A team of researchers has discovered a 4-billion-year-old piece of Earth’s crust beneath Western Australia . The fragment, which would be larger than Ireland, would have survived episodes of great upheaval and changes that occurred on our planet.
The discovered fragment is one of the oldest on Earth. It is surpassed by the rocks of the Canadian Shield, on the eastern shore of Hudson Bay, which are 4.3 billion years old. This data is very important and it is estimated that our planet is 4.54 billion years old. Because the Earth’s crust is constantly churning and being pushed into the mantle by plate tectonics, most of the planet’s rocky surface was formed in the last two billion years.
The piece recently found in Australia would have an age of about 4,000 million years. This suggests, according to Maximilian Droellner, a researcher at Curtin University (Australia) who participated in the study, that something special happened at that time in the history of our planet.
“Comparing our findings with existing data, it appears that many regions around the world experienced a similar timing of early crustal formation and preservation,” said Droellner. “This suggests a significant change in Earth’s evolution about four billion years ago, when meteor bombardment subsided, the crust stabilized, and life on Earth began to establish itself.”
The ancient shard is near an area that holds the oldest minerals on Earth, in Jack’s Hills. There, scientists have discovered very small minerals called zircons , which are believed to be 4.4 billion years old. Although the rocks that once contained them have been eroded away, the tiny zircons have managed to survive. The rocks surrounding Jack’s Hills are not new either, as some of those known as Terrana Narryer are 3.7 billion years old.
Sediment features near the region hinted that there might be an even older crust buried under younger surface rocks and sediments. The researchers then decided to analyze the zircons in the sediments of the Scott Coastal Plain, south of Perth. These sediments, in particular, are eroded from deeper rocks on the Australian continent.
To conduct their study, the researchers applied powerful lasers to the zircons and then analyzed the composition of two pairs of radioactive elements that the lasers had released, uranium and lead and lutetium and hafnium. The versions of these elements found in zircons break down over billions of years. The relative amounts of each isotope tell researchers how long elements have been decaying, providing a “clock” indicative of the age of zircons. The dating revealed that the rocks containing these minerals formed between 3.8 and 4 billion years ago.
To find out where the minerals came from, scientists looked at data collected by satellites . As the earth’s crust is not the same thickness everywhere, gravity varies slightly from one place to another. By measuring the variation in gravity, you can find out how thick the crust is in different areas. The gravitational data obtained revealed a thick segment of crust in southwestern Western Australia, which is likely to be where the ancient crust was buried.
As the researchers have published in their study, the crustal fragment covers an area of 100,000 square kilometers and is buried tens of kilometers below the surface. Gold and iron deposits exist at the edge of the ancient crust, indicating that this old fragment may have been important for the formation of rocks and minerals in the region.
The researchers point out in their study that if they can understand how this crust was formed 4 billion years ago, they could obtain more information about the formation of the continents . This period laid the foundations for the planet as we know it today, but few traces of the earliest Earth have survived the constant turmoil of the planet’s surface.
“This chunk of crust has survived multiple mountain building events between Australia, India and Antarctica,” Droellner said.
Reference: Droellner, M., Kirkland, C. et. al. A persistent Hadean–Eoarchean protocrust in the western Yilgarn Craton, Western Australia. 2022. Terra Nova. DOI: https://doi.org/10.1111/ter.12610
