A new model of ancient Earth has revealed that continents in the earlier period appeared and then suddenly disappeared without leaving little to no trace. Results of the research, published in the journal Precambrian Research, can be used to produce more accurate models of oil maturation in sedimentary basins.
During the course of the latest study, the researchers found that Earth’s continental crust may have been thicker, much earlier than current models suggest, with continents possibly present as far back as four billion years.
“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,” said Dr Derrick Hasterok, geoscientist at the University of Adelaide. “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.”
In order to build the model, Hasterok and his student Matthew Gard compiled 75,800 geochemical samples of igneous rocks of different ages from different continents. After estimating radioactivity in the rocks, the researchers were able to create a model of average radioactivity from four billion years ago to today.
Dr Hasterok explained that all of the rocks contain natural radioactivity responsible for heat and temperate as they decay. Interestingly, the rocks associated with continental crust have higher radioactivity than the oceanic rocks. For example, a rock which is four billion years old, will have about four times more radioactivity compared to a rock if it was created today. To their surprise, the researchers found an unexpected deficit in the level of radioactivity in rocks older than about two billion years. When they corrected the results for higher heat production, 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.”