The sediments deposited in the oceans and the seas are derived from three major sources: 1. Terrigenous Sources 2. Volcanic Eruptions 3. Marine Plants and Animals.
Source # 1. Terrigenous Materials:
The continental rocks are disintegrated and decomposed due to various types of weathering and thus fine to coarse sediments are formed. These sediments of continental origin are called terrigenous materials which are brought to the rivers through surface wash, rain-wash, rills, gullies and small rivulets.
Ultimately terrigenous sediments are brought by the rivers to the oceans and the seas. ‘Some 15,000 million to 20,000 million tones per year of solid material are discharged through the rivers to the oceans. To this can be added a total of about 4000 million tones of soluble material.
This means that for every cubic metre of water reaching the sea an average of about half a kilogram of sediments is carried away from the continents’.
Since there is much variation in the size and shape of terrigenous materials, there is marked gradation of these materials when they are deposited in the oceans, i.e., coarser and larger sediments (boulders, cobbles and pebbles) are deposited near the coast and the size of sediments becomes smaller and finer away from the coast. Very fine sediments are kept in suspension in the offshore regions. On the basis of size, composition, and chemical characteristics terrigenous sediments are divided into gravels, sands and silt, clay and muds.
The diameter of gravels ranges from 2 mm to 256 mm. There is marked gradation in the size of gravels. The following are sub-types of gravels on the basis of diameter of particles (figures in the brackets indicate diameter), boulders (256 mm), cobbles (64 mm), pebbles (4 mm), granules (2 mm) etc.
Since these sediments are very large in size, these are deposited near the coast on the continental shelves. These sediments are further reduced in size due to further disintegration caused by sea waves. Gravels are brought to the oceans by the rivers.
The sediments varying in diameter from 1 mm to 1/16 mm are termed sands.
On the basis of size of grains sands are classified into five types viz., (figures in the brackets denote diameter):
(i) Very coarse sands (1 mm),
(ii) Coarse sands (0.5 mm),
(iii) Medium sands (0.25mm),
(iv) Fine sands (0.125mm), and
(v) Very fine sands (0.0625 mm).
The disintegration and commination of continental rock fragments into fine sediments produces sands which are deposited in the oceans by rivers, surface wash and winds. There is marked gradation of sand deposits in the oceans i.e., coarser sands are deposited close to the coast while fine sands are deposited away from the coast.
Silt, Clay and Mud:
The finer sediments ranging in diameter from 1/32 mm to 1/8192 mm are grouped under the category silt, clay and mud (silt=1/32 mm to 1/256 mm, clay= 1/256 mm to 1/8192 mm). Mud is still finer than clay. Clay is significant cementing element. These materials are brought from the continents by the rivers. Clay and mud are deposited in calm seawater. Generally, these deposits are found at the depth of 100 to 1000 fathoms (600 to 6000 feet).
Murray has divided mud into three types on the basis of colour:
(i) Blue Mud:
Blue mud includes the materials derived through the disintegration of rocks rich in iron sulphide and organic elements. These are generally found at greater depth of the continental shelves. The original colour of blue mud is bluish black and it contains 35 per cent of calcium carbonate. Blue mud predominates in the Atlantic Ocean, Mediterranean Sea, Arctic Sea and enclosed seas.
(ii) Red Mud:
The sediments derived through the communication of rocks rich in iron oxides (FeO) form red mud. The reddish colour is mainly due to the dominance of iron content. It contains 32 per cent of calcium carbonate. The deposit of red mud is confined mostly to the Yellow Sea, Brazilian coast, and the floors of the Atlantic Ocean.
(iii) Green Mud:
Green Mud is formed due to chemical weathering wherein the colour of blue mud is changed to green mud due to reaction of seawater. It contains green silicates of potassium and glauconite (form of iron) which constitutes 7 – 8 per cent of total mineral composition whereas calcium carbonate ranges from 0 to 56 per cent.
The deposits of green mud are found along the Atlantic and Pacific coasts of N. America, off the coasts of Japan, Australia and Africa. These are generally found at the depth of 100 to 900 fathoms (600 to 5,400 feet).
Source # 2. Volcanic Materials:
Volcanic materials deposited in the marine environment are derived from two sources:
(i) Volcanic eruptions on the land-the volcanic materials through violent central eruptions become very fine due to collision among themselves and due to further disintegration. Fine volcanic materials nearer to the coastal lands are blown by wind and are carried to the oceans while volcanic materials of distant places are brought by the rivers via overland flow, rainwash, rills and small rivulets, and
(ii) Volcanic eruption in the oceans and the seas- in such cases volcanic materials are directly deposited. Volcanic materials resemble blue mud and are grey to black in colour.
Source # 3. Organic Materials:
The source of organic materials is sea itself. They include skeletons of marine organisms and plant remains.
These materials are grouped into two categories viz.:
(i) Neretic matter, and
(ii) Pelagic matter.
