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An overview of weathering, erosion, and soil formation. It discusses the two types of weathering - mechanical and chemical, and their impact on Earth materials. The document also explains the importance of studying these processes and how they contribute to the formation of sediments and mineral resources. Additionally, it touches upon the role of climate, particle size, and mineralogy in the rate of weathering.
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Weathering is the physical breakdown and chemicalalteration of Earth materials at or near the surface.
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Parent material is rock altered by weathering, eitherbroken into smaller pieces or some of itsconstituent minerals altered or dissolved.
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Erosion refers to the wearing away of soil and rockby water, wind, or ice.
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Transport refers to themovement of erodedmaterial from its placeof origin to a new sitewhere it is deposited assediment.
Marshak (2003)
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Weathering, erosion, and soil are good examples ofthe continuous interactions of the atmosphere,hydrosphere, and biosphere with Earth materials.
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Weathering, an essential part of the rock cycle,plays an important role in the formation ofsediments, the raw materials for sedimentary rocks.
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Weathering forms soils on which we depend forfood production and isimportant for formation ofsome mineral resources.
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Erosion degrades soils anderoded sediments degraderivers and streams.
Wicander and Monroe (2002)
Weathering
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Mechanical weathering – physical breakdown ofrocks without changing the composition (freezethaw)
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Chemical weathering – breakdown of minerals bychemical reactions with air or water
Source:
Photograph courtesy of USDA Soil Conservation Service.
Montgomery (2003)
Wicander and Monroe(2002)
Mechanical Weathering
Mechanical weathering occurs when physical forcesbreak Earth materials into smaller pieces. The graniteshown here, for instance, has mechanically weatheredto produce smaller pieces of granite and individualgrains of the minerals comprisingthe granite, quartz andfeldspars.
North Carolina
Wicander and Monroe (2002)
Pressure Release
In deep, underground mines such as some of the diamondmines of South Africa, rock sometimes expands violentlyin response to the pressure released by removal of themine rock. These rock bursts kill 20 miners/year.
Chemical Weathering
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Chemical weathering refers tothe processes that decomposerocks and minerals.
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In some instances, minerals arechemically altered such that newminerals are formed.
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Other minerals may completelydissolve and their ions takeninto solution.
Chernicoff & Whitney (2002)
Wicander and Monroe (2002)
Solution is a type of chemical weathering reaction whereininteraction of a solid with a liquid separates the ions of the solidsuch they come to reside in the liquid and the solid is thusdissolved. Water can easily dissolve some minerals because thewater molecule has an asymmetric shape. The two hydrogenatoms are not evenly spaced around the oxygen atom to whichthey are bonded. The side of the water molecule where thehydrogen atoms are located has a slight positive charge and theother side a slight negative charge.
Wicander and Monroe (2002)
Chemical Weathering - Solution
When soluble minerals such as halite (NaCl) come in contact with water, thepositively charged sodium ions are attracted to the negative end of the watermolecules, and negatively charged chloride ions are attracted to the positiveend of the water molecules. In this way the ions are pulled from the structureof the mineral, surrounded by water molecules, and the solid dissolved.
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Oxidation is a chemical weathering reaction whereby metalelements like iron combine with oxygen to form oxides.Oxidation is important for the weathering of ferromagnesianminerals such as olivine, amphibole, and biotite. When the ironin these minerals oxidizes hematite is commonly formed by thefollowing chemical reaction.
4Fe + O
2
2Fe
O 2
3
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The acid mine drainage so common in coal mining areas isproduced by oxidation of pyrite (FeS
), a mineral common in 2
coal and associated rocks. The resulting iron oxide mineralsform a reddish or yellowish sludge, while the sulfuric acidproduced acidifies the drainage water.
iron oxygen
hematite
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In the chemical weathering reaction known ashydrolysis, hydrogen ions (H+) contained in waterreplace positive ions in a mineral. As a result, a newmineral forms and the replaced ions are dissolved inthe water. In this way clay minerals, which are sheetsilicates, are formed from framework silicates suchas feldspars. 2KAlSi
8
+^
Al
Si 2
4
+^
2
orthoclase
hydrogen
ion
water
clay mineral(kaolinite)
potassium
ion
silica ion
Weathering
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Montgomery (2003)
Wicander and Monroe (2002)
Particle Size Rate of Weathering
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Chemical weathering affects particle surfaces, and the greater thesurface area, the more effective is weathering. Smaller particles havegreater surface areas compared to their volumes than do largerparticles.
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As mechanical weathering splits a block into smaller pieces, totalvolume is unchanged, but total surface area increases as doessusceptibility to chemical weathering.
Chernicoff & Whitney (2002)
Climate & weathering
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Where temperature is lowand/or liquid water scarce,such as in arctic or aridregions, the intensity ofchemical weathering is muchreduced and the depth towhich it reaches is onlycentimeters, or at best a fewmeters.
Climate and soil type
Pipkin and Trent (1997)
