Earth formed about 4.6 billion years ago during the Hadean Eon, a period characterized by extreme heat that likely made the planet uninhabitable. By around 4.35 billion years ago, it is believed that the Earth had cooled sufficiently to allow for the formation of the first crust and possibly the emergence of life. However, this early chapter in Earth’s history is shrouded in mystery due to the scarcity of preserved geological records, posing challenges in reconstructing the planet’s appearance during the Hadean Eon and leaving many questions unanswered.
The oldest known terrestrial rocks on Earth, located in northern Quebec’s Nuvvuagittuq Greenstone Belt, date back more than 4 billion years and provide invaluable insights into the origins of our planet. The Hadean Eon marks the first period in the geological timescale, spanning from Earth’s formation around 4.6 billion years ago until about 4.03 billion years ago. Among the earliest materials dated by scientists are zircon minerals discovered in Western Australia, formed as early as 4.4 billion years ago. While the host rock of these zircons has eroded away, the minerals’ durability has allowed them to survive, offering clues about the environment of the Hadean and the formation and evolution of the Earth’s crust.
Chemically, the zircons suggest that they were formed from magmas produced by the melting of sediments deposited at the bottom of an ancient ocean, indicating that Earth may have cooled rapidly enough to allow liquid water oceans to form early in its history. Other studies of the Hadean zircons imply that the earliest crust was mafic, rich in magnesium and iron. Despite previous speculation, the existence of this crust was not confirmed until later research identified preserved rocks within the Nuvvuagittuq Greenstone Belt, first proposed as remnants of the Hadean in a 2008 study.
Located in northern Quebec, the Nuvvuagittuq Greenstone Belt comprises predominantly metamorphosed volcanic rocks. The most common rock in this region, the Ujaraaluk formation, translates to “big old solid rock” in Inuktitut. The age of 4.3 billion years for these rocks was suggested following the detection of neodymium-142 variations, an isotope formed during the Hadean via the radioactive decay of samarium-146. This dating technique had previously been utilized for meteorites and lunar rocks but had not been applied to Earth rocks until that time.
The proposed age of the Nuvvuagittuq rocks has since sparked significant debate among researchers, with some interpreting the presence of younger zircons within the belt to assign an age of around 3.78 billion years, thus placing them in the Archean Eon instead. A more concerted effort to determine the true age of these rocks came in the summer of 2017 when a team returned to the Nuvvuagittuq belt to collect intrusive rocks known as metagabbros, which cut through the Ujaraaluk formation. This geological relationship established that the Ujaraaluk rocks must precede the intrusive metagabbros, providing a basis for dating them accurately.
Collaboration with experts in geochronology involved applying two independent samarium-neodymium age dating methods, resulting in consistent outcomes indicating the metagabbros were 4.16 billion years old. Since these metagabbros intrude the Ujaraaluk rocks, the latter must be even older, firmly placing them within the Hadean Eon. Research into the Nuvvuagittuq rocks represents a unique opportunity to delve into the earliest history of Earth, aiding our understanding of how the first continents formed and the evolution of Earth’s environment towards becoming habituable.
-Raja Aditya
