Since the dawn of the planet, the Earth had been a giant ball of water surrounded by an atmosphere rich in methane. That was up until 2.4 billion years ago, when the first landmass abruptly rose from the depths of the ocean.
Researchers at the University of Oregon say that changes in the mantle's temperature and viscosity have caused a large swath of land to emerge. The Earth's early mantle was too soft and hot to support large mountain ranges and plateaus. But around 2.4 billion years ago, the mantle cooled and hardened, giving sudden rise to what is now called Kenorland, the first supercontinent.
"Crust needs to be thick to stick out of water," says lead researcher and geologist Ilya Bindeman. "The thickness depends on its amount and also on thermal regulation and the viscosity of the mantle."
Kenorland, The World's First Supercontinent
The findings coincide with the theory that Kenorland first came to being around 2.7 billion years ago. However, it completely contradicts the previous belief that the supercontinent took its time to form between 3.5 billion and 1.1 billion years ago.
As the new landmass emerged, it consumed carbon dioxide in the atmosphere due to weathering. This, along with the high temperatures of Earth's surface, led to the arrival of more complex life forms, such as plants, algae, and fungi. The timing also coincides with the shift from the Archean Eon, which was dominated by one-celled creatures such as archaea and bacteria, to the Proterozoic Eon, when prokaryotes abound.
As the land was exposed to physical and chemical weathering, it created a sink of greenhouse gases that disrupted the Earth's radiative balance. The bright swath of land also reflected light coming from the sun, adding more fuel to the radiative-greenhouse imbalance. This ultimately led to the Earth's first snowfall and a glacial period that is believed to have lasted 2.4 billion to 2.2 billion years ago.
"What we speculate is that once large continents emerged, light would be reflected back into space and initiate runaway glaciation," says Bindeman.
The Great Oxygenation Event
The sudden increase in atmospheric carbon dioxide also caused more oxygen, leading to the Great Oxygenation Event that eventually paved the way for more life to flourish on Earth.
The researchers arrived at their findings by analyzing 278 samples of shale, the world's most common sedimentary rock, collected from various parts of the Earth. They were able to identify when the crust was exposed to physical weathering after detecting almost imperceptible traces of rainwater. The researchers also compared the ratio of rare oxygen 17 and 18 with the more common oxygen 16 to identify signs of chemical weathering.
The study is published in the journal Nature.
