Ancient rocks reveal evidence of the first continents and crust recycling processes on Earth

MADISON — Parts of the ancient Earth may have formed continents and recycled crust through subduction far earlier than previously thought.

New research led by scientists at the University of Wisconsin–Madison has uncovered chemical signatures in zircons, the planet’s oldest minerals, that are consistent with subduction and extensive continental crust during the Hadean Eon, more than 4 billion years ago. The findings challenge models that have long considered Earth's earliest times as dominated by a rigid, unmoving “stagnant lid” and no continental crust, with potential implications for the timing of the origin of life on the planet.  

The study, published Feb. 4 in the journal Nature, is based on chemical analyses of ancient zircons found in the Jack Hills of Western Australia. These sand-sized grains preserve the only direct records of Earth’s first 500 million years and offer rare insight into how the planet’s surface and interior interacted as continents first formed.

The conclusions come from measurements of trace elements within individual zircon grains using the WiscSIMS, a powerful instrument housed on the UW–Madison campus that can analyze microscopic objects one-tenth the diameter of a human hair. The UW team developed new procedures for analysis of certain elements that they couldn’t assess previously. 

These elements are essentially fingerprints of the environments where the zircons formed, allowing the scientists to distinguish zircons that formed in magmas that originated in the Earth's mantle beneath Earth’s crust from those associated with subduction and continental crust. Because zircons lock in their chemistry when they crystallize and are highly resistant to alteration, they preserve uniquely reliable records of early Earth processes, even after several billion years.

“They’re tiny time capsules and they carry an enormous amount of information,” says John Valley, a professor emeritus of geoscience at UW–Madison who led the research.

Valley says that the chemistry of zircons found in the Jack Hills clearly shows that they originated from a much different source than other Hadean zircons found in South Africa, which carry a chemical signature typical of more primitive rocks originating within the Earth's mantle.

“What we found in the Jack Hills is that most of our zircons don’t look like they came from the mantle,” says Valley. “They look like continental crust. They look like they formed above a subduction zone.”

Together, the two groups of zircons suggest that early Earth was not dominated by a single tectonic style, according to Valley. 

“I think the South Africa data are correct, and our data are correct,” Valley says. “That means the Hadean Earth wasn’t covered by a uniform stagnant lid.”

Importantly, the type of subduction that could have produced the Jack Hills zircons is not necessarily the same as in modern plate tectonics. Valley described a process in which mantle plumes of ultra-hot rock rose, partly melted and pooled at the base of the crust, creating circulation that could draw surface materials downward.

“That is subduction,” he says. “It’s not plate tectonics, but you have surface rocks sinking down into the mantle.”

This matters because subduction carries water-rich surface rocks down to hotter depths, where they can cause melting and form magmas that produce granitic rocks.

“If you have material on the surface, the surface had liquid water in the Hadean,” Valley says. “And when you take that material down, it’s wet and dehydrates. The water causes melting and that forms granites.”

Granites and related rocks are fundamental building blocks of continents. They’re less dense than other common rocks found under Earth’s oceans. This creates buoyant continents that rise higher above the ocean basins, providing stable environments on the Earth’s surface.

“This is evidence for the first continents and mountain ranges,” Valley says.

The results suggest that early Earth was geologically diverse, with different tectonic styles operating simultaneously in different regions.

“We can have both a stagnant-lid-like environment and a subduction-like environment operating at the same time, just in different places,” Valley says.

That complexity could reshape how scientists think about the planet’s first billion years, and the implications extend beyond tectonics. Subduction and continent formation influence when dry land first appeared and how surface environments evolved.

“What everybody really wants to know is, when did life emerge?” Valley says. “This doesn’t answer that question, but it says that we had dry land as a viable environment very early on.”

The oldest accepted microfossils are about 3.5 billion years old, but the Jack Hills zircons push evidence for potentially habitable surface conditions much earlier.

“We propose that there was about 800 million years of Earth history where the surface was habitable, but we don’t have fossil-evidence and don’t know when life first emerged on Earth,” Valley says.

As scientists continue to hunt for evidence of what the earliest Earth was like, Valley says the latest results are an example of the power of improving and refining laboratory techniques.

“Our new analytical capabilities opened a window into these amazing samples,” he says. “The Hadean zircons are literally so small you can’t see them without a lens, and yet they tell us about the otherwise unknown story of the earliest Earth.”

This research was supported by the European Research Council under the European Union’s Horizon H2020 research and innovation program (856555) and the National Science Foundation (EAR-2320078, EAR-2136782). 

END