Early Earth Had Continents That Have Disappeared Entirely Without A Trace

The Jack Hills in Western Australia are known as the oldest rocks on Earth, but they may not have been the first to form, just the first ones with low enough radioactivity to survive. Robert Simmon/NASA Earth Observatory/Public Domain

It's not quite Atlantis, but a new model of the early development of continental crusts proposes Earth really did once have lost continents, they were just a few billion years early to support a fabled civilization. The idea comes from comparing the age and radioactivity of rocks across the planet

Efforts to study the Earth when life first appeared requires trips to a few remote locations where rocks date back that far. The conventional explanation for this has been that the continental crust formed slowly, and for a long time Earth's surface was largely oceanic crust, which gets recycled every few hundred million years. In this view, an alien passing the planet in its first billion years would have found just a few bits of continent like Western Australia’s Jack Hills on what was otherwise a water-world.

Dr Derrick Hasterok of the University of Adelaide has challenged this idea and authored papers in Precambrian Research and Lithos, suggesting Earth's continental crust was much thicker, much earlier than current models imply, and continents could have existed as far back as 4 billion years ago before disappearing and leaving little trace.

Dr Hasterok's studies observe an inverse relationship between the age of rocks and their radioactivity. Although this has been observed before, the inclusion of 76,000 rock samples made it hard to ignore.

The world has very few rocks more than 3 billion years old - marked as brown on this map, but that doesn't mean the world then had little continental rock. Derrick Hasterok
Measures of radioactive heat production reveals a reverse pattern, with the oldest rocks showing relatively low radioactivity. Derrick Hasterok

Hasterok argues the relationship exists because all the most radioactive early crust has disappeared. “Rocks that are fairly radioactive produce heat,” Hasterok told IFLScience. In the early days of Earth, when radioactive isotopes were more abundant, this heat was four times greater. The most radioactive rocks melted, which Hasterok said “Destroys some degree of the knowledge of the old one,” leaving us the impression there was no crustal material there beforehand.

Where a large part of a continent was highly radioactive, Hasterok added, “The rocks became weak and tectonic processes could pull them apart more easily.” Entire proto-continents may have been destroyed, leaving no trace at all.

Allowing for these processes, Hasterok concludes, the early Earth had more and thicker continental crust than we give it credit. What he doesn’t yet know, and hopes to learn with further study, is whether it had an area a few times the preserved rocks – which would still be less than a single existing continent – or it the extent and thickness of continents 2-4 billion years ago approached what exists today.

Hasterok came to these conclusions by chance. He was attempting to model the radioactivity of parts of Antarctica we have been unable to study directly. Heat from the more radioactive pockets melts glaciers from below, contributing to the speed with which they move. In an effort to improve our understanding of this effect, and its influence on ice melt, Hasterok found himself exploring a field he described as “Much more interesting.”

 

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