Zealandia: The story of the hidden continent beneath New Zealandby Sally Blundell
Of this, about 94% is under water. We always had a hunch it was there. Part of Captain James Cook’s task was to search for a Terra Australis Incognita, based on the fanciful notion that landmass in the north had to be balanced by something in the south. He searched in vain, sailing disappointedly over a landscape of canyons and plateaus. In 1857, colonist Charles Hursthouse surmised that New Zealand was part “of a great continent which has been submerged”. Half a century later, improved bathymetric (underwater topographic) maps showed New Zealand was not a few lumps of land jutting up from the deep ocean crust but part of a much larger plateau.
In 1995, American geophysicist Bruce Luyendyk used the name Zealandia to describe a vaguely defined area east and south of Australia. But it was not until last year that a team of scientists, led by GNS Science geologist Nick Mortimer, recently honoured by the Royal Society Te Apārangi for his work in this field, excavated submerged continental crust to conclude that Zealandia was not a collection of drowned continental fragments but a large, hard-edged, unified landmass made up of continental granite, rhyolite, greywacke and schist. Its clear boundaries, size, elevation above oceanic crust and type of rock all gave credibility to Zealandia’s status as the world’s youngest, smallest, eighth (seventh if you combine Europe and Asia) continent. “Decades ago people used the adjective ‘continental’ to describe the undersea area around New Zealand down to the subantarctic islands,” says Mortimer. “But we have changed that adjective into a noun.
“The idea that continents extend out to continental shelves – that they don’t just stop at the shoreline – is something quite new to non-geologists. Once you accept that and you look at a world map, it is quite obvious there is a relatively high standing area of sea floor all around New Zealand. There is a coherence and a tangible reality to it – rather than being remote islands in a modern-day context or in a Gondwana reconstruction context, Zealandia is something you can draw a line around. It is a definite part of the Gondwana jigsaw puzzle.”
The story of that puzzle begins about 85-100 million years ago, when Zealandia began to peel away from southern Gondwana in a 4000km-long ribbon of land, slowly unzipping northwards up the eastern coast of Australia to form what is now the Tasman Sea, and at its narrowest point just 25km from the Queensland continental shelf edge, the Cato Trough. Unlike Madagascar, Mauritia and other land fragments in the Indian Ocean, Zealandia did not shred itself into tiny pieces. Rather, it broke away as a distinct low-lying forested landmass. But it was also thinning, and as it thinned, it began to sink. By about 25 million years ago it was mostly, some say completely, under the sea.
Since then, the land has lifted and twisted. Our current 270,000sq km landmass, comprising 254 islands larger than eight hectares and more than 400 smaller ones, is the result of ongoing processes of stretching and uplifting, as the Pacific Plate to the east and the Australian Plate to the west grind into and past one another from East Cape to Fiordland. East of the North Island, the oceanic floor of the Hikurangi Plateau, part of the 3500km-long Tonga-Kermadec-Hikurangi trench system, is bending and cracking as it dives, or subducts, under the continental crust of the Australian Plate. South-west of the South Island, the oceanic crust of the Australian Plate plunges, slightly less dramatically, beneath the Pacific Plate.
Although some parts of the country, such as the Marlborough Sounds, are being drawn under the sea through this process, elsewhere these subduction zones are creating new continental crust, widening and stretching our landmass by an average of 3-5cm a year. In 1855, the 8.2 magnitude Wairarapa earthquake uplifted land along the coast as much as 6.4m. The 7.8 earthquake in Kaikōura in 2016 caused a 1-5m uplift along the coastline.
“Continental crust forms largely in subduction zones,” says GNS Science geologist and palaeontologist Hamish Campbell. “This is the process happening off the east coast of the North Island and southernmost South Island – they are like factories in which new continental crust is forming.”
Offshore, the extensive apron of continental crust, unaffected by erosion and mostly beyond the reach of subduction zones, is expected to hold a wealth of resources – research by Niwa says the potential annual income from New Zealand’s offshore hydrocarbons could be in the tens of billions of dollars. Mortimer doesn’t expect a resource rush – offshore mining is costly – but further exploration of submerged Zealandia will help us better understand our mineral and hydrocarbon make-up and offer a rare 60-million-year window into our ancient past.
An understanding of Zealandia, says Mortimer, whose email signature ends in “Dunedin 9016, New Zealand, Zealandia”, will also affect our sense of place in the world. “We live perched on the topmost mountain range of a continent, the Himalayas or the Andes of Zealandia, if you like. We have sorted out the ‘what and where’ aspects of Zealandia, at least at the big-picture level. The next steps will be to answer the more interesting scientific questions, the ‘how and why’. In the meantime, getting Zealandia better known, in schools and in popular culture so that it becomes an everyday word, is important.”
This article was first published in the September 29, 2018 issue of the New Zealand Listener.
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