The geology of coal
At various times in the geologic past, the Earth had dense forests in low-lying wetland areas. Due to natural processes such as flooding, these forests were buried underneath soil.
As more and more soil deposited over them, they were compressed. The temperature also rose as they sank deeper and deeper. As this process continued, the plant matter was protected from breakdown, usually by mud or acidic water. This trapped the carbon in immense peat bogs that were eventually covered and deeply buried by sediments.
Under high pressure and high temperature, dead vegetation was slowly converted to coal. This coalification starts with dead plant matter decaying into peat. Then, over millions of years, the heat and pressure of deep burial causes the loss of water, methane and carbon dioxide and an increase in the proportion of carbon. In this way, first lignite (also called ‘brown coal’), then sub-bituminous coal, bituminous coal, and lastly anthracite (also called ‘hard’ or ‘black’ coal) may be formed.
The wide, shallow seas of the Carboniferous Period provided ideal conditions for coal formation, although coal was formed in most geological periods. Coal is known from Precambrian strata, which predate land plants – this coal is presumed to have originated from algae.
South Africa’s coal resources are contained in the Ecca deposits, a stratum of the Karoo Supergroup, and date from the Permian period between 280 and 250 million years ago. In general terms, they are largely located in the north-eastern quarter of the country. The coal measures are generally shallow, largely unfaulted and lightly inclined, making their exploitation suitable for opencast and mechanised mining.
The largest reserves of coal are found in Europe (30%), North America (28%), East Asia (13%), South East Asia (12%) and South and Central Asia (11%). By comparison, Africa’s resources, almost exclusively in South Africa, are estimated at a mere 3.5%.