Weathering and Soils: Processes, Factors, and Formation, Exams of School management&administration

Download Weathering and Soils: Processes, Factors, and Formation and more Exams School management&administration in PDF only on Docsity! Weathering and Soils 5 Weathering and Soils begins with a brief examination of the external processes of weathering, mass wasting, and erosion. The two forms of weathering, mechanical and chemical, are investigated in detail-including the types, conditions, rates, and net effect of each. The soils section of the chapter begins with a description of the general composition and structure of soil. After examining the factors that influence soil formation, development, and classification, soil erosion, as well as some ore deposits produced by weathering, are presented. Learning Objectives After reading, studying, and discussing the chapter, students should be able to:  List the processes on Earth that are continually removing materials from higher elevations and moving them to lower elevations.  Describe the two types of weathering and the ways that they are different.  Discuss the factors that determine the rate at which rock weathers.  Describe soil and list the factors that control soil formation.  Explain the concept of the soil profile and how it relates to soil formation.  Discuss the characteristics of the soil horizons in a typical soil profile.  List and briefly discuss the major soil types.  Relate the process of weathering to the formation of some ore deposits. Chapter Summary  External processes include (1) weathering—the disintegration and decomposition of rock at or near Earth’s surface; (2) mass wasting—the transfer of rock material downslope under the influence of gravity; and (3) erosion—the removal of material by a mobile agent, usually water, wind, or ice. They are called external process because they occur near Earth’s surface and are powered by energy from the Sun. By contrast, internal processes, such as volcanism and mountain building, derive their energy from Earth’s interior.  Mechanical weathering is the physical breaking up of rock into smaller pieces. Rocks can be broken into smaller fragments by frost wedging (where water works its way into cracks or voids in rock, and, upon freezing, expands and enlarges the openings), unloading (expansion and breaking due to a great reduction in pressure when the overlying rock is eroded away), and biological activity (by humans, burrowing animals, plant roots, etc.).  Chemical weathering alters a rock’s chemistry, changing it into different substances. Water is by far the most important agent of chemical weathering. Dissolution occurs when water-soluble minerals such as halite become dissolved in water. Oxygen dissolved in water will oxidize iron-rich minerals. When carbon dioxide (CO2) is dissolved in water it forms carbonic acid, which accelerates the decomposition of silicate minerals by hydrolysis. The chemical weathering of silicate minerals frequently produces (1) soluble products containing sodium, calcium, potassium, and magnesium ions, and silica in solution; (2) insoluble iron oxides; and (3) clay minerals. 37 38 CHAPTER 5  The rate at which rock weathers depends on such factors as (1) particle size—small pieces generally weather faster than large pieces; (2) mineral makeup—calcite readily dissolves in mildly acidic solutions, and silicate minerals that form first from magma are least resistant to chemical weathering; and (3) climatic factors, particularly temperature and moisture. Frequently, rocks exposed at Earth’s surface do not weather at the same rate. This differential weathering of rocks is influenced by such factors as mineral makeup and degree of jointing.  Soil is a combination of mineral and organic matter, water, and air—that portion of the regolith (the layer of rock and mineral fragments produced by weathering) that supports the growth of plants. About one-half of the total volume of a good-quality soil is a mixture of disintegrated and decomposed rock (mineral matter) and humus (the decayed remains of animal and plant life); the remaining half consists of pore spaces, where air and water circulate. The most important factors that control soil formation are parent material, time, climate, plants and animals, and slope.  Soil-forming processes operate from the surface downward and produce zones or layers in the soil called horizons. From the surface downward, the soil horizons are respectively designated as O (largely organic matter), A (largely mineral matter), E (where the fine soil components and soluble materials have been removed by eluviation and leaching), B (or subsoil, often referred to as the zone of accumulation), and C (partially altered parent material). Together the O and A horizons make up what is commonly called the topsoil.  