CHAPTER 8 - WEATHERING AND SOIL

Overview

Weathering describes the mechanical (no compositional change) and chemical (chemical change) destruction of rocks. Erosion involves the picking-up and physical removal (transportation) of weathering products. Mechanical and chemical weathering usually occur together, and their effects are ubiquitous. Spheroidal weathering of boulders and differential weathering of outcrops are two of the more interesting examples.

Mechanical weathering is caused by frost wedging, frost heaving, pressure release (producing sheet joints and exfoliation), and to a lesser extent plant growth, burrowing animals, crystallization pressure, temperature induced expansion and contraction. Chemical weathering reflects mineral instability. Oxidation produces iron compounds in particular. Natural acids promote chemical weathering, with rainwater, as carbonic-acid, being the most common. Hydrogen easily substitutes for other positive ions in mineral-structures disrupting them and forming new minerals. Minerals, such as calcite, may dissolve completely, while others, like the feldspars and ferromagnesians, produce clays. Chemical weathering is most intense where there is abundant liquid water, and less effective in deserts or cold climates. Quartz, clays and iron oxide are common solid weathering products, while dissolution contributes ions to ground water and ultimately the ocean. Dissolved silica and-bicarbonate are common cementing agents in sedimentary rocks.

Soil is the weathered, unconsolidated material resting on bedrock. It contains organic material and promotes plant growth and reflects climate. As soils mature, distinct soil horizons (O, A, B, and C) appear. Distinctive soil types such as "hardpans" and laterites may be used to interpret paleoclimatic changes.

Table 8.3 provides brief descriptions of the 9 soil orders of Canada used for mapping and classification purposes and their approximate equivalents utilized by the U.S. Soil Conservation Service.

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 LFH horizon (O horizon in the U.S.) 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 may never be 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 detailed classification schemes used for soils in Canada and the U.S. are provided in Table 8.3.

Boxes

8.1 - ENVIRONMENTAL GEOLOGY - ACID RAIN: Acidity, or pH, is measured on a scale which ranges from 0 to 14, with a value of 7 as neutral. Values below 7 are acid, and ordinary rain has a pH of 5.5 to 6.0 because it mixes with carbon dioxide to form carbonic acid. Burning fossil fuels emits carbon-dioxide, nitrogen and sulfur that contribute to even more acidic rain, with pH values as low as 2. Acid rain accelerates chemical weathering and adversely effects plants and animals. The damaging effects of acid rain are most pronounced in areas of eastern Canada underlain by siliceous rocksof the Canadian Shield. Many lakes in these regions have significantly reduced fish populations.

Short Discussion/Essay

1. Explain the difference between weathering and erosion.
2. Cite examples illustrating the difference between mechanical and chemical weathering.
3. What is the difference between soil and regolith?
4. Why are clays so abundant as weathering products?
5. Why is all rain acid?

Longer Discussion/Essay

1. How do rates of weathering vary among the various rock groups? Why do they vary?
2. How does the mineralogy and grain size of a parent rock influence its susceptibility to weathering?
3. Why do rates of weathering vary with climate?
4. Explain why soils reflect climate.
5. How are mechanical and chemical weathering related and how do they influence each other in the weathering history of any rock sequence?

Selected Readings

Berner, R.A. and Lasaga, A.C. 1989. "Modeling the geochemical Carbon cycle," Scientific American 260: 74-81.

Birkeland, P.W. 1998. Soils and Geomophology. 2nd ed., New York: OxfordUniversity Press.

Agriculture Canada, 1987. The Canadian System of Soil Classification (2nd edition) Agriculture Canada Publication 1646, 164pp.

Hodder, A.P. W. 1997. "A geotechnical approach to weathering simulation," Journal of Geoscience Education, 45(4): 359-362

Railsback, L.B. 1993. "A geochemical view of weathering and the origin of sedimentary rocks and natural waters," Journal of Geological Education 41: 404-411.

Web sites with information on Canadian soils and classification systems: