Continents grew early in Earth's history.

As biographers of the planet, geoscientists find the first chapters the most difficult to write. Although they can date the origin of the globe to 4.5 billion years ago, they know precious little about what the world looked like during its early history. A new study of Australian rocks has removed some of the mystery: It shows that continents sprouted quite rapidly on the young planet.

"Our results suggest that the continents could have been around much earlier than people currently think," says Paul J. Sylvester, a geochemist at the Australian National University in Canberra. He and his colleagues describe their findings in the Jan. 24 Science.

According to standard theories of Earth's development, continental rock of the type seen today didn't exist when the surface of the infant planet cooled following its fiery birth. Initially, the world's outer skin was basaltic rock-the same material that makes up the floor of today's oceans and erupts from some volcanoes. Over time, remelting of the basalt created rocks richer in silica, giving rise to the first continental crust.

The oldest known continental rocks date back to almost 4.0 billion years ago, indicating that continents had begun to form by that time. Geoscientists have long believed that the amount of continental rock has grown slowly since that date, with little or no destruction.

Sylvester and his colleagues addressed the question of continental growth by studying 2.7-billion-year-old basaltic rocks collected from western Australia. Because basalt arises from molten mantle, these rocks provide a glimpse of the ancient mantle's chemistry.

To gauge how much crust had grown by 2.7 billion years ago, they measured the ratio of two elements, niobium (Nb) and uranium (U). Geologists believe that Earth's mantle began with a Nb-U ratio of 30, the value measured in meteorites that typify the material from which Earth formed. Over the course of geologic history, the creation of continental rocks removed uranium and many other elements from the mantle and concentrated them in the crust. As a result, the mantle now has a Nb-U ratio of 47, the continental crust a ratio of 10.

Sylvester's team found that the ancient Australian basalts had a Nb-U ratio near 47, indicating that the mantle had already lost much of its uranium to continental rocks by 2.7 billion years ago. If such uranium depletion is typical of the mantle at that time, it implies that the continents must have grown quite quickly. Earth apparently took less than half its history to form a volume of continental rock equivalent to today's.

Much of this early crust must have disappeared, says Sylvester, because most continental rocks are younger. Erosion may have worn down the ancient continents and carried this rock into the oceans. From there, the material would have been dragged down into the mantle. New crust is constantly forming from the mantle, but the total volume of continental rock has remained the same for at least 2.7 billion years.

Before rewriting Earth's early history, scientists will need to study basalts from other parts of the world and from older periods to see whether they have Nb-U ratios similar to those found in western Australia, says Sylvester.

Samuel A. Bowring, a geologist at the Massachusetts Institute of Technology, cautions that researchers must also use other methods to resolve the question of continental evolution. "It's not clear to me that Nb-U is the panacea for understanding crustal growth," he says.

Despite such reservations, researchers remain excited by the prospect of filling in the details of Earth's early history. In the same issue of Science, A. W. Hofmann of the Max Planck Institute for Chemistry in Mainz, Germany, comments that "a crack has been made in one of the more intractable problems of understanding the history of the ground we live on."