A new examination of some of the oldest rocks in the world suggests that the first continents on Earth were unstable, and sank back into the mantle before making their way out again and reforming.
This could explain some of the more puzzling characteristics of cratons, extremely old and stable parts of the lithosphere (the crust and uppermost mantle) that have survived continental changes over eons and record Earth’s ancient history.
The new findings could help us understand Earth’s changing geology over its 4.5-billion-year lifespan.
“The rocks in the core of the continents, called cratons, are more than three billion years old,” explains geologist Fabio Capitanio of the Monash University School of Earth, Atmosphere and Environment in Australia.
“They formed in the early Earth and hold the secret to how continents and the planet 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 specific set of circumstances.
There are several lines of evidence that suggest the continents may have formed from the interior out, around cratonic cores. But the formation mechanism of the cratons themselves is hotly debated.
Cratons, of which around 35 are currently known, are 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 a strangely diverse mix of materials, minerals with a range of ages, compositions, and sources.
This heterogeneity, or diversity, is suggestive of recycling and reworking, previous research has found.
Capitanio and his team conducted computational modeling to simulate the evolution of Earth during the first billion years of its existence, to observe the thermal and chemical evolution of the cratonic lithospheric mantle. In addition, they ran a set of test simulations to work out how sensitive their model was to different parameters.
The results showed that the first continental blocks to emerge on Earth were unstable, sinking back into the mantle. There, they melted and became mixed in with the molten material until dissolved.
However, some pieces can stay down there for a long time before floating back up, building up underneath the lithosphere in layers, giving it buoyancy and rigidity.
Because some of those older pieces of rock can stay in the mantle for long periods of time, this can explain the heterogeneity of the cratonic composition: older rocks from different places mixed in with younger rocks.
In fact, there could still be some of those pieces still down there, waiting to float back up.
The team has named this mechanism ‘massive regional relamination’ (MRR). Because it so neatly fits with the observed composition of cratons, the team says that it may have been a key component of continent formation on early Earth.
Given that continents are thought to be very important for the emergence and ongoing existence of life on Earth, figuring out how they formed has implications, not just for our own planet, but for the search for habitable worlds outside the Solar System.
“Our work is important in two ways,” Capitanio says.
“First, cratons are where important metals and other minerals are stored/found. And second, they tell us how the planets formed and changed in the past, including how the continents came to be and how they supported life, and how the atmosphere formed and changed as a result of the planets’ tectonics.”
The research has been published in PNAS.