A 4-billion-year-old chunk of the Earth’s crust the size of Ireland is hidden beneath Western Australia, a new study reveals.
This part of the crust is among the oldest on Earth, but not the oldest. This honor goes to him Canadian Shield rocks on the east coast of Hudson Bay, which have been dated to 4.3 billion years old. (Earth is 4.54 billion years old.) Because Earth’s crust is constantly being shuffled and pushed back into the mantle by plate tectonics, most of the planet’s rocky surface was formed within the last two billion years.
However, the oldest crust that has been discovered, like the newly found part in Western Australia, tends to date back to about 4 billion years ago. This suggests that something special happened in that era of Earth’s history, said study co-author Maximilian Droellner, a doctoral student at Curtin University in Australia. declaration.
“When we compare our findings with the existing record, it appears that many regions around the world experienced a similar time of early crustal formation and preservation,” Droellner said. “This suggests a significant change in Earth’s evolution about four billion years ago, after the meteorite bombardment faded, the crust stabilized, and life on Earth began to emerge.”
Connected: Earth’s outer shell rose during an uplift 3 billion years ago
The hidden part of the ancient crust is close to where the oldest minerals on Earth were previously found. In Australia’s Jack Hills, researchers have discovered tiny minerals called zircon dating back 4.4 billion years. These minerals have survived even when the rocks that once supported them have eroded away. The rocks around the Jack Hills, known as the Narryer Terrane, aren’t new either: Some date back 3.7 billion years.
Geochemical shifts in sediments near this region suggested that there may be even older crust buried beneath younger rocks and sediments at the surface. So Droellner and his colleagues decided to test zircons in sediments from the Scott Coastal Plain, south of Perth. The sediments in this area are eroded from the deepest rocks on the Australian continent.
To do this, the researchers vaporized the zircons with powerful lasers, 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 trapped in these zircons decay over billions of years. The relative amounts of each version, or isotope, tell researchers how long the elements have been decaying, providing a “clock” on the age of the zircons.
This dating revealed that the rocks bearing these minerals were formed between 3.8 billion and 4 billion years ago.
To learn where these minerals came from, researchers turned to data collected by satellites in Earth’s orbit. Because the Earth’s crust varies in thickness, gravity varies slightly across the planet’s surface. By measuring these changes in gravity, scientists can understand how thick the crust is in different places. These gravity data revealed a thick segment of crust in the southwestern part of Western Australia, which is likely the site of buried ancient crust.
The old crust covers an area of at least 38,610 square miles (100,000 square kilometers), the researchers wrote in their paper, published online June 17 in the journal. New land. It is buried “tens of kilometers” below the surface, Droellner said. The boundary of the ancient crust is associated with gold and iron ore deposits, the researchers found, hinting at the importance of this very old crust in controlling the formation of rocks and minerals in the region.
Understanding how the crust formed 4 billion years ago could help researchers understand how the continents first formed, the researchers write. This period set the stage for the planet as it is today, but few hints of the earliest Earth have survived the constant upheaval of the planet’s surface.
“This part of the crust has survived multiple mountain-building events between Australia, India and Antarctica,” Droellner said.
Originally published in Live Science.