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===Biological effects on mechanical weathering=== |
===Biological effects on mechanical weathering=== |
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Living organisms may contribute to mechanical weathering (as well as chemical weathering, see 'biological' weathering below). [[Lichen]]s and [[moss]]es grow on essentially bare rock surfaces and create a more humid chemical microenvironment. The attachment of these organisms to the rock surface enhances physical as well as chemical breakdown of the surface microlayer of the rock. On a larger scale, seedlings sprouting in a crevice and plant roots exert physical pressure as well as providing a pathway for water and chemical infiltration. |
Living organisms may contribute to mechanical weathering (as well as chemical weathering, see 'biological' weathering below). [[Lichen]]s and [[moss]]es grow on essentially bare rock surfaces and create a more humid chemical microenvironment. The attachment of these organisms to the rock surface enhances physical as well as chemical breakdown of the surface microlayer of the rock. On a larger scale, seedlings sprouting in a crevice and plant roots exert physical pressure as well as providing a pathway for water and chemical infiltration. |
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=={{anchor|Chemical weathering}}Biogeochemical weathering== |
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[[File:Weathering Limestone State College PA.jpg|thumb|left|Comparison of unweathered (left) and weathered (right) limestone.]] |
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Chemical weathering changes the composition of rocks, often transforming them when water interacts with minerals to create various chemical reactions. Chemical weathering is a gradual and ongoing process as the mineralogy of the rock adjusts to the near surface environment. New or ''secondary minerals'' develop from the original minerals of the rock. In this the processes of [[Redox|oxidation]] and [[hydrolysis]] are most important. Chemical weathering is enhanced by such geological agents as the presence of water and oxygen, as well as by such biological agents as the acids produced by microbial and plant-root metabolism. |
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The process of mountain block uplift is important in exposing new rock strata to the atmosphere and moisture, enabling important chemical weathering to occur; significant release occurs of Ca<sup>2+</sup> and other ions into surface waters.<ref>Hogan, C. Michael (2010) [http://www.eoearth.org/article/Calcium?topic=49557 "Calcium"], in A. Jorgenson and C. Cleveland (eds.) ''[[Encyclopedia of Earth]]'', National Council for Science and the Environment, Washington DC</ref> |
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===Dissolution and carbonation=== |
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[[File:GoldinPyriteDrainage acide.JPG|thumb|A [[pyrite]] cube has dissolved away from host rock, leaving [[gold]] behind]] |
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Rainfall is [[acid]]ic because atmospheric [[carbon dioxide]] dissolves in the rainwater producing weak [[carbonic acid]]. In unpolluted environments, the rainfall [[pH]] is around 5.6. [[Acid rain]] occurs when gases such as sulfur dioxide and nitrogen oxides are present in the atmosphere. These oxides react in the rain water to produce stronger acids and can lower the pH to 4.5 or even 3.0. [[Sulfur dioxide]], SO<sub>2</sub>, comes from volcanic eruptions or from fossil fuels, can become [[sulfuric acid]] within rainwater, which can cause solution weathering to the rocks on which it falls. |
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Some minerals, due to their natural solubility (e.g. [[evaporite]]s), oxidation potential (iron-rich minerals, such as [[pyrite]]), or instability relative to surficial conditions (see [[Goldich dissolution series]]) will weather through [[Solvation|dissolution]] naturally, even without acidic water. |
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One of the most well-known solution weathering processes is [[carbonation]], the process in which atmospheric carbon dioxide leads to solution weathering. Carbonation occurs on rocks which contain [[calcium carbonate]], such as limestone and chalk. This takes place when rain combines with [[carbon dioxide]] or an [[organic acid]] to form a [[weak acid|weak]] [[carbonic acid]] which reacts with calcium carbonate (the limestone) and forms [[calcium bicarbonate]]. This process speeds up with a decrease in temperature, not because low temperatures generally drive reactions faster, but because colder water holds more dissolved carbon dioxide gas. Carbonation is therefore a large feature of glacial weathering. |
