Chapter Overview

In this chapter, you will study several visible signs of weathering in the world around you, including the cliffs and slopes of the Grand Canyon and the rounded edges of boulders. As you study these features, keep in mind that weathering processes made the planet suitable for human habitation. From the weathering of rock eventually came the development of soil, upon which the world's food supply depends.

How does rock weather? You learned in chapters 3 and 4 that the minerals making up igneous rocks crystallize at relatively high temperatures and sometimes at high pressures as magma and lava cool. Although these minerals are stable when they form, most of them are not stable during prolonged exposure at Earth's surface. In this chapter, you see how minerals and rocks change when they are subjected to the physical and chemical conditions existing at Earth's surface. Rocks undergo mechanical weathering (physical disnitegration) and chemical weathering (decomposition) as they are attacked by air and water. Your knowledge of the chemical composition and atomic structure of minerals will help you understand the reactions that occur during chemical weathering.

Weathering processes create sediments (primarily mud and sand) and soil. Sedimentary rocks, which form from sediments, are discussed in chapter 6. In a general sense, weathering prepares rocks for erosion and is a fundamental part of the rock cycle, transforming rocks into the raw material that eventually becomes sedimentary rocks.

Learning Objectives

1. Weathering creates sediment and soil by either mechanical or chemical processes. Erosion is the pick-up and removal of weathering products (transportation).

2. Mechanical weathering causes physical disintegration without compositional change. Frost wedging, frost heaving, and pressure release cause most mechanical weathering. Sheeting and exfoliation domes develop as a result of pressure release. Plant growth, burrowing organisms and salt crystal development are additional mechanical weathering processes. Differential weathering is also a common result of mechanical weathering

3.Chemical weathering causes rock decomposition and new mineral formation that reflects mineral instability because conditions of formation of the original minerals are significantly different from those of the earth's surface. Oxygen and slightly acidic rainwater (carbonic acid) are the agents of chemical weathering. The feldspars and ferromagnesian minerals weather to clay minerals, while quartz does not weather chemically. Calcite dissolves adding calcium and bicarbonate ions to ground water, while some silica is produced by chemical weathering of feldspars. This dissolved load may eventually be carried to the ocean.

4. Soil is the layer of unconsolidated weathered material on top of bedrock. Clay minerals make important contributions in holding water and nutrients on their surfaces because they are negatively charged.

5. Soil horizons develop as it matures. The O horizon is the top layer and consists of plant litter and other organic material. The A horizon is the next layer and is characterized by leaching downward. The B horizon is the zone of accumulation for material leached from the A horizon. The C horizon is transitional from soil to un-weathered bedrock. Most soils are residual, but transported soils can be deposited by ice, wind, and running water. Soils thicken with time.

6. The character of the soil depends on the parent material. Soils forming on granite are sandy, while those forming on basalt are never sandy. Soil types reflect climate. Soils containing large amounts of aluminum and iron oxides are found in wet climates. Soils formed in arid climates are thinner and contain higher concentrations of calcite. Hardpans form in either wet or dry climates and have thick B horizons. Laterites form in tropical regions, have thick A horizons and may be mined for aluminum. The Soil Conservation Service utilizes a detailed classification of soils provided in Table 5.3.

Related Readings

Birkeland, P. W. 1984. Soils and Geomorphology. New York: Oxford University Press.

Buol, S. W., F. D. Hole, R. J. McCracken, and R. J. Southard. 1997. Soil Genesis and Classification. 4th ed. Ames, IA: Iowa State University Press.

Colman, S. M., and D. P. Dethier. 1986. Rates of Chemical Weathering of Rocks and Minerals. New York: Academic Press.

Gauri, K. L. 1978. The Presentation of Stone. Scientific American 238(6): 126-36.

Krauskopf, K. K., and D. K. Bird. 1995. Introduction to Geochemistry. 3d ed. New York: McGraw-Hill.

Ollier, C. D., and C. Pain. 1996. Regolith, Soils, and Landforms. New York: W. H. Freeman.

Reinhardt, J., and W. R. Sigleo. 1988. Paleosols and Weathering through Geologic Time: Principles and Applications. Geological Society of America Special Paper 216.

Robinson, D. A., and R. G. Williams, eds. 1994. Rock Weathering and Landform Evolution. New York: John Wiley and Sons.

Singer, M. J., and D. N. Munns. 1986. Soils. New York: Macmillan.

Soil Survey Staff, 1998. Keys to Soil Taxonomy. 8th ed. Natural Resources Conservation Services. U.S. Department of Agriculture.

Wild, A. 1993. Soils and the Environment: An Introduction. New York: Cambridge University Press.

Answers to EOC Questions

Following are answers to the End of Chapter Questions for Chapter 5:

Boxed Readings