(ORDO NEWS) — A new study of some of the world’s most ancient rocks suggests that the first continents on Earth were unstable and sank back into the mantle, then resurfaced and reformed.
This may explain some of the mysterious characteristics of cratons – extremely old and stable parts of the lithosphere (crust and upper mantle) that have survived the change of continents over many centuries and captured the ancient history of the Earth.
New finds could help us understand how Earth’s geology has changed over 4.5 billion years.
“The rocks at the core of the continents, called cratons, are over three billion years old,” explains geologist Fabio Capitanio of Monash University’s School of Earth, Atmosphere and Environment in Australia.
“They formed early in the development of the Earth and hold the secret of how the continents and the planet have changed over time.”
We don’t really know how continents formed. No other planet in the solar system has anything like them, so it seems obvious that there must be a certain set of circumstances.
There is some evidence that suggests that continents may have formed from the inside out around cratonic cores. However, the mechanism of formation of the cratons themselves is hotly debated.
Cratons, of which about 35 are currently known, are buoyant and rigid compared to other parts of the lithosphere, which ensures their stability.
But their composition is unusual compared to the more recent lithosphere, consisting of a strangely varied mixture of materials, minerals of different ages, compositions and sources.
This heterogeneity, or diversity, is indicative of recycling and reuse, as previous studies have shown.
Capitanio and his team performed computational simulations to simulate the evolution of the Earth during its first billion years of existence to trace the thermal and chemical evolution of the cratonic lithospheric mantle.
In addition, they ran a number of test simulations to find out how sensitive their model is to various parameters.
The results showed that the first continental blocks that appeared on Earth were unstable and sank back into the mantle. There they melted and mixed with the molten material until completely dissolved.
However, some pieces can remain there for a long time before resurfacing, accumulating in layers under the lithosphere, giving it buoyancy and rigidity.
Since some of these older pieces of rock can remain in the mantle for long periods of time, this may explain the heterogeneity of the composition of the cratons, with old rocks from different locations mixing with younger ones.
In fact, some of these pieces may still be down there, waiting for them to float back up.
The team called this mechanism “massive regional relamination” (MRR). Because it matches so well with the observed composition of cratons, the team argues that it could have been a key component in the formation of continents on the early Earth.
Given that the continents are considered very important for the emergence and continued existence of life on Earth, figuring out how they formed is important not only for our planet, but also for the search for habitable worlds outside the solar system.
“Our work is important in two respects,” says Capitanio.
“Firstly, cratons are a place of storage / location of important metals and other minerals. Secondly, they tell us about how planets formed and changed in the past, including how continents appeared and how they supported life, as well as how the atmosphere was formed and changed as a result of planetary tectonics.
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