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The [[Stoichiometry|reactions]] as follows: |
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::CO<sub>2</sub> + H<sub>2</sub>O → H<sub>2</sub>CO<sub>3</sub> |
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:carbon dioxide + water → carbonic acid |
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::H<sub>2</sub>CO<sub>3</sub> + CaCO<sub>3</sub> → Ca(HCO<sub>3</sub>)<sub>2</sub> |
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:carbonic acid + calcium carbonate → calcium bicarbonate |
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Carbonation on the surface of well-jointed limestone produces a dissected [[limestone pavement]]. This process is most effective along the joints, widening and deepening them. |
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===Hydration=== |
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[[File:Iddingsite.JPG|thumb|left|[[Olivine]] weathering to [[iddingsite]] within a [[Mantle (geology)|mantle]] [[xenolith]]]] |
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[[Mineral hydration]] is a form of chemical weathering that involves the rigid attachment of H+ and OH- ions to the atoms and molecules of a mineral. |
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When rock minerals take up water, the increased volume creates physical stresses within the rock. For example, [[iron oxide]]s are converted to [[iron hydroxide]]s and the hydration of [[anhydrite]] forms [[gypsum]]. |
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[[File:Weathering 9039.jpg|thumb|A freshly broken rock shows differential chemical weathering (probably mostly oxidation) progressing inward. This piece of [[sandstone]] was found in [[Moraine|glacial drift]] near [[Angelica, New York]]]] |
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===Hydrolysis on silicates and carbonates=== |
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[[Hydrolysis]] is a chemical weathering process affecting silicate and carbonate minerals. In such reactions, pure water ionizes slightly and reacts with silicate minerals. An example reaction: |
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::Mg<sub>2</sub>SiO<sub>4</sub> + 4 H<sup>+</sup> + 4 OH<sup>−</sup> ⇌ 2 Mg<sup>2+</sup> + 4 OH<sup>−</sup> + H<sub>4</sub>SiO<sub>4</sub> |
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:[[olivine]] ([[forsterite]]) + four ionized water molecules ⇌ ions in solution + silicic acid in solution |
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This reaction theoretically results in complete dissolution of the original mineral, if enough water is available to drive the reaction. In reality, pure water rarely acts as a H<sup>+</sup> donor. Carbon dioxide, though, dissolves readily in water forming a weak acid and H<sup>+</sup> donor. |
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::Mg<sub>2</sub>SiO<sub>4</sub> + 4 CO<sub>2</sub> + 4 H<sub>2</sub>O ⇌ 2 Mg<sup>2+</sup> + 4 HCO<sub>3</sub><sup>−</sup> + H<sub>4</sub>SiO<sub>4</sub> |
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:[[olivine]] ([[forsterite]]) + carbon dioxide + water ⇌ Magnesium and bicarbonate ions in solution + silicic acid in solution |
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This hydrolysis reaction is much more common. [[Carbonic acid]] is consumed by [[silicate]] weathering, resulting in more [[alkaline]] solutions because of the [[bicarbonate]]. This is an important reaction in controlling the amount of CO<sub>2</sub> in the atmosphere and can affect climate. |
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[[Aluminosilicate]]s when subjected to the hydrolysis reaction produce a secondary mineral rather than simply releasing cations. |
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::2 KAlSi<sub>3</sub>O<sub>8</sub> + 2 H<sub>2</sub>CO<sub>3</sub> + 9 H<sub>2</sub>O ⇌ Al<sub>2</sub>Si<sub>2</sub>O<sub>5</sub>(OH)<sub>4</sub> + 4 H<sub>4</sub>SiO<sub>4</sub> + 2 K<sup>+</sup> + 2 HCO<sub>3</sub><sup>−</sup> |
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:[[Orthoclase]] (aluminosilicate feldspar) + carbonic acid + water ⇌ [[Kaolinite]] (a clay mineral) + silicic acid in solution + potassium and bicarbonate ions in solution |
