Since the advent of echo-soundings, an enormous amount of information has become available regarding the main submarine features of the seas and oceans. Each of the major land masses is surrounded by a flat, gently sloping zone known as the Continental Shelf which extends from the coast out to a depth of 50–100 fathoms. The width of the Shelf varies from less than 1 mile up to several hundred miles, but 10–100 miles is usual. Outside the Shelf, the slope of the sea bed steepens and passes into the Continental Slope which descends relatively rapidly from the edge of the Shelf down to depths in the region of 2,000–3,000 fathoms. The average gradient on the Slope is normally much steeper than the gradient of the Shelf. At the foot of the Slope, the seaward gradient flattens out, sometimes abruptly, into the Ocean Basin. This is a wide, undulating, but relatively flat zone lying at 2,000–3,000 fathoms and covering most of the central parts of the major oceans.
The surface of the Shelf, which is dominantly flat, is diversified only by local banks and reefs, but the Slope is more irregular, being cut in many areas by the large marine valleys known as submarine canyons. These tend to occur in Slope areas of relatively steep gradient and generally run from the edge of the Shelf to the foot of the Slope. The canyons may reach a width of 5–10 miles, and the bottom of a canyon may reach a depth of 500 fathoms below the general level of the Slope on either side. In a few areas the Slope is also diversified by ridges and basins running more or less parallel to its trend.
The monotony of the ocean floor is diversified in many places by the presence of the Oceanic Ridges, submarine mountain ranges up to several hundred miles in width, several thousand miles in length, and rising from the level of the surrounding ocean floor to depths of only 1,500–1,000 fathoms or less. In other parts of the ocean there occur “seamounts”, isolated submarine mountains which rise from the ocean floor to within a few hundred fathoms of the surface. These seamounts may be as much as 20–30 miles across at the base, and the sides of a seamount usually have quite gentle gradients.
Another important but localised group of features lying at intermediate depths are the “continental borderlands”. These occur near to or immediately adjoining continental blocks, and include a series of broad submarine plateaux and ridges, 100–500 miles wide and lying at depths of 200–800 fathoms, together with intervening depressions. Some of these features have smooth surfaces, but others are irregular with numerous subsidiary banks and basins.
The greatest depths in the ocean are not found over wide areas of the ocean floor, but are concentrated in the Trenches which are large elongated depressions 50–150 miles wide and up to several hundred miles long. The Trenches, which occur mainly in the Pacific Ocean and are located along various parts of the periphery of the Ocean Basin, may descend to depths over 2,000 fathoms below the general level of the nearby ocean floor. The greatest depth recorded to date, the Challenger Deep (5,940 fathoms), occurs in the Mariana Trench. This deep was discovered in 1951 by H.M.S. Challenger II.
The bathymetry of the sea floor around New Zealand provides examples of most of the main types of feature. The coast is surrounded by the Shelf, which varies in width from one area to another. The narrowest portions of the Shelf are found off the east coast between Kaikoura and Cape Kidnappers where the width varies from less than 1 mile up to about 15 miles, and off Fiordland where the variation is from 1–4 miles. Around other parts of the coast, the Shelf is much more extensive, being generally 10–40 miles wide, while in western Cook Strait and south of Stewart Island the width increases to over 100 miles.
The gradient of the Slope varies a great deal between different areas, there being a broad correlation between the steepness of the Slope and the narrowness of the Shelf. In the Fiordland sector, the Slope is very steep, with a maximum gradient of about 1 in 4; the Slope is also relatively steep off the east coast between Kaikoura and East Cape, although in this sector there occur a number of submarine ridges and basins which locally reverse the general gradient. Apart from these two areas, most of the Slope is relatively gentle, but locally there occur steeper zones, for instance, the upper part of the Slope east of Otago Peninsula.
Several submarine canyons cut the Slope between Banks Peninsula and East Cape. The largest of these are the Pegasus Canyon (north of Banks Peninsula), the Cook Strait Canyon, and the Madden Canyon off Porangahau. The Cook Strait Canyon has an unusual shape as compared with most submarine canyons in other parts of the world, with its numerous branches and other irregularities, while the Madden Canyon with its great headward expansion is also exceptional. A group of smaller submarine canyons occurs east of Otago Peninsula.
Between Kaikoura and East Cape, the Slope flattens out rather abruptly at 1,500–2,000 fathoms into the Hikurangi Trench, a feature of low relief which is replaced by the much deeper Kermadec Trench to the north-east. In the sector between East Cape and North Cape, the Slope descends slowly and somewhat irregularly into the South Fiji Basin and the Havre Trough, while it flattens out at intermediate depths opposite the prominent Kermadec Ridge and the less conspicuous Colville Ridge. West of Fiordland, the Slope flattens out rapidly at about 2,000 fathoms into the wide and flat Tasman Basin. North-west of the North Island, and south-east of the South Island, the Slope fades out at shallow or intermediate depths and passes into two large continental borderlands. That to the north-west comprises the Lord Howe Rise, New Caledonia Basin, and Norfolk Ridge, while that to the south-east comprises the Chatham Rise, Bounty Trough, and Campbell Plateau. All of the continental borderland features are of major dimensions, and the majority cover an area equal to or greater than the North or South Islands. Along their outer margins they grade downwards into the adjoining ocean basins.
