As the Californian heat soared outside in the summer of 2018, Rose Turnbull sat in the cool of a windowless basement, sorting out grains of fine sand. A New Zealand-based geologist, Turnbull was in the lab of a colleague at California State University, Northridge, trying to find tiny zircon crystals, which she hoped would help unlock the secrets of the mysterious eighth continent of Zealandia, also known by its Maori name Te Riu-a-Maui.
The task required a trained hand and a little elbow grease, or rather nose grease. Turnbull demonstrates on Zoom, lifting the closed tweezers to the outside of his nose to scoop up some oil, which keeps the kernels from scintillating across the room when plucked.
The crystals came from rocks collected from the islands of New Zealand, which are among the few pieces of the nearly two million square miles of Zealandia that overlook the sea. Recently recognized by scientists, Zealandia is the most submerged continent, the thinnest and youngest ever. Turnbull, who works at the research and advisory group GNS Science in New Zealand, and his colleagues wanted to learn more about the processes that have shaped this unusual landmass.
What they found surprised them: Hidden under the east coast of New Zealand’s South and Stewart Islands, lingers a piece of a billion-year-old supercontinent. The finding suggests that Zealandia may not be as young as it was once thought, which could strengthen the case for its continental status.
“The continents are kind of like icebergs,” says study author Keith Klepeis, a structural geologist at the University of Vermont. “What you see on the surface is not really the full extent of the beast.”
The discovery, described in the journal Geology, can help solve a puzzle that has long puzzled scientists. Most continents contain a core of rock known as a craton, a kind of geological core that is at least a billion years old that acts as a stable base on which the continents are built. Until now, however, the oldest continental crust found on Zealandia has been dated to around 500 million years ago, which is relatively young in geological terms. So if Zealandia is a continent, why did its craton seem to have disappeared?
This new fragment of ancient rock could be part of Zealandia’s missing piece. The discovery “ticks the last box,” says Turnbull. “We are sitting on a continent.”
The work is also part of the larger puzzle of how Zealandia – or any continental crust – was formed, says study author Joshua Schwartz, a granite geologist at California State University, Northridge.
“This layer above the Earth that we call the crust, this thin layer is where all the action of life takes place,” he says. The continental crust is where we live, cultivate, draw water, extract minerals, etc. “Basically our whole life is built on the crust.”
Find the lost continent
Scientists have been tracking Zealandia for decades, but defining it as a continent has proven difficult. “The dirty secret of geology is that there isn’t really a hard and fast definition of a continent,” says Schwartz.
A major element is the composition of the rocks: the seabed around New Zealand is not made up of the magnesium and iron laden rocks that make up most of the oceanic crust. Instead, rocks are silica-rich types, like granite, which are more commonly found in the continental crust. The rocks extend over a large area which is also much thicker and elevated compared to the more typical oceanic crust that surrounds it.
A team of scientists led by Nick Mortimer of New Zealand’s GNS Science made these points and more when they made a compelling case for calling Zealandia a continent in 2017. However, Mortimer and his team mentioned one quirk: the absence of any obvious craton.
“It’s weird,” Klepeis says. The continental crust is more floating than its oceanic counterparts, so it tends to resist the processes that recycle surface rocks back into the mantle. The stable cratonic core of these rocks provides a base from which continents can develop over time, as the slow march of plate tectonics sends island arcs and other landmasses to pile up along their edges.
For example, Schwartz says of his family vacation in New Mexico, “I’m just south of the Wyoming craton.” This area of rock, some of which dates back over 3 billion years, is one of the many cratons that make up the stable interior of North America. The Santa Fe rocks beneath Schwartz’s feet, however, have rejoined the mainland more recently, as a series of islands collided with the ancient coast.
So far, it seemed that Zealandia’s oldest crust had taken shape around 500 million years ago, when the continent formed the edge of the supercontinent Gondwana. Zealandia contains evidence of older rock, including pieces of mantle 2.7 billion years old, but older crust has been elusive.
The new study focuses on 169 samples from New Zealand’s South and Stewart Islands. Some Turnbull and his team had collected from multiple trips to the area, and others from the Country Rock Catalog, so the collection sites stain the southern island pair in their entirety.
Back in the lab, they crushed the rocks and sorted the grains by density and magnetism until all that was left was fine sand composed mostly of zircon crystals. Turnbull then selected thousands of zircons, transferring them to microscope slides, which were then quenched in epoxy and polished before chemical analysis could finally begin.
“It’s a complete process,” says Turnbull.
History in crystals
As the data came in, an unexpected story emerged. The researchers used a method in which they modeled the age not only of the zircons, but also of the bedrock that melted to form them. The ages they recorded revealed that a strip of zircons along the eastern edge of the two southern islands came from underground rocks dating back to 1.3 billion years ago.
At that time, all of the world’s landmasses were heading for a slow-motion collision that would ultimately form the supercontinent named Rodinia. According to the team, this global crush and split likely generated pockets of magma that would become the very ancient rock slab that now lurks deep beneath New Zealand, a cratonic fragment that Zealandia later built upon.
The zircons also appear to bear marks of the eventual separation of the child Zealandia from its parent supercontinent.
This is because the crystals have low amounts of an isotope of oxygen called O-18. This chemical fingerprint is rare in zircons embedded in granite, as the team discovered. For these rocks to form, “a ton of different things have to come together,” says Juliana Troch, a geochemist specializing in magma generation at the Smithsonian National Museum of Natural History in Washington, DC.
A key ingredient is heat, which helps imprint O-18 signatures from the percolation of water onto the surrounding rock. According to the team, a scorching mantle plume beneath Rodinia may have weakened parts of its crust, causing it to rupture 750 million years ago and leaving behind O-18 imprints in the zircon bedrock.
The crystals themselves – and the rocks that surround them – would not form until 500 to 100 million years ago, when fiery explosions of volcanism partially melted these pieces of hidden Rodinian crust. The drops of magma slowly rose upward, crystallizing into granites studded with zircon. Tectonic changes eventually brought these tiny time capsules to the surface, where Turnbull and his team accidentally picked them up.
“It’s a classic thing with science,” says Turnbull. “The things that we discovered are things that we didn’t necessarily intend to find out.”
A nascent continent
Oddly enough, while the discovery suggests Zealandia’s crust is much older than previously thought, it is still considerably younger than its mainland cousins. All of today’s major continents (Africa, Europe, Asia, Australia, North America, South America and Antarctica) are home to rocks over 3 billion years old. There is currently no hard age limit that defines continents and cratons, but their generally long histories are a testament to the expected strength of these landforms, says Schwartz.
Maybe Zealandia is just a young continent. “You see the process of creating a continent around the [Rodinian] fragment, “he says. Turnbull agrees, adding,” It’s like the birth of a craton. “
However, more work is needed to finalize the picture of Zealandia’s origins. The study’s conclusions come from traces of what lies below and not bits of Rodinia in hand, so there is still some uncertainty in the precise steps that led to the curious chemistries the team found, explains Alex McCoy-West, a geochemist at James Cook University in Australia. .
“It would be great if we actually found this real evidence,” he says.
Still, the work promises to help scientists better understand the dance of Earth’s continents as they waltzed across the planet, periodically combining into supercontinents, then tearing each other apart.
“This study highlights that you can still get pieces of this very ancient history from much, much younger rock,” says Jack Mulder, a geologist at the University of Queensland, who was not on the team. study.
And there is much more to be found within Zealandia’s limits, adds Turnbull. “It just makes you want to keep going out and exploring.”