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Sea floor geology

by  Keith Lewis, Scott D. Nodder and Lionel Carter

Like the land, New Zealand’s sea floor is dramatic and dynamic. It is studded with active volcanoes, gouged with channels and smothered by huge landslides. It is constantly moving: the seabed is being sheared and destroyed in trenches up to 10 kilometres deep.


Zealandia: the New Zealand continent

A drowned continent

The land area of New Zealand is a small part of a continent of nearly 4 million square kilometres (almost half the size of Australia, or about the size of western Europe). However, 93% of the New Zealand continent, sometimes called Zealandia, is underwater.

The continent is unusually long and thin. It stretches from latitude 19° south (north of tropical New Caledonia) to 56° south (south of New Zealand’s bleak subantarctic islands).

Continents and the deep ocean basins around them represent the two main levels that make up the earth’s surface:

  • continents are generally close to sea level
  • abyssal ocean basins are usually about 5 kilometres deep.

These two levels are separated by a steep zone known as the continental slope.

The rocks that make up continents and the rocks that make up ocean basins are different in origin and composition.

Ocean basins

The floors of deep ocean basins are made up of heavy, basalt volcanic rocks. These form on spreading ridges in the middle of oceans. Spreading ridges have huge cracks in the earth’s crust where molten rock is squeezed out. The cooled rock spreads outward on both sides of the ridge, moving away from it and across the basin floor as if on a conveyor belt.

At the speed a fingernail grows, the rocks move on the conveyor belt towards the edges of the oceans. They become older, heavier and colder the further they are from the ridge. When they reach a plate boundary, ocean floor rocks may sink back (subduct) into the earth’s interior.

Around the edge of the Pacific Ocean, subduction occurs in many places, marked by deep ocean trenches. Parallel to trenches are lines of volcanic activity where water-rich ocean floor melts, and the magma moves up to the surface. The circle of volcanoes around the ocean is known as the Pacific Ring of Fire.

Continental rocks

Compared to ocean basins, continents are very stable. When the conveyor belt of the ocean floor reaches a continent and sinks (subducts) into the earth’s interior, the sediment that has formed on the floor over millions of years is scraped off onto the continent. Continental rocks can be extremely old – ten times older than the oldest oceanic rocks. This is because continents have grown over hundreds of millions of years, from oceanic sediments that are added to the edges.

Continents are generally 25–30 kilometres thick, and are light and buoyant. They float on the earth’s mantle (the layer of semi-molten rock beneath the earth’s crust). Because of their buoyancy and thickness, continents ride high on the mantle, generally above the level of water in the oceans.

The restless seabed

Pushed by the ever-moving ocean basins, much of the shallower seabed around New Zealand is also moving. Submarine faults rip the seabed apart, leaking hot mineral-rich fluids, or they push it together to form new land, or tear it sideways. Fault movements generate earthquakes that trigger submarine landslides hundreds of times bigger than those on land.

A continent of two ridges

The New Zealand continent is principally made up of two almost parallel ridges. These are largely underwater and trend north-west through the southern Pacific Ocean. The western ridge includes Lord Howe Rise and the Campbell Plateau. The narrower eastern ridge forms New Caledonia, Norfolk Ridge, the Northland peninsula of New Zealand and the Chatham Rise.

Both ridges form sea floor 1,000–1,500 metres deep, with occasional rocky islets rising above the water. These islands help define the edge of New Zealand’s huge Exclusive Economic Zone. The ridges are continental rock, but are deeper than ordinary continents because they are thinner than normal (only about 20 kilometres thick), and therefore float lower on the earth’s mantle. These continental rocks pulled away from the edge of the great southern continent of Gondwana when the Tasman Sea opened, between 60 million and 85 million years ago.


Active plate boundaries

New Zealand’s long, thin, drowned continent forms an unusual-shaped seabed. Even more striking is the manner in which this continent and the surrounding ocean basins are being torn and crushed along the boundary where two of the earth’s largest crustal plates meet. Along this boundary, the Pacific Plate to the east and the Australian Plate to the west grind into and past one another. This has created a line of extremely deep trenches, volcanic ridges and, above the sea, snow-capped mountains.

Kermadec Trench–Ridge system

The Kermadec Trench forms a great gash in the surface of the earth. From the 5-kilometre-deep Southwest Pacific Basin to the east, the trench gradually drops to more than 10 kilometres below the water’s surface. The Kermadec Trench marks where the Pacific Plate meets the Australian Plate. The Pacific Plate bends and cracks as it plunges under the Australian Plate – a process known as subduction.

Lying to the west, and towering higher than Mt Everest above the trench, the Kermadec Ridge rises to 1 kilometre below the ocean’s surface. This ridge marks the edge of the Australian Plate. It has been pushed upward by the Pacific Plate driving beneath it, and has been built up by volcanism.

