Basics of Environmental Science

124 / Basics of Environmental Science

million years ago, during the Silurian Period, but most dates from the Carboniferous, about 300

million years ago. Tectonic movements have since transported it to most parts of the world from

Pangaea, the former supercontinent in which all the present continents were united and where it

formed (ALLABY, 1993, pp. 143–151).

Coal and peat contain ‘volatiles’, substances that are given off as gases when the material is heated

in the absence of air, and the quality of the fuel is determined by the proportion of volatiles it

contains: the lower the proportion the more energy the fuel will release when burned. Peat contains

more than 50 per cent volatiles, lignite (a soft, brown coal) about 45 per cent, and anthracite about 10

per cent. Anthracite is the highest quality, and very hard. Bituminous coal, the most abundant type

and the one most widely used domestically, has 18–35 per cent volatiles.

Petroleum forms by a somewhat similar process. Organic material is buried by sediment, usually in

a river delta, and is then trapped between two layers of impermeable rock. Many oil deposits are

found beneath anticlines: rock strata that have been folded upwards into domed shapes. A similar

structure occurs where a large mass of salt, deep below the surface, rises slowly through the less

dense material surrounding it and the dense rock sinks to replace it. The process is called ‘diapirism’

and the salt dome it produces a ‘diapir’. Oil is often found in ‘salt-dome traps’. The material is then

strongly compressed and heated. The resulting fluid fills all the pore spaces within the porous rock

around it (ALLABY, 1992, pp. 162–163).

Some of the carbon and hydrogen comprising the organic matter form methane (CH 4 ), associated with

both coal and oil. In coal mines, methane can cause fires, but when associated with oil it can be

extracted and used as the fuel known as ‘natural gas’ (to distinguish it from ‘town gas’, mainly carbon

monoxide (CO), obtained by heating coal and formerly an important industrial and domestic fuel).

Coal occurs in seams of varying thickness and at varying depths. There are four ways in which

access can be gained to the seams and the coal extracted, illustrated in Figure 3.17. Seams at

great depth are approached by sinking a shaft vertically from the surface, with associated shafts

for ventilation. Most traditional British mines were shaft mines of this type, working deep seams.

Figure 3.17 Types of coal mines