Chapter 6 Weathering, Erosion, and Soil
Introduction Rocks and minerals disintegrate and decompose by the processes of physical and chemical weathering. This breakdown occurs because the parent material reacts with its new physical and chemical environment transforming it into a new equilibrium state. Fig. 6.1, p. 136
Introduction How does weathering differ from erosion? Weathering is the mechanical and chemical alteration of Earth materials at or near the surface. Erosion involves removing weathered materials from their place of origin-by running water or wind, for example.
Alteration of Minerals and Rocks The products of weathering include soluble salts, ions in solution, and solid particles. These products of weathering can be eroded and become sedimentary rock at another location or be modified in place to become soils. Fig. 6.1, p. 136
Alteration of Minerals and Rocks Weathering and erosion take place at different rates Rocks are not compositionally and structurally homogenous Different parts of a rock will weather at different rates producing differential weathering Geo-inSight 9., p. 151
Mechanical Weathering - Disaggregation of Earth Materials Mechanical processes (Physical Weathering) Frost action Pressure release Thermal expansion and contraction Crystal growth Activities of organisms The products of mechanical (physical) weathering are chemically the same as their parent materials. Fig. 6.9d, p. 143
Mechanical Weathering - Disaggregation of Earth Materials Frost Action When water freezes in cracks in rocks it expands and contracts when it thaws, thus exerting pressure and opening the cracks wider. Repeated freezing and thawing disaggregates rocks into angular pieces that may tumble downslope and accumulate as talus. Fig. 6.2a, p. 137
Fig. 6.3 a, p. 138 Fig. 6.4, p. 138 Mechanical Weathering - Disaggregation of Earth Materials Pressure Release and Sheet Joints Sheet joints are fractures more or less parallel to exposed rock surfaces, especially rocks now at the surface that formed under great pressure at depth.
Fig. 6.3 a, p. 138 Fig. 6.4, p. 138 Mechanical Weathering - Disaggregation of Earth Materials Pressure Release and Sheet Joints These joints form in response to pressure release; that is, when the rocks formed deep with the Earth, they contained energy that is released by outward expansion at the surface.
Mechanical Weathering - Disaggregation of Earth Materials Thermal Expansion and Contraction Extreme heating and cooling in deserts, or forest fires, might crack rocks Dark minerals absorb heat better than surrounding light-colored minerals, so differential expansion and cracking can occur Lab experiments indicate that this process is probably minor
Mechanical Weathering - Disaggregation of Earth Materials Growth of Salt Crystals Growth and expansion of salt crystals in cracks in rocks can widen the cracks Most common in hot deserts and along rocky marine coasts where breaking waves create salt spray
Mechanical Weathering - Disaggregation of Earth Materials How do organisms contribute to mechanical and chemical weathering? Any organic activity, such as tree roots growing in cracks, contributes to mechanical weathering. Organic acids, and the tendrils of mosses and lichens, aid in the chemical alteration of parent material. Fig. 6.5b, p. 139
Chemical Weathering - Decomposition of Earth Materials Chemical weathering: Decomposes rocks and minerals by chemical alteration Unlike mechanical weathering, the chemistry and mineralogy changes during weathering Hot and wet environments accelerate chemical weathering Chemical weathering occurs in all environments, except, possibly, permanently frozen polar regions
Chemical Weathering - Decomposition of Earth Materials Chemical processes Solution Oxidation Hydrolysis Fig. 6.7, p. 141
Chemical Weathering - Decomposition of Earth Materials The parent material is transformed into weathering products, including: ions in solution, water-soluble salts and insoluble solids, such as clay mineral Fig. 6.6, p. 140
Chemical Weathering - Decomposition of Earth Materials Important agents of chemical weathering Oxygen Carbon dioxide Water Acids Organisms Fig. 6.6, p. 140
Chemical Weathering - Decomposition of Earth Materials Solution rocks dissolve Carbonate Rocks Calcite (CaCO ³ ) in limestone is nearly insoluble in neutral or alkaline solutions, but it rapidly dissolves in acidic solutions. The atoms making up the minerals dissociate, that is, they separate and the rock dissolves.
