WEATHERING PROCESSES

Several times during the past 500 million years, shal- low seas covered large regions of North America and all

In environments with high rainfall, the abundant water other continents. At times, those seas were so poorly

dissolves and removes most of the soluble ions from soil connected to the open oceans that water did not circulate

and rock near the Earth’s surface. This process leaves the freely between them and the oceans. Consequently, evap-

relatively insoluble ions in the soil to form residual oration concentrated the dissolved ions until salts pre-

deposits . Both aluminum and iron have very low cipitated as marine evaporites. Periodically, storms

solubilities in water. Bauxite, the principal source of flushed new seawater from the open ocean into the shal-

aluminum, forms as a residual deposit, and in some in- low seas, providing a new supply of salt. Thick marine

stances iron also concentrates enough to become ore. evaporite beds formed in this way underlie nearly 30 per-

Most bauxite deposits form in warm, rainy, tropical or cent of North America. Table salt, gypsum (used to man-

subtropical environments where chemical weathering ufacture plaster and sheetrock), and potassium salts (used

occurs rapidly, such as those of modern Jamaica, Cuba, in fertilizer) are mined extensively from these deposits.

Guinea, Australia, and parts of the southeastern United

342 CHAPTER 19 GEOLOGICRESOURCES

tions too low to be mined at a profit. Over millions of years, ground water and rain can weather the deposit and carry off most of the dissolved rocks and minerals in so- lution. In some cases, however, the valuable metals react to form new minerals in the zone of weathering and are not removed. In this way, the metals may become concentrated by factors of tens or hundreds to create rich supergene ore lying above a low-grade mineral deposit. These mineral deposits are easily mined because they lie at the Earth’s surface. Supergene ore caps once covered many of the great porphyry copper deposits of the United States, but because they were so rich and easily mined, most are now gone.

Weathering also forms immense amounts of clay, a nonmetallic resource described in Chapter 6. One clay mineral, called kaolin, is the primary constituent of porce- lain and other ceramics. Another, smectite, is mixed with

Figure 19–9 In this banded iron formation from Michigan, the red bands are iron minerals and the dark layers are

water and other minerals to make drilling mud, a clay- chert. (Barbara Gerlach/Visuals Unlimited)

rich slurry used to cool and lubricate drill bits in the drilling of deep wells. Several types of clay are used to line sanitary landfills and irrigation ditches and ponds because wet clay is impermeable.

States (Fig. 19–10). Some bauxite deposits are found to- day in regions with dry, cool climates. Most of them, however, formed when the regions had a warm, wet cli-

METAMORPHIC PROCESSES mate, and they reflect climatic change since their origin.

Recall from Chapter 8 that a metamorphic rock forms Weathering processes also can enrich metal concen-

when heat and pressure alter the mineralogy and texture trations in mineral deposits formed by other processes.

of any preexisting rock. Metamorphism can also expel For example, a disseminated hydrothermal deposit may

water from rocks to create hydrothermal fluids, which, in contain copper, lead, zinc, and silver, but in concentra-

turn, deposit metal ores. Thus, some hydrothermal ore deposits are of metamorphic origin.

Metamorphic processes also form several types of nonmetallic mineral resources. Graphite, the main com- ponent of the “lead” in pencils, forms when metamorphism alters the carbon in some organic-rich rocks. Asbestos is a commercial name for two different minerals formed by metamorphism. Metamorphism also forms marble, a valuable building stone and sculptor’s mate- rial, from limestone.

WEATHERING PROCESSES