Most of the sea bed is covered by sedimentary deposits, although there are numerous areas, particularly on the Shelf and on the Oceanic Ridges, where outcrops of solid rock occur. Except for limited regions near the coast, where the sea bed is within reach of divers, direct examination of the sediments and rock outcrops by a geologist is not possible without the use of a diving machine such as a bathyscaphe. Such a machine is very expensive and, although samples could be collected by remote control, the crew themselves could see the rock or sediments only through the window of the observation sphere. In practice, geological information may be gained by a series of methods which cover most of the information which can be obtained with a bathyscaphe, and a great deal which cannot. These methods include collecting samples of the sea bed, submarine photography, and geophysical methods, mainly seismic, gravity, and magnetic surveys.
Rock outcrops on the Shelf occur most frequently near the coast and around the outer edge of the Shelf. The rock types may include igneous, sedimentary, or metamorphic varieties, and are generally similar to those on the nearby land mass, although the sedimentary rock types tend to be more common than on land. Apart from the outcrops, the Shelf is covered with sediments of various types. Gravel and sand predominate around the intertidal zone, and may also be found in limited zones near the Shelf margin. Over the remainder of the Shelf, the sediments consist of sand and mud in different proportions: the ratio of sand to mud tends to decrease away from the coast, but may locally increase near the edge of the Shelf. The material of the sediments is normally composed of erosional debris from the land mass, but in the tropics large parts of the Shelf may be covered with coral reefs and calcareous sediments derived from these. Opposite the mouth of large rivers, sandy and muddy river-derived sediments may spread right across the Shelf, especially when the sediment supply is abundant. In these cases, there is usually a delta projecting out some distance across the Shelf. In glaciated regions, the Shelf may have a hummocky surface due to glacial erosion and to the deposition of till and other glacially derived sediments; the latter usually contain a high proportion of pebbles and sand.
Deposition on the Shelf takes place in several ways. Near the coast sediment is churned into suspension by waves and washed out across the Shelf by wave-generated currents and turbulence; this outward movement of sediment is assisted by the “turbidity effect”, that is, the tendency of sediment-laden water near the coast to move down the gradient of the Shelf by virtue of its effective density, which is greater than that of clear sea water. This is a weaker form of the same process that produces “turbidity currents” on the Slope and in submarine canyons. The wave-turbulence effect becomes weaker away from the coast, and this also applies to the turbidity effect as far as the Shelf margin. The weakening of these two processes away from the coast allows the deposition of sediment on the upper and central Shelf, but near the Shelf margin there occur tidal and oceanic currents which tend to wash away recent sediment and prevent deposition, thus allowing rock outcrops and areas of older sediment to appear. In addition, tidal streams may be strong locally in constricted channels, and correspondingly effective in preventing present-day deposition of sediment, as in central parts of Cook Strait.
The essential form of the Shelf is believed to be due to erosion and deposition governed by “wave-base”, that is, the depth below sea level at which erosion or sediment transport ceases to be effective. This depth is normally different for erosion and for deposition, and it depends greatly on the nature of the available sediment and the underlying bedrock. Nevertheless, the present depth of the Shelf is too great for it to have been controlled entirely by present-day sea level. The Shelf was probably formed mainly during the later phases of the Pleistocene glaciation, when the ice-caps of the world were much larger than at present. A significant proportion of the water in the oceans was locked up in these ice-caps, and sea level is believed to have fallen by 200–300 ft on several occasions during the Pleistocene.
Slope sediments are predominantly mud, which may be greenish, bluish-grey, yellow, red, or black, depending on the source of the material and conditions of deposition. These muds grade outwards into one or other of the various types of oceanic sediment. Some submarine canyons contain predominantly muddy sediment, but others contain sand, and outcrops of rock occur along many canyon walls.
Transport by currents belonging to the main system of oceanic circulation is more significant than on the Shelf. Wave-generated turbulence is weaker, and less sediment is directly stirred up by waves, but the steeper gradients facilitate turbidity flow. In some areas, moreover, particularly within submarine canyons, the gradient is sufficient to allow the occurrence of submarine landslips. A heavy storm on the Shelf, producing a mass of water heavily laden with sediment, or a big submarine landslip, may in fact generate a self-propagating “turbidity current”, in which the downward velocity of the turbid water is sufficient to keep the contained sediment in suspension. These turbidity currents usually flow into and down submarine canyons, and are believed to be responsible for the sandy sediments sometimes found in the canyons. It is probable, in fact, that some of the canyons have actually been eroded by turbidity currents, although others can be shown to represent river valleys submerged by local subsidence of the earth's crust.
