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Kids are frequently taught that seven continents exist: Africa, Asia, Antarctica, Australia, Europe, North America, and South America.
Geologists, who look at the rocks (and tend to ignore the humans), group Europe and Asia into a supercontinent — Eurasia — making for a total of six geologic continents.
But according to a new study of Earth's crust, there's a seventh geologic continent called "Zealandia," and it has been hiding under our figurative noses for millennia.
The 11 researchers behind the study say that New Zealand and New Caledonia aren't merely island chains. Instead, they're both part of a single, 4.9 million-square-kilometer (1.89 million-square-mile) slab of continental crust that's distinct from Australia.
"This is not a sudden discovery, but a gradual realization; as recently as 10 years ago we would not have had the accumulated data or confidence in interpretation to write this paper," the researchers wrote in GSA Today, a journal of the Geological Society of America.
Ten of the researchers work for institutions within the new continent; one works for a university in Australia. But other geologists are almost certain to accept the team's continent-sized conclusions, says Bruce Luyendyk, a geophysicist at the University of California at Santa Barbara, who wasn't involved in the study.
"These people here are A-list earth scientists," Luyendyk told Business Insider. "I think they've put together a solid collection of evidence that's really thorough. I don't see that there's going to be a lot of pushback, except maybe around the edges."
Why Zealandia is almost certainly a new continent
The concept of Zealandia isn't new. In fact, Luyendyk coined the term in 1995.
But Luyendyk says it was never intended to be a new continent. Rather, the name was used to describe New Zealand, New Caledonia, and a collection of submerged pieces and slices of crust that broke off a region of Gondwana, a 200 million-year-old supercontinent.
"The reason I came up with this term is out of convenience," Luyendyk said. "They're pieces of the same thing when you look at Gondwana. So I thought, 'Why do you keep naming this collection of pieces as different things?'"
Researchers behind the new study advanced Luyendyk's idea a huge step further: They took decades' worth of newer evidence and examined it with four criteria that geologists use to deem a slab of rock a continent:
1. Land that pokes up relatively high from the ocean floor.
2. A diversity of three types of rocks: igneous (spewed by volcanoes), metamorphic (altered by heat/pressure), and sedimentary (made by erosion).
3. A thicker, less dense section of crust compared with surrounding ocean floor."
4. Well-defined limits around a large enough area to be considered a continent rather than a microcontinent or continental fragment."
Geologists had determined that New Zealand and New Caledonia fit the bill for items one, two, and three. After all, they're large islands that poke up from the sea floor, are geologically diverse, and are made of thicker, less dense crust.
This eventually led to Luyendyk's coining of Zealandia and the description of the region as "continental," since it was considered a collection of microcontinents, or bits and pieces of former continents.
The authors say the last item on the list — a question of "Is it big enough and unified enough to be its own thing?" — is one that other researchers had skipped over, though by no fault of their own. At a glance, Zealandia seemed broken up.
However, the new study used recent and detailed satellite-based elevation and gravity maps of the ancient seafloor to show that Zealandia is indeed part of a unified region. The data also suggests Zealandia spans "approximately the area of greater India" — larger than Madagascar, New Guinea, Greenland, or other pieces of crust.
"If the elevation of Earth's solid surface had first been mapped in the same way as those of Mars and Venus (which lack [...] opaque liquid oceans)," they wrote, "we contend that Zealandia would, much earlier, have been investigated and identified as one of Earth's continents."
The geologic devil's in the details
The authors point out that while India is big enough to be a continent — and probably used to be — it's now part of Eurasia because it collided and stuck to that continent millions of years ago.
Zealandia, meanwhile, has not yet smashed into Australia. A piece of seafloor called the Cato Trough still separates the continents by 25 kilometers (15.5 miles).
Another wrinkle for Zealandia is its division into northern and southern segments by two tectonic plates: the Australian Plate and the Pacific Plate. This split makes the region seem more like a bunch of continental fragments than a unified slab.
But the researchers say that Arabia, India, and parts of Central America have similar divisions yet are still considered parts of larger continents.
"I'm from California, and it has a plate boundary going through it," Luyendyk said. "In millions of years, the western part will be up near Alaska. Does that make it not part of North America? No."
What's more, the researchers wrote, rock samples suggest Zealandia is made of the same continental crust that used to be part of Gondwana and that it migrated in ways similar to Antarctica and Australia.
The samples and data also show that Zealandia is not broken up. Instead, plate tectonics have thinned, stretched, and submerged Zealandia over of millions of years.
Today, only about 5% of it is visible — which is part of the reason it took so long to discover.
"The scientific value of classifying Zealandia as a continent is much more than just an extra name on a list," the scientists wrote. "That a continent can be so submerged yet unfragmented makes it a useful and thought-provoking geodynamic end member in exploring the cohesion and breakup of continental crust."
Luyendyk said he believes the distinction won't end up as merely a scientific curiosity. He thinks it could have larger, real-world consequences.
"The economic implications are clear and come into play: What's part of New Zealand, and what's not part of New Zealand?" he said.