Formation of Earth's continents began much earlier than scientists had proposed

Earth may have started recycling its crust and forming continents far earlier than many scientists expected. A new study links chemical clues inside ancient crystals to short, intense bursts of early subduction and rapid crust growth.

The team analyzed tiny pockets of ancient magma sealed inside green olivine crystals and paired the chemistry with computer models of early Earth. The results point to active plate behavior in the Hadean – not a quiet planet capped by a still outer shell.

Lead author Adrien Vezinet, from Grenoble Alpes University, led the international collaboration that combined high precision geochemistry with geodynamic simulations.

Earth recycles its crust

Subduction happens when one tectonic plate sinks beneath another into the mantle. This process drives earthquakes, volcanoes, and mountain building.

It is a cornerstone of how our planet reshapes its surface and cycles materials from the crust back into the interior, as explained by the U.S. Geological Survey’s overview of subduction.

This conveyor belt also helps make a buoyant continental crust. Without it, continents would be smaller, thinner, and far less stable.

Very few clues from early Earth

The Hadean is the earliest slice of Earth’s history, from about 4.6 to 4.0 billion years ago, and it left precious few rocks we can hold in our hands.

A USGS fact sheet summarizes how little survives from that time and why it is often treated as an informal interval in geological time.

Because the Hadean rock record is sparse, scientists turn to indirect clues locked inside younger rocks that preserve older signals.

Ancient lava locks early secrets

South Africa’s Barberton Greenstone Belt preserves volcanic rocks more than 3.2 billion years old, offering a rare window into early Earth.

This UNESCO site is especially valuable for reconstructing the planet’s earliest conditions because its rocks include lava called komatiite, which erupted at extremely high temperatures.

That lava crystallized into olivine capable of trapping tiny droplets of the original melt.

These droplets, known as melt inclusions, are sealed pockets of magma that can record a magma’s composition before later alteration. When carefully screened, they serve as powerful archives of magmatic processes.

Building on this archive, researchers recently examined inclusions from 3.27-billion-year-old komatiites.

The Grenoble group measured strontium isotopes and trace elements within the inclusions. The Potsdam group developed geodynamic models to test what kinds of tectonic behavior could explain those chemical fingerprints.

Crystals and continent formation

The melt inclusions revealed an exceptionally low 87Sr/86Sr value of 0.69932 ± 0.00024, a model age of 4.31 ± 0.19 billion years, and canonical trace element ratios of Nb/U 36.9 ± 1.5 and Ce/Pb 16.7 ± 1.1.

These numbers indicate a mantle source that had been stripped of certain elements by earlier crust extraction.

“We report an unprecedented, unradiogenic Sr mantle source component,” wrote Adrien Vezinet, lead author at ISTerre, Université Grenoble Alpes.

From these values and mass balance, the authors estimate that up to 80 percent ± 16 percent of the mass of today’s continental crust – and much of the continents that formed from it – may have been extracted by the late Hadean. That is a startlingly high fraction for such an early time.

Reading Earth’s early chemistry

Geochemists use two “steady” ratios to track how Earth’s crust formed over time. These ratios, Nb/U and Ce/Pb, usually stay constant in mantle melts but shift whenever continental material is created and recycled.

The elevated Nb/U and Ce/Pb seen in these inclusions fit the pattern expected if large volumes of continental crust had already formed and the residues had been pushed back into the mantle.

The strontium isotopes add timing, pointing to sources isolated since around 4.31 billion years ago. Together, the isotope and trace element signals argue for a much livelier early mantle than many models assumed.

Models simulate early tectonics

The geodynamic side of the work explored different tectonic styles. The model that best matched the geochemical data produced 40 to 70 percent of the present continental crust mass during the Hadean.

It also required repeated episodes of short, intense subduction. The preferred scenario describes “a fluctuating mobile-lid tectonic regime,” wrote Vezinet.

In the simulations, large mantle plume upwellings repeatedly weakened the lid and kicked off subduction for tens of millions of years at a time.

Continents formed relatively fast

This picture challenges the idea that early Earth was stuck in a rigid, stagnant lid for hundreds of millions of years.

Instead, slabs likely sank and were recycled in pulses, seeding the mantle with the chemical signals the team captured in the inclusions.

It also reframes how quickly continents can form when the mantle is hot and wet enough to encourage subduction.

If much of the continental mass was already in place by the end of the Hadean, that affects models of early climates and ocean chemistry. It also changes how scientists view the settings where life’s building blocks assembled.

Caution in early Earth estimates

The authors are clear about limits. The highest crustal fraction estimates assume the whole mantle participated, so the true value may sit lower. Even so, it would still be high enough to favor active recycling very early.

Even so, the convergence of the inclusion chemistry and the models provides a coherent story. Early Earth was not quiet, and the fingerprints of that vigor survived inside crystals that formed billions of years later.

The study is published in Nature Communications.

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