Hikurangi Trough and Plateau

The Kermadec Trench shoals southward and merges with the 3-kilometre-deep, mud-filled Hikurangi Trough east of the North Island. This is where the unusually shallow oceanic floor of the Hikurangi Plateau is subducting beneath the continental crust of the Australian Plate.

Puysegur Trench

South-west of Fiordland, the Puysegur Trench marks the zone where the oceanic crust of the Australian Plate plunges beneath the Pacific Plate. The topography is not as dramatic as the Kermadec Trench, as the plates largely slide north–south past one another, rather than over and under. The north–south sliding of the Australian and Pacific plates continues to the South Island, where the movement is taken up by the Alpine Fault.

Seabed volcanoes

Along the western side of the Kermadec Ridge is a line of mainly submerged volcanic cones, with names like Rumble I, II and III. The volcanoes run from the Kermadec Islands to White Island and ultimately Mt Ruapehu.

Between the volcanoes of the Kermadec Ridge and the Colville Ridge, 100 kilometres to the west, is the Havre Trough. Three kilometres deep, the trough is where the earth is being torn apart between the Kermadec and Colville ridges by molten rock rising above the deeply diving Pacific Plate. The trough continues ashore into the Rotorua–Taupō area of New Zealand, where the land is also being stretched. The sea floor of the Havre Trough is particularly complex, with many ridges and lines of extinct volcanoes serving to record the trough’s formation.

South of New Zealand, volcanic activity is rare. Only the extinct volcanic Solander Island is located near present volatile plate-boundary processes. The Solander Trough is smooth and old, and unrelated to the spreading movement found in basin sea floors.


The continental shelf

Like most land masses, New Zealand is surrounded by a continental shelf. Fishermen have long known that there is a gently sloping seabed that drops away (at a point called the shelf break), usually at a depth of 100–160 metres. The shelf extends for only a few hundred metres off Fiordland, but is over 100 kilometres in western Cook Strait.

The shelf seabed is of great interest to New Zealand: it supports near-shore fisheries and is the most accessible area to exploit marine minerals.

Effect of the ice ages

During the last ice age, when much of the world’s water was frozen in polar ice-caps, the sea was about 120 metres below its present level. What is now the continental shelf was, in the ice age of 20,000 years ago, a bleak windswept plain that was smoothed by wave action. The shelf grew outward as rivers dumped sediment there.

Nearer to the present-day shoreline, as sea levels rose due to melting ice, parts of the continental shelf were cut by waves. In places where the land has since risen, old wave-cut platforms and cliff lines were preserved as terraces, and can now be seen in many places on the New Zealand coast.

It is about 7,000 years since the sea stopped rising at the end of the last ice age. The continental shelf, including old river valleys such as the Marlborough Sounds, is now drowned.

Two meanings of ‘continental shelf’

Seafarers have long understood the continental shelf to be the flat seabed extending from the shore to where the sea floor drops away more steeply onto the continental slope. This is sometimes confused with a more recent legal use of the same term.

Since the 1980s international lawyers have worked to clarify a nation’s jurisdiction of the seabed that surrounds the land. The legal concept of an ‘extended continental shelf’ encompasses the whole area underlain by continental crust, and includes not only the shelf and the slope, but also some distance across the deep ocean floor. Because much of the continent of Zealandia is submerged, this allows New Zealand to have jurisdiction over a very large offshore area.


Continental slopes, canyons and landslides

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.

Grand canyons

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.

Submarine landslides

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.


Abysses

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.

Abyssal rivers

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.


How sediment forms

Almost the entire ocean floor is covered with different types of sediment: mud, sand and gravel. This is formed of material from land, the skeletons of plankton and seabed-dwelling animals, chemical reactions, and air-borne dust.

Sediment from land

Material from land makes up 75% of seabed sediment. Most of this is carried to the sea by rivers and dumped on the nearby continental shelf and slope. Some is carried further offshore by wind and ocean currents. Sediment is also deposited through flows of mud and sand (turbidity currents) and submarine landslides, such as those that occur off Kaikōura.

Dead plankton and sea life

In the deep ocean, sediment from land makes up only 20% of the seabed cover. Most is made up of minute organisms or plankton that have died in surface waters and sunk. In the oceans there is a continuous rain of these tiny creatures, which include diatoms, coccoliths, radiolarians and foraminifers.

Debris from once-living sea creatures settles on the continental shelf, especially where the supply of sediment entering the sea from land is low. These deposits commonly contain shells, bryozoans, algae and tube worms. Such deposits are currently forming extensively on the Chatham Rise, Campbell Plateau and Lord Howe Rise, and will one day be limestone.

Clay

In the deep ocean basins beyond the New Zealand continent, at depths of 4–5 kilometres, red and brown clays dominate the sea floor. They contain fine particles carried from land by wind and ocean currents, ash from volcanic eruptions, and particles from space. At these depths material from once-living sea creatures is sparse because of the dissolving power of very cold water.

