Physical Resources / 131Most modern quarries and open-cast mines are very large and, because they exist to detach and
remove rock, cannot avoid devastating their sites. Nowadays planning consents require such sites to
be restored when operations cease, but many older, abandoned quarries remain. The disfig-urement
they cause is not permanent, although it is only fair to point out that most older quarries were much
smaller than modern ones and produced building stone, sand, or gravel in modest amounts for local
use. Quarries scar the land, but they do not poison it and in time plants colonize the bare ground. An
unrestored quarry site is rarely of any agricultural use and so usually it remains undisturbed and
eventually may mature into a place of considerable interest to naturalists and conservationists.
Mineral mining, as opposed to the quarrying of rock, is much more disruptive, because it involves
separating the desired minerals from the valueless minerals with which they are associated. The
minerals themselves may be gemstones. Sapphires, oriental emeralds, and rubies are all aluminium
oxides (Al 2 O 3 ), differing from one another because of colours imparted by impurities; beryl is a
compound of beryllium, aluminium, silicon, and oxygen (Be 3 Al 2 Si 6 O 18 ); and diamond is a form of
pure carbon. All are minerals and their high monetary value indicates their rarity: if they were common
they would be cheap. When a rare substance is separated from the commoner substance containing
it, a residue remains and this can cause environmental difficulties.
Metals are separated from their ores, an ore being a body of rock containing that metal in a compound,
called an ore mineral, in a concentration high enough to be extracted economically. The concentration
of the metal within its ore mineral may be quite high. Chalcocite (Cu 2 S), for example, is 80 per cent
copper and the best-quality uranite, or pitchblende (UO 2 ), is 85 per cent uranium, but the concentration
of the metal within the ore (the rock containing the ore mineral) is very different. Iron ores are
widespread and abundant, and iron is seldom extracted from ores containing less than 25 per cent of
the metal, but scarcer metals commanding a high market price can be extracted economically from
ores containing as little as 1 per cent or even less, for example in the case of copper. This means that
in the case of iron, up to 75 per cent of the rock, and with some metals as much as 99 per cent of it,
is useless waste for which some means of disposal must be found.
Mine waste cannot be returned easily to the hole whence it came. Unless the mine is exhausted,
the waste would bury extractable ore, but in any case the waste no longer fits the hole. As rock,
the minerals were tightly compressed; once they have been broken, crushed, and processed
further to remove the desired ore mineral they consist of small particles with spaces between
them. This greatly increases the bulk of the material, and there may be a truly vast amount of it.
At Bingham, Utah, for example, copper is extracted from a hole 3.2 km in diameter and 900 m
deep; this is large enough to accommodate two Empire State Buildings, one on top of the other,
with room to spare. A lead mine in Missouri has two underground machinery repair shops 10
km apart, one with a floor area of 1.5 ha and the other 2.2 ha. Dry rock removed from these
holes and expanded by processing is usually tipped to build hills of ‘tailings’. Wet process
residues are stored in ponds.
Tailings must be treated with respect, because the minerals they contain are in the form of tiny
fragments, with a vastly increased surface area, and they are exposed to water. Subsequent chemical
reactions can release very acid liquids, sometimes containing other toxic metal compounds. Dry
tailings can be blown as dust, also causing contamination. Today, most governments set stringent
regulations for the containment of mine tailings, so they cause little pollution, but it was not
always so. In 1945, Britain had more than 1000 ha of land left totally derelict after overlying
topsoil was mixed with tailings and dumped following iron-ore mining in what was otherwise
good-quality agricultural land (MELLANBY, 1992, p. 47), and in south-eastern Tennessee an area
of 145 km^2 remains barren to this day because of copper mining in the last century (RAVEN
ET AL., 1993, p. 331).