Just like people, continents break up and drift apart. And just like messy human relationships, sometimes an ex that you thought you had forgotten resurfaces unexpectedly. Happily though, today's case does not involve any mean texts or tweets, but the remains of an ancient supercontinent, the discovery of which sheds light on the tumultuous and mysterious beginnings of this planet.
The ancient remains in question belong to Mauritia, which was one of the pieces that splintered from the massive supercontinent Gondwana to form a sandwich with India and Madagascar. We know that this ancient threesome eventually separated with India and Madagascar going their separate ways but whatever happened to Mauritia?
For the clues to its whereabouts, one needs to look at rocks found on the present-day island of Mauritius, an island in the Indian Ocean off the east coast of Africa. But we're not just talking about any rocks, we're looking at those containing zircon. This mineral is significant because it occurs mainly in granites from the continents and predates the formation of young volcanic islands such as Mauritius. In other words, zircon should not be found here at all – it's old and the island is young. Lead author Lewis Ashwal, a geologist at the University of the Witwatersrand in South Africa explains why:
"Mauritius is an island, and there is no rock older than nine million years old on the island. However, by studying the rocks on the island, we have found zircons that are as old as three billion years."
The theory is that when India and Madagascar started moving apart, Mauritia found itself being pulled both ways, stretching and growing thinner until it broke apart in chunks, which sank into the ocean. These chunks would have contained zircon and though the chunks themselves disintegrated when they came into contact with the lava from the undersea volcanos, the mineral survived and ended up being embedded in the cooling magma, which eventually formed the rocks that make up present-day Mauritius's surface.
"The fact that we have found zircons of this age proves that there are much older crustal materials under Mauritius that could only have originated from a continent," says Ashwal.
Incidentally, this is not the first time that zircons have been found on Mauritius. Ashwal was also part of a 2013 study that discovered traces of it in beach sand. Back then, however, some critics had cast doubt on these findings, arguing that they could have been either blown in by the wind, or carried in on vehicle tires or scientists' shoes. As Jérôme Dyment, a geologist at the Paris Institute of Earth Physics and one of the critics of the 2013 study put it: "Finding zircons in sand is one thing, finding them within a rock is another one ... Finding the enclave of deep rocks that, according to the author's inference, bring them to the surface during an eruption would be much more convincing evidence."
With these latest findings, Ashwal can now put those criticisms to rest. "The fact that we found the ancient zircons in rock (6-million-year-old trachyte), corroborates the previous study and refutes any suggestion of wind-blown, wave-transported or pumice-rafted zircons for explaining the earlier results."
The study of the earth's formation is an ongoing one and these latest findings are key to helping scientists refine their understanding of what the world looked like back in the day and how continents break apart due to plate tectonics. In this case, it shows that the break-up of Gondwana was not a simple or straightforward process and that there might be more fragments waiting to be discovered in the Indian Ocean.
"According to the new results, this break-up did not involve a simple splitting of the ancient super-continent of Gondwana," says Ashwal. "Rather, a complex splintering took place with fragments of continental crust of variable sizes left adrift within the evolving Indian Ocean basin."
Ashwal talks about his work in the video below.
The findings were published in Nature Communications.
Source: Wits University