Coal

1. Coal • The specification states that you should be able to: • Describe and explain the origin of coal and coal seams as part of a cyclothem. Describe the development of rank and the physical properties of lignite, bituminous coal and anthracite. • Describe the methods of extracting economic deposits of coal by opencast and underground mining. Describe the geological problems that can make coal mining uneconomic. • Describe the broad structure and distribution of coalfields in the British Isles. Describe the effects of mining and understand the social and economic consequences of mining operations. Understand that coal is an example of a non-renewable energy resource.

2. Deltas and Coal • All coal was once living matter, typically plants. • The Coal Measures in the Carboniferous formed from large equatorial forests. • During that period the conditions were perfect for the formation of coal. • Low relief • High water table • Anaerobic conditions • Slow subsidence

3. Coal Forming Environments • In plan view you can see that deltas are divided into: • Distributary channels. • Swamps/flood plain.

4. Distributary channels: • These diverge from the main river channel and distribute the water and sediment over the delta top. • The channel will be channel shaped and will have medium/coarse sediment in the channel but usually coarse/medium sandstone. • It will have cross bedding/lamination due to current ripple formation.

6. Delta Sediments • These beds are called topset beds and can be seen on the cross-section (A). • These deposits get finer as the delta progresses into the sea as the energy drops. • At the front slope of the delta there are finer sandstones with cross-lamination, these are Foreset beds (B). • Where the delta front levels out onto the seabed the sediment is a finer still possibly siltstones or even marine limestones and these are the Bottomset beds (C).

7. Swamps: • In between the distributary channels are swamps and slightly elevated areas with trees/vegetation. • These swamps are stagnant water and are therefore anaerobic/anoxic. • This means that the vegetation does not break down and so peat can accumulate. • This peat ultimately forms coal on burial/diagenesis.

8. Cyclothems • These occur where there is a repeated sequence of sedimentary rocks. • The most common kind of cyclothem occurs where deltas advance (prograde) and retreat (retrograde) repeatedly. • Why would deltas do this: • Sea level changes. • Changes in the sediment supply. • Isostasy.

9. Cyclothems • The cyclothem shows the growth of the delta then it’s decay. • The coal and seat earth (fine sand with roots which represents the soil in which the plants grew) formed. • Where did they form? • On the delta top in the swamps. • The delta retreats and erodes until limestones form offshore in clear sea (bottom set beds).

10. Cyclothems • The delta then advances (progrades) the limestones get covered by fine sediments such as marine shales at the end of the delta (still bottom set beds). • This is followed by slightly coarser siltstones which are part marine part fresh water (still bottom set). • Then as the delta advances further the delta slope with its fine sandstones appears (foreset beds). • Because the sands are transported by a river current it is cross bedded/laminated.

11. Cyclothems • Again the delta advances so that the delta top appears. • This produces coarse cross bedded sandstones in the distributary channels (top set). • Then we are back to the coal and seat earth in the swamps. • The whole thing then repeats.

12. Coal Rank • The first stage of coal formation is always peat. • Different types of coal exist and whichever is formed depends on the depth and T of burial. • Peat Lignite and brown coal bituminous coalanthracite • Low T and P  High T and P • Anthracite has undergone the greatest compaction, therefore contains a higher % of C and a lower % of H, O. • Peat contains lower % of C and higher % of H, O and N. • The term rank can be applied to the sequence of coals formed. • Rank is related to the amount of C present.

14. Coal Rank 2 • Therefore anthracite is the highest rank of coal. • The higher rank coals have a higher calorific value (or heat producing ability) and are therefore more efficient.

16. Peat • Cellulose in the plant matter has partially broken down; therefore it no longer looks like normal plant material. • Resistant materials that cannot decompose will accumulate e. g. lignin (25% of wood) plus bacterial remains. • As more dead organic matter and sediment collects on top of the peat, so the peat layer gets buried deeper. • Peat areas and swamps are very good at preserving animals that fall into them because of the reducing environment and incomplete decomposition.

17. Peat 2 • As peat is buried it gradually undergoes great compression and T change, if sufficiently high coal will form. • During this process as in diagenesis of sediment the peat becomes compacted and hardened, most of the water present will be squeezed out. • Approximately 5m. of peat forms 1m. of coal.

19. Lignite • Lignite still contains recognisable remains of plant matter. • Itresembles peat in outward appearance except it is harder. • It is classified as a “soft coal”. • It has a low calorific value. • When burnt it tends to produce much pollution.

20. Bituminous coal: • Layers of dull and bright coal. • Harder than previous types. • Classified with anthracite as a “hard coal”.

21. Anthracite: • Very hard and bright. • Can appear iridescent. • Also quite light because of the high C content.

22. U. K. Coal • Most U. K. coalfields are of Carboniferous age. • However, there are some Tertiary coals in Devon (tend to be soft). • S. Wales coalfield produces more than 50% anthracite, mainly open cast. • Most U. K. coal is bituminous with only small areas of anthracite except S. Wales and Kent.