Scientists have dated the rocks, found in the western Slave Province of the Northwestern Territories, to 4.06 billion years, making them hundreds of millions of years older than the previous oldest rocks and half a billion years younger than the Earth itself.
Sam Bowring, professor of geology at the Massachusetts Institute of Technology in Cambridge, Massachusetts, said the find opens a new window on a little- known period."What is remarkable about these rocks is that they are not remarkable. They look like any other rocks on Earth today, which shows how the production of rocks hasn't changed in that time."
Geologists once thought no rocks could be older than 3.8 billion years, because the Earth had suffered meteorite bombardment before then, suggesting nothing on its surface could have survived.
But in the Eighties scientists who analysed minerals in the ancient rocks of Western Australia discovered crystals more than 4 billion years old. The search began to find rocks that could be this old.
The Canadian rocks are igneous - they were formed under the volcanic conditions of the Earth's interior - and contain microscopic "time capsules" of zircon crystals. Two kinds of uranium isotopes within the crystals degrade into isotopes of lead at a known rate, giving scientists two clocks to gauge the age of the material to within a couple of million years.
The oldest rocks to contain signs of life date to 3.8 billion years and were found in sedimentary deposits in Greenland. Sedimentary rocks are formed under watery, low-temperature conditions which are conducive to the survival of living organisms. Professor Bowring said he hopes to search the same part of Canada for similar sedimentary rocks which may possess fossilised remains of the earliest life forms to have evolved.
"There could be sedimentary rocks dating back to then, although no one knows when the oceans were in place which helped them to form," he said. If we find sedimentary rocks as old as 4 billion years I think we'll find evidence of life in them."
Ian Williams, a geologist at the Australian National University in Canberra and co-discoverer of the rocks, said: "The real interest in the old rocks will be to study their chemical and isotopic compositions, which will provide direct information on the rock-forming processes that were operating 4,000 million years ago and how the earliest continents formed.
"Even though these are the oldest rocks known, hidden within some of their zircon crystals we have found remnants of smaller zircons that are even older. These must have come from the rock from which these oldest rocks were formed."
The latest find could also help to establish whether the Earth's crust undergoes continual turnover or whether it is the result of a gradual accumulation of new material.