Coal may be defined as a compact stratified mass of plant debris which has been modified chemically and physically by natural agencies, interspersed with smaller amounts of inorganic matter. In situ and Drift are the two major theories of coal formation.
The natural agencies causing the observed chemical and physical changes include the action of bacteria and fungi, oxidation, reduction, hydrolysis and condensation - the effect of heat and pressure in the presence of water.
Many factors determine the composition of coal.
- Mode of accumulation and burial of the plant debris forming the deposits.
- Age of the deposits and the geographical distribution.
- Structure of the coal forming plants, particularly details of structure that affect chemical composition or resistance to decay.
- Chemical composition of the coal forming debris and its resistance to decay.
- Nature and intensity of the peat decaying agencies.
- Subsequent geological history of the residual products of decay of the plant debris forming the deposits.
Major in situ coal fields generally appear to have been formed either in brackish or fresh water, from massive plant life growing in swamps, or in swampland interspersed with shallow lakes. The development of substantial in situ coal measures thus requires extensive accumulations of vegetable matter that is subjected to widespread submersion by sedimentary deposits.
Accumulations of vegetable matter and associated mineral matter, generally clays and sands, are balanced by the subsidence, or motion of the Earth’s surface, in the area on which these materials are accumulating. Hence, coal formed like this has bands of coal and inorganic sedimentary rocks arranged in a sequence.
It was the difference in coal properties of Gondwana coals that led to the formation of the drift theory. The mode of deposition of coal forming can be explained as said below:
- Coal is formed largely from terrestrial plant material growing on dry land and not in swamps or bogs.
- The original plant debris was transported by water and deposited under water in lakes or in the sea.
- There will not be a true soil found below the seam of coal.
- The transported plant debris, by its relative low density even when water logged, was sorted from inorganic sediment and drifted to a greater distance in open water. The sediments, inorganic and organic, settled down in regular succession.
- The process of sedimentation of the organic and inorganic materials continues until the currents can deposit the transported vegetation in the locations.
- These deposits are covered subsequently by mineral matters, sand, etc. and results in coal seams.
- The depositions can also stop for a particular period and again begin to happen depending upon the tidal and current conditions.
- The coal properties vary widely due to the varied types of vegetation deposited.