Zealandia is a mostly submerged continent in the south-west Pacific, and New Zealand sits on its highest parts. About 94% of it is underwater, and it covers roughly 4.9 million square kilometres, about two-thirds the size of Australia. That sounds like something from a science-fiction film, but it is a real geological structure, and it sits remarkably close to home: according to 1News, the edges of Australia’s and Zealandia’s continental crust come within about 25 kilometres of each other across the Cato Trough, off the Queensland coast. When I first wrote about it in 2020, I got one important thing wrong about why it sank, so here are the questions people most often ask, answered properly.
What is Zealandia?
Zealandia, also known as Te Riu-a-Māui, is a large, continuous region of continental crust. It is not just a boundary someone drew around New Zealand. Geologists have spent decades collecting rock samples, mapping the seafloor and measuring the crust beneath it, and in a landmark 2017 paper in GSA Today, a team led by GNS Science geologist Nick Mortimer argued that it should be regarded as a continent rather than a collection of islands and fragments. As the authors put it, this was not a sudden discovery but a gradual realisation, built on years of accumulated data. New Zealand is the largest part above water, and New Caledonia is the other big piece; the rest is ocean floor. It is the smallest, thinnest and youngest continent, and by far the most submerged, as GNS Science puts it.
How much of Zealandia is underwater, and how deep?
About 94% is submerged. Its typical, or modal, elevation is around 1,100 metres below sea level, according to the 2017 paper, so much of the continent lies under roughly a kilometre of water, though the depth varies a lot across such a huge area. That was one of the clues that helped geologists recognise it. The seabed is high compared with the surrounding deep ocean floor, and underneath it is continental crust rather than the thin crust found beneath the deep oceans. The thickness varies enormously, too: GNS Science measures it from about 10 kilometres in the offshore New Caledonia Trough to more than 40 kilometres under the higher parts of the South Island, compared with about 7 kilometres for typical ocean crust.
When did Zealandia sink?
There was no single moment when Zealandia disappeared beneath the ocean. It subsided over a very long time. Zealandia was once part of Gondwana, the ancient supercontinent that also contained the land that became Australia, Antarctica, Africa, South America and India. During the Late Cretaceous, roughly 100 to 80 million years ago, tectonic forces began pulling the region apart, in a complicated sequence of stages, and new ocean crust began forming around it. Zealandia did not fall into the ocean; its crust was stretched and thinned, and it settled lower over millions of years.
Why did Zealandia sink?
This is where the earlier version of this article got it wrong: Zealandia did not sink because of global warming. As it was pulled away from Gondwana, its continental crust was stretched and thinned, and thinner crust sits lower than thicker crust because of isostasy, the balance between the crust and the denser rock beneath it. Think of floating wood: a thick slab rides high in the water, but thin it out and more of it sits below the waterline. Something similar happened to Zealandia. The 2017 paper concluded that its submergence is mainly the result of widespread Late Cretaceous crustal thinning, followed by that isostatic adjustment. Later thermal and tectonic changes played a part too, but modern sea-level rise has nothing to do with it; Zealandia was well on its way down tens of millions of years before humans existed.
Why is it considered a continent?
Geologists do not decide something is a continent because of how much dry land it has. The 2017 paper set out the characteristics that set Zealandia apart from ordinary ocean crust. It covers about 4.9 million square kilometres and forms a coherent, well-defined region. It is made predominantly of continental crust, with a wide range of continental rocks, including silica-rich granites, schists and greywackes found on its islands and dredged from the seabed. Its crust is substantially thicker than ocean crust, it stands higher than the surrounding deep ocean floor, and it is separated from Australia by oceanic crust and clear geological boundaries. As Mortimer told New Zealand’s TVNZ, quoted by Gulf News, if you could pull the plug on the oceans, everyone would see mountain chains and a big, high-standing continent.
There is one important qualification. No international body keeps an official list of continents or approves new ones, Gulf News noted, so saying Zealandia is “officially the eighth continent” goes too far. The accurate version is that a substantial body of geological research treats Zealandia as a continent, and the 2017 paper made the formal scientific case.
Is New Zealand the eighth continent?
Not quite. New Zealand is part of Zealandia, not the whole of it. Its North and South Islands hold some of the continent’s highest ground, and the rest extends beneath the Pacific, so calling New Zealand itself the eighth continent mixes up the country with the much larger geological structure it sits on. New Zealand’s dramatic landscape is not just a leftover from Gondwana, either. It has been reshaped since by the collision of the Australian and Pacific plates, which is why it has such high mountains and active faults; the continent beneath it is still changing.
How do you map a hidden continent?
Piece by piece, over decades. Scientists combined rock samples dredged from the seafloor with seismic surveys, gravity and magnetic measurements, satellite-derived maps of the ocean floor, and geological mapping on New Zealand and the other exposed land. The hardest part was the north. A voyage aboard Australia’s research vessel RV Investigator collected rock samples from the Fairway Ridge in the Coral Sea, and the resulting study in the journal Tectonics, according to the US Geological Survey’s listing, completed the reconnaissance geological mapping of the entire Zealandia continent, onshore and offshore. GNS Science announced the completed map in 2023. That does not mean every rock and fault is now known, but it does mean there is a continent-scale geological map of a place almost entirely hidden beneath the sea.
Is Zealandia Atlantis?
No, although the comparison is understandable. Atlantis comes from the writings of the ancient Greek philosopher Plato, who described a legendary island civilisation that vanished beneath the sea. Zealandia is a real geological structure, not a city, and it did not disappear in a single catastrophe; its crust thinned and subsided over geological timescales, long before anyone was around to see it. Both stories involve land beneath the ocean, and that is where the connection ends.
Lost, or just overlooked?
In geological terms, Zealandia is a lost continent, but “lost” is a little misleading. It did not vanish suddenly, and scientists had been studying pieces of it for decades. What changed was understanding what all those pieces added up to. It is also a reminder that “why” matters as much as “what”: the crust thinned and sank long before anyone was around to warm the planet, a very different process from today’s sea-level rise, which I unpacked in the difference between climate change and global warming. And if you like hidden forces behind things we take for granted, my post on the barycentre shows how the Moon and Earth circle a point deep underground. Next time you look at a map of New Zealand, you are really looking at the small part of a much bigger continent that happens to be sticking out of the Pacific.
Sources
- Mortimer et al: Zealandia, Earth’s hidden continent, GSA Today (2017)
- GNS Science: New hidden continent mostly underwater, scientists say
- GNS Science: Crustal structure of Zealandia
- 1News: Scientists excited by gigantic new continent Zealandia
- Gulf News: New Zealand part of sunken lost continent, scientists say
- US Geological Survey: Reconnaissance basement geology and tectonics of North Zealandia, Tectonics