Newly discovered crystals expelled from volcanic eruptions on the island of Mauritius are billions of years older than the island itself, and are thought to be the remnants of an ancient microcontinent known as Mauritia.

The proposed 'lost continent' would have once connected Madagascar and India in the Gondwana supercontinent, but likely disappeared into the Indian Ocean around 84 million years ago. Now researchers have found what appears to be a piece of Mauritia, dredged up by ancient volcanoes and hidden below the surface.

"Earth is made up of two parts - continents, which are old, and oceans, which are 'young'," says one of the team, Lewis Ashwal from the University of the Witwatersrand in South Africa.

"Mauritius is an island, and there is no rock older than 9 million years old on the island. However, by studying the rocks on the island, we have found zircons that are as old as 3 billion years."

The island of Mauritius is located about 2,000 kilometres (1,200 miles) off the southeast coast of the African continent. 

Thought to be a relatively new landmass, the island was formed by gigantic underwater volcanic eruptions between 8 and 9 million years ago, and now belongs to the Mascarene Islands archipelago, along with the Saint Brandon, Réunion, and Rodrigues islands.

Ashwal and his team dug up the piece of crust from a rocky outcrop in Mauritius that had been covered by molten lava during volcanic eruptions at the time of the island's formation. 

When they analysed the material within, they found tiny zircon crystals that were billions of years older than the landmass itself.

The fact that the crystals were made from zircon - a common gemstone that can come in colourless, yellow, red, brown, blue, and green hues - was enough to tip off the researchers that what they were looking at did not come from the ocean below.

zircon-febOne of the zircon crystals. Credit: University of the Witwatersrand

Zircons are known to form mainly from the granites of ancient continents that once spread across Earth's surface, and once the team dated the crystals to between 2.5 and 3 billion years, they say it's a strong indication that they belonged to the long-lost continent of Mauritia.

"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.

This isn't the first time that ancient zircons have been found on the island of Mauritius - back in 2013, a separate team found traces of zircon in beach sand that were also billions of years old. 

But the find was controversial, because critics argued that these minerals could have blown onto the beach from elsewhere. 

Now that we have similar crystals that were buried by ancient lava deposits, scientists can make a much stronger case for them being the remains of a lost continent. 

"The fact that we found the ancient zircons in rock corroborates the previous study, and refutes any suggestion of wind-blown, wave-transported, or pumice-rafted zircons for explaining the earlier results," says Ashwal.

The find now paints a picture of an ancient continent being splintered into fragments, some of which ended up being anchored below the current landmass that makes up the island of Mauritius today.

It also explains the mystery of why some parts of the Indian Ocean have been found to have stronger gravitational fields than others - a potential indication of thicker crusts.

It's expected that more chunks of the ancient Gondwana supercontinent will be dredged up in the coming years. 

As Alice Klein reports for New Scientist, several pieces of continent have recently been found off the coast of Western Australia and underneath Iceland. 

"It's only now as we explore more of the deep oceans that we're finding all these bits of ancient continents around the place," Martin Van Kranendonk from University of New South Wales in Australia, who wasn't involved in the study, told Klein.

The research has been published in Nature Communications.