Chemical Weathering - Decomposition of Earth Materials Solution rocks dissolve Important acids in nature Carbonic acid forms from CO 2 interacting with water Organic acids from organisms Nitric acid from acid rain Sulfuric acid from acid rain and the weathering of sulfide minerals
Chemical Weathering - Decomposition of Earth Materials Oxidation rocks rust Rocks such as sandstone may contain iron minerals that will breakdown when exposed to oxygen. The atoms making up the minerals dissociate; that is, they separate as the rock rusts away. Geo-InSight 4, p. 150
Chemical Weathering - Decomposition of Earth Materials Hydrolysis breakdown to clays Potassium Feldspar During hydrolysis hydrogen ions react with and replace positive ions in potassium feldspar. The result is clay minerals and substances in solution such as potassium and silica.
Chemical Weathering - Decomposition of Earth Materials Hydrolysis breakdown of orthoclase (K-feldspar) to kaolinite clay 2KAlSi 3 O 8 + 2H + + 2HCO 3 - + H 2 O orthoclase hydrogen bicarbonate ion water Al 2 Si 2 O 5 (OH) 4 + 2K + + 2HCO - 3 + 4SiO 2 clay (kaolinite) potassium ion bicarbonate ion silica Equation, p. 141
Chemical Weathering - Decomposition of Earth Materials Factors That Control the Rate of Chemical Weathering Mechanical weathering enhances chemical weathering by breaking material into smaller pieces, thereby increasing the surface area for chemical reactions. Because chemical weathering is a surface process, the more surface exposed, the faster the weathering Fig. 6.8, p. 142
Chemical Weathering - Decomposition of Earth Materials Factors That Control the Rate of Chemical Weathering The stability of igneous minerals during chemical weathering is the inverse of Bowen's Reaction Series (Chapter 4). Quartz is most stable and less susceptible to chemical weathering, whereas olivine is the least stable. Table 6.1, p. 142
Chemical Weathering - Decomposition of Earth Materials Spheroidal Weathering Angular rocks tend to round during chemical weathering. Chemical weathering preferentially attacks the sharp corners of fractured rocks rather than the center. Fig. 6.9, p. 143
Soil and Its Origin Regolith Layers of weathering products on the Earth's surface, includes: Sediment - weathered materials brought in by erosion Soil - organic-bearing regolith that supports plant life Materials largely weather in place, but some soils may contain sediments brought in from other locations
Soil and Its Origin Soils Supports plant life Contains humus - organic debris from decayed organisms, resistant to weathering, important plant nutrient Contains clays, sands, and/or other mineral matter Residual or transported Has layers or soil horizons
Soil and Its Origin Soils Soils consist of weathered materials, air, water, humus and also the plants which they support. Fig. 6.10a, p. 146
Soil and Its Origin Soil Profiles Soil formation produces horizons. The most common horizons in descending order are O, A, B, and C. These horizons differ from one another in texture, structure, composition and color. Fig. 6.10b, p. 146
Soil and Its Origin Soil Profiles O horizon Thin layer of humus and partially decayed plants Fig. 6.10b, p. 146
Soil and Its Origin Soil Profiles A horizon Top soil, thick in fertile soils More organically rich than underlying horizons Most important layer for plant growth, including crops Fig. 6.10b, p. 146
Soil and Its Origin Soil Profiles B horizon Subsoil Zone of accumulation, especially of clays Fig. 6.10b, p. 146
Soil and Its Origin Soil Profiles C horizon Partially weathered inplace bedrock Other horizons, such as the E, are less common Fig. 6.10b, p. 146
O A E Loose leaves and organic debris Partly decomposed organic debris Topsoil; dark in color; rich in organic matter Zone of intense leaching or eluviation B Subsoil, zone of accumulation Transition to C C Partly weathered parent material Parent material Stepped Art Fig. 6-10b, p. 146
