Origin of Coal

Coal is a black solid or sedimentary rock, which is combustible in nature. It has a large amount of carbon in it - almost 50% of its weight. The formation of coal takes a long long time. The first coal-bearing rock is said to have appeared about 350 million years ago. This period was known as the carboniferous period or the “coal-bearing” period. Also, there are extensive coal deposits from the Cretaceous age i,e about 144 million years. Let us take a look at the coal origin and formation.

Biological Origin of Coal

Coal is an organic rock that originated by the accumulation of plant material and also subsequent physical and chemical alteration of the material over a long period of time. The coal has inorganic elements too in the form of mineral matter or ash. The plant materials that eventually form the coal have some primary constituents which have different properties and different decomposition rates. These are:

Different Property and Decomposition Rates

Earlier, the scientists used to believe that as the plant matters are made up of cellulose mostly and cellulose is present in double amounts of lignin, cellulose is the mother substance of coals. But, later, through scientific studies, it was found that when bacterial degradation of peat swamp happens, cellulose decomposes very fast to carbon dioxide, methane, and aliphatic acids. Hence, it was proposed that Lignin was the mother substance of the coals. This is known as the Lignin Theory. 

Evolution of Humic Matter

Wax and resins are the constituents of the plant matter which are more resistant to decomposition and decay. They are fossilized with no change during the coal formation process. The other constituents of the plant matter - cellulose, lignin, and proteins, and minerals are converted into 2 parts - Humic Substance and Inert Carbonaceous Matter. The remnants of the organic matter in peat swamps are first in the solid state which is converted into colloidal material and then in the liquid or gaseous state. 

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Order of Decomposition

In the process of coal formation, first, the hydrogen is removed, then the nitrogen, and then the carbon. Carbon is most stable amongst hydrogen, nitrogen, and carbon. When the biochemical decomposition of vegetal matter happens, the result is carbon enrichment. 

Mode of Deposition of Coal

Let us understand the process of coal deposition in detail. 

• Coal is formed mainly from the terrestrial plant material that grows on dry land. 

• The plant debris gets transported by water and then gets deposited under the water in the water bodies. 

• The sediments - organic and inorganic, get settled down gradually.

• This sedimentation process continues till the deposit is covered completely with minerals and sand which results in coal seams.

• Coal has a wide variety because of the varied types of vegetation deposition. 

• The plant debris that accumulates in wet and fluviatile conditions is deposited and is buried by sand, mud, and silt. 

• After the deposition, the metamorphosis of the wood occurs because of temperature and pressure effects. It produces various types of coal.

• The initial transformation of debris material includes various kinds of decay and degradation because of bacterial and fungal action. Slow atmospheric oxidation also takes place.

• The organic material which is water-saturated, spongy, and plant-derived, known as peat, is known as the precursor of coal. This is called peatification - the primary transformation.

• The secondary transformation is a slow process. It includes the aging of Peat deposits under substantially anaerobic conditions, elevated temperatures, and higher pressures. This process is known as Coalification. 

• The Coalification process is the progressive transformation of peat to higher coals.

• Day to day increasing deep burial has a requirement of younger sediment to advance coalification to the bituminous and anthracite stages.

• The pressure that is exerted by the weight of overlying sediments and the heat that is increased by the depth and length of exposure, determines the degree of coalification and the rank of coal.

Factors Determining the Composition of Coal

The factors that determine the composition of coal are:

• The mode of accumulation and burial of the plant debris forms the deposits.

• The age and the geographical distribution of the deposits.

• Structure of the coal forming plants, their chemical composition, and resistance to decay.

• The nature and the intensity of the peat decaying agencies. 

• The geological history of the residual products of the decay of the plant debris that forms the deposits.

Origin of Coal Theories

There are two theories proposed for the mode of accumulation of the plant debris to transform into coal. These theories are - In - situ (Autochthonous) Theory and Drift (Allochthonous) theory.

In - Situ or Autochthonous Theory

There is a popular theory that the plants which compose the coal were accumulated in the large freshwater swamps or peats for many years. This popular theory is known as the autochthonous theory. According to this theory, the coal seams are observed most where the forest grew. The forest land was sinking slowly and the plant matters went underwater gradually. These plant matters did not decompose and were not destroyed. As time passed by, more land submerged and gradually the whole forest was underwater. Again, with the passage of time, the land of the forest came out of the water and this cycle remained repeating. And this eventually resulted in the formation of coal strata and seams.

Drift or Allochthonous Theory

This theory states that the coal strata that is accumulated from plants which are rapidly transported and deposited under flood conditions. As this theory assumes the transportation of plant debris, it is called an allochthonous theory. According to this theory, the plant debris is transported with the flowing water from one place to another and finally gets deposited in the place like swamps, lakes, and seas. These places have suitable conditions for the supply of sediments. An example of this theory is the Indian Coal Seams.