Geography
Weathering
Weathering refers to the process by which rocks and minerals are broken down into smaller pieces through exposure to natural elements such as water, wind, and temperature changes. There are two main types of weathering: mechanical (physical) weathering, which involves the physical breakdown of rocks, and chemical weathering, which involves the alteration of rock composition through chemical reactions.
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eBook - PDFBiogeochemistry
An Analysis of Global Change
- William H. Schlesinger(Author)
- 2012(Publication Date)
- Academic Press(Publisher)
However, about 75% of the rocks now exposed on land are sedi-mentary rocks that have been subjected to geologic uplift (Blatt and Jones 1975). These sedimentary rocks are subject to further Weathering reac-tions with acid volatiles, in accord with Siever's basic equation. In this chapter we will review the basic types of rock Weathering on land and the processes that drive the Weathering reactions. Rock Weathering is important for the release of biochemical elements that have no gaseous forms (e.g., Ca, Κ, Fe, and Ρ). Reactions between soil waters and the minerals found in soil determine the availability of these elements to biota and the losses of these elements in runoff. Conversely, land biota and soil microbes affect rock Weathering and soil development. In this chapter, we will examine soil development in the major ecosystems on Earth. Finally, we will examine the rates of Weathering in an attempt to determine the supply of biochemical elements on land and the global loss of Weathering products to rivers and the ocean. Rock Weathering Upon uplift and exposure, all rocks undergo Weathering, a general term that encompasses a variety of geological processes by which parent rocks are broken down. Mechanical Weathering is the fragmentation of materi-als with no chemical change; in laboratory terminology it is equivalent to a physical change. Chemical Weathering occurs when parent rock materials 74 Processes and Reactions react with acidic and oxidizing substances. Usually chemical Weathering involves water, and mineral constituents are released as dissolved ions. Chemical Weathering also includes the formation of new, secondary min-erals that are more stable at the physical conditions on the surface of the Earth. Mechanical Weathering includes wind abrasion and rock splitting by the freezing of water and by the growth of roots in rock crevices. Mechanical Weathering is important in extreme and highly seasonal climates and in areas with much exposed rock.
eBook - PDFBiogeochemistry
An Analysis of Global Change
- William H Schlesinger(Author)
- 2012(Publication Date)
- Academic Press(Publisher)
These sedimentary rocks are subject to further Weathering reac-tions with acid volatiles, in accord with Siever's basic equation. In this chapter we will review the basic types of rock Weathering on land and the processes that drive the Weathering reactions. Rock Weathering is important for the release of biochemical elements that have no gaseous forms (e.g., Ca, K, Fe, and P). Reactions between soil waters and the minerals found in soil determine the availability of these elements to biota and the losses of these elements in runoff. Conversely, land biota and soil microbes affect rock Weathering and soil development. In this chapter, we will examine soil development in the major ecosystems on Earth. Finally, we will examine the rates of Weathering in an attempt to determine the supply of biochemical elements on land and the global loss of Weathering products to rivers and the ocean. Rock Weathering Upon uplift and exposure, all rocks undergo Weathering, a general term that encompasses a variety of geological processes by which parent rocks are broken down. Mechanical Weathering is the fragmentation of materi-als with no chemical change; in laboratory terminology it is equivalent to a physical change. Chemical Weathering occurs when parent rock materials 74 Processes and Reactions react with acidic and oxidizing substances. Usually chemical Weathering involves water, and mineral constituents are released as dissolved ions. Chemical Weathering also includes the formation of new, secondary min-erals that are more stable at the physical conditions on the surface of the Earth. Mechanical Weathering includes wind abrasion and rock splitting by the freezing of water and by the growth of roots in rock crevices. Mechanical Weathering is important in extreme and highly seasonal climates and in areas with much exposed rock. Fragmented rock often forms the lower horizons of soil profiles.
