Continental slopes are the relatively steep inclines between the continental shelf and the surrounding ocean basins. New Zealand’s continental slopes vary enormously, depending mainly on their age and origin. Off Fiordland, the slope falls precipitously from a narrow shelf to the ocean basin. There, the continental slope is higher than, but not as steep as, Mt Cook (3,754 metres). Off western Cook Strait and Canterbury the continental slope steps down to a marginal plateau and from there to the deep ocean basin.
Passive pulled-apart and blanketed slopes
Continental slopes around New Zealand are typically inclined at a moderate angle of three to six degrees. Most are ancient. Some formed when Zealandia broke away from Gondwana about 85 million years ago.
These slopes survive much as they were when formed. However, slopes that are close to land have been extensively modified either by blanketing mud or the pushing and tearing of plate boundaries. The slope to the west of Cook Strait is like the edge of a huge underwater rubbish tip, built up over millions of years with mud and debris flowing out from rivers.
Active sheared and bulldozed slopes
The boundary between the Pacific Plate and the Australian Plate runs through New Zealand. Off the Fiordland coast the Australian Plate tears past and dips (subducts) beneath the Pacific Plate and this has created the steep (10°) continental slope.
Plate boundaries do not always produce steep slopes. Off the eastern North Island, a gentle continental slope (2°) with numerous ridges and troughs has been formed. Slices of mud up to several kilometres thick are scraped off the continuously subducting Pacific Plate and crumpled onto and under the East Coast of the North Island. Rocks of the North Island have also been crumpled to form the coastal hills of southern Hawke’s Bay and Wairarapa.
Huge canyons that rival the size of any on land cut deep into the continental slope around New Zealand. Most have been carved by avalanches of sand and mud dumped at the canyon heads. Avalanches must have been very common during glacial ages, when rivers would have carried sediment right to the top of the slope.
New Zealand’s most extensive canyon system, around 1.5 kilometres deep and multi-headed, is the Cook Strait Canyon. It is supplied partly by sediments swept from west of the strait by powerful tides. Not far away, the Kaikōura Canyon brings abyssal depths (and sperm whales) to within a stone’s throw of the tiny settlement at Goose Bay. It is one of the few canyons that are presently active, trapping gravel, sand and mud that move along the shore. There are also impressive canyons off Otago, South Canterbury and the West Coast, and in the Bay of Plenty.
In places, New Zealand’s continental slope is not stable, either because rocks are being tilted seawards or because land-derived sediments are collecting there. Lubricated by water, and subject to severe earthquakes, steep underwater slopes are prone to crumbling and slipping. However, even very gentle slopes (some off Hawke’s Bay are only 1°) can slide.
When a section of the seabed collapses it pulls down the water above it. When the sea rebounds, a tsunami is born. Small submarine landslides have come close to capsizing Kaikōura fishing boats in otherwise calm seas. One near Gisborne may have caused the tsunami that caused extensive flooding in 1947. The huge landslide off East Cape, first mapped in 1995, would have generated catastrophic waves. Fortunately it occurred about 170,000 years ago – long before people were living in New Zealand.
In many cases slips are merely tens of metres thick, but in a few places submarine landslides have been on a truly massive scale. One such slide, the Ruatoria Debris Avalanche, occurred on a 3-kilometre-high slope off East Cape. Over 3,000 cubic kilometres of rock collapsed (equivalent to the area of Coromandel Peninsula falling from Mt Cook). It left a 30-kilometre-wide amphitheatre and a tangle of blocks – some 17 kilometres across and 1.5 kilometres high – littering the abyssal sea floor for up to 50 kilometres beyond the toe of the slope.