The lost continent we sit upon is much older than first thought

New Zealand's striking landscape has been shaped by volcanic activity dating back over 500 million years. Video / Tāmaki Paenga Hira Auckland War Memorial Museum

The sunken continent that New Zealand sits upon is much older than first thought - and has now been conclusively linked to ancient supercontinents that existed more than a billion years ago.

While scientists have known about it for decades, it's only been in recent years that Te Riu-a-Māui / Zealandia has been publicly recognised as the planet's youngest, smallest, thinnest and most submerged continent.

At one relatively brief point in the Earth's history, its sprawling mass once stood above the waves, dispersing animals and plants from here to New Caledonia and the east of Australia.

Most groups of dinosaurs, including the towering titanosaurs, roamed its rolling, mountainless, low-lying landscape.

About 94 per cent of the continent today lies underwater – if we drained the oceans, we could see it unfurling some 4.9 million sq km across the South Pacific.

Source / GNS Science

Now, tiny mineral grains taken from granite rocks collected in Fiordland and Stewart Island have widened our understanding of Zealandia - and also led a potential breakthrough in ancient continental reconstructions.

Until the fresh findings, just published in the journal Geology, New Zealand's oldest rocks – in the Tasman District – were dated at about 500 million years old and linked to the supercontinent Gondwana.

However, the new records show that some granite rocks in Fiordland and Rakiura/Stewart Island can trace their origins back much further in time and are likely linked to an earlier supercontinent called Rodinia that existed one billion years ago.

"This represents a major shift in thinking and it comes down to the [isotopic] analysis of tiny grains of the mineral zircon in specialist laboratories in Australia and Germany," said the study's lead author, Dr Rose Turnbull of GNS Science.

Source / GNS Science

A GNS Science-led study published in 2017 put the continent of Zealandia on the world map.

It showed that Zealandia - like other continents - is large, relatively high, has thick crust, and contains rocks like granite and greywacke.

However, all the other continents have rocks that are a billion years old or older, a feature that appeared to be missing from Zealandia.

"This new study ticks that final continental box. There is no longer any doubt that we live on top of a continent."

Granite rocks form by crystallising magmas that were made by melting deep parts of the Earth's continental crust.

This map highlights the tectonic profile of Zealandia. The colours show continental crust in red, orange, yellow and brown hues and oceanic crust in blues. Image / GNS Science

The granites had been brought to the surface by uplift of the Zealandia continent in response to earthquake activity along our plate boundary over millions of years.

Turnbull said a key finding in the study, supported by the Marsden Fund, was the unique isotopic signature measured in microscopic grains of zircon, a mineral that is found in all granites.

The isotopic composition of zircon was used in geology to understand deep time and can be used to reveal what the Earth's crust looked like both at and deep below the surface.

"The isotopic signature of zircon grains from Zealandia tell us that there are ancient billion-year-old rocks still concealed deep in the crust beneath Fiordland and Rakiura/Stewart Island – rocks that were formed as part of the Rodinia supercontinent," Turnbull said.

"This work sheds new light on Zealandia's ancient past – a geological past that has now been shown to be over one billion years old."

Turnbull said the new study is an example of GNS Science's ongoing exploration of the continent of Te Riu-a-Māui / Zealandia and shows that new discoveries are there to be made.

Geologist Dr Rose Turnbull, of GNS Science, analysing zircon crystals from Fiordland at a laboratory at Heidelberg University, Germany. Photo / GNS Science

"To use a human analogy, all of today's eight continents have older ancestors such as Gondwana, Laurasia, and Pangea," she said,

"The new study has enabled scientists to place Zealandia in the 'family tree' of continents descended from Rodinia."

With this new information, Zealandia may yet turn out to be a "missing link" between South China, Australia, and North America – and this opens up the position of South China and Zealandia within Rodinia to new international scrutiny.

Rodinia itself, one of seven supercontinents, was thought to have formed about 1.2 billion years ago and broke up 750 to 550 million years ago.

Geologists tended to focus on zircon grains when investigating past continents because the ultra-durable mineral crystallised in most magmas and could survive many geologic events over billions of years.