No longer available |Learn more- James Petersen, Dorothy Sack, Robert Gabler(Authors)
- 2016(Publication Date)
- Cengage Learning EMEA(Publisher)
Likewise, ions chemically removed from rocks during Weathering are transported in surface or subsurface water to other locations. These ions represent a major source of nutrients for terrestrial as well as aquatic ecosystems, including rivers, ponds, lakes, and the ocean. The several types of rock Weathering fall into two basic cat- egories. Physical Weathering, also known as mechanical weath- ering, disintegrates rocks, breaking larger blocks or outcrops of rock into smaller clasts. Chemical Weathering decomposes rock through chemical reactions that remove ions from the original rock-forming minerals. Disintegration and decomposition of rock matter accomplished in ways influenced by organisms is sometimes referred to as biological Weathering, even though the processes are fundamentally physical or chemical in nature. Many physical and chemical processes contribute to rock Weathering, and water plays an important role in almost all of them. Physical Weathering The mechanical disintegration of rocks by physical Weathering is especially important to landscape modification in two ways. First, the resulting smaller clasts are more easily eroded and trans- ported than the initial larger ones. Second, the breakup of a large rock into smaller rocks encourages additional Weathering because it increases the rock surface area exposed to Weathering processes. Rocks can be physically weathered in several ways; even a person breaking a rock with a hammer is carrying out physical Weathering. Five principal types of physical Weathering are discussed here. Unloading Most rocks originate under much higher pressure (weight per unit area) than the 1013.2 millibars (29.92 in. Hg; 15 lbs/in. 2 ) of average atmospheric pressure that exists at Earth’s surface. Intrusive igneous rocks solidify slowly deep beneath the surface under great pressure from the weight of the overlying rocks.
eBook - PDF- Tom Freeman(Author)
- 2012(Publication Date)
- Wiley(Publisher)
Weathering 93 6 Weathering A. Weathering defined Definition—Everyone knows about Weathering. It’s the fading and peel- ing of paint. It’s the deterioration of masonry buildings and monuments. It’s the wear and tear on all things exposed to ‘the elements.’ Geologically speak- ing, Weathering is the physical and chemical alteration of rock and sedi- ments through the effects of air, mois- ture, heat, cold, and organic matter. Weathering consists of all those processes that convert rock into regolith—the loose, unconsolidated surface material that rests on the bedrock from which it developed (Fig. 6.1). In engineering parlance, regolith is earth material that can be moved with a bulldozer or front-end loader. (If explosives are required, the material is likely to be called bedrock.) While Figure 6.1 A Bedrock overlain by regolith. B Bedrock overlain by sediments. this is a practical definition of regolith, it includes unconsolidated sediments that are not products of in situ weather- ing, and, so, are not part of the regolith. Genetically, regolith is dynamic in that it is constantly in a state of observable change, whereas bedrock and sediments are, by comparison, relatively stable. Q6.1 The nature of the boundary with bedrock serves to distinguish regolith from sediments. Describe the difference in the boundaries between (A) bedrock and regolith, and (B) bedrock and sediments, in Figure 6.1. The bad and the good—A downside of Weathering is repainting your home and refurbishing masonry. But there’s an upside to Weathering, most impor- tantly the production of soil, without which Planet Earth would be barren. There are two broad types of weather- ing, (a) physical Weathering and (b) chemical Weathering. Topics A. What is the definition of Weathering? What is regolith? B. What is physical Weathering? How does physical Weathering facilitate chemical Weathering? What are four examples of physical Weathering, and what is the definition of each? C.
