Physical Resources / 125A slope mine approaches the seam through an angled shaft and a drift mine approaches horizontally.
Where seams lie too close to the surface for a shaft to be cut to them, the overlying material is
removed to expose the coal. This is an open-cast mine if it exposes a substantial part of the seam all
at the same time and a strip mine if the seam is exposed and worked in sections.
All coal mining produces wastes, consisting of soil and rock that must be removed to gain access to
the coal and rock mixed with the coal that must be separated from it. It is possible to store this waste,
in ‘spoil heaps’, until the mine is exhausted then return it to below ground, but this is by no means
the general practice and mining more commonly produces large, black spoil heaps. These are composed
of finely crushed material with almost no soil and little in the way of plant nutrients. The heaps often
contain large amounts of iron pyrites (FeS), producing very acid conditions (pH 2.0–4.0), and acid
liquor, also containing metals, may leach from the heap into nearby watercourses, where it causes
severe pollution. It is possible to reclaim mining spoil heaps. If they are treated with lime to reduce
the acidity and soil and fertilizer supplied, a cover of grass can be established, leading in time to a
more diverse plant community (MELLANBY, 1992, pp. 44–45).
Open-cast and strip mining can be even more destructive. In the past, large areas of attractive
countryside were stripped of their soil (the ‘overburden’), which was dumped in large heaps,
and when the seam was exhausted the site was just left, utterly devastated. In some countries
this is still the practice, but in many the planning consent stipulates that when operations cease
the overburden must be returned to the surface and the site restored to a state better than its
original condition. The effect is not always so destructive as it may appear in the older industrial
countries, such as Britain. Coal seams suitable for open-cast or strip working often occur close
to deeper seams that have been mined in the past to feed nearby industries, leaving land already
in a state of industrial dereliction that restoration can improve once mining has ceased. In strip
mining, restoration begins long before mining ceases, the reclamation of each strip starting as
soon as the extractive machinery has moved on to the next strip. Indeed, British planning
regulations are now stringent, and open-cast mining has little adverse long-term effect on areas
of conservation or wildlife importance.
As well as the gaseous pollutants released when it is burned, coal combustion also produces ash.
This can cause disposal problems, not least because it contains heavy metals.
Oil and natural gas are held in their traps under pressure. A hole drilled through the cap rock releases
the pressure and they rise to the surface. The depth of drilling determines the part of the reservoir
that is tapped, since the gas lies above the oil. Figure 3.18 shows how oil and gas are held within
structural traps. Because all operations are conducted from the surface and no overburden has to be
removed, oil and gas mining generate no spoil heaps. Such environmental damage as they cause
arises from spillages of oil around the well or in transit to the refineries where it is processed.
Coal and oil may well be forming at the present time, but at a rate much lower than that at which they
are being consumed. They are, therefore, non-renewable for all practical purposes. This being so, it
is commonly assumed that one day they will be economically exhausted, oil first because it is much
less abundant than coal. Certainly oil is being used rapidly, and in 1994 the United States imported
more petroleum and its products than it produced from its own resources, for the first time becoming
a net importer (ABELSON, 1995).
Impending shortages, combined with the environmental problems arising from the combustion of
fossil fuels, have stimulated a search for alternatives, but all is not necessarily as it seems. Some
people suggest that valuable resources should be conserved for the benefit of future generations,
but consider what has happened to the coal industry. There is probably more than 45 billion
tonnes of coal lying beneath Britain and a century ago it was being mined intensively and much