Geologists at the University of the Witwatersrand in Johannesburg, South Africa, have confirmed the existence of a small piece of an ancient continent, called Mauritia, beneath modern-day Mauritius.
This is Indian Ocean topography showing the location of Mauritius as part of a chain of progressively older volcanoes extending from the presently active hot-spot of Reunion toward the 65-million-year-old Deccan traps of northwest India. Image credit: Wits University.
Located between India and Madagascar, the tropical islands of the Seychelles and Mauritius attract many tourists. But they are of great interest for researchers as well.
Whereas the geology of the Seychelles shows old granitic rocks, indicating that the Seychelles archipelago was formerly part of Madagascar and India, Mauritius is of young volcanic origin.
Beneath modern-day Mauritius a hotspot heated the oceanic crust, thus inducing melting of the rocks and leading to volcanism. The volcanic activity that created the island started only 9 million years ago.
“Earth is made up of two parts – continents, which are old, and oceans, which are young,” said Wits University Professor Lewis Ashwal.
“On the continents you find rocks that are over 4 billion years old, but you find nothing like that in the oceans, as this is where new rocks are formed.”
“Mauritius is an island, and there is no rock older than 9 million years old on the island.”
However, in 2012, Prof. Ashwal and his colleagues found tiny crystals of the mineral zircon that are as old as two billion years.
“Zircons are minerals that occur mainly in granites from the continents. They contain trace amounts of uranium, thorium and lead, and due to the fact that they survive geological process very well, they contain a rich record of geological processes and can be dated extremely accurately,” the researchers explained.
“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,” Prof. Ashwal said.
The team’s previous study received some criticism, including that zircons could have been either blown in by the wind, or carried in on vehicle tires or scientists’ shoes.
In the latest research, zircon grains were not sampled from a beach but were extracted from a volcanic rock called trachyte that was collected on the island of Mauritius.
“The fact that we found the ancient zircons in 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,” Prof. Ashwal said.
“There are many pieces of various sizes of Mauritia spread over the Indian Ocean, left over by the breakup of Gondwanaland,” he added.
“According to the new results, this break-up did not involve a simple splitting of the ancient supercontinent of Gondwana, but rather, a complex splintering took place with fragments of continental crust of variable sizes left adrift within the evolving Indian Ocean basin.”
The findings were published this week in the journal Nature Communications.
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Lewis D. Ashwal et al. 2017. Archaean zircons in Miocene oceanic hotspot rocks establish ancient continental crust beneath Mauritius. Nature Communications 8, article number: 14086; doi: 10.1038/ncomms14086
