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On the basis of the size of the rock fragments, the sediments can be broadly classified into gravel, sand and mud. The finest material, which is usually grouped under the general term mud, covers a large part of the continental shelf and of the slope beyond. Muds are of finer texture than the sands. They consist to a large extent of minute particles of various rock forming minerals, quartz being the most abundant Murray distinguishes between the three main classes of mud based on the colors of the sediments - blue mud, red mud and green mud.

Volcanic Deposits. In volcanic regions, the deposits of the continental shelf and slope consist chiefly of product of volcanism. The volcanic material, which is being thrown out from a volcano, is subject to chemical and mechanical weathering. It is then transported to the ocean by the action of running water and wind. These deposits again differ from the ordinary terrigenous deposits in the sense that they consist of the fragments of lava instead of quartz.

Organic Deposits. A great number of animals and plants live and grow on many parts of the continental shelf. Their shells and skeletons settle down on the bottom and may form the greater part of the deposits. The organic shells and skeletons are changed into sand and mud by mechanical and chemical processes. These deposits contain only the calcium carbonate and thus they are different from the ordinary terrigenous deposits.

Pelagic Deposits. These deposits are most conspicuous on the deep sea plains and in the deeps covering about 75 per cent of the oceanic area. With the exception of fine volcanic dust, very little terrigenous material is carried beyond the continental slope. Wherever the supply of terrigenous mud is not much on the slopes, the deposits become more or less pelagic in type. The pelagic deposits are both organic and inorganic in nature. They consist partly of the remains of marine animals and plants and partly of the volcanic dust brought by the wind.

These siliceous remains of organism are of two types. The Neiritic deposits consist of dead skeleton of marine animals and plants. They are abundant in shallow water of low salinity.

The organic group is chiefly represented by a kind of liquid mud, which is known as ooze. The oozes contain shells of several kinds of organisms. In some oozes, the shell is made of calcium carbonate, while in others it is made of silica. Therefore, there are two main kinds of oozes: Calcareous ooze and the Siliceous ooze. These oozes are named after the predominant type of organisms.

The calcareous ooze is either the pteropod ooze or the globigerina ooze. The siliceous ooze comprises the diatom ooze and the radiolarian ooze. Pteropod ooze is very rarely found deeper than l000 m and it occurs in shallow waters between 400-500 m. It predominates in the western and eastern pacific and in small patches near Azores; Antilles, Canary Islands and Indian Ocean.

Globigerina ooze contains 30% of carbonate of lime. Its colour varies from white, yellow to grey. This is perhaps the most widespread in warm and temperate waters of the Atlantic, along the eastern and western continental shelves of the Indian Ocean and in the eastern pacific.

Radiolarian ooze is dirty grey powder, siliceous remains of radiolaria shells. It is the characteristic of deep water and abundant in the tropical regions of the Pacific Ocean. Diatom ooze is the plant shells of very microscopic size and made of silica. It is yellow or cream coloured. It occurs in high latitudes. In the Antarctic Ocean south of 40 degrees S and north pacific have broad bands of the deposits. The ooze is found between 300-1000 m. Sometime extending up to 2000 m.

In addition to the organic oozes, there is another type of deposit called the Red Clay. It consists mainly of inorganic material, which is apparently of volcanic origin. Silicon and aluminium dioxide are the chief constituents. It also contains iron, manganese, phosphorus and radium. Red clay is the most widely spread pelagic deposits. It covers about 38 per cent of the total oceanic area. It covers more than half of the Pacific Ocean.

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