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/electric generation purposes. Under suitable geological conditions dead matter is formed successively into:
Other classifications of coal have been identified and these fall in the sub-bituminous and bituminous ranges. They include:
Coal was formed during the Carboniferous Period, which lasted from about 354 to 290 million years ago. It was mainly formed from tree-like plants that grew in warm humid swamps:
During the Carboniferous Period early conifer trees began to appear, which were closely related to the Cordaites, now extinct.
Ferns (Filicopsida), horsetails (Equisetaceae) and club mosses (Lycopodiaceae and Selaginellaceae) are three families of plants that have survived. Ferns reproduce by means of minute spores that are usually situated on the undersides of leaves. Horsetails reproduce by means of minute spores that develop in cones at the tip of some stems. Horsetails have hollow, ribbed, roughish, leafless, jointed stems, the joints being covered by toothed sheaths. The joints often bear whorls of similarly ribbed, jointed and leafless branches. Club mosses reproduce by means of minute spores that develop in erect cigar-shaped cones.
Coal miners found plant fossils during their work and sometimes they would take these home to show their families. The fossils illustrated below are typical of those found by miners.
The fossil illustrated below is a Sphenopteris or seed fern that grew during the Carboniferous Period. Seed ferns varied in size from ground cover plants to trees and they were among the earliest seed-bearing plants but they have no descendants.