Posted in: 24/08/2022 – 15:36
A new study has found that a 4-billion-year-old piece of Earth’s crust the size of Ireland lies beneath the dirt in western Australia.
This piece of crust is among the oldest on Earth, if not the oldest. This is thanks to the Canadian Shield rocks on the eastern shore of Hudson Bay, which are 4.3 billion years old (Earth is 4.54 billion years old). Because the Earth’s crust is constantly agitated and pushed back into the mantle by plate tectonics, most of the planet’s rocky surface has formed over the past billion years.
However, the oldest crust discovered, such as the newly discovered piece in Western Australia, is about 4 billion years old. This indicates that something special happened at that point in Earth’s history, study co-author Maximilian Druelner, a doctoral student at Curtin University in Australia, said in a statement.
When comparing the scientists’ findings with the existing data, Druelner added, it appears that many regions around the world experienced a similar timing of early crust formation and preservation. This indicates a major change in the evolution of the Earth about four billion years ago, as meteorite bombardment waned, the crust settled and life on Earth began to establish.
The hidden piece of ancient crust is located in Jack Hills in Australia near where the oldest minerals on Earth were previously found. The researchers discovered a minute mineral called zircon dating back 4.4 billion years. These minerals have survived even as the rocks that once held them have eroded. Some of these rocks around the Jack Hills, known as Narryer Terrane, date back 3.7 billion years.
Geochemical hints in sediments near this area indicate that there may be older crust buried under the new rocks and sediments at the surface. So Droelner and his colleagues decided to test zircon in sediments from the Scott Coastal Plain, south of Perth. The sediments in this plain are eroded from the deep rocks of the Australian continent.
To do this, the researchers induced zircon to evaporate with powerful lasers, then analyzed the composition of two pairs of radioactive elements released by the laser: uranium, lead, lutetium, and hafnium. Copies of the elements trapped in these zircons fade over billions of years. The relative amounts of each copy tell the researchers how long the elements decay, providing an “hour” about the life of the zircon.
This date revealed that the rocks that hold these minerals formed between 3.8 billion and 4 billion years ago.
To learn more about the source of these minerals, researchers turned to data collected by Earth-orbiting satellites. Because the Earth’s crust varies in thickness, gravity varies slightly across the planet’s surface. By measuring these differences in gravity, scientists can tell how thick the crust is at different locations. This gravity data revealed a thick patch of crust in the southwestern part of Western Australia, likely the site of buried ancient crust.
The ancient crust covers an area of at least 38,610 square miles (100,000 square kilometers), the researchers write in their paper, published online June 17 in the journal Terra Nova. Droelner said the crust is buried “tens of kilometers” below the surface of the Earth. The researchers found that the ancient crust boundary is linked to deposits of gold and iron ore, hinting at the importance of this very old crust in controlling the formation of rocks and minerals in the area.
Understanding the formation of the crust 4 billion years ago could help researchers understand how continents formed for the first time, the researchers said. This period paved the way for the planet as it is today, but few hints of the early Earth have survived the constant turbulence of the planet’s surface.