Earth was formed around 4.6 billion years ago, during a time called the Hadean Eon. The name "Hadean" comes from the Greek god of the underworld and was given to this period because of the extreme heat Earth had in its early years.

By 4.35 billion years ago, Earth may have cooled down enough for its first solid crust to form—and possibly even for the first signs of life to appear. But scientists still know very little about this time because rocks from the Hadean Eon are extremely rare. Most of them have eroded away or been recycled deep inside the planet, making it hard to study how Earth looked or changed in its very early days.

Now, a team of researchers has found solid evidence that the oldest rocks on Earth are located in northern Québec, Canada. These rocks are more than 4 billion years old and help scientists understand what our planet was like just after it formed.

Zircons and the search for early earth clues

The Hadean Eon lasted from about 4.6 billion to 4.03 billion years ago. One of the few things scientists have from this time are zircon crystals, which were found in Western Australia. These tiny minerals, formed as early as 4.4 billion years ago, are very strong and survived even though their original rocks did not.

By studying these zircons, scientists learnt important details about the early Earth. For example, they suggest that Earth cooled faster than expected and that liquid water oceans may have formed early. The zircons also point to the Earth's earliest crust being mafic, meaning it had lots of magnesium and iron.

In 2008, a study led by Jonathan O’Neil, then a PhD student at McGill University, claimed that some of Earth’s earliest rocks still existed in northern Québec, in a place called the Nuvvuagittuq Greenstone Belt. These rocks were thought to be 4.3 billion years old, based on the presence of a rare isotope called neodymium-142—created only during the Hadean Eon.

Confirming the age of the ancient rocks

The Nuvvuagittuq Greenstone Belt is found in the Nunavik region of northern Québec, above the 55th parallel. It contains rocks known as the Ujaraaluk formation, a name that means "big old solid rock" in Inuktitut, the local Indigenous language. Most of these rocks are metamorphosed volcanic rocks, rich in magnesium and iron.

However, some scientists disagreed with the claim that these rocks were from the Hadean Eon. They believed they were younger, around 3.78 billion years old, placing them in the Archean Eon, the next time period after the Hadean.

To clear this doubt, the research team went back to the Nuvvuagittuq belt in the summer of 2017. This time, they collected new samples of intrusive rocks called metagabbros that cut across the Ujaraaluk rocks. If these newer rocks could be dated, the team could then confirm that the Ujaraaluk rocks must be even older.

The project was led by Chris Sole, a master’s student at the University of Ottawa. Other team members included undergraduate students David Benn and Joeli Plakholm and French geochronologist Jean-Louis Paquette. The team used two different dating methods that look at how samarium and neodymium isotopes change over time in rocks.

Both methods gave the same result: the metagabbros are 4.16 billion years old. Since these rocks cut through the Ujaraaluk formation, it proves that the Ujaraaluk rocks are even older—putting them firmly in the Hadean Eon.

A rare window into earth’s past

These rocks are the only preserved remains from the Hadean Eon, giving scientists a unique chance to learn how Earth’s first crust formed and how the planet became habitable for life. By studying them, researchers can better understand the formation of the first continents, the cooling of the planet, and possibly how early life began.

“Studying the Nuvvuagittuq rocks, the only preserved rocks from the Hadean, provides a unique opportunity to learn about the earliest history of our planet. They can help us understand how the first continents formed and how and when Earth's environment evolved to become habitable.”

The inputs for this article are taken from a piece written by Hanika Rizo, Associate Professor, Department of Earth Sciences, Carleton University, and Jonathan O'Neil, Professor, Earth and Environmental Sciences, University of Ottawa.

(Disclaimer: Hanika Rizo and Jonathan O'Neil receive funding from the Natural Sciences and Engineering Research Council of Canada [NSERC].)

This article is based on information shared on The Conversation under a Creative Commons license. 

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