The rocks may be remnants of Earth's primordial crust, which formed on the planet's surface as it cooled following the birth of the solar system, according to Jonathan O'Neil of McGill University in Montreal.
"Maybe it was the original crust, and before that there was no stable crust on the Earth. That's a big question," O'Neil said in a telephone interview.
The expanse in northern Quebec, measuring about 4 square miles (10 square km), is made up of the volcanic rock basalt. To determine the age of the rocks, geochemists used isotopic dating methods analyzing the elements samarium and neodymium.
The scientists, who describe the discovery in the journal Science, said studying these rocks can give clues about what the planet was like early in its history. The solar system, including the Earth, was formed about 4.57 billion years ago. These rocks date from roughly 290 million years later.
Richard Carlson of the Carnegie Institution of Washington said certain characteristics of the rocks suggest that water was already present on the Earth's surface. Scientists debate when oceans first appeared and whether water formed on the planet or was brought here when icy comets struck it.
The nature of the rocks also give clues as to temperatures when they formed, Carlson said.
"Probably when the planet formed it was a cauldron, but even this early in Earth history it had cooled down to something not dramatically different from today -- probably hotter but not dramatically hotter," Carlson said.
The scientists did not find direct evidence of life in the rocks. The earliest life is thought to have been bacteria.
"We know that probably the right environment was there for life to be on the Earth -- so liquid water and all it takes to have life. Now was there life? This is a big question mark," O'Neil said.
The previously known oldest rocks, in Canada's Northwest Territories, are 4.03 billion years old.
While some tiny mineral grains from western Australia date from 4.36 billion years ago, no complete rocks have been found older than these newly identified ones, the scientists said.
The rocks, found in an area called the Nuvvuagittuq greenstone belt, are a pinkish, brownish color.
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