Earth’s first continents emerged from the oceans around 3.3–3.2 billion years ago — at least 700 million years earlier than previously thought, a study has concluded.
Researchers from Monash University studied the sedimentary and igneous rocks of an ancient continental fragment in India called the Singhbhum Craton.
The ancient beach sediment, which dates back around three billion years ago, was evidence that continental landmasses were being eroded.
Moreover, biochemical analysis of the granites formed at the base of the craton indicates that the continental crust began thickening out 3.5 billion years ago.
The team stated that the buoyant crust was thick enough to have emerged from the oceans around 3.2 billion year ago and formed one of the oldest landmasses.
The subduction–collision processes of today’s continents were not involved in the emergence of the first continents, the team explained.
In fact, plate tectonics likely did not kick into action until around 2.94–2.78 billion years ago, they believe.
Earth’s first continents formed around 3.3–3.2 billion years ago — at least 700 million years earlier than previously thought, a study has concluded. Pictured: outcrops of 3 billion-year-old granites in India’s the Singhbhum Craton, which is an ancient continental fragment
Priyadarshi, an Australian petrologist from Monash University, and his collaborators conducted the investigation.
The team stated in The Conversation that “The Emergence of the Continents was a pivotal point in the History of Life on Earth” and that they were the humble home of most people.
“But it’s not known when the continental landmasses were first discovered on Earth and what tectonic processes created them.”
In their study, Dr Chowdhury and colleagues analysed sandstone deposits preserved within the Singhbhum Craton that were once ancient rivers, estuaries and beaches.
The accumulation of gravel and sand along the coastlines creates beaches. This material is then eroded by rain and wind, and transported to the ocean by rivers.
This is why beaches sediments and river and estuary sediments can be used to provide information about continental landmasses that they are formed from.
By studying tiny grains of a mineral called Zircon which contains tiny amounts uranium, the researchers were able to determine the age and geological period for each of these sandstones.
You can determine the age of the zircon crystals by measuring their ratio to uranium.
The team’s uranium–lead dating indicated that the ancient sandstones in the Singhbhum Craton were deposited around 3 billion years ago, making them some of the oldest-known beach deposits in the world.
This means that beaches were formed at the same time as a continental landmass. It must have existed in India, which we know now, at least 3 billion year ago.
The researchers noted that sedimentary rocks approximately this age can also be found in older cratons of Australia (the Pilbara, Yilgarn and South Africa) as well.
They said that this suggests multiple continental landmasses could have formed around the world at this point.
Researchers found evidence of erosion of continental landmasses through the formation of sandstone-horizons (pictured), that once served as beaches and date back around 3 Billion years.
In the second part of their study, the team turned their attention to working out how the first continents rose above the oceans — a feat which would have required them to increase in thickness beyond a critical threshold.
According to the team, “A distinctive feature of continents” is their buoyant crust. This allows them to float on top Earth’s mantle just like corks in water.
“Like icebergs,” the continent’s top with thick crust (typically greater than 45 km) [30 miles]thick) sticks out over the water, while continental blocks with crusts that are thinner than approximately 40 kilometers have smaller pieces. [25 miles]’Do not sink.
Most ancient continents — the Singhbhum Craton included — are built up of granite, formed from the melting of pre-existing rocks at the base of the crust.
Analysis of granitoids (TTG in the above) from the craton revealed a transition from predominantly sodium to a more potassium-based geochemistry in the period from around 3.53 to 3.1 billion years ago — indicating a progressive maturation of the crust. Furthermore, the rocks become increasingly depleted in heavy rare earth elements (HREE) and yttrium, which the team explain indicates that the granites were forming at greater pressures — more than tripling in the period from 3.5–3.25 billion years ago — and, by extension, greater depths
Analysing the granitoids of the craton showed a shift from primarily sodium to more potassium-based geochemistry over the time period of 3.53 billion to 3.1 trillion years ago. This indicates a gradual maturation of crust.
Furthermore, the rocks become increasingly depleted in heavy rare earth elements and yttrium, which the team explain indicates that the granites were forming at greater pressures — more than tripling in the period from 3.5–3.25 billion years ago.
With these rocks forming at the bottom of the continental crust, this indicates an increase in the thickness of the Singhbhum Craton from around 20 to 28 miles over the same time-span.
“We estimate that the Singhbhum Craton’s continental crust was approximately 50 kms in diameter by 3 billion years back. [31 miles]Researchers explained that it is thick enough to buoyantly rise over sea level.
All findings of this study have been published in Proceedings of the National Academy of Sciences.
A step-by-step diagram showing how the protocrust of 3.6 billion years ago (bottom) gradually built up, causing the continent to emerge from the ocean around 3.2 billion years ago before plate tectonic subduction finally kicked in around 2.94–2.78 billion years ago
Monash University scientists studied the sedimentary rocks and igneous rock of the Singhbhum Craton, an Indian ancient continental piece.