The neretic matter includes skeletons of marine organisms and plant remains while pelagic matter consists of remains of different types of algae. The skeletons of animals and dead plants are subjected to decomposition and chemical changes. Thus, they are changed to mud and sands and are ultimately deposited on the sea floor.
Neretic matter is deposited mostly on the continental shelves and are generally covered by terrigenous materials. These include shells of molluscs and their fragments, skeletons of radiolaria and spicules of sponges, calcareous and siliceous plant remains.
Pelagic deposits consist of matter derived from algae and are mostly in the form of liquid mud, generally known as ooze.
Pelagic materials are oozes which are divided into two groups on the basis of lime and silica contents as follows:
(i) Calcareous Oozes:
Calcareous oozes contain lime content in abundance and are seldom found at greater depth because of their high degree of solubility. They are generally found at the sea floor between the depths ranging from 1000 fathoms (6000 feet) to 2000 fathoms (12000 feet).
On the basis of principal organisms calcareous oozes are further divided into two sub-types viz.:
(a) Pteropod ooze, and
(b) Globigerina ooze.
(a) Pteropod Ooze:
Most of the pteropod oozes are formed of floating pteropod molluscs having thin shells of generally conical shape with average diameter of half inch. It contains 80 per cent calcium carbonate and is mostly found in the tropical oceans and seas at the depth of 300-1000 fathoms.
It decreases with greater depths and practically disappears beyond 2000-fathom depth. It is found mostly in the regions of corals. The main location of pteropod ooze includes the western and eastern parts of the Pacific Ocean, surroundings of Azores, Canary Island, Antiles, mid- Mediterranean submarine ridge and Indian Ocean.
(b) Globigerina Ooze:
Though this ooze is formed from the shells of a variety of foraminifera but most of such oozes are formed of germs called globigerina. When this deposit is dried up it becomes dirty white powder. Besides milky white colour, it is also blue, grey, yellow and green in colour.
The chemical composition reveals 64.46 percent of calcium, 1.64 percent of silica and 3.33 percent of minerals. Globigerina is found mostly in the tropical and temperate zones of the Atlantic Ocean, on the eastern and western continental shelves of the Indian Ocean and in the eastern Pacific Ocean. It is generally found between the depths of 2000 to 4000 fathoms and becomes absent at greater depths.
(ii) Siliceous Ooze:
When silica content dominates, the ooze becomes siliceous in nature. Silica is derived from a group of protozoa or radiolarians and benthic animals mainly sponges. This ooze does not dissolve as compared to calcareous ooze because of less calcium carbonate and dominance of silica. Thus, such oozes are found in both warm and cold water at greater depths.
This group is further divided into two subtypes on the basis of dominance of a particular organism:
(a) Radiolarian Ooze:
Radiolarian ooze is formed by the shells of radiolaria and foraminifera. It changes to dirty grey powder when dried. Silica predominates but calcium carbonate is also present (ranging between 5 to 20 percent, average being 4 percent). Lime content decreases with increasing depth and it absolutely disappears at greater depth. This ooze is found upto the depth of 2000 to 5000 fathoms in the tropical oceans and seas. It covers the largest areas in the Pacific Ocean.
(b) Diatom Ooze:
Diatom ooze is formed of the shells of very microscopic plants containing silica in abundance. It also contains some clay. Calcium content varies from 3 to 30 percent. It is blue near the land and the colour changes yellow or cream away from the land. It becomes fine coherent white powder when dried. Diatom ooze is very frequently found at greater depth in high latitudes. Significant area of this deposit includes the zone around Antarctica and a belt from Alaska to Japan in the N. Pacific at the depth of 600-2000 fathoms.
(4) Inorganic Materials:
Majority of inorganic elements are basically precipitates which fall down from above. These elements fall on the land as well as in the oceans. Some of the inorganic elements are transported from the land to the oceans by various agencies. The inorganic precipitates include dolomite, amorphous silica, iron, manganese oxide, phosphate, barite etc. Besides, glauconite, phosphorite, feldspar, phillipsite and clay minerals are also found. The organic and inorganic materials are so mixed together due to chemical processes that it becomes very difficult to isolate them from each other.
Red clay, previously considered to be of organic origin, is the most significant inorganic matter and very important member of pelagic deposits. It covers the largest area of deep sea deposits. Silicates of alumina (85.35 percent) and oxides of iron are the chief constituents of red clay. Besides, calcium (6.7 per cent), siliceous organisms (2.39 per cent) and a few minerals are also present. It also contains decomposed volcanic material.
It may be pointed out that red clay contains more radioactive substances than any other marine deposit. It is soft, plastic and greasy in character. It becomes reddish brown powder when dried. Red clay is widely distributed at the greatest depth in all the oceans. Its dominant locations include the zone between 40°N and 40° S in the Atlantic Ocean, eastern part of the Indian Ocean and the North Pacific Ocean covering 129 million km2 of area.