Oceanic crust lives fast and dies young, usually being dragged down a subduction zone after 250m years or so. By contrast, continental crust lives to a ripe old age, with some of the oldest continental crust on Earth – dating to 4bn years – found in Canada and Greenland.
But geologists have long been puzzled as to why there isn’t more continental crust bobbing around. Now new research indicates that some of it might be hidden underneath ocean-island volcanoes.
Mauritius is a young volcanic island which emerged around 9m years ago. But it hides a secret. Last year, geologist Lewis Ashwal from the University of Witwatersrand in South Africa, discovered tiny zircon crystals hiding within Mauritian volcanic rocks, and these crystals turned out to be 3bn years old. Zircons are found mainly in granite on continents, and they are super-hardy, surviving immense pressures and temperatures relatively unscathed.
In this case Ashwal thinks that these zircons are the remnants of a continental fragment which was pulled apart around 85m years ago and subsequently buried under volcanic lavas. When the volcanism that created Mauritius began, around 9m years ago, Ashwal reasons that it swept up some of these hardy zircon crystals as the lava punched through the ancient crust sitting on the sea floor. His findings are published in Nature Communications.
Meanwhile, it isn’t just ancient continental crust that we have trouble keeping track of. Last month scientists confirmed that Earth had an extra continent surrounding New Zealand (now known as Zealandia) which had been overlooked until now because 94% of it lies beneath the ocean.
