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A summer of ordinary geologic investigation in a remote area of Canada has reaped an unexpected scientific bonanza: nearly 4-billion-year-old rocks, the oldest known on Earth.

Samuel Bowring, head of the U.S. and Australian team that found the ancient stones, told the National Science Foundation Wednesday that two rocks collected in Canada's Northwest Territories are about 3.96 billion years old."We've now pushed 100 million years farther back in terms of rock we can study, into a period of Earth's history about which we know almost nothing," said Bowring, an assistant geology professor at Washington University in St. Louis.

The granitelike rocks, which do not look unusual to the average observer, were removed from outcroppings north of the Great Slave Lake. Bowring's team spent from 1983 to 1989 collecting samples from the region, which is near the middle of the North American continental plate where geologists have suspected they might find ancient rocks.

The scientists dated the two rocks in question by studying the chemical makeup of tiny zirconium crystals chipped off the main blocks, which weighed about 5 pounds and 50 pounds.

The device used to perform the analysis was Australia's Sensitive High Mass-Resolution Microprobe, one of the most precise dating instruments in the world. Results of the study, which looked at the ratio of uranium to lead in the crystals, pinpointed the date within about 3 million years.

Previously, a rock found in Greenland was believed to be the world's oldest, dating back 3.82 billion years. Most scientists think Earth and nearby planets formed 4.5 billion years ago from material left as the cooling sun shrank.

"With this new discovery, we are a giant step further in our search for knowledge of what happened in the Earth's first 800 million years," said Bowring, whose findings will be published next month in the journal Geology.

He added, "The question now is whether rocks from 4.6 to 3.8 billion years were mostly destroyed by meteorite impact or whether they never formed in great volume."

Thomas Wright, a geologist with the National Science Foundation, agreed the discovery is of "far-ranging significance" for researchers trying to figure out Earth's development.

Scientists do not know exactly when Earth evolved from a uniform lump of elements into its current structure of a superheated core, intermediate mantle and rocky crust, Wright said.

"This (new finding) has nudged that process back by at least 100 million years," he said, adding it is likely that even older rocks exist in the remote Canadian region.