The rise of the continents was a crucial moment in the history of the life of our planet, mainly because they represent our home. But it is not yet clear exactly when these landmasses arose on Earth and what tectonic processes built them.
Now, a team of researchers from the University of Monash (Australia) has studied the sedimentary and igneous rocks of an ancient continental fragment in India called the Singhbhum craton , near Calcutta, discovering evidence of erosion of the continental masses in the form of sediments ancient beach sites dating back about 3 billion years. Furthermore, biochemical analysis of the granites formed at the base of the craton indicates that the continental crust began to thicken 3.5 billion years ago.
The main conclusion is that the first continents of the Earth emerged from the oceans between 3,300 and 3,200 million years ago, at least 700 million years earlier than previously thought, according to the study collected by the journal Proceedings of the National Academy of Sciences and long before the 2.5 billion years estimated by previous research.
When the continents rise above the oceans, they begin to erode. The wind and rain break the rocks into grains of sand, which are carried downstream by rivers and accumulate along the coasts to form what we know as beaches. These processes have been operating in this way for billions of years.
So by scanning the rock record for signs of ancient beach deposits, geologists can study episodes of continent formation that occurred in Earth’s distant past.
“A unique feature of the continents is their thick, floating crust, which allows them to float on the Earth’s mantle, like a cork in water,” the authors explain.
For this study, the authors traveled to the Singhbhum craton, located in eastern India. Previously, “pockets” of ancient sedimentary rocks had been found in it, and the team wanted to determine their exact ages and the nature of how they were formed. To date the rocks, they examined them for tiny crystals called zircons, which contain the radioactive element uranium using a method known as mass spectrometry . By measuring the ratio of lead to uranium, it is possible to determine how long ago the zircon crystals formed.
The team’s uranium and lead dating indicated that the ancient sandstones at this site were deposited about 3 billion years ago, making them some of the oldest beach deposits in the world.
But what force caused the craton to emerge from the waters?
To determine this, the researchers took samples of igneous rocks from the cratons, that is, rocks formed through the crystallization of scorching magma that lie just below the sedimentary rocks within the craton, forming a kind of base.
The chemical composition of these igneous rocks encodes details about the stress and temperature at which they were first formed. Thus, taking into account these chemical compositions, they recreated the circumstances that shaped the rocks and their subsequent process.
“We estimate that around 3 billion years ago, the Singhbhum Craton’s continental crust had grown to be about 50 kilometers thick, making it buoyant enough to start rising above sea level,” the researchers explained.
Geologists believe that the weathering of the cratons would have caused nutrient runoff, supplying the ocean with phosphorus and other building blocks for early life. It was the first step in making the planet more livable.
Referencia: Priyadarshi Chowdhury et al, Magmatic thickening of crust in non–plate tectonic settings initiated the subaerial rise of Earth’s first continents 3.3 to 3.2 billion years ago, Proceedings of the National Academy of Sciences (2021). DOI: 10.1073/pnas.2105746118
