Many people know that the land masses on which we all live represent simply 30% of Earths surface, and the rest is covered by oceans.
The development of the continents was a turning point in the history of life in the world, not least because they are the modest abode of many human beings. However its still not clear exactly when these continental landmasses initially appeared in the world, and what tectonic processes constructed them.
Our research, published in Proceedings of the National Academy of Sciences, approximates the age of rocks from the most ancient continental fragments (called cratons) in India, Australia and South Africa. The sand that produced these rocks would when have actually formed some of the worlds very first beaches.
We conclude that the very first big continents were making their way above sea level around 3 billion years back– much earlier than the 2.5 billion years estimated by previous research.
A 3-billion-year-old beach
When continents rise above the oceans they begin to erode. Wind and rain break rocks down into grains of sand, which are transported downstream by rivers and accumulate along coastlines to form beaches.
These processes, which we can observe in action throughout a journey to the beach today, have actually been operating for billions of years. By scouring the rock record for signs of ancient beach deposits, geologists can study episodes of continent development that occurred in the remote past.
Left: sandstone formations (with ruler for scale); right: tiny images of zircon grains. Credit: Author offered
The Singhbhum craton, an ancient piece of continental crust that makes up the eastern parts of the Indian subcontinent, consists of several developments of ancient sandstone. These layers were initially formed from sand deposited in rivers, beaches and estuaries, which was then buried and compressed into rock.
We identified the age of these deposits by studying microscopic grains of a mineral called zircon, which is maintained within these sandstones. This mineral includes small quantities of uranium, which really gradually becomes lead via radioactive decay. This allows us to approximate the age of these zircon grains, using a technique called uranium-lead dating, which is well matched to dating extremely old rocks.
The zircon grains expose that the Singhbhum sandstones were transferred around 3 billion years earlier, making them a few of the earliest beach deposits on the planet. This also recommends a continental landmass had actually emerged in what is now India by at least 3 billion years back.
Remarkably, sedimentary rocks of approximately this age are likewise present in the earliest cratons of Australia (the Pilbara and Yilgarn cratons) and South Africa (the Kaapvaal Craton), recommending several continental landmasses may have emerged around the world at this time.
Rise above it
How did rocky continents manage to increase above the oceans? A special feature of continents is their thick, buoyant crust, which permits them to drift on top of Earths mantle, simply like a cork in water. Like icebergs, the top of continents with thick crust (normally more than 45km thick) sticks out above the water, whereas continental blocks with crusts thinner than about 40km remain immersed.
If the secret of the continents increase is due to their thickness, we need to understand how and why they started to grow thicker in the first location.
Granites are some of the least dense and most resilient kinds of rock (pen included for scale). Credit: Author offered
The majority of ancient continents, including the Singhbhum Craton, are made from granites, which formed through the melting of pre-existing rocks at the base of the crust. In our research study, we discovered the granites in the Singhbhum Craton formed at significantly higher depths in between about 3.5 billion and 3 billion years ago, suggesting the crust was ending up being thicker during this time window.
Because granites are one of the least dense kinds of rock, the ancient crust of the Singhbhum Craton would have become progressively more resilient as it grew thicker. We determine that by around 3 billion years earlier, the continental crust of the Singhbhum Craton had grown to be about 50km thick, making it resilient enough to begin increasing above sea level.
The rise of continents had a profound impact on the environment, atmosphere and oceans of the early Earth. And the disintegration of these continents would have provided chemical nutrients to seaside environments in which early photosynthetic life was flourishing, causing a boom in oxygen production and ultimately helping to develop the oxygen-rich environment in which we grow today.
Erosion of the early continents would have likewise helped in sequestering co2 from the environment, causing global cooling of the early Earth. The earliest glacial deposits likewise happen to appear in the geological record around 3 billion years earlier, quickly after the very first continents emerged from the oceans.
Composed by:
We identified the age of these deposits by studying microscopic grains of a mineral called zircon, which is preserved within these sandstones. This permits us to approximate the age of these zircon grains, using a strategy called uranium-lead dating, which is well fit to dating very old rocks.
How did rocky continents handle to rise above the oceans? An unique function of continents is their thick, buoyant crust, which permits them to drift on top of Earths mantle, just like a cork in water. Like icebergs, the top of continents with thick crust (normally more than 45km thick) sticks out above the water, whereas continental blocks with crusts thinner than about 40km remain immersed.
Priyadarshi Chowdhury– Postdoctoral research study fellow, Monash University
Jack Mulder– Research Associate, The University of Queensland
Oliver Nebel– Associate Professor, Monash University
Peter Cawood– Professor and ARC Laureate Fellow, Monash University
Originally published on The Conversation.