eBook - PDF- Q Zaruba(Author)
- 2012(Publication Date)
- Elsevier(Publisher)
Chapter 6 Weathering OF ROCKS In all foundation excavations carried to the bedrock a surface layer of weathered rock is first encountered; undisturbed fresh rocks very rarely occur at or near the earth's surface. It is therefore necessary for the engineer to know the properties of weathered rocks, which usually differ from the character of fresh rock, and the processes that caused physical and chemical alteration of the rock mass. The thickness and nature of the weathered zone differ, depending on the kind of rock, the Weathering agents, duration of Weathering processes, the slope of the terrain, and on the rate and mode of displacement of weathered material. The term 'Weathering' generally denotes processes of mechanical disintegration or chemical decomposition of rocks caused by natural agents, man included. Alterations of rocks are produced mainly by temperature changes, insolation and rapid cooling of the rock, frost action, chemical action of percolating water, effects of plant roots and smoke gas, etc. The type of Weathering is controlled mainly by climatic conditions. In arid regions, for example, mechanical (or physi-cal) Weathering effected by temperature changes predominates and fine-weathered material is removed chiefly by wind. In polar regions and high mountains freeze-and-thaw is the main Weathering agent. In temperate and particularly tropical zones, a warm and humid climate supports deep-reaching chemical Weathering, which results in chemical decomposition of rock components. If climatic conditions of a given area remain unchanged for a long time, a typical profile through the weathered layer develops near the surface. The composition and depth of this profile correspond to climatic conditions and the kind of parent rocks. In some favourable places old Weathering crusts covered by younger sedi-ments have been preserved. From these fossil-Weathering crusts the climate of the respective geological period can be inferred.
eBook - PDF- F. G. Bell(Author)
- 2013(Publication Date)
- Butterworth-Heinemann(Publisher)
Chapter 9 Weathering of rocks and rock masses Weathering of rocks is brought about by physical disintegration, chemical decomposition and biological activity. The Weathering process is primarily controlled by the presence of discontinuities in that they provide access for the agents of Weathering. Some of the earliest effects of Weathering are seen along discontinuity surfaces. Weathering then proceeds inwards until the whole of a discontinuity bounded block is affected. The agents of Weathering, unlike those of erosion, do not themselves provide for the transportation of debris from a rock surface. Therefore unless this rock waste is otherwise removed it eventually acts as a protective blanket, preventing further Weathering taking place. If Weathering is to be continuous, fresh rock exposures must be constantly revealed, which means that the weathered debris must be removed by the action of gravity, running water, wind or moving ice. 9.1 Rate of Weathering The rate at which Weathering proceeds depends not only upon the vigour of the Weathering agent but also on the durability of the rock mass concerned. This, in turn, is governed by the mineralogical composition, texture and porosity of the rock on the one hand, and the incidence of discontinuities within the rock mass on the other. Most studies regarding the rate at which Weathering occurs have been made upon stone used for construction purposes and have involved measuring the rate at which the surfaces of stones have been removed (Table 9.1). In addition, a number of tests have been used to simulate and accelerate the rate of Weathering of construction stone. The problem with such tests is the difficulty in relating the results obtained to the natural performance of the rocks concerned. Nevertheless according to Fookes et al. (1988) the rate of Weathering declines with time where a residual layer is developed at the surface of the rock and, if surface reaction is involved, then the rate of Weathering is linear.
eBook - PDFEarth Science
An Introduction
- Mark Hendrix, Graham Thompson, Mark Hendrix(Authors)
- 2020(Publication Date)
- Cengage Learning EMEA(Publisher)
Copyright 2021 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 228 CHAPTER 10 Weathering, Soil, and Erosion In mountains or deserts at midlatitudes, temperature may fluctuate from −5°C to +25°C during a spring day. This 30-degree difference is probably not sufficient to frac- ture rocks. In contrast to small daily or annual temperature changes, fire heats rock by hundreds of degrees. If you line a campfire with granite stones, the rocks commonly break as you cook your dinner. In a similar manner, forest fires or brush fires occur frequently in many ecosystems, producing cracked rock that is an important agent of mechanical Weathering. CHEMICAL Weathering Rock is durable over a human lifetime. Over geologic time, however, air and water chemically attack rocks near Earth’s surface. The most important processes of chemical Weathering are dissolution, hydrolysis, and oxidation. Water, acids and bases, and oxygen in the atmosphere or in surface water or groundwater cause these processes to decompose rocks. Dissolution We are all familiar with the fact that some minerals dissolve readily in water and others do not. If you put a crystal of halite (rock salt, or table salt) in water, the crystal will rapidly dissolve to form a solution. The process is called dissolution. Halite dissolves so rapidly and completely in water that the mineral is rare in natural, moist environments. On the other hand, if you drop a crystal of quartz into pure water, only a tiny amount will dissolve and nearly all of the crystal will remain intact.
