5 MINERAL RESERVES
䊳 19.5 MINERAL RESERVES
Mining depletes mineral reserves by decreasing the amount of ore remaining in the ground; but reserves may increase in two ways. First, geologists may discover new mineral deposits, thereby adding to the known amount of ore. Second, subeconomic mineral deposits—those in which the metal is not sufficiently concentrated to be mined at a profit—can become profitable if the price of that metal increases, or if improvements in mining or re- fining technology reduce extraction costs.
Figure 19–10 Bauxite forms by intense weathering of Consider an example of the changing nature of re- aluminum-bearing rocks. (H. E. Simpson/USGS)
serves. In 1966 geologists estimated that global reserves
Coal 343
most intensively used metals, although they account for consumption of iron was about 280 million tons per year.
of iron were about 5 billion tons. 1 At that time, world
only 14 percent of world population. Assuming that consumption continued at the 1966 rate,
Currently, the United States depends on 25 other the global iron reserves identified in 1966 would have
countries for more than half of its mineral resources. been exhausted in 18 years (5 billion tons/280 million
Some must be imported because we have no reserves of tons per year ⫽ 18 years), and we would have run out of
our own. We have reserves of others, but we consume iron ore in 1984. But iron ore is still plentiful and cheap
them more rapidly than we can mine them, or we can today because new and inexpensive methods of process-
buy them more cheaply than we can mine them. ing lower-grade iron ore were developed. Thus, deposits
that were subeconomic in 1966, and therefore not counted as reserves, are now ore.
䊳 19.6 COAL
The three major fossil fuels are coal, petroleum, and nat- THE GEOPOLITICS OF METAL RESOURCES
ural gas. All form from the partially decayed remains of The Earth’s mineral resources are unevenly distributed,
living organisms. Humans began using coal first because and no single nation is self-sufficient in all minerals. For
it is easily mined and can be burned without refining. example, almost two thirds of the world’s molybdenum
Coal-fired electric generating plants burn about 60 reserves and more than one third of the lead reserves are
percent of the coal consumed in the United States. The located in the United States. More than half of the alu-
remainder is used to make steel or to produce steam in minum reserves are found in Australia and Guinea. The
factories. Although it is easily mined and abundant in United States uses 40 percent of all aluminum
many parts of the world, coal emits air pollutants that produced in the world, yet it has no large bauxite de-
can be removed only with expensive control devices. posits. Zambia and Zaire supply half of the world’s
Large quantities of coal formed worldwide during cobalt, although neither nation uses the metal for its own
the Carboniferous Period, between 360 and 285 million industry.
years ago, and later in Cretaceous and Paleocene times, Five nations—the United States, Russia, South Africa,
when warm, humid swamps covered broad areas of low- Canada, and Australia—supply most of the mineral re-
lying land. Coal is probably forming today in some sources used by modern societies. Many other nations
places, such as in the Ganges River delta in India, but the have few mineral resources. For example, Japan has al-
process is much slower than the rate at which we are most no metal or fuel reserves; despite its thriving econ-
consuming coal reserves. As shown in Figure 19–11, omy and high productivity, it relies entirely on imports
widespread availability of this fuel is expected at least for both.
until the year 2200.
Developed nations consume most of the Earth’s min- eral resources. Four nations—the United States, Japan,
COAL FORMATION
Germany, and Russia—consume about 75 percent of the When plants die in forests and grasslands, organisms
1 B. Mason, Principles of Geochemistry, 3rd ed. New York: John
consume some of the litter, and chemical reactions with
Wiley, 1996, Appendix III.
oxygen and water decompose the remainder. As a result,
High estimates of initial reserves of coal
Low estimates of initial reserves of coal
(billions of metric tons per year) 0 1800
Figure 19–11 Past and predicted global coal supplies based on two different estimates of reserves. Shaded area shows coal already consumed. (Adapted from M. King Hubbard)
344 CHAPTER 19 GEOLOGICRESOURCES
(a) Litter falls to floor of stagnant swamp (b) Debris accumulates, barrier forms, decay is incomplete
(c) Sediment accumulates, organic matter is converted to peat
(d) Peat is lithified to coal
Figure 19–12 Peat and coal form as sediment buries organic litter in a swamp.
little organic matter accumulates except in the topsoil. In The mine produces 230,000 tons of copper a year and some warm swamps, however, plants grow and die so
smaller amounts of gold, silver, and molybdenum (Fig. rapidly that newly fallen vegetation quickly buries older
19–14). Most modern coal mining is done by large power plant remains. The new layers prevent atmospheric oxy-
shovels that extract coal from huge surface mines (Fig. gen from penetrating into the deeper layers, and decom-
position stops before it is complete, leaving brown, par- tially decayed plant matter called peat. Commonly, peat is then buried by mud deposited in the swamp.
Plant matter is composed mainly of carbon, hydro- gen, and oxygen and contains large amounts of water. During burial, rising pressure expels the water and chem- ical reactions release most of the hydrogen and oxygen, and the proportion of carbon increases. The result is coal , a combustible rock composed mainly of carbon (Fig. 19–12).