Plate tectonics is responsible for dramatic events like earthquakes and volcanos, but it's also believed to have played a crucial role in keeping our climate stable and in the development of life on our planet. The internal movements have created supercontinents and global oceans in regular cycles over hundreds of millions of years, and now scientists propose that a more complex cycle might be taking place.
In a paper published in the journal Precambrian Research, the international team of researchers suggest two main ways for the formation of supercontinents. In one, the ocean crust remains intact, while in the other the Earth’s interior gobbles the crust back inside.
The creation of supercontinents happens roughly every 600 million years. There was Nuna (1.6 billion-1.4 billion years ago), Rodinia (900-700 million years ago), and finally Pangea (320-170 million years ago). When a supercontinent breaks, the internal ocean comes rushing into the fracturing continent. When the continents come back together, the team propose two repeating ways for how the reconnection occurs.
In the “introversion” scenario, once the internal ocean has taken hold and the continents are separated, the continents stop moving away from each other. The phenomenon of subduction (a plate going under the other) suddenly starts in the internal ocean. This event has the continents getting pulled back in together and the crust of the new ocean, which formed after the break-up, being taken back in by the Earth. This is seen as an incomplete break-up.
In the “extroversion” scenario, the breaking up of the continent and the formation of the internal ocean lead to the opposite. The external ocean begins to shrink, with a ring of fire forming. A ring of fire is a hot spot of volcanoes and earthquakes along the edge of a tectonic plate, like the one currently around the Pacific Ocean. The continents are then pulled further apart until they collide with each other once again on the “other side”. The superocean’s crust is subsequently swallowed in by the Earth and a new superocean layer forms.
"We found that supercontinents appear to assemble through two alternating processes of extroversion and introversion," lead author Professor Zheng-Xiang Li, from Curtin University, said in a statement.
"More intriguingly, these two alternating processes determine not only whether the superocean survives, but also whether the circum-superocean Ring of Fire – like the present-day Pacific Ring of Fire – survives."
Professor Li added: "Such alternating ways of supercontinent assembly, along with the survival or regeneration of the superocean and the Ring of Fire, led to the presence of an Earth cycle twice as long as the 600-million-year supercontinent cycle and influenced the formation of some of the planet's resources."
In the model proposed by the researchers, Nuna broke up and then turned into Rodinia via the introversion method, so the crust of the global ocean didn’t change during that period. But after Rodinia split, it then took the extroversion path, with its ocean lost to the Earth’s interior.
While the idea is certainly intriguing and there's supporting evidence, it's possible there isn't a true double-cycle but instead two potential options. Currently, the former superocean, now the Pacific, is shrinking, which would be seen as an extroversion movement. But maybe this trend will change and things will go back according to the introversion approach. Either way, there is still 50 to 200 million years to go before the next supercontinent.