Soil and Its Origin Factors That Control Soil Formation Climate - the most important factor in soil formation because chemical processes operate faster where it is warm and wet Fig. 6.11, p. 146
Fig. 6.11, p. 146
Soil and Its Origin Factors That Control Soil Formation Types of soils: Humid Forest Forest soils develop in humid climates such as the eastern United States and much of Canada. Well-developed A horizon Iron- and aluminum-rich B horizon Fig. 6.12c,d, p. 147
Soil and Its Origin Factors That Control Soil Formation Types of soils: Hot desert Hot desert soils have thin O and A horizons because of the lack of plant growth B horizon often contains irregular masses of calcite, called caliche Desert soils are often alkaline Fig. 6.12a, p. 147
Soil and Its Origin Factors That Control Soil Formation Types of soils: Hot desert Fig. 6.13, p. 148
Soil and Its Origin Factors that Control Soil Formation Types of Soils: Laterites Red tropical soils Severe chemical weathering Fig. 6.14a, p. 149
Soil and Its Origin Factors that Control Soil Formation Types of Soils: Laterites Thin and infertile A horizons. Rapid decomposition prevents the formation of fertile A horizons Not a good soil for agriculture. Fig. 6.12c, p. 147
Soil and Its Origin Factors that Control Soil Formation Types of Soils: Laterites Thick B horizon rich in insoluble clays, aluminum oxides and iron oxides B horizon may be mined as an aluminum ore (bauxite) Fig. 6.12c, p. 147
Fig. 6.12, p. 147
Soil and Its Origin Other Factors That Control Soil Formation Parent material Organic activity Relief and slope Time Fig. 6.7, p. 141
Expansive Soils and Soil Degradation Expansive Soils Some soils contain clays that considerably expand when wet and contract when dry. With expansive soils, the change in volume is 6% or more. Expansive soils destroy the foundations of buildings, roadways, and other structures. Construction projects should avoid these soils. If alternative sites cannot be found, the soils may need to be removed, which can be expensive.
Expansive Soils and Soil Degradation Soil Degradation - Any process that leads to loss in soil fertility, including: erosion, contamination with pollutants, and the destruction of soils nutrients Fig. 6.17, p. 153
Expansive Soils and Soil Degradation Soil Degradation Soil erosion is caused mostly by sheet and rill erosion. Erosion is often accelerated by human activities, such as construction, agriculture, ranching, and deforestation. Fig. 6.16, p. 153
Expansive Soils and Soil Degradation Soil Degradation Depletion of soil nutrients by extensive farming Salinization: Accumulation of salts in soils from excessive irrigation Other pollutants: Improper disposal of wastes, excessive use of pesticides and fertilizers, and oil and chemical spills Compaction of soils by livestock and heavy equipment, which makes the soils too hard for plant growth
The Dust Bowl An American Tragedy Overgrazing, deforestation, and excessive plowing in the Midwest led to wind erosion, destroying valuable top soils during the drought of the 1930s. Fig. 6.15a,c, p. 152
Expansive Soils and Soil Degradation Protecting soils Crop rotation to maintain soil nutrients Contour plowing Terracing No-till planting Fig. 6.17, p. 153
Weathering and Resources Intense chemical weathering may concentrate valuable mineral resources Residual concentrations bauxite and other valuable ore deposits are concentrated by selective removal of soluble substances during chemical weathering
Weathering and Resources Residual concentrations Bauxite forms in laterite soils in the tropics. Bauxite is a valuable aluminum ore. Bauxite occurs in areas where chemical weathering is so intense that only aluminum oxides and other very insoluble compounds accumulate in soils.
Weathering and Resources Residual concentrations Gossans - hydrated iron oxides formed on the surface by oxidation of iron Sulfide minerals oxidize, leach and the metals concentrate as ore deposits of iron, copper, lead, and zinc beneath the gossan
End of Chapter 6