eBook - PDFThe Changing Earth
Exploring Geology and Evolution
- James Monroe, Reed Wicander(Authors)
- 2014(Publication Date)
- Cengage Learning EMEA(Publisher)
■ Weathering yields materials that may become soil or sedi-mentary rock. ■ Mechanical Weathering processes include frost action, pressure release, thermal expansion and contraction, salt crystal growth, and the activities of organisms. The parti-cles yielded retain the composition of the parent material. ■ Chemical Weathering by solution, hydrolysis, and oxida-tion results in a chemical change in parent material and proceeds most rapidly in hot, wet environments. ■ Mechanical Weathering contributes to chemical Weathering by breaking parent material into smaller pieces, thereby exposing more surface area. ■ Soils possess horizons designated, in descending order, as O, A, E, B, and C, which differ from one another in texture, composition, structure, and color. ■ The important factors controlling soil formation are climate, parent material, organic activity, relief and slope, and time. ■ Soils in humid regions are darker and more fertile than those of semiarid regions. Laterite is soil that forms in the tropics where chemical Weathering is intense. ■ Soil degradation results from erosion as well as from physical and chemical deterioration. Human activi-ties, such as construction, agriculture, deforestation, waste disposal, and chemical spills, contribute to soil degradation. ■ Chemical Weathering is responsible for the origin of some mineral deposits, such as residual concentrations of iron, lead, manganese, and clay.
eBook - PDFPhysical Geology
The Science of Earth
- Charles Fletcher(Author)
- 2017(Publication Date)
- Wiley(Publisher)
4. Changes in the moisture content of soils. Organisms influence the moisture content of soils, and thus enhance Weathering. Shade from leaves and stems, the presence of root masses, and high levels of organic material in soil all increase the amount of water in the soil. This higher moisture content, in turn, enhances physical and chemical Weathering processes. Products of Weathering Weathering occurs because conditions at Earth’s surface are different from the high-temperature and high-pressure con- ditions prevailing within the crust, where most igneous and Biological Weathering Involves Both Chemical and Physical Processes, and Sedimentary Products 199 metamorphic rocks and min- erals are formed. The products of Weathering are new minerals that are in equilibrium with sur- face conditions, and therefore are more likely to resist Weathering themselves. Chemical Weathering yields Weathering products that are sig- nificant components of the rock cycle and the sedimentary family of rocks. Some of these products are new sedimentary minerals that result from crystallization, such as sedimentary quartz, various types of clay, calcite, and hematite. Other Weathering prod- ucts include dissolved compounds and some gases. Sedimentary quartz, hema- tite, and calcite are important natural cements that precipitate from groundwater and bind sed- imentary particles to form solid sedimentary rock. Clays are sheet silicates that trap water between layers.
eBook - PDF- James Petersen, Dorothy Sack, Robert Gabler, , James Petersen, James Petersen, Dorothy Sack, Robert Gabler(Authors)
- 2021(Publication Date)
- Cengage Learning EMEA(Publisher)
Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 430 C H A P T E R 1 5 • W E A T H E R I N G A N D M A S S W A S T I N G salts. Abundant salts, high humidity, and contact with seawater make salt Weathering of rocks very effective in marine coastal locations. The other principal climatic variable, tem- perature, also influences dominant types and rates of Weathering. Most chemical reactions proceed faster at higher temperatures. Low lati- tude regions with humid climates consequently experience the most intense chemical weather- ing. In the tropical rainforest, savanna, and monsoon climates, chemical Weathering is more significant than physical Weathering, soils are deep, and landforms appear rounded. Although chemical Weathering is somewhat less extreme in the midlatitude humid climates, its influence is apparent in the moderate soil depth and rounded forms of most landscapes in those regions. In contrast, the landforms and rocks of both arid and cold regions, where physical Weathering dominates, tend to be sharper, angu- lar, and jagged, but this depends to some extent on rock type, and rounded features may remain the rock are major influences on the effective- ness of the various Weathering processes. Under- standing how specific rocks weather in natural settings is important for explaining the charac- teristics of regolith, soil, and relief, and it is also important in our cultural environment because Weathering affects building stone as well as rock in its natural setting. 15-3a Climate Factors In almost all environments, whether biological or not, physical and chemical Weathering pro- cesses operate together, even though one of these two categories usually dominates. Water plays a role in most of the physical Weathering processes, but it is essential for all types of chemical Weathering. Also, chemical Weathering increases as more water comes into contact with rocks.
