A new study of some of the world’s oldest rocks suggests that Earth’s first continents were unstable, sank back into the mantle before making their way back up and getting better.
This may explain some of the most puzzling characteristics of the craton, the very ancient and stable parts of the lithosphere (the crust and upper mantle) that have survived continental changes over the ages and chronicle the ancient history of the Earth.
TNew discoveries could help us understand Earth’s changing geology over its 4.5 billion-year lifespan.
“The rocks at the core of the continents, called kratons, are more than three billion years old,” explains geologist Fabio Capitanio of Monash University’s School of Earth, Atmosphere and Environment in Australia.
“They were formed at the beginning of the Earth and they hold the secret of how the continents and planet have changed over time.”
We don’t really know how the continents formed. No other planet in the solar system has anything like them, so it seems clear that there must be a definite set of circumstances.
There is a lot of evidence to suggest that continents may have formed from the inside out, around the insect cores. But the mechanism of kraton formation itself is hotly debated.
Cratons, which around 35 currently knownbuoyant and solid compared to other parts of the lithosphere, which gave it stability. But its composition is unusual compared to the more recent lithosphere, which consists of a strange mixture of materials and minerals with a range of ages, compositions, and sources.
This asymmetry, or variety, Suggest recycling and rework, previous research found.
Capitanio and his team performed computational modeling to simulate Earth’s evolution during its first billion years of existence, observing the thermal and chemical evolution of the cratonic lithosphere. In addition, they ran a set of test simulations to see how sensitive their model was to different parameters.
The results showed that the first continental masses that appeared on Earth were unstable, and sank back into the mantle. There they melted and mixed with molten materials until they melted.
However, some pieces can stay there for a long time before they float back up, piling up under the lithosphere in layers, giving them buoyancy and rigidity.
Since some of those ancient rock fragments can remain in the mantle for long periods of time, this could explain the heterogeneity of the crustal structure: ancient rocks from different places mixed with younger rocks.
In fact, there are still some of those pieces at the bottom, just waiting to float up again.
The team dubbed this mechanism “massive regional transport” (MRR). Because it fits so well with the observed composition of the craton, the team says it may have been a key component of the continent’s formation early on Earth.
Given that the continents are believed to be very important for the emergence and continued existence of life on Earth, discovering how they formed has implications not only for our planet, but also for the search for habitable worlds outside the solar system.
“Our work is important in two ways,” says Capitanio.
“First, the cratons are where important minerals and other minerals are stored/found. Second, they tell us how planets formed and changed in the past, including how continents arose and how they supported life, and how the atmosphere formed and changed as a result of planetary tectonics.”
The search was published in PNAS.
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