Throughout its 4.5 billion-year history, the Earth’s crust — the thin outer layer of the planet — has been recycled several times, and most of the original crust has long been forced deep below the surface. The oldest portions of the crust that exist today are about 2.7 billion years old — much younger than the ancient crust that formed the “ancestors” of the modern-day continents.

A team of researchers from the University of Ottawa and the Carnegie Institution for Science has now discovered samples of Earth’s crust that existed over 4.2 billion years ago, when our planet was still in its formative years. The discovery, made after analysis of samples collected from a large mass of exposed rock in northwestern Quebec known as the Canadian Shield, has been detailed in a study published Friday in the latest edition of the journal Science.

“The Earth is always ‘recycling’ and re-melting its crust, erasing records of its early beginnings. We knew that the craton in northern Quebec had been formed by partial fusion of an ancient precursor crust, but for the first time we have specific information about the age and the nature of this older source,” lead author Jonathan O’Neil from the University of Ottawa’s Department of Earth and Environmental Sciences said in a statement.

Cratons are the oldest portions of Earth’s continents that have remained stable for over a billion years. Cratonic areas are present in every single continent.

In order to date the rocks obtained from near the eastern shore of the Hudson Bay in Northwestern Quebec, the researchers tracked the decay of an isotope called samarium-146. This isotope, which existed only for about 500 million years after the Earth formed, decayed into neodymium-142. By analyzing the amount of this element in the rocks and by comparing it with the amount present in newer rocks, the researchers were able to calculate their true age.

“Our findings suggest that the primordial crust involved in forming our craton was actually more than 4.2 billion years old and similar in composition to oceanic crust, which is very different from continental crust,” O’Neil said. “Re-melting or recycling of this primitive crustal ancestor produced the younger rocks of the Archean craton, which grew and stabilized to form the core of this part of the North American continent.”