Too Much Stretching Doomed Earth's Eighth Continent

Scientists have finished mapping what has been called Earth's eighth continent, revealing how it formed and why it largely sank to the depths of the ocean. Above the waves, all that's left of the continent known as Zealandia are the South Pacific islands of New Caledonia and the two islands that make up New Zealand. But the landmass, 94% drowned, covers nearly 2 million square miles—the equivalent of about two-thirds of Australia, which lies to the northwest. In 2019, scientists mapped the geology of the southern third of Zealandia, finding rock samples that suggested it had been stretched in two stages, per the BBC. For this study, published in Tectonics, researchers mapped the northern two-thirds of the continent to form a complete picture.

They dredged rocks including "pebbly and cobbly sandstone, fine-grain sandstone, mudstone, bioclastic limestone, and basaltic lava from a variety of time periods," per Popular Mechanics. A mix of granite and volcanic pebbles were dated to 110 million to 130 million years ago, sandstone to 95 million years ago, and basalts to 40 million years old, reports PM and the BBC. This allowed researchers from GNS Science of New Zealand to compile a history of Zealandia before and after it formed an estimated 80 million years ago. They believe that in the rupture of the supercontinent Gondwana, land encompassing Australia and East Antarctica broke off Zealandia's northwest around 83 million years ago, creating the Tasman Sea that now separates Australia and New Zealand, per Science Alert.

A few million years later, Zealandia and West Antarctica, both of which show evidence of internal deformation, split to create the Pacific Ocean. Zealandia—now "the world's smallest, thinnest and youngest continent"—is thought to have survived for tens of millions of years as an island to the southeast of Australia, per the BBC. However, it would've had a low profile due to the thinning of the continental crust through all the stretching it endured. This stretching, in a direction that varied by up to 65 degrees, may have ultimately triggered a break that "resulted in the creation of ocean floor that makes up the lower parts of the Zealandia continent," per a release. The continent is thought to have largely vanished an estimated 25 million years ago. (More continents stories.)