Although there are hundreds of soil types and subtypes worldwide, the three very generic types are (1) pedalfer—characterized by an accumulation of iron oxides and aluminum-rich clays in the B horizon; (2) pedocal—characterized by an accumulation of calcium carbonate; and (3) laterite—deep soils that develop in the hot, wet tropics that are poor for growing crops because they are highly leached.  Soil erosion is a natural process; it is part of the constant recycling of Earth materials that we call the rock cycle. Once in a stream channel, soil particles are transported downstream and eventually deposited. Rates of soil erosion vary from one place to another and depend on the soil’s characteristics as well as such factors as climate, slope, and type of vegetation.  Weathering creates ore deposits by concentrating minor amounts of metals into economically valuable deposits. The process, often called secondary enrichment, is accomplished by either (1) removing undesirable materials and leaving the desired elements enriched in the upper zones of the soil, or (2) removing and carrying the desirable elements to lower zones where they are redeposited and become more concentrated. Bauxite, the principal ore of aluminum, is one important ore created as a result of enrichment by weathering processes. In addition, many copper and silver deposits result when weathering processes concentrate metals that were formerly dispersed through low-grade primary ore. Weathering and Soils 41 Answers to the Review Questions 1. Weathering, mass wasting, and erosion are all integral processes involved in the rock cycle because of their relationship to the formation of sedimentary rocks. In the rock cycle, the first step in the formation of sedimentary rocks involves the derivation of sediment from pre-existing igneous, metamorphic, or sedimentary rocks. Solid rock is transformed into sediment by the collective processes of weathering, mass wasting, and erosion. Therefore, the external processes provide the important first step towards the formation of sedimentary rocks. 2. A rock exposed to mechanical weathering would be broken into smaller and smaller pieces. Therefore, the same rock following mechanical weathering would look identical to the original rock, only in smaller fragments or pieces. Chemical weathering involves various processes that convert the minerals in a rock into new minerals or release them to the surrounding environment. Because of these chemical changes, the same identical rock above, having been chemically weathered, will most likely look considerably different than the mechanically weathered rock. 3. Reduction in particle size produces an enormous increase in surface area of the material being weathered. For very small particles, water, oxygen, and other important chemical-weathering agents have direct access to nearly the entire grain, while uncracked interiors of coarser particles are protected, at least temporarily, from contact with the main chemical-weathering agents. 4. Exfoliation domes form when the reduction in pressure that accompanies unloading leads to sheet ing. Fractures typically develop parallel to the surface and give these exhumed granite masses a domed shape. Continued weathering causes the slabs produced by sheeting to separate and spall off. Examples include Stone Mountain, Georgia, and Half Dome and Liberty Cap in Yosemite National Park. 5. (a) Moisture and warm temperatures accelerate rates of chemical reactions, thus chemical weathering will predominate. Also, mechanical weathering processes dependent on freezing and thawing will not be operative. (b) Basalt would probably weather more rapidly. Ferromagnesian minerals would be rapidly oxidized and decomposed under these conditions, and basalts have much larger percentages of these minerals than granite. 6. In general, high temperatures do raise chemical weathering reaction rates, but most of these reactions take place in an aqueous (watery) media or on the moist surfaces of rock and soil particles. Reaction rates decrease drastically under very dry conditions. 7. Carbonic acid is a very weak acid formed by the solution of carbon dioxide (C02) in water. Carbon dioxide is a minor component of the atmosphere but is often enriched in soil gases by the oxidation of organic matter. The common cations of feldspars (K = +1, Na = +1, and Ca = +2) are fairly soluble in acidic solutions, so reaction of carbonic acid with potassium feldspar causes the feldspar to chemically decompose. Potassium, sodium, and calcium are then released to the soil and precipitated as minerals. 42 CHAPTER 5 8. Soil is an interface where different parts of the Earth system interact. It forms where the solid Earth, the atmosphere, the hydrosphere, and the biosphere meet. Over time, the material of soil develops in response to complex environmental interactions among the different parts of the Earth system. 9. Different soils are likely to form from the same parent material if the climates are different. Other factors, which would contribute to differences, include the nature of the vegetation, the slopes, and the length of time the soils have been forming. Similar soils from different parent materials would result if the above-named factors were essentially the same in each situation. 