Most people are aware that only 30 percent of the Earth's surface is made up by land masses. The rest is covered in oceans.
Not least because they are the humble home of most people, the emergence of the continents was an important moment in Earth's history. It is not yet clear when and how these continents first appeared on Earth.
The results of our research were published in Proceedings of the National Academy of Sciences. They estimate the age of rocks found in the oldest continental fragments (called "cratons") in South Africa, India and Australia. These rocks were once formed by the sand that made them.
Our research has shown that the first continents above sea level were formed around 3 billion years ago. This is much earlier than previously estimated 2.5 billion years.
(Author supplied)
Left: Sandstone formations (with ruler to scale); right: Microscopic images zircon grains.
A beach that is 3 billion years old
As continents rise above the oceans, they begin to erode. Rain and wind break down rocks into grains of sand. These are then transported downstream by rivers to form beaches.
These processes can be observed in action on a beach trip today. They have been going on for billions upon billions of years. Geologists can examine episodes of continent formation from the distant past by looking at the rock records for evidence of ancient beach deposits.
Many formations of ancient Sandstone can be found in the Singhbhum Craton, an ancient section of continental crust, which forms the eastern part of the Indian subcontinent. These layers were formed originally from sand that was deposited on beaches, estuaries, and rivers. It was then buried and pressed into rock.
The age of these deposits was determined by studying microscopic grains made of zircon, a mineral that is well preserved in sandstones.
The tiny amounts of uranium in this mineral slowly transforms into lead by radioactive decay. This allows us to determine the age of the zircon grains using a technique called "uranium-lead Dating", which is useful for dating very old rocks.
Zircon grains show that the Singhbhum Sandstones were formed around 3 billion years ago. They are therefore some of the oldest known beach deposits. This suggests that a continent-sized landmass existed in India at least 3 billion years before the present.
(Author supplied)
Above: Granites are among the most buoyant and dense types of rock (pen included to scale).
It is interesting to note that sedimentary rocks approximately this age are also found in the oldest cratons of Australia (the Pilbara & Yilgarn Cratons) and South Africa's Kaapvaal Craton, suggesting that multiple continents may have emerged at the same time.
Rise above it
How do rocky continents rise above the oceans and remain afloat? One of the most distinctive features of continents is their buoyant crust. This allows them to float on top Earth's mantle like a cork in water.
The top of continents with thick crust (typically greater than 45km or 28 miles thick) rises above the water. Continental blocks with thinner crusts than 40 km are submerged.
If the secret to the rise of continents is their thickness, then we must understand why and how they began to thicken.
Granites are the most ancient continents, including Singhbhum Craton. They were formed by melting pre-existing rock at the crust's base.
Our research revealed that the Singhbhum Craton's granites were formed at increasing depths between 3.5 billion to 3 billion years ago. This suggests that the crust was thickening during this period.
Granites are one of most dense rock types, so the Singhbhum Craton's ancient crust would have been buoyant as it grew in thickness. The continental crust of the Singhbhum Craton was approximately 50 km thick by 3 billion years ago. This made it buoyant enough for it to rise above sea level.
The climate, atmosphere, and oceans of the Earth's early days were profoundly affected by the rise of the continents.
These continents' erosion would have provided chemical nutrients to coastal areas in which early photosynthetic lives were flourishing. This led to an increase in oxygen production, which ultimately helped to create the oxygen rich atmosphere that we enjoy today.
The erosion of early continents would also have contributed to the sequestration of carbon dioxide from the atmosphere, which led to global cooling of early Earth.
The geological record also records the appearance of the first glacial deposits around 3 billion years ago. This is just after the formation of the continents from the oceans.
Priyadarshi, Postdoctoral Research Fellow, Monash University; Jack Mulder and Jack Mulder, Research Associates, The University of Queensland; Oliver Nebel and Peter Cawood, Professors and ARC Laureate Fellows, Monash University.
This article was republished by The Conversation under Creative Commons. You can read the original article.