eBook - PDFGeomorphology
The Mechanics and Chemistry of Landscapes
- Robert S. Anderson, Suzanne P. Anderson(Authors)
- 2010(Publication Date)
- Cambridge University Press(Publisher)
The strength of a rock mass reflects both the strength of the intact rock mass, as most easily quantified with uniaxial compressive strength, and the number, extent, and orientation of fractures in the rock. A number of schemes to quantify these weathered rock characteristics exist. We discussed the rock mass strength classification of Selby. For many geomorphologists, Weathering can be summarized as the process (or perhaps, in generous moments, the suite of processes) that detaches fragments from intact bedrock, releas-ing material to the mobile blanket of debris found over much of Summary 209 the Earth’s surface. The production of mobile debris is import-ant, since it introduces material into the mobile and chaotic layer that can creep or slide or otherwise move downslope. However, as the change from fixed to free is neither a purely chemical nor a purely physical transformation, the process (or suite of processes) is difficult to study. Current models suggest that the production rate of mobile regolith is controlled by the thickness of the mobile layer, either as an exponentially declin-ing rate with increasing depth, or as a rate that rises to a maximum at an intermediate depth, before falling exponen-tially with depth. There is a desperate need for models that explicitly incorporate the physical, chemical, and biological processes that we have cataloged in this chapter that can then be employed to understand these depth dependences. Think of the Earth’s surface as a Critical Zone in which water, air, rock, and living systems react and interact with each other. Weathering processes – the mechanisms that break and breakdown rock and minerals – set the scene for the behavior of the Critical Zone. Water and gases (including dissolved gases) move in pore spaces and along fractures generated by Weathering. Soil creep and landsliding are possible in mobile debris.
- I.B. Campbell, G.G.C. Claridge(Authors)
- 1987(Publication Date)
- Elsevier Science(Publisher)
Chapter 5 PHYSICAL Weathering AND ROCK DISINTEGRATION INTRODUCTION The major processes of physical Weathering and rock disintegration in Ant- arctica are glacial action; the action of water in various forms, as liquid, ice and vapour; salt Weathering; insolation and wind action. The regolith is formed through the combination of these processes. Though many of them have not been intensively studied in Antarctica, some, such as cavernous Weathering, salt Weathering, patterned ground movements and ventifact formation, have been considered in some detail. The effectiveness and the extent of chemical Weathering declines as the dis- tance from tropical regions increases (Peltier, 1950), consequently physical Weathering processes that induce rock decay gradually assume a much more sig- nificant role with increasing latitude, due to lower temperatures and a lesser availability of moisture. In Antarctica, the combination of extremes of very low moisture availability and very low temperatures has produced distinctive soil and landscape features, to the extent that Antarctica can be regarded as a distinct morphogenetic region. The nature and extent of chemical Weathering in Antarctic soils has been studied by a number of workers. Their conclusions, discussed in detail in Chapter 6, show that the degree of chemical Weathering is relatively small and that the regolith is very largely a product of physical or mechanical Weathering processes. While physical Weathering is, thus, the dominant process of rock decay in Antarctica, and its various manifestations are important in the formation of the regolith and subsequent soil formation, it is much less intensive than in alpine or subalpine zones of more temperate areas such as New Zealand or South Amer- ica. Processes such as frost wedging, frost cracking, exfoliation and transport of products of disintegration are much more effective in temperate areas where there is an abundant moisture supply.
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