There are four stages in the coal formation. They are peat, lignite, bituminous and anthracite. These stages depend upon the conditions to which the plant remains are subjected after they were buried. Greater the pressure and heat, the higher the rank of coal. Higher-ranking coal is denser and contains less moisture and gases and has a higher heat value than lower-ranking coal.
Further Reading:
- NCERT Notes: Geography- Some major minerals and their characteristics
- NCERT Notes: Geography-Physical characteristics of minerals
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