What is Coal?
If plant remains fall onto water-logged soil (e.g. a swamp or bog) they are prevented from oxidising and instead accumulate to form peat. The original plants are rich in organic compounds and contain about 50% carbon. Peat is a loose aggregate of this plant matter with a carbon content around 60%, and a high water content. It is found just below the surface and can be cut out, allowed to dry, and burnt as fuel.
If the peat becomes buried under later sediments, causing it to be compressed and heated, then water, carbon dioxide and methane escape and the proportion of carbon increases. After a moderate amount of compression lignite (or brown coal) is formed. Lignite contains in the region of 65% carbon. When first mined it is too wet to burn readily but after drying it can be used as a fuel. However it gives a very smoky flame and a strong sulphurous odour so is not popular.
If lignite is further compressed and heated then over time it becomes coal. Coal contains from 75% to over 90% carbon and since burning carbon releases more energy than burning the other compounds present in peat, a given weight of coal releases more heat when burnt than the same weight of peat does.
When was Coal Formed?
Peat has been forming since land plants first appeared 450 million years ago, and is still being formed today in swamps such as that below. If in the future this peat is buried under sediment it may turn into coal.
A modern-day peat swamp, the Great Dismal Swamp in North Carolina. Image from 'Geology Today' (1999).
However most of the coal deposits in the world were laid down as peat during the Carboniferous and Permian periods, from 360 to 245 million years ago. The swamp plants at that time were different to modern plants, being mostly giant tree ferns over 10 metres tall, whose fossilised remains are sometimes found in the coal deposits. An excellent example of a fossilised tree stump and its root system can be seen in the Manchester Museum, on Oxford Road.
Typical Carboniferous period plant fossils from coal deposits. Image from 'The Hamlyn Guide to Minerals, Rocks and Fossils' (1977).
Annularia was in fact the leaves of a type of fern known as Calamites, which grew to 20m tall. Its descendants the horsetail ferns still grow in Bury today.
On the left a drawing showing the likely appearance of Calamites, and on the right a modern horsetail fern, about 0.5m tall.
How Exactly Does Coal Form?
During the Carboniferous period there was considerable erosion of the land masses and the eroded sediment was washed out into the surrounding seas by rivers. This produced huge fertile river deltas just above sea level and covered with vegetation.
The area which is now western Europe during Carboniferous times.
This shows land areas (in green) which were eroding away towards the seas (blue) and depositing sediments (orange) around their margins. The outline of the modern coast is shown for clarity but the coastline was very different during the Carboniferous period.
A 10 metre thickness of peat yields only a 1 metre thick coal seam, so the amount of coal deposited implies that the plant growth must have been very luxuriant. Even so it is estimated that it took 7000 years of plant growth to produce a 1m coal seam. Possibly the average world climate was warmer than today but certainly the land which is now Britain was at that time on or just slightly north of the equator and had tropical conditions. The swamps that grew on the coastal deltas may have looked like this:
Artist's impression of a Carboniferous swamp. Image from the British Museum.
There were rhythmic changes in sea level over relatively short timescales during the Carboniferous period, probably caused by melting and reforming of polar ice sheets as the climate alternately warmed and cooled. As the sea level rose the swamps and peat deposits on the land margins were flooded by sea water and at first covered with a fine mud. This has by now turned into a rock, shale, containing marine deposits. As the sea became deeper the shells of marine creatures dropped to the bottom and formed a layer of limestone. Later, as sea levels dropped, brackish (slightly salty) or fresh-water lagoons formed and the mud which collected in these lagoons became another layer of shale. Next coarser material was washed down from the interior of the continent by rivers, perhaps because of an increase in the rate of erosion, and this left a layer of sandstone and sandy shale. Finally a soil developed in which the roots of the swamp plants could grow and the next layer of peat began to be laid down. A cross section through a typical sequence of sediments looks like this:
Typical strata layers produced by a single cycle of sea level change. The 'non-marine' strata formed when the land was above sea level and the 'marine' strata formed when the land was submerged. Image from 'Building Planet Earth' (2000).
Although the sediments were originally deposited at close to sea level they would gradually sink below sea level due to compaction of the lower layers, and some downward displacement of the whole crust by the weight of sediment.
The final stage is that later geological processes such as collisions between continental plates raised the coal measures until they were on dry land.