4 billion-year-old chunk of Earth's crust found below Australia

A 4-billion-year-old piece of Earth's crust the size of Ireland is lurking beneath Western Australia, new research finds. 

This piece of crust is among the oldest on Earth, though not the oldest. That honor goes to rocks of the Canadian Shield on the eastern shore of the Hudson Bay, which have been dated to 4.3 billion years old. (The Earth is 4.54 billion years old.) Because Earth's crust is constantly being churned up and pushed back into the mantle by plate tectonics, most of the planet's rocky surface was formed within the last couple billion years. 

However, the oldest crust that has been discovered, like the newly found chunk in Western Australia, tends to date back around 4 billion years. That suggests something special occurred in that era of Earth history, study coauthor Maximilian Droellner, a doctoral student at Curtin University in Australia, said in a statement.

"When comparing our findings to existing data, it appears many regions around the world experienced a similar timing of early crust formation and preservation," Droellner said. "This suggests a significant change in the evolution of the Earth some four billion years ago, as meteorite bombardment waned, crust stabilized and life on Earth began to establish."

Related: Earth's outer shell ballooned during a growth spurt 3 billion years ago

The hidden piece of ancient crust is near where the oldest minerals on Earth have previously been found. In Australia's Jack Hills, researchers have discovered tiny minerals called zircons dating back 4.4 billion years. These minerals have survived even as the rocks that once held them have eroded away. The rocks around the Jack Hills, known as the Narryer Terrane, are no newbies, either: Some date back 3.7 billion years. 

Geochemical hints in the sediments near this region suggested that there might be even older crust buried under newer rocks and sediments at the surface. So Droellner and his colleagues decided to test the zircons in sediments from the Scott Coastal Plain, south of Perth. The sediments on this plain erode out of deeper 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 freed, 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, tells researchers how long the elements have been decaying, providing a "clock" on the age of the zircons. 

This dating revealed that the rocks holding these minerals formed between 3.8 billion and 4 billion years ago. 

To learn about where these minerals came from, the researchers turned to data collected by Earth-orbiting satellites. Because Earth's crust varies in thickness, gravity varies slightly across the surface of the planet. By measuring these variations in gravity, scientists can figure out how thick the crust is in different locations. This gravity data revealed a thick segment of crust in the southwestern part of Western Australia, likely to be the site of the 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 Terra Nova. 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 the formation of crust 4 billion years ago can 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 hints of the earliest Earth have survived the constant upheaval of the planet's surface.

"This piece of crust has survived multiple mountain-building events between Australia, India and Antarctica," Droellner said.

Originally published on Live Science.