In the Ocean Basins, the distance from land is so great that land-derived sediment accumulates very slowly and consists almost entirely of clay. As a result, the main character of the sediment in many parts of the ocean is determined largely by the presence of the accumulated skeletons of planktonic micro-organisms, which frequently grow in great profusion. The Foraminifera, which have calcareous shells, are abundant in temperate and tropical latitudes, and form a large proportion of the deep-sea sediment known as “foraminiferal ooze” (also called “globigerina ooze” after one of the dominant groups of Foraminifera). This covers the sea floor for wide areas around New Zealand. At depths below about 2,500 fathoms, however, the calcium carbonate of the shells becomes soluble in sea water, and so the shells cannot remain in the sediment. Only the land-derived clay is left, and the sediment is known as “red clay”, although its colour is usually pink or brown. In some parts of the tropics, notably in the Pacific, the siliceous organisms known as Radiolaria grow in great abundance, and form the sediment known as “radiolarian ooze”. Diatoms (minute siliceous plants) are abundant in both Arctic and Antarctic waters, and form extensive areas of “diatomaceous ooze”, although here the situation is complicated by the presence of glaciers and ice-caps; melting icebergs introduce large quantities of rock-flour and glacial erratics into the sediments of the surrounding seas.
Core samples from the Ocean Basins occasionally show layers of sand intercalated with the typical deep-sea oozes. These sands are believed to have been deposited by exceptionally powerful turbidity currents, which still had sufficient velocity after reaching the base of the Slope to travel and spread out for several hundred miles over the ocean floor.
Rock outcrops on the Oceanic Ridges are mainly submarine lava flows, which are nearly always composed of basalt; this is also by far the commonest type of lava on the oceanic volcanic islands.
On isolated submarine banks, the sediment may consist predominantly of gravel, sand, or mud, depending on the depth of the bank, its distance from shore, and the strength of currents in the area. There is a general tendency, however, for sediments on banks to contain a high proportion of calcareous biogenetic material, which is composed mainly of shells and shell fragments in the coarser types of sediment, and Foraminifera in the finer-grained types. Two unusual types of sedimentary material which form under rather special conditions are phosphorite, which takes the form of brownish granules or nodules composed of a calcium phosphate mineral, and glauconite, a potassium iron silicate which occurs in the form of small granules with a green or greenish-black colour. Glauconite and phosphorite both tend to occur in places where sediment is accumulating very slowly, such as continental borderlands or submarine banks and ridges situated on the outer Shelf or on the Slope; the Chatham Rise is an excellent example of this kind of environment. These minerals form on the sea bed or just within the sediment, which is usually a sand or mud containing numerous Foraminifera.
New Zealand Shelf sediments consist basically of gravel and sand near the open coast, sand and mud in various proportions on the central part of the Shelf and near river mouths, and gravel, sand, and mud in various proportions near the margin of the Shelf and in constricted channels where strong currents are active, for instance, Cook Strait and Foveaux Strait. The near-shore sediments and those on the central part of the Shelf are evidently being deposited at the present day, but the relatively coarse sediments near the Shelf margin and in constricted channels were deposited during an earlier phase of sedimentation, being late Pleistocene to early Flandrian in age. The areas where these older sediments are found are zones where no present-day deposition is taking place.
The derived pebbles and sand which make up the Shelf sediments of New Zealand consist mainly either of very resistant rocks like greywacke, or of types whose great abundance outweighs their relatively non-resistant character, such as the Taupo rhyolitic pumice. Metamorphic fragments are abundant off some parts of the South Island coast. Formations of Tertiary age consist mostly of non-resistant rocks which are rapidly broken up by erosion and transport, and are rarely found as recognisable fragments on the Shelf. In a few areas, for example, Foveaux Strait, shells and shell fragments make up a considerable proportion of the sediment.
The sediments on the New Zealand Slope, over most parts of the continental borderlands, and in the nearby ocean basins, consist mainly of terrigenous mud and foraminiferal ooze in various proportions. The foraminiferal component predominates at a considerable distance from land, and here the sediment is whitish in colour; near the coast, on the other hand, the percentage of terrigenous mud rises, and the sediment takes on a pink, brown, or green tinge, depending on the colour of the terrigenous admixture. South of the latitude of Auckland the colour is usually pale green, but north of this latitude pale pink or brown colours are typical. On the higher parts of the continental borderlands, particularly in the case of isolated banks, coarse shell-fragment sand tends to occur instead of foraminiferal muds.
The sediments of the Chatham Rise are unusual in several respects. The typical sediment is a foraminiferal sand or silt, but there occur places where cobbles and pebbles of igneous, metamorphic, and sedimentary rocks are present in and below the finer material. These pebbles may have been rafted on to the Rise by icebergs during the Pleistocene. There also occur pebbles of phosphatised limestone, and small granules of phosphatic glauconite.
by Henry Moir Pantin, B.A.(CANTAB.), PH.D.(CANTAB.), New Zealand Oceanographic Institute, Department of Scientific and Industrial Research, Wellington.