London, Dec 31 (ANI): A model of the early Earth has suggested that until around 2.5 billion years ago, our planet was almost completely covered by water.
Today, some 28 per cent of Earth's surface is above sea level. Exactly how the ratio of land to sea has varied through Earth's history is unclear, but it is generally agreed that the amount of continental crust has increased over time.
Now, according to a report in New Scientist, calculations by Nicolas Flament of the University of Sydney, Australia, and colleagues suggest that Earth was a water-world until about 2.5 billion years ago, with land making up only 2 to 3 per cent of its surface.
The team assumed that back then Earth's mantle was up to 200 degrees Celsius hotter than it is now, mainly because there was then a larger quantity of radioactive elements decaying and producing heat.
A hotter mantle would have made the crust beneath the oceans hotter and thicker than it is today, buoying it up relative to the continents.
The resulting shallower ocean basins would have held less water, leading to the flooding of what is now land.
In addition, the hotter mantle would cause the continental crust of the time to spread laterally, making it lower-lying and flatter than today, and so more likely to flood.
Then, as the mantle cooled, land would have gradually appeared as the oceans became deeper and regions of high relief on the continental crust formed.
According to the team, this transition may help to explain why levels of oxygen in the atmosphere rose around this time.
During the water-world period, any oxygen produced by photosynthesizing bacteria would have been quickly used up through reactions with decaying organic matter in the oceans.
When the newly emerged land eroded, it produced sediment that, once washed into the oceans, would have buried the organic matter, preventing any further reactions with oxygen, and so allowing it to build up in the atmosphere.
This would have allowed oxygen-breathing organisms to flourish, said the team.
The eroded sediment would also have caused an explosion in life by fertilizing the oceans with phosphorus - an important nutrient, and newly formed coastal regions would have provided plenty of shallow habitats for photosynthesis. (ANI)