Most people know that the land masses on which we all live represent just 30% of Earth’s surface, and the rest is covered by oceans.
The emergence of the continents was a pivotal moment in the history of life on Earth, not least because they are the humble abode of most humans. But it’s still not clear exactly when these continental landmasses first appeared on Earth, and what tectonic processes built them.
Our research, published in Proceedings of the National Academy of Sciences, estimates the age of rocks from the most ancient continental fragments (called cratons) in India, Australia and South Africa. The sand that created these rocks would once have formed some of the world’s first beaches.
We conclude that the first large continents were making their way above sea level around 3 billion years ago – 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 start 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 during a trip to the beach today, have been operating for billions of years. By scouring the rock record for signs of ancient beach deposits, geologists can study episodes of continent formation that happened in the distant past.
The Singhbhum craton, an ancient piece of continental crust that makes up the eastern parts of the Indian subcontinent, contains several formations of ancient sandstone. These layers were originally formed from sand deposited in beaches, estuaries and rivers, which was then buried and compressed into rock.
We determined the age of these deposits by studying microscopic grains of a mineral called zircon, which is preserved within these sandstones. This mineral contains tiny amounts of uranium, which very slowly turns into lead via radioactive decay. This allows us to estimate the age of these zircon grains, using a technique called uranium-lead dating, which is well suited to dating very old rocks.
The zircon grains reveal that the Singhbhum sandstones were deposited around 3 billion years ago, making them some of the oldest beach deposits in the world. This also suggests a continental landmass had emerged in what is now India by at least 3 billion years ago.
Interestingly, sedimentary rocks of roughly this age are also present in the oldest cratons of Australia (the Pilbara and Yilgarn cratons) and South Africa (the Kaapvaal Craton), suggesting multiple continental landmasses may have emerged around the globe at this time.
Rise above it
How did rocky continents manage to rise above the oceans? A unique feature of continents is their thick, buoyant crust, which allows them to float on top of Earth’s mantle, just like a cork in water. Like icebergs, the top of continents with thick crust (typically more than 45km thick) sticks out above the water, whereas continental blocks with crusts thinner than about 40km remain submerged.
So if the secret of the continents’ rise is due to their thickness, we need to understand how and why they began to grow thicker in the first place.
Most ancient continents, including the Singhbhum Craton, are made of granites, which formed through the melting of pre-existing rocks at the base of the crust. In our research, we found the granites in the Singhbhum Craton formed at increasingly greater depths between about 3.5 billion and 3 billion years ago, implying the crust was becoming thicker during this time window.
Because granites are one of the least dense types of rock, the ancient crust of the Singhbhum Craton would have become progressively more buoyant as it grew thicker. We calculate that by around 3 billion years ago, the continental crust of the Singhbhum Craton had grown to be about 50km thick, making it buoyant enough to begin rising above sea level.
The rise of continents had a profound influence on the climate, atmosphere and oceans of the early Earth. And the erosion of these continents would have provided chemical nutrients to coastal environments in which early photosynthetic life was flourishing, leading to a boom in oxygen production and ultimately helping to create the oxygen-rich atmosphere in which we thrive today.
Erosion of the early continents would have also helped in sequestering carbon dioxide from the atmosphere, leading to global cooling of the early Earth. Indeed, the earliest glacial deposits also happen to appear in the geological record around 3 billion years ago, shortly after the first continents emerged from the oceans.
- Priyadarshi Chowdhury – Postdoctoral research 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
Adapted from an article originally published on The Conversation.
Amazing new Continents are fragile at birth? WOW! Like newborn babies of all Species? Huh!
Compared to this and later developments in Earth’s story, hominids after a couple of million years are a mere fleck of inconsequential dust that might turn out to be one of the shortest living species ever.
oh well, what’s half a billion years….give or take? SCIENCE!!!!
We do not understand the life of our planet. It did not create itself from gas and dust. The big bang put all the matter we see directly into the quark plasma state from the pressure and friction from a massive collision. Everything has been cooling ever since. Our galaxy was a single mass, then, through entropy, our solar system was a single mass, and lastly, our planet and moon were a single mass of quark plasma. This plasma forms all the naturally occurring elements all by itself from the surface inward. Black holes are pure quark plasma that has not cooled to create elements yet. Black holes start this process by first fusing the quarks and the dark matter of space, which is made of electron neutrinos, into neutrons. This let’s the black hole create the first light it will have. Then, the neutrons break down to hydrogen atoms at which point the hydrogen then fuses with the constantly forming neutrons and creates helium using the beta minus decay reaction. This process continues creating heavier and heavier elements until the light of the star goes out and a crust forms. This begins to answer the mystery of this article. The land formed first before our planet created all the water. Mountains were floating chunks of rock in a sea of magma. Once the surface hardened, the atmosphere was formed. The water out planet possesses was made no differently than the water coming out of a car tailpipe. The heat of the core and the hydrocarbons the Earth created in the crust turned into a runaway water producing machine that created it for millions of years. The water level was once as high as Mt Everest which explains why fossils are found in mountains. The land didn’t come up but the water slowly evaporated because gravity is dependent on the energy of the core of the planet. Our oceans were once deeper than all the land on the Earth. It is the entropy of our planet that is making it lose its water just like Mars and the moon have lost theirs. The continents didn’t necessarily rise up through the water but the water simply evaporated down to the land.
