Continents on early Earth rose from the sea, then disappeared

According to a new model, Earth's first continents emerged much earlier than previously thought. After emerging from the planet's primordial oceans, the continents disappeared without a trace.

Researchers in Australia designed the model of rock radioactivity to better understand the history of Earth's crust. Simulations showed Earth's crust was much thicker, much earlier than previous models predicted.

"We use this model to understand the evolving processes from early Earth to the present, and suggest that the survival of the early crust was dependent on the amount of radioactivity in the rocks -- not random chance," Derrick Hasterok, geoscientist at the University of Adelaide, said in a news release. "If our model proves to be correct, it may require revision to many aspects of our understanding of the Earth's chemical and physical evolution, including the rate of growth of the continents and possibly even the onset of plate tectonics."

To build their model, scientists catalogued 75,800 geochemical signatures of igneous rocks. Each rock sample was tagged with an estimated date and level of radioactivity. Scientists designed their model to simulate the history of average radioactivity across Earth's continents over the last 4 billion years.

"All rocks contain natural radioactivity that produces heat and raises temperatures in the crust when it decays -- the more radioactive a rock the more heat it produces," said Hasterok. "Rocks typically associated with the continental crust have higher radioactivity than oceanic rocks. A rock four billion years old would have about four times as much radioactivity when it was created compared with today."

In rocks older than two billion years, Hasterok and his colleagues were surprised to find low radioactivity levels. When they tweaked their model to account for greater heat output, which boosted radioactivity levels, they erased the disparity.

"We think there would have been more granite-like -- or continental-type -- rocks around but because of the higher radioactivity, and therefore higher heat, they either melted or were easily destroyed by tectonic movement," Hasterok said. "That's why these continental crusts don't show in the geological record."

The models showed continents emerged from Earth's oceans as the planet's crust thickened. But even before Earth's earliest continents emerged, the new simulations showed Earth's had begun to develop substantial but unstable continental crust much earlier than previously thought.

Scientists shared the predictions of their new radioactivity model in the journal Precambrian Research.

"What this new model allows us to do is help predict rock radioactivity in places where we have few or no samples, like Antarctica, where we cannot access samples, which could be very important in assessing the stability of ice sheets and the threshold of temperature changes needed for global warming to impact glacial melting," said Martin Hand, a professor of earth sciences at the University of Adelaide.

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