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UMD GEOL 342 - Weathering

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GEOL 342 Sedimentation and StratigraphyLecture 2: Weathering1 February 2005Assoc. Prof. A. Jay KaufmanWeatheringGiven that most of the igneous continental mass is composed of feldspars, whathas happened to these minerals when we look at clean quartz sand at the beach? The physical andchemical processes of breakdown of rocks and minerals are known as weathering. It is the simpleconsequence of exposing pre-existing rocks to the conditions at the Earth’s surface, which isconsiderably different from where they formed. Regolith is the general term for particles ofbroken down rock, including gravel, sand, silt, and clay.Bowen’s reaction scalePhysical weatheringThere are four major mechanisms of physical weathering, including 1)freeze-thaw. 2) insolation, 3) stress release, and 4) organic activity. Inaddition, the action of the wind, rivers, glaciers, and landslides add to thephysical breakdown of rock and the landscape.Freeze-thaw (ice-wedging) – when water freezes a 9-10% expansion ofvolume occurs, which can create forces as great as several kilograms persquare centimeter in cracks and fissures of a rock. This is greater than thetensile strength of most rocks. The process is most efficient where fracturesare abundant and where there the daily temperature range hovers aroundthe 0oC mark. Similar processes occur when evaporite minerals crystallize incracks and crevices.800oC1400oC1200oC1050oC1Insolation – when rocks and minerals are exposed to changing temperaturesstresses are generated causing differential expansion and contraction. This ismost likely in desert regions where temperatures fluctuate greatly, or mayoccur following wildfires. In moist climates, the hydration and dehydration ofclay minerals aids in insolation.Expansion of Clay Minerals by Volume:Ca-montmorillonite (45-185%)Na-montmorillonite/bentonite (1400-1600%)Illite (15-120%)Kaolinite (5-60%)Stress release – rocks that are buried deep beneath the surface experiencehigh confining pressures, so when overburden is removed the rock expands.A series of expansion cracks or joints develop roughly parallel to the surface,which results in the exfoliation and spheroidal weathing of rock in onionskin-like sheets. Organic activity – the activities of organisms and plants that live on and inrocks promotes their physical breakdown. These organisms also leave behindorganic acids, which promote the chemical attack on minerals in rocks.2In addition, physical weathering processes promote chemicalweathering of rocks and minerals by greatly increasing the exposed surfacearea, which may be attacked by water and other acids. Chemical weatheringChemical weathering proceeds through complete dissolution (e.g.,halite and calcite) or through the conversion of one mineral into another(e.g., feldspar → clay). The new, and generally finer-grained, minerals formwhen selected components are removed and carried away. Chemicalweathering involves a variety of chemical reactions, including 1) hydrolysis,2) hydration, 3) simple solution, and 4) oxidation.Simple solutionSiO2 + 2H2O → H4SiO4 (6 ppm)H2O + CO2 → H2CO3H2CO3 + 2CaCO3 → Ca2+ + 2HCO3-NaCl + H2O → Na+ + Cl-Volume = 1 m3surface area = 6 m2Volume = .53 x 8 = 1 m3surface area = 12 m23Hydration and DehydrationThe addition or removal of water during weathering is known ashydration or dehydration. These processes produce minerals that are inbetter equilibrium with the environment.CaSO4.H2O → CaSO4 + 2H2OFe2O3 + 3H2O → 2Fe(OH)3HydrolysisHydrolysis is the replacement of cations in a mineral structure by hydrogen ions fromwater or other acids. The cations are released to solution and the mineral is converted to anentirely different mineral, or it dissolves completely. Most silicates weather through hydrolysisreactions. Mg2SiO4 + 4H+ → 2Mg2+ + H4SiO4 While mafic minerals such as olivine and pyroxene tend to dissolve completely, felsicminerals like orthoclase and plagioclase dissolve partially and leave behind a residue of fine-grained, and easily transportable, clays.KAlSi3O8 + H+ → Al2Si2O5(OH)4 + K+ + H4SiO4OxidationWhen minerals gain oxygen from the atmosphere they lose electronsand become “rusted.” Conversely, when a mineral becomes reduced it gainselectrons.Fe2S + O2 → Fe2O3 + 2S(Fe2+)2SiO6 + O2 + H2O → 4Fe3+(OH)3 + H4SiO4The Controls on Weathering4Climate is perhaps the primary control on the type and intensity ofweathering around the world. For example, higher temperatures promotechemical weathering as the increase in kinetic energy facilitates reactions.Thus chemical weathering will predominate in warmer (and wetter) climates.Insofar as hydrolysis is an important weathering process, theavailability of hydrogen ions is a critical constraint. The activity of hydrogenions in solution is known as pH and is controlled by the acidity or alkalinity ofthe solution. Acids are the key to hydrolysis reactions as they release protons(H+) to replace cations in the mineral lattice. pH ranges from 1-14, but mostnatural waters range from 4-9. Fresh water is typically acidic (<7) becausewater absorbs atmospheric CO2 (and SO2) as it falls as rain. In contrast,seawater has a pH of ~8.2, so little submarine hydrolysis occurs. Its alkalinitypromotes the precipitation of carbonate minerals.Oxidation of mineral depends on the free availability of electrons,which is more easily understood as free oxygen. The oxidation potential of asolution is known as the Eh, which is measured in volts; higher Eh solutionsare more oxidizing and lower Eh solution are more reducing (usually physically isolated from theatmosphere).Natural weatheringSource rock breaks down in a complex fashion depending on 1)physical and chemical weathering and 2) climate, as well as 3) sourcecomposition, 4) drainage, and 5) topographic relief.Source composition5Recall Bowen’s reaction scale, and recognize that the simple crystallattices (e.g., olivine, pyroxene) are less tightly bonded together. Theseminerals also contain abundant cations (i.e., iron, magnesium, and calcium)that are subject to chemical weathering, and are readily absorbed by roots inbiological weathering.


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