Found: A Lost Continent! (Really)

Feb 02, 2017

You'd think it would be hard to misplace an entire continent, what with the mountains and trees and all that other hard-to-miss stuff. Now, however, it seems that one of Earth's continents indeed went missing. The good news is, it's at last been found, lying below the waters of the Indian Ocean, beneath the tiny, 790 sq. mi. (2,040 sq. km) island of Mauritius.

Mauritius long drew the attention of geologists and other scientists because of one curious feature: its strong gravitational pull. Earth's gravity is not completely uniform at all places across the globe, but can be slightly stronger or weaker, depending on the density of the material in the local crust. The moon is dotted by many such "mascons" — or mass concentrations — the remains of heavy metal meteorites that long ago crashed onto the surface and buried themselves where they hit. The Earth has a few too, but it is typically plate tectonics — the constant movement and reformation of the crust — that is responsible.

If Mauritius sits atop a mascon, it was likely a result of crustal motion that caused an existing landmass to shatter and sink entirely from view. The island then rose back up to mark the burial site of the lost land like a giant tombstone. That theory was formally advanced by geoscientist Lewis Ashwal of the University of Witwatersrand in Johannesburg, in 2013. In a brand new paper, published in Nature Communications, Ashwal seals the deal with a new study of 13 small grains of zircon.

Mauritius is a mere eight million years old — babyhood on a geological scale — which means its overall geology should be similarly young. However 2 billion-year-old samples of the mineral zircon have been collected on the island's beaches. Ashwal and his colleagues collected more samples and used uranium-lead dating techniques, which relies on the concentration of the two elements in the zircon to determine its age, and found the samples were even older — dating back roughly three billion years. The zircon could not have been formed along with the much younger Mauritius; instead, it was likely blown up onto dry land when undersea volcanoes erupted, blasting out much older rocks from subsurface crust.

Ashwal and his colleagues now conclude that a great swatch of land between India and Madagascar once existed, forming part of the ancient supercontinent of Gondwanaland. Roughly 200 million years ago, when Gondwanaland broke apart, that portion of the overall landmass fractured into ribbons and sank beneath the water. The compressed, sunken land created the mass concentrations, subsequent volcanoes created Mauritius, and still later eruptions deposited the telltale zircon.

"Our findings confirm the existence of continental crust beneath Mauritius," the researchers straightforwardly write. That continent may be gone for good, but the tiny bits of zircon remain to tell its story.

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