How zircon led researchers to a lost continent under the Indian Ocean

The idea that "country borders can be drawn, and re-drawn, but continents are forever," turns out to be merely a convincing illusion.

Scientists have detected remnants of a lost continent hiding under the island country of Mauritius in the Indian Ocean. While the phrase may conjure up images of the fabled Atlantis, this lost land is a leftover fragment of the ancient supercontinent known as Gondwana. This discovery helps fill in details of a poorly understood chapter of our planet’s history.

Today's seven continents may seem as stable as could be, but that’s only how they look from our perspective as geologically short-lived beings. Half a billion years ago, experts believe the currently far-flung continents clustered together into two supercontinents, Gondwana and Laurasia.

After merging to form the more well known Pangea, they split again, Gondwana splintering into Africa, Australia, South America, and Antarctica, and sending India on its Himalaya-forming collision course with Asia that continues to this day.

Lewis Ashwal of Wits University in South Africa is one of the geologists trying to understand this continental dance. It’s like doing a giant jigsaw puzzle, but the pieces keep changing shape, shattering apart, and sinking into the ocean.

But Dr. Aswhal has located one of those lost pieces, according to his paper published Tuesday in Nature Communications. The key was noticing something out of place on the island of Mauritius.

“Earth is made up of two parts — continents, which are old, and oceans, which are young," Ashwal said in a press release. Lighter rocks tend to stay on top of the continents, where they can lay for eons, while heavier materials tend to sink into the ocean floor where they are drawn into the production of new land that drives continental drift.

Mauritius is rather young in geologic terms, having been recently formed from lava deposited by volcanic eruptions. It was amongst that new rock that Ashwal found something very, very old.

“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,” he said.

The mineral zircon is a favorite of geologists because it often contains the radioactive element uranium, which decays into lead at a predictable rate, allowing for precise dating.

The presence of the billions-of-years-old minerals embedded in rock thousands of times younger was the smoking gun. Something ancient was lurking underneath. “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,” Ashwal said.

Gondwana’s breakup seems to have been a messy one. The new results suggest that some of the leftovers of that fracturing formed an “undiscovered continent” called “Mauritia,” whose pieces are spread over what is now the Indian Ocean. Most may have sunk into the sea, but at least some part of the ancient crust seems to remain under Mauritius.

And the latest paper fits with other suggestive evidence. Traces of ancient zircon had been found on the island before, but incorporated into loose beach sand, which critics pointed out could have blown from elsewhere one the wind, or even caught a ride on scientists’ shoes and vehicles. The new paper took great care to avoid any possibility of such contamination.

The study contributes to our evolving understanding of Earth's hidden past. “We are studying the break-up process of the continents, in order to understand the geological history of the planet,” Ashwal said.