircons, which are smaller than a speck of sand, can offer a window back in time to about 4.4 billion years ago, when the Earth was a mere 150 million years old because they are extremely resistant to chemical changes.

The research team analyzed the ratios of different isotopes of lithium (which have different atomic weights and number of neutrons per atom) in zircons from the Jack Hills in Western Australia. They compared the lithium fingerprints of those zircons to those from continental crust and rocks similar to those found in Earth's mantle, the molten layer sandwiched between the crust and core.

The results of the analysis, detailed in a recent online issue of the journal Earth and Planetary Science Letters, provide evidence that the young Earth already had the beginnings of continents, relatively cool temperatures and liquid water by the time the Australian zircons formed.

But the lithium signatures also hold signs of rock exposure on Earth's surface and breakdown by weather and water, suggesting that early rocks were destroyed by intense weathering.

The early Earth is thought to have had extremely high levels of carbon dioxide ─ perhaps 10,000 times as much as today. Carbon dioxide in the atmosphere can combine with water to create acid rain.

"At [those levels of carbon dioxide], you would have had vicious acid rain and intense greenhouse [effects]. That is a condition that will dissolve rocks," said study team member John Valley. "If granites were on the surface of the Earth, they would have been destroyed almost immediately ─ geologically speaking ─ and the only remnants that we could recognize as ancient would be these zircons."

(Agencies)