Caution Urged
University of Colorado's Mojzsis said the data used to reach the 4.28-billion-year-old date "meets the highest standards"—but he is unconvinced the rocks themselves are that old.
The rocks could be derived from 4.28-billion-year-old sediments, he noted, similar to rocks that form from pre-existing, older rocks.
"We need to be cautious about this and, frankly speaking, skeptical," he said.
Mojzsis added that he is currently looking for a rock in the Hudson Bay region that contains 4.28-billion-year-old zircon, which he said would help resolve the debate.
Mojzsis's research is funded, in part, by a grant from the National Geographic Society's Committee for Research in Exploration. (National Geographic News is owned by the National Geographic Society.)
If the rocks themselves are shown to be 4.28 billion years old, he added, they would represent the only known piece of crust that has survived from the Hadean, the earliest era of Earth's history.
Most pieces of Earth's crust have been mashed up, melted, and reformed time and again through the process of plate tectonics.
Pinpointing when Earth's crust-mantle and plate tectonics systems got underway remains an active area of geological research, Mojzsis added.
The Hudson Bay rocks, he said, reinforce the idea that the early Earth "was one that had dynamic rock cycling and volcanism and formation of crust from the earliest days."
http://news.nationalgeographic.com/news/2008/09/080925-oldest-rocks.html