How Coal Is Formed
The fossil fuel coal is formed when peat is altered physically and chemically. This ongoing process is known as “coalification” and can take as long as 400 million years to complete in some cases. During the coalification process the peat undergoes numerous changes as a result of bacterial decay, compaction, heat, and time. Typically peat deposits are varied and can contain everything from fairly pristine plant parts such as bark, roots, and spores to decaying plants, decaying products, and even charcoal if the peat caught fire during the accumulation process. These peat deposits will usually be formed in a waterlogged environment where plant debris can accumulate easily such as peat bogs and swamps. In these particular environments, the accumulation of plant debris will exceed the rate of the bacterial decay of the debris.
In order for the peat to become coal, the matter must be buried by sediment. It is this burial that will compact the peat and squeeze water from the mass. Over long periods and time and prolonged heat, the complex hydrocarbon compounds in the peat will break down and alter in a number of interesting ways. Gases such as methane will be expelled from the mass and as other elements disperse, it will become richer in carbon (the element that largely characterises coal). The longer the mass is subjected to heat and pressure the higher the level of carbon will be in the eventual coal. In order of the coal rank (starting with the type that is produced the most quickly) the type of coal produced is -
- Lignite
- Sub-bituminous coal
- Bituminous coal
- Anthracite
- Graphite (pure carbon)
For more on these types of coal see our coal faq
It is estimated that over time 3 to 7 feet of compacted plant matter was required to form only 1 foot of bituminous coal. The formation of coal is an ongoing process and some of the youngest coal is only 1 million years old. Even though coal is continuing to be made, it is being used at a far faster rate so in several 100 years, if we continue to use coal at the same rate as we do now, we will have used up the world’s supply of coal.