By 2.7 billion years ago, the cooling Earth had formed a crust of continental rock about as thick as today's terra firma. The finding, reported in today's Science,* appears to solve a long-running mystery of when the ground we live on formed.
The crust congeals from molten rock that wells up from the mantle, cools, and thickens. It's "a thin scum on top of the mantle," says geochemist Albrecht Hofmann of the Max Planck Institute for Chemistry in Mainz, Germany. Geologists understand the mechanics of crust formation, but they have had little idea whether most of it formed at once or over billions of years.
Clues to the early history of the planet's surface come from the crust's composition: It's enriched in niobium and uranium, elements deposited by cooling magma. Paul Sylvester and colleagues at Australian National University in Canberra argue that the relative abundances of the two elements in the crust and magma at different points in Earth's history can indicate when and how much crust was formed on ancient Earth. They discovered that basalt rock formed from the mantle 2.7 billion years ago in Western Australia has roughly the same concentration of niobium and uranium as mantle rock formed more recently--meaning that the amount of Earth's crust then was probably the same as now. "We think that a significant amount of crust was already extracted from the mantle 2.7 billion years ago," says Sylvester.
"It's a great observation," says Harvard geologist Roberta Rudnick, who liked the approach of measuring niobium and uranium levels. She says, however, that she has qualms that the basalt samples may have been contaminated with uranium that had migrated from crust. "But this is the sort of way we're going to find out when the crust separated from the mantle," she says.