Coal is the most abundant fossil fuel. It is thought to be fossilized vegetation. At least 20 ft of compacted vegetation is necessary to produce a 1 ft seam of coal. The compacted vegetation - in the absence of air and in the presence of high pressure and temperature - is converted into peat (a very low-grade fuel), then brown coal, then lignite, then subbituminous coal, then bituminous coal, and finally anthracite coal.
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