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.
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



In plan view you can see
that deltas are divided
into:
Distributary channels.
Swamps/flood plain.
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.
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).
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.
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.
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).
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.
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.
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.
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.
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.
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.
Lignite





Lignite still contains
recognisable remains
of plant matter.
It resembles 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.
Bituminous coal:



Layers of dull and
bright coal.
Harder than previous
types.
Classified with
anthracite as a “hard
coal”.
Anthracite:



Very hard and bright.
Can appear
iridescent.
Also quite light
because of the high C
content.




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.