A 4-billion-year-old chunk of the Earth’s crust the size of Ireland is hiding beneath Western Australia, new research has found.
This piece of crust is the oldest on Earth, though not the oldest. That respect goes Rocks of the Canadian Shield on the eastern shore of Hudson Bay, which is 4.3 billion years old. (Earth is 4.54 billion years old.) Because Earth’s crust is churned by plate tectonics and pushed back into the mantle, most of the planet’s rocky surface was formed within the last two billion years.
However, the oldest crust found in Western Australia, like the newly discovered fragment, is about 4 billion years old. Study coauthor Maximilian Drollner, a doctoral student at Curtin University in Australia, suggests something special happened during that era of Earth’s history. statement.
“Comparing our findings with existing data, it appears that many regions around the world experienced similar times of early crust formation and preservation,” Drolner said. “This suggests a significant change in Earth’s evolution about four billion years ago, when meteorite bombardment decreased, the crust stabilized and life began to establish itself on Earth.”
Related: Earth’s outer shell ballooned up 3 billion years ago
A hidden piece of ancient crust is near where the oldest minerals on Earth were found. In Australia’s Jack Hills, researchers have discovered a tiny mineral called zircon 4.4 billion years old. These minerals survive in rocks once they have been eroded. The rocks surrounding the Jack Hills, known as the Narrier Terrane, are not new at all: some are 3.7 billion years old.
Geochemical signals in the sediments near the area suggested that there may be new rocks and older crust buried beneath the sediments at the surface. So Drolner and his colleagues decided to test zircon in sediments from the Scott Coastal Plain, south of Perth. The sediments in this plain come out of the deep rocks of the Australian continent.
To do this, the researchers vaporized the zircons with powerful lasers, then analyzed the composition of the two pairs of radioactive elements liberated by the lasers, uranium and lead and lutetium and hafnium. The versions of these elements trapped in these zircons decay over billions of years. The relative amount of each variant, or isotope, tells researchers how long the elements have been decaying, providing a “clock” on the age of zircons.
This dating revealed that the rocks containing these minerals were formed between 3.8 billion and 4 billion years ago.
To learn about where these minerals came from, researchers turned to data collected by Earth-orbiting satellites. Because Earth’s crust varies in thickness, gravity varies slightly across the planet’s surface. By measuring these differences in gravity, scientists can determine how thick the crust is in different locations. These gravity data revealed a thick section of crust in the southwestern part of Western Australia, possibly 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, Drolner said. The boundaries of ancient crust are associated with gold and iron ore deposits, the researchers found, indicating the importance of this much older crust in controlling the formation of rocks and minerals in the region.
Understanding the formation of the crust 4 billion years ago could help researchers understand how the continents first formed, the researchers wrote. This period set the stage for the planet as it is today, but few signs of the early Earth have survived the constant upheaval of the planet’s surface.
“This part of the ridge has survived many mountain-building events between Australia, India and Antarctica,” Drelner said.
Originally published in Live Science.