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===Oxidation=== |
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[[File:PyOx.JPG|thumb|Oxidized [[pyrite]] cubes]] |
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Within the weathering environment chemical [[oxidation]] of a variety of metals occurs. The most commonly observed is the oxidation of Fe<sup>2+</sup> ([[iron]]) and combination with [[oxygen]] and water to form Fe<sup>3+</sup> hydroxides and oxides such as [[goethite]], [[limonite]], and [[hematite]]. This gives the affected rocks a reddish-brown coloration on the surface which crumbles easily and weakens the rock. This process is better known as '[[rusting]]', though it is distinct from the rusting of metallic iron. Many other metallic ores and minerals oxidize and hydrate to produce colored deposits, such as [[chalcopyrite]]s or CuFeS<sub>2</sub> oxidizing to [[copper hydroxide]] and [[iron oxide]]s. |
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===Biological weathering=== |
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A number of plants and animals may create chemical weathering through release of acidic compounds, i.e. the effect of moss growing on roofs is classed as weathering. Mineral weathering can also be initiated and/or accelerated by soil microorganisms. [[Lichens]] on rocks are thought to increase chemical weathering rates. For example, an experimental study on hornblende granite in New Jersey, USA, demonstrated a 3x – 4x increase in weathering rate under lichen covered surfaces compared to recently exposed bare rock surfaces.<ref>{{cite journal|doi=10.1016/j.chemgeo.2011.10.009|title=Effect of lichen colonization on chemical weathering of hornblende granite as estimated by aqueous elemental flux|date=2012|last1=Zambell|first1=C.B.|last2=Adams|first2=J.M.|last3=Gorring|first3=M.L.|last4=Schwartzman|first4=D.W.|journal=Chemical Geology|volume=291|pages=166}}</ref> |
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[[File:lava z14.jpg|thumb|right|Biological weathering of [[lava]] by [[lichen]], [[La Palma]].]] |
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The most common forms of biological weathering are the release of [[chelating]] compounds (i.e. organic acids, siderophores) and of acidifying molecules (i.e. protons, organic acids) by plants so as to break down [[aluminium]] and [[iron]] containing compounds in the soils beneath them. [[Bacterial decay|Decaying]] remains of dead plants in soil may form organic acids which, when dissolved in water, cause chemical weathering.<ref>{{cite book|last=Chapin III|first=F. Stuart|author2=Pamela A. Matson |author3=Harold A. Mooney |title=Principles of terrestrial ecosystem ecology|date=2002|publisher=Springer|location=New York|isbn=9780387954431|pages=54–55|edition=[Nachdr.]}}</ref> Extreme release of chelating compounds can easily affect surrounding rocks and soils, and may lead to [[podsol]]isation of soils.<ref>{{cite book|last=Waugh|first=David|title=Geography : an integrated approach|date=2000|publisher=Nelson Thornes|location=Gloucester, U.K.|isbn=9780174447061|page=272|edition=3rd}}</ref> |
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The symbiotic [[Mycorrhiza|mycorrhizal fungi]] associated with tree root systems can release inorganic nutrients from minerals such as apatite or biotite and transfer these nutrients to the trees, thus contributing to tree nutrition.<ref>{{cite journal|author=Landeweert, R. |author2=Hoffland, E. |author3=Finlay, R.D. |author4=Kuyper, T.W. |author5=van Breemen, N. |pmid=11301154|date=2001|title=Linking plants to rocks: Ectomycorrhizal fungi mobilize nutrients from minerals|volume=16|issue=5|pages=248–254|journal=Trends in ecology & evolution|doi=10.1016/S0169-5347(01)02122-X}}</ref> It was also recently evidenced that bacterial communities can impact mineral stability leading to the release of inorganic nutrients.<ref>{{cite journal|author=Calvaruso, C.|author2=Turpault, M-P.|author3=Frey-Klett, P.|doi=10.1128/AEM.72.2.1258-1266.2006|title=Root-Associated Bacteria Contribute to Mineral Weathering and to Mineral Nutrition in Trees: A Budgeting Analysis|date=2006|journal=Applied and Environmental Microbiology|volume=72|issue=2|pages=1258–66|pmid=16461674|pmc=1392890 }}</ref> To date a large range of bacterial strains or communities from diverse genera have been reported to be able to colonize mineral surfaces and/or to weather minerals, and for some of them a plant growth promoting effect was demonstrated.<ref>{{cite journal|author=Uroz, S.|author2=Calvaruso, C.|author3=Turpault, M-P|author4=Frey-Klett, P.|title=Mineral weathering by bacteria: ecology, actors and mechanisms|journal=Trends Microbiol|date= 2009|volume=17|issue=8|pages=378–87|doi=10.1016/j.tim.2009.05.004|pmid=19660952}}</ref> The demonstrated or hypothesised mechanisms used by bacteria to weather minerals include several oxidoreduction and dissolution reactions as well as the production of weathering agents, such as protons, organic acids and chelating molecules. |
