Rocks and Sediments
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.