Minerals and chemical processes

Sediment may form from new mineral growth around submarine volcanoes and hot-water vents, or from chemical processes in the sea water. Such sediments comprise less than 1% of the seabed, but can give rise to locally unique and important habitats. The green mineral glauconite, for example, occurs where there is little sediment entering the sea from the land. This occurs on Chatham Rise, where there is enough of it to form large areas of greensands. Chatham Rise also hosts particles so rich in phosphate that they have economic potential. In the deep ocean, fist-sized nodules of manganese and iron form beneath currents flowing along the base of the Campbell Plateau.

Sediment movement

Where there are strong currents and waves, such as in Cook Strait, sediment is easily moved. Boulders and pebbles roll and tumble, gradually becoming smooth and round. Moving water also washes out fine material, leaving only grains of similar size. As gravel and sand have large grains, these often accumulate in beach and continental shelf environments.

Ocean currents and tides, especially when intensified by storms, can bring about marked changes in the sea floor. In contrast, the protected waters of harbours, estuaries and fiords encourage silt and clay to settle.

As water depth increases further from the New Zealand coast, the effects of waves and currents decrease. Gravel and sand beaches give way to finer-grained deposits, and at depths below about 30 metres the seabed is often covered with mud.


New Zealand sea-floor sediment

Because New Zealand’s marine and terrestrial environments are so variable and dynamic, sediments off the coast are also diverse. The land mass itself produces almost 1% of all the sand and mud entering the world’s oceans. This outflow is caused by rapid uplift of the mountain ranges, frequent earthquakes, high rates of erosion, and an abundance of soft rocks that are easily eroded.

The continental shelf around New Zealand is covered mainly with sediment from the land, except at the northern and southern extremities. Here, a lack of major rivers means that there is no way for material to be washed to the sea, and shelly sediment from once-living sea creatures prevails.

Diversity

In Cook and Foveaux straits, powerful tides and waves sweep away much of the mud, leaving gravel, coarse sand and shells. In contrast, the sea floor off the eastern North Island is swamped with mud because currents are relatively weak and the supply of sediment from land is the largest in New Zealand.

Off the western North Island, black, iron-rich sand has been formed by wave action on volcanic rock.

Canyons, channels and plateaux

The continental shelf off Otago, Cook Strait, North Canterbury and elsewhere is dissected by submarine canyons. These steep-sided gorges siphon off sediment and guide the turbid flows of sand and mud (called turbidity currents) into deeper water.

Off Cook Strait, submarine canyons merge into the deep Hikurangi Channel. This guides rapidly flowing mud and sand 2,000 kilometres across the 3-kilometre-deep Hikurangi Plateau, then onto the deep Pacific Ocean floor. These currents overflow the channel spreading sandy mud over everything they pass.

In contrast, south of New Zealand, the Campbell Plateau is isolated from sediment coming from land. Sediment there is dominated by calcium carbonate, sand and mud.


Exploring the sea bed

Sampling devices

The type of equipment used by New Zealand scientists to sample marine sediment depends on the needs of the survey and the nature of the seabed. Dredges are used to collect rock and gravel, while corers and grabs recover sand and mud. These devices are normally operated from ships, such as the Tangaroa, owned by the National Institute for Water and Atmospheric research. In very shallow waters, divers can collect samples with hand-held corers.

Dredges and grabs

Dredges are large metal buckets and cages that are dragged by a steel wire across rough, hard areas of the sea floor. They are used on fields of manganese nodules and underwater volcanoes.

Grabs have jaws that close when they hit the sea floor, and are useful for recovering muddy material.

Corers and drills

Corers are used to extract a core – a cylindrical sample of sediment. Corers consist of a square or round barrel up to 50 metres long. A heavy weight rests on the barrel, pushing it into the sediment. These are known as gravity corers. Some have a piston inside the barrel to create a vacuum that assists entry into the seabed. This technique enables longer cores than those collected by simple gravity. Some corers have a door that snaps shut to hold in the sediment while the barrel is withdrawn from the seabed.

Hydraulic drilling techniques are used in the deep ocean to obtain very long core samples. Submersibles and remotely operated vehicles are also used.

Observation

The seabed, and the plants and animals that live there, can be filmed from survey vessels. Instruments known as benthic landers or tripods can be placed on the sea floor and remotely operated by computers, to make long-term observations.


Hononga, rauemi nō waho

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How to cite this page: Keith Lewis, Scott D. Nodder and Lionel Carter, 'Sea floor geology', Te Ara - the Encyclopedia of New Zealand, http://www.TeAra.govt.nz/mi/sea-floor-geology/print (accessed 14 December 2019)

He kōrero nā Keith Lewis, Scott D. Nodder and Lionel Carter, i tāngia i te 12 Jun 2006