Continents are a lot like people. They drift about, break old bonds, develop new ones, go around in circles.
There was, of course, the great joint family, Pangea, about 299 million years ago, when almost all the land on Earth was a single fertile mass. In about 250 million years, all the continents will likely fuse again.
What’s interesting, in the present, is that we are still discovering what happens when continents split. Proof of this lies in the recently discovered continent of Zealandia.
Zoom out for a quick recap: About 200 million years ago, Pangea broke up into two supercontinents. One of these, Gondwana, broke up further, about 150 million years ago, causing one chunk — now India and Africa — to drift north, and another — now Australia and Antarctica — to stay south.
Tectonic activity would tear these apart over time. In the midst of which, about 80 million years ago, the world’s thinnest and youngest continent was born, as a ribbon-like strip twisted away from the edges of the southward-drifting chunk, caught fire, and eventually sank.
This ribbon only broke the surface as a series of small plateaus, and so, for all of recorded history, were considered a group of islands, which we know as New Zealand.
Now it turns out those islands have been on a continent all their own, spread across 4.9 million sq km (about half the size of the US and more than half the size of Australia).
How did we miss it?
Until recently, it was assumed that much of the seabed around New Zealand was oceanic crust. It was only in the 1990s that researchers studying the ocean floor here discovered that this wasn’t an oceanic plate, but a continental one.
The difference? An ocean crust consists largely of heavy, dense basaltic rock. Continents consist largely of the lighter, less dense granite, which coalesced on the surface of a still-molten Earth, billions of years ago, to form land masses.
Zealandia was first identified as a large land mass in 1995, by American geophysicist Bruce Luyendyk, who also gave it its name. But its identity as a full continent remained unproven, and it fell off the radar again.
It was only when a group of 11 scientists from New Zealand, Australia and New Caledonia published a study in 2017 that it was officially confirmed. With its well-defined plateau rising above the ocean floor, and a total area greater than 1 million sq km isolated from other continents, it met the criteria for “continent”.
“We wrote the report as a ‘tidy-up’ paper, not with a blockbuster announcement in mind,” says geologist Nick Mortimer, laughing. He works with New Zealand’s Institute of Geological and Nuclear Sciences (GNS Science), and led the 2017 study. “Continents are the largest solid objects on Earth. That we had missed one, hidden in plain sight, until now, was quite surprising.”
Where exactly is it?
Most of Zealandia lies about 2 km deep in the South Pacific Ocean.
In a symbolic nod to the largely sidelined native inhabitants of this region, it has been given a Maori name too: Te Riu-a-Māui, or “the hills, valleys and plains of Māui”, a Polynesian folk hero.
In September, Zealandia became the first continent to be fully mapped, out to its underwater limits. Details usually stop at the high-tide mark, Mortimer explains. “Few maps or globes show the bathymetry of the ocean seabed. Our rock map of Zealandia is a first reconnaissance, but it is a world first.”
What made Zealandia sink?
The mapping has revealed that the crust, in some places, is just 10 km thick, which, geologists say, could account for why much of it remains underwater. Most continents have a crust about 40 km thick across most of their area.
The only portions of this one visible above water are New Zealand, New Caledonia and the Australian territories of Lord Howe Island and Ball’s Pyramid, making it the only continent on Earth where far more surface area is submerged than is visible.
What made it so thin?
By 2023, researchers from GNS Science had studied rocks dredged up from the crust and formed a picture of Zealandia’s early years.
Giant volcanic regions typically erupt as supercontinents split, Mortimer says. A well-known example is the Deccan Traps, large parts of present-day Maharashtra, Gujarat, Goa, Madhya Pradesh and Rajasthan that were covered in lava, as India and Africa tore away from each other.
Here, a rift in southeast Gondwana birthed the Tasman Sea, which sits between Australia and New Zealand. Another rift led Zealandia to split from West Antarctica. A giant volcanic region, the size of New Zealand itself, then emerged between Zealandia and Antarctica, between 100 and 60 million years ago.
For about 40 million years, as magma flooded out of cracks and fissures, Zealandia stretched and thinned “like pizza dough”, Mortimer has said.
This activity eventually ceased, Zealandia cooled, and as its thin crust solidified, it sank.
But… but…
Numerous questions remain. Why did the thin ribbon not crumble, as the Seychelles microcontinent did? What flora and fauna lived on it before it sank? Could the rocks hold revelatory fossil remains?
“We are satisfied at the moment to have put Zealandia on the map,” Mortimer says. But as the youngest continent is studied further, he admits, it is likely to throw up more surprises.