Story: Coal and coal mining

Rank of coal

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 is reduced, 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

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How to cite this page:

Alan Sherwood and Jock Phillips, 'Coal and coal mining - The nature of coal', Te Ara - the Encyclopedia of New Zealand, http://www.TeAra.govt.nz/en/table/7408/rank-of-coal (accessed 7 December 2019)

Story by Alan Sherwood and Jock Phillips, published 12 Jun 2006, updated 7 Sep 2016