Coal was formed in wide, low-lying equatorial swamps crossed by large rivers and covered by forests of primitive trees. Here, the remains of trees and plants were saved from biodegration and oxidation by mud and water. Coal is usually black in colour but sometimes it occurs as a brownish-black colour. There are four broad ranks or types of coal depending upon its age. Commencing with the youngest and lowest carbon content, these are: lignite, sub-bituminous, bituminous and anthracite. Anthracite is classified as a metamorphic rock because of its subsequent exposure to elevated pressures and temperatures. Bitumen obtained from bituminous coal is a black viscous material generally referred to as 'tar'. Coal is primarily composed of carbon, along with a range of other elements, particularly sulphur. It has been estimated that a coal seam 1 foot thick needed as much as 7 to 10 feet of peat thickness to commence with.
Coal is formed by natural geological processes applying pressure to dead matter. It is principally used for steam/electricity generation purposes. Under suitable geological conditions dead matter is formed successively into:
Lignite or brown coal.
Coal was formed during the Carboniferous Period, which was from about 358.9 to about 298.9 million years ago. It was mainly formed from tree-like plants that grew in warm humid swamps:
Stigmaria root system in Manchester Museum. This fossil was discovered in a quarry at Clayton, Bradford, Yorkshire, in 1886.
During the Carboniferous Period early conifer trees began to appear that were cone-bearing seed plants closely related to seed plants called Cordaites, which are now extinct.
Four families of plants that have survived are:
Coal miners found plant fossils during their work and sometimes they would take these home to show their families. The two fossils illustrated below are typical of those found by miners.
Above left: Calamite. A genus of extinct tree-like horsetails to which modern horsetails (genus Equisetaceae) are closely related. These plants had distinct nodes, similar to modern horsetails, and their branches and leaves emerged in whorls from these nodes. Their upright stems were woody and connected by an underground runner system. Above right: Sphenopteris or seed fern. These ferns varied in size from small plants to trees and they were among the earliest seed-bearing plants but they have no descendants.