Noted: The zircons also contain enough uranium that they can be precisely dated by the decay of that uranium. In 2001, two groups, one led by Dr. Harrison and the other by John W. Valley of the University of Wisconsin, reported that the Australian zircons formed during the Hadean period as long ago as 4.4 billion years and were later embedded in the younger, 3-billion-year-old rocks.
Office of Strategic Communication