The rank of coal is determined by how deeply the original plant matter was buried. Plant material buried close to the surface becomes peat, while plants buried very deeply become semi-anthracite coal. As the depth increases, the proportion of water in the coal decreases and the proportion of carbon increases. These changes also affect the volume of the coal. As it becomes higher in rank, water and gaseous hydrocarbons are driven off, and the coal becomes more compressed.
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Source: GNS Science