Some mineral deposits are formed, improved, or preserved by geologic
processes that occur in arid lands as a consequence of climate. Ground
water leaches ore minerals and redeposits them in zones near the water
table. This leaching process concentrates these minerals as ore that can
be mined. Of the 15 major types of mineral deposits in the Western
Hemisphere formed by action of ground water, 13 occur in deserts.
Evaporation in arid lands enriches mineral accumulation in their lakes.
Playas may be sources of mineral deposits formed by evaporation. Water
evaporating in closed basins precipitates minerals such as gypsum, salts
(including sodium nitrate and sodium chloride), and borates. The
minerals formed in these evaporite deposits depend on the composition
and temperature of the saline waters at the time of deposition.
Significant evaporite resources occur in the Great Basin Desert of the
United States, mineral deposits made forever famous by the “20-mule
teams” that once hauled borax-laden wagons from Death Valley to the
railroad. Boron, from borax and borate evaporites, is an essential
ingredient in the manufacture of glass, ceramics, enamel, agricultural
chemicals, water softeners, and pharmaceuticals. Borates are mined from
evaporite deposits at Searles Lake, California, and other desert
locations. The total value of chemicals that have been produced from
Searles Lake substantially exceeds $1 billion.
The Atacama Desert of South America is unique among the deserts of the
world in its great abundance of saline minerals. Sodium nitrate has been
mined for explosives and fertilizer in the Atacama since the middle of
the 19th century. Nearly 3 million metric tons were mined during World
War I.
Valuable minerals located in arid lands include copper in the United
States, Chile, Peru, and Iran; iron and lead-zinc ore in Australia;
chromite in Turkey; and gold, silver, and uranium deposits in Australia
and the United States. Nonmetallic mineral resources and rocks such as
beryllium, mica, lithium, clays, pumice, and scoria also occur in arid
regions. Sodium carbonate, sulfate, borate, nitrate, lithium, bromine,
iodine, calcium, and strontium compounds come from sediments and
nearsurface brines formed by evaporation of inland bodies of water,
often during geologically recent times.
[Illustration: This open-pit mine in the Sonoran Desert near Ajo,
Arizona, has exposed an elliptical copper deposit about 1,000 meters
long and 750 meters wide. The copper ore mined here is in a bed that
averages 150 meters in thickness (photograph by Peter Kresan).]
The Green River Formation of Colorado, Wyoming, and Utah contains
alluvial fan deposits and playa evaporites created in a huge lake whose
level fluctuated for millions of years. Economically significant
deposits of trona, a major source of sodium compounds, and thick layers
of oil shale were created in the arid environment.
[Illustration: Trona mine at Searles Lake, California (photograph by
John Keith).]
Some of the more productive petroleum areas on Earth are found in arid
and semiarid regions of Africa and the Mideast, although the oil
reservoirs were originally formed in shallow marine environments. Recent
climate change has placed these reservoirs in an arid environment.
Other oil reservoirs, however, are presumed to be eolian in origin and
are presently found in humid environments. The Rotliegendes, a
hydrocarbon reservoir in the North Sea, is associated with extensive
evaporite deposits. Many of the major U.S. hydrocarbon resources may
come from eolian sands. Ancient alluvial fan sequences may also be
hydrocarbon reservoirs.
[Illustration: The Sahelian drought that began in 1968 was responsible
for the deaths of between 100,000 and 250,000 people, the disruption of
millions of lives, and the collapse of the agricultural bases of five
countries (photograph by Daniel Stiles, UNEP).]