What is Coal?
Accumulated, compacted, and altered plants from the sedimentary rock is called coal.
What are the Theories of Coal Formation?
Two theories proposed for the mode of accumulation of plant materials that transform into coal are:
- In-situ Theory
- Drift Theory
In-situ (Autochthonous) Theory of Coal Formation
What is the In-situ Theory?
The popular theory is that the plants that compose the coal accumulated in large freshwater swamps or peat bogs over many thousands of years. This theory, which supposes the growth-in-place of vegetable material, is called the autochthonous theory. According to this theory, the coal seams are observed where the forest grew. As the land sank slowly, the accumulated vegetation matter went underwater slowly. It did not decompose and get destroyed. Over time, the rate of sinking of land increased, and coal forests were submerged under water. Again, land along with the coal forest emerged out of the water after sufficient time, and this cycle went on again and again, which is responsible for the formation of coal strata and seams.
Evidence Supporting the In-situ Theory:
- Relative purity & consistency in thickness and composition of coal seams, indicating minimal transport.
- Erect & rooted fossil tree trunks with roots in the underlying clays.
- Poor presence of alkalies and lime in under clays below coal seams.
- Wide lateral extent of coal belts.
- Balanced accumulations of vegetable matter and mineral matter, suggest subsidence.
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.
Drift Theory (Allochthonous) of Coal Formation
What is the Drift Theory?
This theory suggests that coal strata accumulated from plants that had been rapidly transported and deposited under flood conditions. Because this theory assumes the transportation of vegetable debris, it is called the allochthonous theory. According to this theory, the plant material was transported with the flowing water from one place to another and finally got deposited in a place of swamps, lakes, seas, and estuaries having suitable conditions like a supply of sediments. The coal seams of India are of drift origin.
Evidence Supporting the Drift Theory:
- High ash content of coals.
- Scarcity of plant fossils in the roofs of coal seams.
- Digitation/Splitting of coal seams.
- Lateral transition of coal to carbonaceous shales.
- Current peat/brown coal formation in river deltas (e.g., Ganga, Mississippi).
- Fish remains in coal, suggesting open waters like coastal/marine environments.