You have a vivid imagination. Have you considered writing fiction for children? You have a way of presenting your ideas as if they are actually factual. That is a talent that not everyone has.
It’s called a paradigm shift. It explains things that have no explanation. At one time, you would have thought the Earth was the center of the universe and scoffed at someone saying it wasn’t. Now, I am explaining that a planet doesn’t start its life as a cloud of gas and dust and you scoff at that. Unfortunately, what you believe doesn’t follow the laws of physics while my theory does. You believe what I said is nonsense because of what is called conventionalism which is what describes why humans weren’t able to accept the fact that the Earth wasn’t the center of the universe. Naturally, what I say will seem foreign to you when you think everything already has answers.
“Unfortunately, what you believe doesn’t follow the laws of physics while my theory does.”
No, what I’m troubled by is your sense of being absolutely right, implying that you are the only one in the world who has figured it out. I don’t share your certitude that your view of reality is more faithful to physics than what I was taught. It is not just a paradigm shift.
Full of pseudoscience and fury, signifying nothing.
so its confirmed then
eastern parts of the indian subcontinent singhbhum craton an ancient piece of continental crust most ancient continents including the singhbhum craton are made of granites which formed at increasing greater depths through the melting of preexisting rocks at the base of the crust implying the crust was becoming thicker during this time window
the plate tectonics are taking place the proto eastern indian plate is meeting the proto african plate and wham bam crash and bash the proto african plate is being subducted submerged beneath the proto indian plate and as the push and the shove and the heave and the heft take place the two fronts buckle like any front row set of all black rugby front line a haka ing
and the proto africa front row plate pushes the proto indian front row plate up into the atmosphere air 1 kilometre two three and four ten twenty thirty and forty and when they reach fifty its time to surface for air from the watery turf from where we were
continental blocks with crusts thinner than about 40 kilometres remain submerged below water
the emergence of the continents how did rocky continents manage to rise above the oceans with their thick buoyant crust which allows them to float above the earths viscous mantle just like a cork in water just like bog in a marsh like icebergs the top of the continents with thick crust typically more than 45 kilometres thick sticks out above the water
ask any front row forward what power is required what speed is needed what stength is required what stamina is given to drive them there plates over the top and push them back to their own try line
there is a fine line between geology rugby and making a beach to enjoy the sunshine
“The zircon grains reveal that the Singhbhum sandstones were deposited around 3 billion years ago, …”
No! Three billion years is the age of crystallization of the granite containing zircons, not the younger beaches. Granites are emplaced at a depth of several miles. For the granites to be eroded, producing sand grains, there would have to be tectonic uplift over a period of tens of millions of years, followed by subsequent erosion over a similar length of time. It has taken about 50 million years for the granodiorite core of the Sierra Nevada to be exposed to significant erosion.
Therefore, the beach sands are tens of millions of years younger than the zircons they contain!
What was responsible for the thickening of the crust over 500 million years if there was nothing above the water until 3 billion years ago? It couldn’t have been beach sands! Possibly, it was rhyolite or quartz latite eruptions of ash and lava. More likely, it was mafic eruptions, allowing the magma to differentiate into a silica-rich magma that became granite. Although, the picture shown looks like a migmatite, indicating an extreme metamorphism, rather than a complete melting and crystal-settling in the magma chamber.
“The main detrital zircon age populations overlap with the major phases of granitoid magmatism in the craton (Fig. 1B).”
It’s a long analysis, with lots of details involved. I’m not particularly swayed by this result since there are many similar and some point to older continents.
3.2 Gyrs ago is a popular dating for onset of plate tectonics [“Scientists Pin Down When Earth’s Crust Cracked, Then Came to Life”, Quanta Magazine] but many or most papers prefer 3.6+ Gyrs [“Earth’s Oldest Minerals Date Onset Of Plate Tectonics To 3.6 Billion Years Ago”, Astrobiology; “New study helps pinpoint when earth’s plate subduction began”, Phys Org; “Earth’s crust is way, way older than we thought”, Live Science].
How did the oceans form from the inferno that was before that?