A new examination of some of the world’s oldest rocks suggests Earth’s early continents were unstable and sank into the mantle before rolling off and reforming.
This could explain some of the most puzzling features of cratons, extremely old and stable parts of the lithosphere (the crust and upper mantle) that have survived continental changes over the eons and record Earth’s ancient history.
Jhe new findings could help us understand how the Earth’s geology has changed over its 4.5 billion year lifespan.
“The rocks at the heart of continents, called cratons, are more than three billion years old,” says geologist Fabio Capitanio from Monash University School of Earth, Atmosphere and Environment in Australia.
“They formed at the beginning of Earth and hold the secret to how the continents and the planet have changed over time.”
We do not really know how the continents were formed. No other planet in the solar system has anything like them, so it seems clear there must be a specific set of circumstances.
There are several lines of evidence that suggest continents may have formed from the inside out, around cratonic cores. But the mechanism of formation of the cratons themselves is hotly debated.
Cratons, of which approximately 35 are currently knownare buoyant and rigid compared to other parts of the lithosphere, which has given them their stability. But their composition is unusual compared to the more recent lithosphere, made up of an oddly diverse mix of materials, minerals of varying ages, compositions and sources.
This heterogeneity, or diversity, suggests recycling and reworking, according to previous research.
Capitanio and his team conducted computer modeling to simulate the evolution of the Earth during the first billion years of its existence, in order to observe the thermal and chemical evolution of the cratonic lithospheric mantle. Additionally, they ran a set of test simulations to determine the sensitivity of their model to different parameters.
The results showed that the first continental blocks to emerge on Earth were unstable, falling back into the mantle. There they melted and mixed with the molten material until dissolved.
However, some pieces may stay there for a long time before rising, accumulating under the layered lithosphere, giving it buoyancy and rigidity.
Because some of these older pieces of rock can remain in the mantle for long periods of time, this may explain the heterogeneity in cratonic composition: older rocks from different locations mixed with younger rocks.
In fact, there might still be some of those pieces out there, waiting to come back up.
The team named this mechanism “massive regional relamination” (MRR). Because it matches the observed composition of cratons so perfectly, the team says it may have been a key part of the formation of continents on early Earth.
Since continents are considered very important for the emergence and continued existence of life on Earth, understanding how they formed has implications, not only for our own planet, but for the search for habitable worlds in outside the solar system.
“Our work is important in two ways,” says Capitanio.
“First, cratons are where important metals and other minerals are stored/found. And second, they tell us how planets formed and changed in the past, including how continents came to be and how they supported life, and how the atmosphere formed and changed due to the tectonics of the planets.”
The research has been published in PNAS.