Auckland: Raised from the sea

By John Roughan

As Auckland merges to create a supercity, the Herald looks back at how Auckland has changed over the years. Click here to view the full series.

Science and Maori legend agree on one thing: New Zealand was fished out of the sea comparatively recently.

The ancient slice of Gondwanaland that broke away and sank beneath the sea during its long journey on spreading crust, came to acquire layer after layer of marine silt.

The layered rock formations now visible on Auckland's coastal cliffs and many other places are sediments, mudstones and sandstones that accumulated on the deep ocean floor in about 1000 to 2000 metres of water, between 22 and 18 million years ago.

If Maui the legendary fisherman hauled the land from the deep he had some seismic help. Far below the ocean the Earth's hot mantle had altered its convection in a way that created a new boundary between the Australian and Pacific plates.

As they ground together the vigorous and sustained tectonic activity gradually forced the land mass upwards and more of New Zealand's higher ground emerged from the sea.

The submarine land mass reached its lowest point 23 million years ago and has been rising ever since, thrust up by the collision of the plates and bolstered by the subsequent Pacific Ring of Fire volcanic eruptions near the boundary.

These eruptions do not include the relatively small Auckland volcanoes whose cones are visible today. They came much later. The volcanoes that did most to build the Auckland region were ancient giants, not unlike Ruapehu today, and were located several kilometres off the west coast. The Waitakere Ranges are all that remain of a portion of the eastern flank of one of them.

A similar-sized eruption occurred a little to the north, off Kaipara. A chain of volcanoes, all of similar age, that formed an arc not unlike the modern Tonga-Kermadec Arc, stretched through Northland and beyond to the north.

While the entire Waitemata basin from Wellsford almost to Huntly was under water an amazing submarine event occurred. Between 25 and 20 million years ago, a huge section of sea floor, up to 4000 metres thick and 35,000 square kilometres in area, slid down over submarine Northland from the northeast.

The huge gravity slide is known to geologists as the Northland allochthon (allochthon is Greek for "foreign, out of place") and represents a very substantial movement of sea floor over a distance of more than 100 kilometres. The rocks involved are between 90 and 25 million years old.

The Waitemata basin, lying just south of the toe of the allochthon, was slowly filled with sediment eroded from it. At the same time it was being raised by the sustained collision of the Australian and Pacific plates.

But if much of this extraordinary history happened beneath the sea, how can science explain the survival of kauri and other plant species that date from Gondwanaland, and the tuatara that walked on the ancient land mass with the dinosaurs?

Biologists such as George Gibbs suspect Zealandia must have carried more "ghosts of Gondwana", as his 2009 book was called, than geologists allow.

Geologist Hamish Campbell says the Zealandian continent would have had a cargo of Gondwanan animals and plants, but most if not all, would have perished as Zealandia slowly subsided beneath the sea. Unusually among its Gondwanan relatives, New Zealand has no snakes, crocodiles and other species that travelled with other continents. It has one of the world's highest proportions of endemic fauna, found nowhere else. But its plants are much less endemic since pollen, spores and seeds can more easily travel across sea.

"Most (and possibly all) of the ancestors of the plants and animals that were to evolve in New Zealand, came from elsewhere, mainly Australia, within the past 20 million years," says Dr Campbell. "They subsequently evolved in their own sweet way and are found nowhere else."

- NZ Herald

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