Part of ancient supercontinent found under New Zealand

In the summer of 2018, geologist Rose Turnbull was in California, sorting through grains of fine sand. She is a researcher based in New Zealand, and she was looking at tiny crystals of zircon to get a better understanding of the eighth continent of Zealandia, known as Te Riu-a-Maui in Maori.

As one of the authors of a recent study published in Geology, she and her colleagues wrote: “Zircon is the foremost deep-time recorder of Earth’s history, preserving a rich archive of isotope information that informs on magma parentage and crust-mantle evolution. Unlike whole rocks, zircon is highly resistant to alteration and weathering.”

The zircon crystals came from rocks collected from the islands of New Zealand, which, according to a National Geographic article, are among the few bits of Zealandia's nearly two million square miles that poke above the sea. “Only recently recognised by scientists, Zealandia is the most submerged, thinnest, and youngest continent yet found.” Turnbull, who works for the research and consulting firm GNS Science in New Zealand, and her colleagues were curious about the processes that shaped the unique landmass.

They were surprised by their discovery, that hidden underneath the eastern side of New Zealand’s South and Stewart Islands was a chunk of a billion-year-old supercontinent. This means that Zealandia may not be as young as once believed, and thus be a stronger candidate to be accepted as a continent.

"Continents are sort of like icebergs," study author Keith Klepeis, a structural geologist at the University of Vermont, tells National Geographic. "What you see at the surface is not really the full extent of the beast." 

There is a catch, however. Most continents are known to have a core of rock called a craton, “a sort of geologic nucleus at least a billion years old that acts like a stable base upon which continents build,” writes National Geographic. Yet with Zealandia, the oldest continental crust was dated back to roughly 500 million years ago - pretty young in terms of geology. That is, where is Zealandia’s craton, if it is supposed to be a continent?

So this is why the discovery of the newfound fragment of ancient rock is so significant. It “ticks the final box,” Turnbull says. “"We are sitting on a continent."

Another one of the study’s authors, Joshua Schwartz, a geologist specialising in granites at California State University, Northridge, says the work is also part of the larger puzzle of how Zealandia came to be.

"That layer on top of the Earth that we call the crust, that thin layer is where all the action for life happens," he says. The continental crust is where we live, grow crops, draw water, mine minerals, and more. "Essentially, all of our life is built on crust."

Seeking Zealandia as a continent

Scientists have known about Zealandia for tens of years, but were not able to define it as a continent, because, as Scwartz puts it, "The dirty secret of geology is that there's no real hard and fast definition of a continent."

A big element is the rock composition around New Zealand’s seafloor. It isn’t made of the magnesium- and iron-packed rocks that you would find in most oceanic crust, but rather what you would find in continental crust, such as rocks rich in silica like granite. They span a big area that’s remarkably thicker and elevated compared to the more typical oceanic crust around it.

A group of scientists led by Nick Mortimer of GNS Science, the same outfit that Turnbull works for, made a case for calling Zealandia a continent in 2017 in an article published in the Geological Society of America's GSA Today. Yet while their article had strong points, they had to admit that they didn’t identify an obvious craton.

Klepeis finds it “weird.” According to National Geographic, “Continental crust is more buoyant than its oceanic counterparts, so it tends to resist the processes that recycle surface rocks back into the mantle. The stable cratonic nucleus of these rocks provides a foundation from which continents can grow over time, as the slow march of plate tectonics sends island arcs and other landmasses piling up along their edges.”

Until now, Zealandia’s oldest crust seemed to have taken shape starting about 500 million years ago, when the continent was sitting at the edge of the supercontinent Gondwana. Zealandia also has hints of older rocks, including “bits of the mantle as old as 2.7 billion years” but more ancient crust has been hard to find.

The new study focuses on 169 samples from New Zealand’s South and Stewart Islands. Some of these came from samples collected by Turnbull and her colleagues while others came from New Zealand’s rock catalog, so the collection is pretty exhaustive.

Turnbull and her team crushed the rocks and sorted the grains by density and magnetics in the lab, until they reached fine sand of mostly zircon crystals. Turnbull then selected thousands of zircons, transferring them to microscope slides, later covered in epoxy and polished to prepare them for chemical analysis.

"It's a full-on process," Turnbull says.

Aged crystals

While they worked on the crystals, the researchers modeled the age not of the final zircons, but the parent rock that melted to form them. They found that a swath of zircons along the eastern edge of the two southern islands came from rocks below the surface that were as old as 1.3 billion years.

1.3 billion years ago all the world’s landmasses were “headed towards a slow-motion collision”, National Geographic notes, “that would ultimately form the supercontinent named Rodinia.” The team suggests that “the global crash and then split possibly created pockets of magma that would become the slab of very ancient rock that now lurks deep beneath New Zealand – a cratonic fragment upon which Zealandia later built.”

Younger than its relatives

So the discovery suggests that Zealandia’s crust is much older than once believed yet it is still younger than other continents. All the other continents – Africa, Europe, Asia, Australia, North America, South America, and Antarctica – go back to more than 3 billion year old.

Geographically, there isn’t an age cutoff that defines continents and cratons, but Schwartz explains that their commonly lengthy histories speak to the expected staying power of these landforms.

It’s possible that Zealandia is simply a young continent. "You’re seeing the process of continent creation around the central [Rodinian] fragment," he says. Turnbull agrees, noting, "It's like the birth of a craton."

More work, it seems, is needed, to bring the picture of Zealandia’s origins into focus, National Geographic concludes. A geochemist at Australia’s James Cook University, Alex McCoy-West says the study’s conclusions come from traces of what lies below and not pieces of Rodinia in hand, so there’s still some uncertainty in the precise steps that led to the curious chemistries the team found. "It would be awesome if we did actually find that true evidence," he says.

"This study highlights that you can still get pieces of that very ancient story from rocks that are much, much younger," says Jack Mulder, a geologist at the University of Queensland, who was not part of the study team.

And there's plenty more to find within Zealandia's bounds, Turnbull tells National Geographic. "It just makes you keen to keep getting out there and exploring."