A new study suggests that Earth's first continents, the cratons or the cratons, formed from the ocean between 3.33 billion and 3.2 billion year ago.
This is a significant shift in the timeframe for when the cratons first emerged from the water. Numerous studies have suggested that craton emergence occurred approximately 2.5 billion years ago.
Ilya Bindeman (a professor of geology at Oregon) said that there was no doubt that continents had been partially sticking out of water as far back as 3.4 billion years ago. She was not part of the new study. Scientists have discovered sedimentary rocks, which are formed from fragments of rocks that have been weathered and eroded over time. These sedimentary rocks were formed when land was first exposed to the oceans of the early Earth.
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Bindeman, an email exchange with Live Science, said that although geologists know that at least some of the cratons were exposed over 3 billion years ago but the precise timing and extent of their emergence remain a matter for debate. He stated that the study authors suggested that entire cratons emerged from the oceans around 3.3 billion years ago. This was despite the fact that the planet did not have the modern-style platetectonics needed to move the crust up.
The new study was published in the journal Proceedings of the National Academy of Sciences (PNAS) on Nov. 8. Priyadarshi Chowdhury, first author and postdoctoral researcher at Monash University's School of Earth, Atmosphere and Environment, Melbourne, Australia, stated that "pockets" of sedimentary rocks from ancient times had been previously found at the craton. The team wanted to find out their ages and how they formed.
Chowdhury explained to Live Science that when we merged all the sedimentary pockets together we discovered that they all kind of formed simultaneously in river-like or beach-like environments. This would indicate that craton was exposed to both air and water simultaneously. "That was the moment when we realized that we were onto something."
The team looked for tiny zircons (radioactive element uranium) in the rocks to date them. Chowdhury stated that zircons are removed from the rocks. "That's a tedious process." You can visualize that zircon grain measurements are akin to finding a needle in a haystack.
This close-up image shows the ancient sedimentary rocks at India's Singhbhum Craton. Image credit: Courtesy Priyadarshi Choudhury
The team collected the zircons and then used mass spectrometry to zap the crystals with lasers to determine their chemical composition. The rate at which uranium is lost to lead is fixed. Therefore, the team examined the ratio of le ad to uranium in each sample to determine the age of the rock. They then estimated that the whole craton was exposed between 3.2 billion and 3.3 billion years ago.
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What were the first forces that drove the Singhbhum Craton from the water? The authors collected igneous rock samples from the cratons to find out. These are rocks that were formed by the crystallization hot magma. Chowdhury stated that these rocks form a kind "basement" and are just below the sedimentary rock in the craton.
He explained that the chemical composition of these igneous rock encodes information about their formation at a particular temperature and pressure. The team used these chemical compositions to create a model that could be used to simulate the conditions under which the rocks formed and then forced through the ocean's surface.
According to the model, portions of the craton were thickened by hot plumes from magma below the crust around 3.5 billion to 3.25 billion years ago. These plumes enriched the craton with buoyant, lightweight materials like sicilia or quartz. Chowdhury stated that the process made the craton physically thicker than the surrounding rock and buoyed it up.
Photo of the site from which scientists took rocks from the Singhbhum Craton, India. Image credit: Courtesy Priyadarshi Choudhury
Other cratons also contain sedimentary rocks with similar ages as the Singhbhum Craton, India. These include the Kaapvaal Craton (South Africa) and the Pilbara Craton (Australia). According to the report, the authors of the study believe that these cratons may have also been formed in full more than three billion years ago. Although this mass emergence of continents may be possible, there are still many questions regarding Earth's past. For example, how much land was actually exposed at once and how long were these landmasses above water? Chowdhury stated that the answers to both these questions are still a mystery.
Bindeman stated that "many rocks that could have emerged" means they were subducted. This means they got pushed beneath a chunk of crust. Eos reported that although continental crust is more susceptible to subduction than crust beneath the ocean, it can still be damaged and deformed by various forces. "The more you look into the geological records, the fewer rocks you will find."
Chowdhury pointed out that even if some cratons had dipped into the ocean shortly after their first appearance above water, it would have caused significant changes in the world's surface.
The first landfall on Earth would have triggered processes such as weathering and erosion. These would then carry elements like phosphorus into oceans, which would provide key ingredients for life on the planet. Chowdhury stated that organisms called cyanobacteria (or blue-green algae) would eventually populate the oceans and, through photosynthesis, bring oxygen into the atmosphere.
Chowdhury stated that oxygen was not a major component in Earth's atmosphere until around 2.45 billion year ago. There are evidence of oxygen whiffs popping up all over the place. Chowdhury speculates that these whiffs may be due to the first appearance of cratons. He suggests that small crops of Cyanobacteria might have been found near landmasses, which may have increased oxygen levels locally.
He also said that the weathering of silica on the ground would have pulled carbon dioxide out of the atmosphere, cooling the planet at a regional level. This is a known side effect of weathering silica. According to Eos, 150 million to 300 million tons (1136-299 million tonnes) of carbon dioxide are pulled from the atmosphere every year by weathering silicate rocks.
Bindeman stated that the debate about when and how the cratons emerged will continue. He said that in theory, discovery of new cratons could help to settle the matter.
Bindeman explained to Live Science that while everyone keeps an eye on a few well-known localities, there are still many unknown outcrops. These outcrops could be hidden in Russia or Antarctica and help complete the picture about when and how Earth's first continents emerged.
Original publication on Live Science
