One of the greatest mysteries around Earth’s ancient history is around hows and whens of land emergence. Even with top-notch technology at our disposal, the geologists have only been able to pinpoint how and when the continents emerged from the ancient water-world.
I accidentally came across the below Twitter post: The Singhbhum region in Jharkhand might have been the world’s first-ever beach, as per the new study.” I am currently based in the Jharkhand state of India and so this Twitter post attracted my attention immediately, and I thought of blogging on this news.
A recent international study led by scientists from India, Australia and South Africa has revealed that the Earth’s first continents or “cratons” may have emerged from the ocean around 3.2 billion years ago. This new finding is at odds with previous estimates, which suggested that the first continents emerged 2.5 billion years ago.
Cratons are a specific kind of continental crust made up of a top layer called a platform and an older layer called a basement. A shield is the part of a craton where basement rock crops out of the ground, and it is relatively the older and more stable section, unaffected by plate tectonics.
The Indian continent was put together by the collision and welding of several cratons, namely Aravalli, Bastar, Bundelkhand, Dharawar, and Singbhum. The Singhbhum Craton, 4,000 sq.km area which primarily covers Jharkhand as well as parts of Odisha, northern Andhra Pradesh, northern Telangana and eastern Maharashtra. It is limited by the Chhota Nagpur Plateau to the north, the Eastern Ghats to the southeast, Bastar Craton to the southwest and alluvium plain to the east.
Now, the existence of weathered and eroded sedimentary rocks more than 3 billion years old led geologists to speculate that partially submerged continents existed around 3.4 billion years ago. This is because the sedimentary rocks could form only after land broke through the surface of the early Earth’s oceans.
And you’ll never guess where the scientists found the evidence to make such a claim: in Singhbhum, Jharkhand! A recent study dates its emergence out of ocean to ~3.2 billion years ago.
The researchers trekked all the way to the Singhbhum Craton, located in eastern India, where “pockets” of ancient sedimentary rocks had previously been found. Priyadarshi Chowdhury from Australia’s Monash University assessed igneous and sedimentary records of the Singhbhum Craton. They also used continental fragments from India, Australia, and South Africa to date some of the planet’s oldest rocks, which are thought to have developed from the world’s first beaches.
From the cratons, researchers took samples of volcanic rocks, which are generated by the crystallisation of hot magma. These igneous rocks are found directly beneath the sedimentary rocks in the craton, forming a kind of foundation.
Interestingly, the findings in this regard, too, were eye-opening. The study didn’t just shatter our beliefs on when continental lands first rose. It found that contrary to previously held beliefs within the scientific community, plate tectonics didn’t contribute to the emergence of the landmass.
Time-integrated petrogenetic modeling of the granitoids quantifies the progressive changes in the cratonic crustal thickness and composition and the pressure–temperature conditions of granitoid magmatism, which elucidates the underlying mechanism and tectonic setting of emersion. The results show that the entire Singhbhum Craton became subaerial ∼3.3 to 3.2 billion years ago (Ga) due to progressive crustal maturation and thickening driven by voluminous granitoid magmatism within a plateau-like setting.
For the continents to emerge above the surface of the water, their crusts need to be at least 45 km in thickness. If their thickness is less than 40 km, the continental blocks would remain submerged. According to the model the scientists created, heated magma plumes beneath the crust caused areas of the craton to thicken to about 50 km and become enriched with buoyant, lightweight elements like silica and quartz around 3.5 billion to 3.2 billion years ago. In comparison to the denser rock surrounding it, this process left the craton “physically thick and chemically light,” buoying the landmass up to float “like a cork in water”.
The new findings shed more light on the formation of the Earth’s earliest continents and how they may have influenced the planet’s atmosphere, oceans and climate. The study was published in “Proceedings of the National Academy of Sciences” earlier this month and can be accessed here.