Coal is primarily formed by plant remains, especially tropical trees. These trees were growing in tropical to sub-tropical swamp landscapes and humid environments where there was a slow surface subsidence. When other sediments were deposited over the plant remains before they could decompose, anaerobic conditions were created that prevented oxidation, biodegradation and disintegration.
After some time, began to plant the remains to carbonise, a gradual process in which the cellulose from the plants is converted into the peat. This conversion is gradual, as superimposition of the beds increase. It has been estimated that 4 meter thick bed of coal was once a 50-60 meter layer of peat.
The formation of the coal from the peat is known as coalification and is dependent on temperature, pressure and time. The grade of coalification depends on both the plant remains and coalification procees, and are determining factors for the quality and the type of coal that is formed. In general one can say that the coal with a young age has a low coalification and low carbon content, while older coal has a higher coalification and higher carbon content and thus higher energy content. Coal can be divided into the following types: Lignite (brown coal), Bituminous Coal and Anthrasite.
Deposits of Coal in Norway
Since plant life first evolved on land in Devonian-period, coal is found primarily in the rocks of Devonian age and younger. On the Norwegian mainland there are very few sedimentary rocks younger than the Carboniferous age.
Jurrasic coal from the Ramså site on Andøya is known to have historically been used in the household and sold by the tons long before it was first investigated by Tellef Dahl in 1867. Sand containing coal particles was discovered during construction of the tunnel South of the Bergen (Bjorøytunnelen) in 1994-95. Coal rocks have been said to have been found on the bottom Beitstad's fjord in Nord-Trøndelag. In addition to this, there is known to be large deposits of coal on the continental shelf.
Coal found on Spitsbergen can be dated back to the Devonian, Carboniferous, Cretaceous and Tertiary periods. The large commercial deposits of coal in the younger sedimentary rocks from the tertiary period are well known. Coal mining has taken place in Svalbard only since the beginning of the 20th century, even though coal deposits were discovered already in the 17th century. The coal located along a large coal seam, called 'fløtser', which is easily visible in the walls. The thickness of the seam vary, and the thickest in the Svea- it can be up to 5 meters thick.
The coal from the Spitsbergen mines is considered high quality, in general, as it contains little sulphur.