10. Climate is the most important factor in soil formation. Temperature and moisture abundance largely control most of the other weathering variables mentioned in Review Question 11. Time, however, is obviously independent of the climatic variable. 11. Slope greatly influences drainage and the amount of erosion that will occur. Because of accelerated erosion on steep slopes, soils are thin. Conversely, in flat bottomlands soils are often waterlogged. Optimum conditions for soil development are flat to undulating upland surfaces. Here erosion is at a minimum and drainage is good. Slope orientation refers to the amount of sunlight received. This affects the soil temperature and moisture conditions that in turn influence the nature of the vegetation and the character of the soil. 12. O - This is a dark, surface layer of decaying plant matter (humus). A - The A horizon (just below the 0 horizon) consists of humus mixed with very tough, chemically resistant mineral grains like quartz that have survived the processes of disintegration, chemical weathering, and leaching associated with the soil-forming process. E - This is a light-colored horizon of resistant mineral grains as in horizon A, but depleted in silt and clay-sized particles that have been transported downward (eluviated) to the B horizon; humus is sparse or absent. B - The B horizon is often called the zone of accumulation. Small size soil particles eluviated downward from higher horizons accumulate in the B-horizon, and chemical constituents leached from the A horizon are precipitated there as well. These include the most insoluble minerals formed in soils such as the iron oxides and hydroxides and clays. This zone is usually red, yellow, or brown, depending on the nature of the finely divided iron oxides in the soil. C - The C horizon is the zone of partly weathered bedrock. Weathered fragments and chips of rock found here are evidence of the unweathered parent rock material at depth. The solum includes all the horizons above the top of the C-horizon. Recently exposed regolith lacks distinctive soil horizons because the soil-forming process is still in its infancy, and horizons have not had enough time to develop. A newly deposited, floodplain sediment is a good example of regolith. 13. These terms denote soil types. The prefix “ped(o)” comes from pedology, the scientific study of soils. In pedalfer, the “al” and “fe” refer to the elements aluminum (Al) and iron (Fe), which accumulate as iron oxides and clays in the B-horizon. In pedocals, the “cal” refers to calcium carbonate (calcite) that precipitates in the B-horizon. The B-horizon in pedocals generally has a whitish tinge because of the Weathering and Soils 43 calcite; finely divided iron oxides impart shades of red, yellow, or brown to the B-horizons of pedalfers. Pedocals form in temperate climatic zones under marginally humid to semiarid conditions. Pedalfers develop in moist, temperate regions where enough water moving through the soil can remove (leach) most constituents from the B-horizon except for the highly insoluble iron oxides and clays. 14. Laterites are the soils most associated with tropical rain forests. Several factors contribute to the fact that laterites are poor soils for growing crops. First, because these soils develop under conditions of high temperature and rainfall, the nutrients have been removed by leaching. Moreover, when cleared of plants in preparation for agriculture, laterites are subject to accelerated erosion and can be baked to brick-like hardness. 15. Soil erosion is a natural process; it is part of the constant recycling of Earth materials that we call the rock cycle. However, soil erosion is a growing problem as human activities expand and disturb more and more of Earth’s surface. 16. Soil erosion contributes excess sediment loads to reservoirs, streams, and rivers, thus degrading water quality and adversely affecting aquatic and riparian habitats. It also diminishes the quality of outdoor recreational activities and raises the cost of maintaining navigational channels and hydroelectric power generation facilities. Blowing dust can seriously degrade air quality, causing health problems and premature failure of machinery and electronics equipment. 17. Bauxite is the primary ore of aluminum. Bauxite results when intense and prolonged chemical weathering leaches most of the soluble elements, leaving the highly insoluble aluminum concentrated in the soil. PowerPoint slides for each chapter of Essentials of Geology accompany the DIGIT disc (ISBN 013-008171-X). There are instructions in the CD's ReadMe file for embedding QuickTime for use in the PowerPoint slides. For additional resources, visit the Essentials of Geology Home Page at http://www.prenhall.com/tarbuck.