...And earth just gets younger

The oldest rocks on earth have just become a little younger, writes Steve Farrar.

Geologists have calculated that the earliest reliable date that can be attached to any known in-situ rock is 3.8 billion years old at the most.

They suggest this record age may never be significantly beaten because it immediately followed an intense period of bombardment by interplanetary debris that, when combined with convective currents in the earth's mantle, would have completely erased the pre-existing crust (see story above).

The calculations, to be revealed at the William Smith millennium meeting "Celebrating the Age of the earth", hosted by the Geological Society on Thursday, will add to an ongoing debate over the age of the earth's surface.

Some teams of experts claim to have evidence of in-situ rock going back 4 billion years. It is even alleged that there are hints of life from this cataclysmic period in our planet's past.

Stephen Moorbath, emeritus professor of isotope geology at the University of Oxford, who led the team, said: "Everything gets going after the impacts stopped - before then it would have been too destructive."

Most scientists agree the earth formed about 4.56 billion years ago and assumed something resembling its current shape and form roughly 110 million years later.

The surface of this young planet would have had to endure an era when huge chunks of debris left over from the formation of the solar system rained down.

Evidence of this period has emerged from analysis of lunar samples, but no scars remain on earth.

Professor Moorbath believes that between them, this destructive rain and the great convective currents in the molten mantle below pulverised the earth's crust and then chewed up what was left and recycled it inside.

Perhaps 100 million years after this time of destruction ended, some 3.8 billion years ago, continental crust could form on a completely rejuvenated planet.

"You then had the opportunity for life to start," Professor Moorbath said.

His team looked at the ratios of different isotopes formed by radioactive decay within the rocks to estimate their likely age.

As well as studying the oldest in-situ rocks, the scientists re-dated rocks from west Greenland in which possible chemical traces of life have been found, pulling the dates back from 3.9 billion to 3.7 billion years.

If there had been any life before this, Professor Moorbath believes the intense bombardment would have wiped it out rapidly.