Coal forms from the physical and chemical alteration of organic peat moss. This process, called coalification, takes hundreds of years to complete. During coalification, peat moss undergoes a series of transformations as a result of bacterial decay, compaction, variations in temperature and time.
Coal formation occurs wherever there are large deposits of peat. These deposits are often located deep within the ground and take many years to accumulate. Peat deposits contain a large portion of moss but accrue many other types of organic materials as they progress through time, including plant and tree leaves, minerals, clay and even animal remains. Some deposits, particularly those situated proximately to volcanoes, contain traces of ash and charred charcoal as well.
The ingredients in coal deposits decay over time; numerous bacteria, which expedite the process, accelerate this process. The breakdown of organic matter is an important step in the process of coalification, but the peat deposit must be buried in sediment for the transformation from peat to coal to take place. Sediment burial squeezes excess water out of the peat, allowing it to take in complex hydrocarbon atoms and heat. A series of chemical reactions follows; the mixture becomes rich in carbon and releases waste products, eventually producing the end product of coal.