How is Coal Formed?

Coal is the altered remains of prehistoric vegetation that originally accumulated in swamps and peat bogs.

The build-up of silt and other sediments, together with movements in the earth’s crust (known as tectonic movements) buried these swamps and peat bogs, often to great depths. With burial, the plant material was subjected to high temperatures and pressures. This caused physical and chemical changes in the vegetation, transforming it into peat and then into coal.

Coal formation began during the Carboniferous Period – known as the first coal age – which spanned 360 million to 290 million years ago.

The quality of each coal deposit is determined by temperature and pressure and by the length of time in formation, which is referred to as its ‘organic maturity’. Initially the peat is converted into lignite or ‘brown coal’ – these are coal-types with low organic maturity. In comparison to other coals, lignite is quite soft and its colour can range from dark black to various shades of brown.

Over many more millions of years, the continuing effects of temperature and pressure produces further change in the lignite, progressively increasing its organic maturity and transforming it into the range known as ‘sub-bituminous’ coals.

Further chemical and physical changes occur until these coals became harder and blacker, forming the ‘bituminous’ or ‘hard coals’. Under the right conditions, the progressive increase in the organic maturity can continue, finally forming anthracite.