CANADA: Oldest rocks in the world

Remnants of the Earth's early crust have been found in a belt of ancient bedrock in northern Quebec, along the eastern shore of Hudson Bay. They have been around since roughly 300 million years after the planet was formed 4.6 billion years ago, says Jonathan O'Neil, a doctoral candidate at McGill University and the lead author of a paper published in the journal Science.

The rocks offer scientists a glimpse of the distant past, and what the young Earth looked like. They suggest that continents formed very early in the Earth's history, says Dr Richard Carlson at the Carnegie Institution in Washington, a member of the team that analysed the rocks and a co-author on the paper.

That is because their chemical composition resembles volcanic rocks in places where tectonic plates crash together today, he says, such as the Cascade mountain range that extends from southern British Columbia into the United States.

Previously, the oldest known rocks were from an outcrop known as Acasta Gneiss in the Northwest Territories, southeast of Great Bear Lake, which are 4 billion years old. Tiny mineral grains within rocks in Western Australia have been dated at 4.36 billion years old.

"But what we have is the actual rock," O'Neil says.

Rocks offer more clues about the processes that shaped the Earth, says Carlson, but it is rare to find remnants of the early crust, because most of it has been mashed and recycled into the Earth's interior by plate tectonics.

Carlson and O'Neil were part of a team that included Don Francis at McGill and Ross Stevenson at the Université du Québec à Montréal.