If a group of Australian and New Zealander researchers have their way, the seven longtime continents will soon be joined by an eighth: a vast, submerged land three times larger than Alaska whose highest peak is New Zealand’s Aoraki (Mount Cook).
This drowned landmass, called “Zealandia” by researchers, lies east of Australia and covers some 1.9 million square miles—an area larger than the Indian subcontinent. The landmass is 94 percent underwater, with just the islands of New Zealand and New Caledonia poking out above sea level.
“Zealandia illustrates that the large and the obvious in natural science can be overlooked,” write the researchers in their study, published in the academic journal GSA Today.
Previously, some researchers had thought of Zealandia as a motley collage of continental crust fragments. Others, including National Geographic, have defined Zealandia as a microcontinent: a well-defined landmass that checks the geologic box of continental crust but is geologically disconnected from the nearest continent—in this case, Australia.
To the research team, led by New Zealand geologist Nick Mortimer, this definition sells Zealandia short. They point out that at 1.9 million square miles across, Zealandia is about six times bigger than Madagascar—enough to clearly distinguish it from other microcontinents. To that end, they propose that qualifying landmasses more than 1 million square kilometers (386,000 square miles) across should be labeled as continents.
By that definition, Zealandia would currently be a continent, even though it’s submerged.
“One of the reasons it’s not been thought of as a continent is because of the coastlines,” says Bruce Luyendyk, a marine geophysicist at the University of California, Santa Barbara and a Zealandia expert. “People think of continents as where people live, but… sea levels become less relevant when you think about it geologically.”
But not everyone is convinced, in part because the everyday definition of “continent” implies a landmass surrounded by water—not underneath it.
“My judgment is that though Zealandia is continental, it is not a continent,” says Christopher Scotese, a Northwestern University geologist and expert on Earth’s ancient geography. “If it were emergent, we would readily identify it with Australia, much like we identify Greenland with North America and Madagascar with Africa,” he adds.
‘Almost’ a Continent?
Mortimer and his colleagues argue that being submerged shouldn’t disqualify Zealandia from being a full-blown continent. For instance, if Antarctica were ice-free, much of its western half would be underwater.
What’s more, the team points out that Zealandia’s crust is indistinguishable from that of a continent’s. For instance, Zealandia’s crust is thicker, elevated above, and less dense than surrounding oceanic crust—a classic hallmark of continental crust. What’s more, core samples drilled from the seafloor reveal telltale signs of continental crust’s rocky diversity.
The landmass also holds vital information on the breakup of Gondwana, a vast southern megacontinent that contained what’s now Antarctica, Australia, South America, and Africa.
More than 100 million years ago, during the Late Cretaceous, Gondwana was well on its way toward breaking up, and in the process, Zealandia stretched and thinned like taffy. In some places, Zealandia is less than seven miles thick as a result—unusually thin for a continent, but still thicker than oceanic crust. Consequently, the stretched-out Zealandia sank down deeper into the underlying mantle, sinking underwater.
The landmass’ geologic novelty, argue Mortimer and his colleagues, is what makes Zealandia so special. If classified as a continent, it’d be the youngest, thinnest, and most submerged, presenting a useful extreme for how continental crust can behave.
Scotese, for one, is reluctant to make the jump Mortimer and his colleagues seek—but he welcomes their effort.
“I appreciate the author's attempt to change the definition of continent, but definitions change slowly,” he says. “Consider the word ‘planet’... Just as Pluto is 'almost' a planet, Zealandia is 'almost' a continent.”