Today for episode 381, we’re going to Zealandia. No, it’s not a quirky TV show modeled after Portlandia. It’s the 7th largest continent on earth.
We’re not talking continents in the geographic sense, really large land masses like Africa and South America, but we are talking about continental material in the geologic sense, even though most of it is submerged beneath the ocean.
Public Domain via WikipediaZealandia is centered on New Zealand, and in some ways, it’s been known for decades, ever since good charts of subsurface bathymetry were created. Those data show a complicated mix of rises and troughs on the sea floor around New Zealand, including the Lord Howe Rise in the Tasman Sea between New Zealand and Australia, the Campbell Plateau east of New Zealand, and other highs and lows.
Although the ocean floor around the globe is diverse, with seamounts and trenches, fault zones and piles of volcanic rocks here and there, on the whole oceanic crust is deep and uniform, or at least varying broadly and predictably. These relatively small-scale high plateaus on the ocean floor around New Zealand are unusual.
Let’s define a continent. Geographically, it usually means a large land mass. “Large” is subjective, but Australia, at about 9 million square kilometers, is considered to be the smallest continent. And the count depends on whether you separate Europe from Asia, or just say Eurasia as one, and other conventions. Many would say that a degree of isolation and separation also defines a continent, so Eurasia should be one, together with Africa, North America, South America, Antarctica, and Australia, for a total of six. Forget all that. We’re looking at places made of continental crust, the generally lighter, silica-rich material in contrast to oceanic crust, denser and more iron-rich.
By that geologic definition, all the conventional continents really are mostly continental crust too. All well and good. There are numerous smaller, separated areas of continental crust that are usually called microcontinents. Madagascar is probably the best example of a microcontinent, but there are lots of them, and you can get into plenty of arguments about whether a piece like, say, Greenland, is or is not fully a part of the North American continent.
So, let’s go to Zealandia. In 2017, Nick Mortimer and colleagues, writing in GSA Today, pretty much laid any argument to rest. There’s abundant evidence to say that there is a large, diverse terrain centered on New Zealand but 94% submerged, that is mostly continental crust. It adds up to about 5 million square kilometers, comparable in size to Arabia and India, sub-continents that are connected in one way or another with full-fledged continents. Zealandia is close to, but clearly separated from Australia, and Mortimer and others make a convincing case that it should be considered the seventh continent, six times larger than Madagascar, the biggest microcontinent.
So if continental rocks are lighter, which accounts for them being for the most part above sea level, why is most of Zealandia under water? The crust there is thinner, so it doesn’t rise as high as that on most continents, and it has been stretched and broken, partly rifted apart during a long complex history.
Zealandia was probably originally part of the supercontinent of Gondwana, attached to eastern Australia and West Antarctica, when those two continents were fully attached to each other about 105 million years ago, in Early Cretaceous time. That margin of Gondwana, thousands of kilometers long, was a subduction zone somewhat like today’s Andes, with an old Pacific oceanic plate diving beneath the Gondwana continental crust. There would have been a volcanic mountain range there, recorded in Zealandia as a surviving string of granitic rocks, batholiths similar to those that developed in western North and South America during Mesozoic and Cenozoic time and continuing to this day.
Gondwana began to be rifted apart by about 85 million years ago, Late Cretaceous time. It appears that one of the first parts to go from this part of the supercontinent was what is now Zealandia, a long narrow ribbon along the coast of the continent. Australia and Antarctica probably began to separate at about the same time, but that was initially a rather slow break-up that wasn’t complete for close to fifty million years.
The idea of a long narrow ribbon of continental crust rifting off a continent has plenty of precedents. During the Ordovician, the Avalonian ribbon rifted away from what is now northwest Africa and ultimately collided with North America, where it forms parts of Newfoundland, Nova Scotia, and New England today. More recently, a string of continental fragments rifted off what is now eastern Arabia and Africa, to collide with Eurasia to form the cores of Turkey, Iran and Afghanistan today. It’s also possible that one or more “ribbon” continents broke off what is now northern India, to begin to amalgamate with Eurasia in today’s Himalayas, even before India itself collided.
The rifting that split Zealandia off Gondwana is recorded in sedimentary rocks. The initial rifting may have begun the crustal thinning and breaking of Zealandia that we see today, but some of that may have developed later. Because Zealandia was (and is) in a zone of complex interactions among small and large continental and oceanic plates, it’s not really surprising that it would have undergone quite a bit of tectonic and crustal modification. Today, the main part of Zealandia that’s above sea level, New Zealand, contains the Alpine Fault, a huge strike-slip fault similar to the San Andreas. The Alpine Fault is ripping Zealandia apart, but even that helps us recognize Zealandia as a small continent. Mortimer and his colleagues identified rock types in the sea floor on both sides of the Alpine Fault – rock types that can be correlated with each other, and that reflect the predicted offsets you’d expect from the Alpine Fault.
I think Mortimer and his colleagues have made the case quite convincingly that we should indeed think of Zealandia as a continent, geologically speaking, even though most of it is under water. 
Mortimer et al. paper Book, Zealandia:Our Continent Revealed, published in 2014. 
—Richard I. Gibson