Chapter Overview

Throughout this book, we have mentioned human use of Earth materials, most of which are nonrenewable. Water is a resource that is an important exception. It is renewable and was discussed separately in Chapters 10 and 11. Our purpose in this chapter is to survey briefly some important geologic resources of economic value.

We first look at energy resources to see which ones might help replace our disappearing supplies of oil. Then we discuss metals and their relation to igneous rocks and plate tectonics and conclude with nonmetallic resources such as sand and gravel.

Learning Objectives

1. The three main categories of geologic resources are: energy, metals and nonmetallic resources. All are nonrenewable. Resources include discovered and undiscovered deposits. Reserves are discovered deposits that can be extracted economically under current conditions.

2. Fossil fuels provide about 90% of U.S. energy, while the nation derives about 41% of its energy from oil, 24% from natural gas, 24% from coal, 9% from nuclear, and 3% from hydroelectric. .

3. Petroleum includes crude oil and natural gas. Oil and natural gas originate in sedimentary environments, and underground oil and gas accumulations require source rock, reservoir rock, a trap, and proper thermal maturity. Traps include structures (anticlines, faults), stratigraphic relations, unconformities, reefs, and salt domes. Oil fields are regions with one or more oil pools. Environmental concerns involving petroleum include spills, natural seeps, subsidence, and air pollution.

4. World oil reserves will last from 40 to 45 years at present rates of consumption. The United States imports approximately 51% of the oil it uses, and its domestic reserves are only 8 years at current rates of production. Natural gas reserves are approximately 30 years, but those estimates may be too low because price decontrols have made some occurrences economic that were not previously (e.g. coal beds and gas hydrates). Heavy crude, oil sands, and oil shale add to U.S. reserves, and have been exploited in Canada, but not to any degree in the United States.

6. Coal provides 24% of U.S. energy needs and 88% of it is used to generate electricity. Ranks of coal from lowest to highest are: peat, lignite, subbituminous, bituminous, and anthracite. The major bituminous coal producing areas in the United States are the Appalachian and interior fields (Michigan to Texas). Far western fields (New Mexico to Montana) produce lignite and subbituminous coal. Coal reserves are adequate for centuries at current rates of production, and the U.S. exports coal.

7. Uranium is used in nuclear reactors, which supply about 9% of U.S. electricity. Uranium occurs in a variety of deposits (black shales and phosphorites), but sandstones in the western United States have produced the bulk of the metal thus far. Reserves are adequate for the foreseeable future, and breeder reactors may extend the amount of uranium that can be used as fuel.

8. Renewable alternative sources of energy include hydroelectric power that contributes about 3% of domestic needs, and power derived from other energy sources: geothermal, solar, wind, tidal, wave, ocean current and oceanic temperature differences, nuclear fusion, and hydrogen dissociated from water.

9. Chemical precipitation of ores in layers is a common occurrence for iron and manganese. Concentration by weathering produces placers. Supergene enrichment develops copper ores as a result of leaching and redeposition by groundwater.

10. Metallic ores are related to plate tectonics. Diverging plate boundaries develop hydrothermal fluids that contain metallic ions and form hot spring deposits in rift valleys along mid-oceanic ridges. Converging plate boundaries may develop ores associated with ophiolites and island arcs.

11. Mining involves both underground and surface activity. Open-pit, strip and placer mines are large surface operations that pose potential environmental problems from waste rock, subsidence, and acid drainage.

12. Important metals for an industrialized economy include iron, copper, aluminum, lead, zinc, silver and gold. An"other" category includes: chromium, nickel, cobalt, manganese, molybdenum, tungsten, vanadium, tin, mercury, titanium, and platinum. Ores of most of these metals are recovered by open pit mining, and those ores may have several of these minerals in association.

13. Nonmetallic resources include sand and gravel, stone (cut or crushed), fertilizers (phosphates, nitrates and potassium compounds), evaporites (rock salt, gypsum, and sulfur), gemstones, asbestos, talc, mica, barite, borates, fluorite, clays, diatomite, glass sand, and graphite.

14. Ocean mining will increase in the future to exploit manganese nodules, and brine deposits. Better technology will also be applied to the search on land.

15. The future will require balancing three positions: 1) maintain or raise standard of living worldwide; 2) maintain environmental quality; 3) conserve resources for the future.

Related Readings

Brobst, D. A., and W. P. Pratt. 1973. United States Mineral Resources. U.S. Geological Survey Professional Paper 820.

Cameron, E. N. 1986. At the Crossroads - The Mineral Problems of the United States. New York: John Wiley and Sons.

Craig, J. R., D. J. Vaughan, and B. J. Skinner. 1988. Resources of the Earth. Englewood Cliffs, NJ: Prentice-Hall.

Dixon, C. J. 1979. Atlas of Economic Mineral Deposits. Ithaca, NY: Cornell University Press.

Guilbert, J. M., and C. F. Park. 1986. The Geology of Ore Deposits. New York: W. H. Freeman.

Hodges, C. A. 1995. Mineral Resources, Environmental Issues, and Land Use. Science 268: 1302-1312.

Jensen, M. L., and A. M. Bateman. 1981. Economic Mineral Deposits. 3d ed. New York: John Wiley and Sons.

Keary, P., and F. J. Vine. 1990. Plate Tectonics and Economic Geology. In Global Tectonics. London: Blackwell Scientific Publications.

McKelvey, V. E. 1986. Subsea Mineral Resources. U.S. Geological Survey Professional Paper 1689-A.

North, F. K. 1985. Petroleum Geology. Boston: Allen and Unwin.

Park, C. F., Jr. 1968. Affluence in Jeopardy: Minerals and the Political Economy. New York: Freeman, Cooper.

Park, C. F., Jr. 1975. Earthbound: Minerals, Energy, and Man's Future. New York: Freeman, Cooper.

Rona, P. A. 1986. Mineral Deposits from Sea-floor Hot Springs. Scientific American 254(1): 84-93.

Sawkins, F. J. 1984. Metal Deposits in Relation to Plate Tectonics. New York: Springer-Verlag.

Selley, R. C. 1985. Elements of Petroleum Geology. New York: W. H. Freeman.

U.S. Bureau of Mines (now part of U.S. Geological Survey).Minerals Yearbook and Mineral Commodity Summary (published annually). Washington, D.C.: U.S. Government Printing Office.

Answers to EOC Questions

Following are answers to the End of Chapter Questions for Chapter 21:

Boxed Readings