Deep basins and trenches
The major ocean basins around New Zealand – the vast South-west Pacific Basin to the east and the Tasman Sea Basin to the west – are abysses. Typically these are 4.5–5 kilometres deep and dotted with extinct volcanic cones and ridges. Where abysses plunge down into the earth’s interior, they form trenches up to twice that depth.
The minor basins to the north of New Zealand, including the South Fiji and Norfolk basins, are shallower (3.5–4.5 kilometres down), as is the newly forming Havre Trough. The New Caledonia Basin and Bounty Trough, wedged between the two parallel ridges of the New Zealand continent, fall away to a maximum depth of 3.5 kilometres. The basin and trough formed when continental crust was pulled apart, making it thinner, and represent an early, failed opening-up of the Tasman Sea.
An oddity is the Hikurangi Plateau, between the North Island and the Chatham Rise, which ranges from 2.5 to 4 kilometres deep. Despite being so shallow, the plateau has volcanic seamounts that are like deep ocean basins. It is thought to be the result of a huge, ancient volcanic outpouring in the ocean basin.
Kermadec and Puysegur trenches
The Kermadec Trench, which extends for over 1,000 kilometres north-north-east of New Zealand’s East Cape, is one of the earth’s great trenches. Its deepest point is just over 10 kilometres below the ocean’s surface, marking the place where the Pacific Plate is pushing under the Australian Plate. At its southern end, the Kermadec Trench shallows to about 5 kilometres and merges into the sediment-flooded Hikurangi Trough, off the east of the North Island.
South-west of Fiordland, the 6-kilometre-deep Puysegur Trench marks where the Australian Plate is pushing under and past the Pacific Plate.
Louisville Seamount Chain
The deep ocean floor is dotted with innumerable extinct volcanic cones that are usually less than 2 kilometres high. Most date from when the surrounding sea floor was formed from a mid-ocean spreading ridge (the place where molten rock is forced out of the earth’s interior). Much larger and younger are the cones of the Louisville Seamount Chain, which extends more than 4,000 kilometres across the South-west Pacific Basin. They are similar in origin, size and bearing to the volcanoes that form the Hawaiian Islands. Like the Hawaiian chain, they are thought to be younger towards the south-east, reflecting the movement of the ocean floor north-westwards over a ‘hot-spot’ deep in the earth. The youngest volcano marks the site of the ‘hot-spot’, south-east of the Chatham Islands.
At the other end of the chain, the oldest seamounts are being dragged down into the Kermadec Trench, forming a barrier that separates the Kermadec Trench from the Tonga Trench.
Most of the canyons that are carved into the continental slope around New Zealand end within a few tens of kilometres of the toe of the slope. A few canyons merge into meandering submarine channels, reminiscent of the Mississippi or the Amazon rivers. These channels, which are typically several hundred metres deep and 5–10 kilometres wide, meander across the deep ocean floor for up to 2,000 kilometres.
The longest, and one of the few presently active channel systems, is the Hikurangi Channel, which is fed by the Kaikōura and Cook Strait canyons. The channel meanders along the Hikurangi Trough east of the North Island, cutting through the Hikurangi Plateau and continuing far out into the South Pacific Basin. It is active only every couple of centuries during catastrophic underwater flash floods. These occur when earthquakes trigger avalanches of the sediment that has been poured into the canyon heads. Once mobile, the avalanches change into dense flows of sand and mud capable of travelling at motorway speeds along the abyssal channel for over 2,000 kilometres.
Lying to the south of New Zealand, the Bounty Channel is probably not presently active. It was fed by three canyons off South Canterbury, mainly during the ice ages, when rivers poured sand and mud directly into the canyon heads.