Earth matured early in its adolescence.

Geoscientists, the biographers of the planet, struggle to chronicle a reluctant 4.5-billion-year-old subject that jealously guards all secrets about its distant youth. By slowly chipping away at the rock record, however, researchers are starting to develop a portrait of Earth in its tumultuous first 2 billion years, the Archean era.

Evidence culled from South Africa now reveals that the planet had a flip-flopping magnetic field as far back as 3.2 billion years ago, reports a trio of geophysicists this week.

"We feel we have an example of the oldest known reversal of Earth's magnetic field. It looks like the magnetic field has behaved similarly since that time," says Paul W. Layer of the University of Alaska, Fairbanks.

Generated by currents of molten iron alloy within Earth's core, the geomagnetic field has a habit of reversing its direction every few hundred thousand years. Although evidence of the field's existence reaches back 3.45 billion years, scientists could not tell when the field had matured to the point of flipping its polarity. Prior to the South African discovery, the oldest reversal record had hailed from rocks about 2.7 billion years old.

Layer and his coauthors, Alfred Kroner of the Johannes Gutenberg University in Mainz, Germany, and Michael McWilliams of Stanford University, report their find in the Aug. 16 Science.

The team uncovered signs of the reversal in northeast South Africa within a pluton-a deposit of once-molten volcanic rock that forced its way up through the crust and then hardened while still underground. Over billions of years, erosion stripped away rock layers above and exposed the innards of the 30-kilometer-wide pluton.

The pluton stores ancient magnetic information because it contains iron- rich particles that aligned themselves with the orientation of Earth's field when the rock was molten. As the deposit gradually cooled 3.2 billion years ago, these particles became locked in place, recording the field's direction.

Layer and his colleagues discovered that rock toward the edge of the pluton showed a magnetic orientation almost directly opposite to that of material in its interior. They reasoned that the magnetic field must have reversed during the several million years it took the entire pluton to cool. Because the outer layer of the pluton cooled first, it locked in the field direction prior to the reversal. Later, the interior recorded the postreversal orientation.

If subsequent volcanic activity had reheated the pluton, it would have reset the magnetic orientation to that of a younger age. But Layer and his colleagues say that they have ruled out this possibility by studying the radioactive decay of elements within the rock. The data indicate that the pluton has escaped severe heat since the time it first cooled, in Earth's adolescence.

Not all geomagnetists share this certainty. Rob Van Der Voo of the University of Michigan in Ann Arbor says that "one has to disprove remagnetization, and that is hard to do. I've looked at hundreds of rock formations and about half of them are probably remagnetized."

Layer's tests get a better reception from Joseph G. Meert of Indiana State University in Terre Haute, who has documented a later Archean reversal in Kenya. "I and others will approach this with caution. But the authors have demonstrated the case against resetting as best they could."

Meert says he would like to see confirming evidence of the reversal from another site, although he holds out little hope of finding other relatively undisturbed rocks from this age.