Zealandia: The geological origins of the “Eighth Continent” are becoming clearer

In 1642, Dutch explorer Abel Tasman and his crew left Jakarta on two ships. Their goal: to map the poorly known regions of the Southern Hemisphere and, if possible, to discover the Terra Australis that I have imagined since ancient times. This continent is roughly represented on maps seventeenthe Century, appears in the imagination of time as a wilderness filled with gold, waiting to be discovered. In December of the same year, Tasman was the first European to see New Zealand – inhabited since then Thirteenthe A century by the Maoris – but set off again without having time to explore. Without knowing it, the Dutchman had already set his eyes on a vanished continent. It would take 375 years, in 2017, for an international team to confirm this: There is an “eighth continent,” called Zélandia, or Te Riu-a-Māui in the Maori language. In a recent study, geologist Rose Turnbull of the GNS Science Institute in Dunedin, New Zealand, and colleagues show that this microcontinent is made up partly of billion-year-old rocks.

If Zelandia has remained hidden for so long, it’s because 94% of its surface, estimated at 4.9 million square kilometers – or about eight times the area of ​​Madagascar (also called the small continent) – is submerged under the ocean. Pacific Ocean, eastern Australia. Only the emerging peaks of New Caledonia, New Zealand and adjacent islands testify to the existence of this vast continental mass. Before sailing, Zélandia was an integral part of Gondwana, a giant subcontinent at the origin of Africa, India, Australia and Antarctica. Only during the break-up of Gondwana, 80 to 100 million years ago, did Zealandia, strictly speaking, form before submerging in the process. Its crust, thinner than that of other continents (20-25 kilometers thick), rises lower above the ocean floor, thereby submerging it.

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Topographic map of the continent of New Zealand.

© Wikimedia commons / Ulrich Lange / Scripps Institution of Oceanography, University of California, San Diego, US

Since the discovery of Zélandia, researchers have estimated its oldest rocks to be 500 million years old. But the granite boulders of Fiordland and Stewart Island in New Zealand tell an entirely different story, much older. These granites were formed by the crystallization of magma from very deep regions of the Earth’s crust, before rising to the surface in a continental elevation, itself the result of seismic activity over millions of years. Rose Turnbull’s team studied the zircon grains found in this granite. This mineral is able to withstand many geological events for billions of years, and therefore can provide information about the first days and the conditions in which it crystallized. Thus the isotopic signature of zircon reveals the presence of billion-year-old rocks at depth, in the crust under Fiordland and Stewart Island. Geologists associate these rocks with the giant continent of Rodinia, which predates the Pangea rocks from which Gondwana arose, since it formed 1.1 billion years ago before disintegrating 750 million years ago. This makes Zealandia the missing link between southern China, Australia and North America in the geological puzzle that was this giant continent.

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