Lost Continents of Early Earth

“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,” says Dr 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.”

But the researchers found an unexpected deficit in the level of radioactivity in rocks older than about two billion years. When they corrected for higher heat production, because of the higher radioactivity that would have been present, the deficit disappeared.

“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. That’s why these continental crusts don’t show in the geological record.

“Our prevailing models suggest that continents eventually grew out of the oceans as the crust thickened. But we think there may have been significant amount of, albeit very unstable, continental crust much earlier.”

Co-author Professor Martin Hand, also from the University of Adelaide, says the new model could have important implications for monitoring the effects of global warming.

“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,” says Martin Hand, Professor of Earth Sciences.

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The researchers say the new radioactivity model also may help in the search for hot rocks with geothermal potential and can be used to produce more accurate models of oil maturation in sedimentary basins.

The studies are published in the journals Precambrian Research and Lithos.

This research aligns with the University’s industry engagement priority in energy, mining and resources and in tackling the grand challenges of sustainable energy and environmental sustainability.

Contacts and sources:
Robyn Mills / Dr Derrick Hasterok
University of Adelaide

Citation: A 4 Ga record of granitic heat production: Implications for geodynamic evolution and crustal composition of the early Earth.
D. Hasterok, M. Gard, G. Cox, M. Hand. Precambrian Research, 2019; 105375 DOI: 10.1016/j.precamres.2019.105375