New date resets geologic clocks.

What if someone discovered that a day lasts 25.8 hours instead of 24? That's precisely the kind of revision scientists are now facing, after finding age problems with one of the more familiar yardsticks in Earth's history.

Researchers often use prominent changes in the planet's magnetic field as reference points for dating rocks. For more than a decade, they have considered the last major change as occurring 730,000 years ago, between the Brunhes and Matuyama geomagnetic periods. But recent experiments suggest this transition occurred 50,000 years earlier -- a finding that could substantially alter ideas about the past.

Geophysicist Michael McWilliams of Stanford University dated the Brunhes-Matuyama transition at 780,000 years ago using the argon-argon technique, a variation of the standard potassium-argon dating method. His finding confirms results reported last year by oceanographers who redated the transition using an entirely different technique, based on counting the number of Earth's orbital oscillations in ocean sediments. The previous age of 730,000 was determined from conventional potassium-argon dating, which can yield inaccurate results when used with certain types of rocks, McWilliams says.

He notes that the new age would solve some problems confronting scientists studying the San Andreas fault. Precise measurements across the fault suggest that the Pacific plate moves past North America at a rate of 48 millimeters per year. But a different type of estimate, based on magnetic lineations in the ocean, puts that speed at 51 millimeters per year. Redating the Brunhes-Matuyama transition eliminates the discrepancy, McWilliams says.