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==Building weathering== |
==Building weathering== |
Revision as of 09:28, 21 September 2015
Physical weathering
Physical weathering, also known as mechanical weathering, is the class of processes that causes the disintegration of rocks without chemical change. The primary process in physical weathering is abrasion (the process by which clasts and other particles are reduced in size). However, chemical and physical weathering often go hand in hand. Physical weathering can occur due to temperature, pressure, frost etc. For example, cracks exploited by physical weathering will increase the surface area exposed to chemical action, thus amplifying the rate of disintegration.
Abrasion by water, ice, and wind processes loaded with sediment can have tremendous cutting power, as is amply demonstrated by the gorges, ravines, and valleys around the world. In glacial areas, huge moving ice masses embedded with soil and rock fragments grind down rocks in their path and carry away large volumes of material. Plant roots sometimes enter cracks in rocks and pry them apart, resulting in some disintegration; Burrowing animals may help disintegrate rock through their physical action. However, such influences are usually of little importance in producing parent material when compared to the drastic physical effects of water, ice, wind, and temperature change. Physical weathering is also called mechanical weathering or disaggregation.
Thermal stress
Thermal stress weathering (sometimes called insolation weathering)[1] results from the expansion and contraction of rock, caused by temperature changes. For example, heating of rocks by sunlight or fires can cause expansion of their constituent minerals. As some minerals expand more than others, temperature changes set up differential stresses that eventually cause the rock to crack apart. Because the outer surface of a rock is often warmer or colder than the more protected inner portions, some rocks may weather by exfoliation – the peeling away of outer layers. This process may be sharply accelerated if ice forms in the surface cracks. When water freezes, it expands with a force of about 1465 Mg/m^2, disintegrating huge rock masses and dislodging mineral grains from smaller fragments.
Thermal stress weathering comprises two main types, thermal shock and thermal fatigue. Thermal stress weathering is an important mechanism in deserts, where there is a large diurnal temperature range, hot in the day and cold at night.[2] The repeated heating and cooling exerts stress on the outer layers of rocks, which can cause their outer layers to peel off in thin sheets. The process of peeling off is also called exfoliation. Although temperature changes are the principal driver, moisture can enhance thermal expansion in rock. Forest fires and range fires are also known to cause significant weathering of rocks and boulders exposed along the ground surface. Intense localized heat can rapidly expand a boulder.
The thermal heat from wildfire can cause significant weathering of rocks and boulders, heat can rapidly expand a boulder and thermal shock can occur. The differential expansion of a thermal gradient can be understood in terms of stress or of strain, equivalently. At some point, this stress can exceed the strength of the material, causing a crack to form. If nothing stops this crack from propagating through the material, it will result in the object's structure to fail.
Frost weathering
Frost weathering, frost wedging, ice wedging or cryofracturing is the collective name for several processes where ice is present. These processes include frost shattering, frost-wedging and freeze-thaw weathering. Severe frost shattering produces huge piles of rock fragments called scree which may be located at the foot of mountain areas or along slopes. Frost weathering is common in mountain areas where the temperature is around the freezing point of water. Certain frost-susceptible soils expand or heave upon freezing as a result of water migrating via capillary action to grow ice lenses near the freezing front.[3] This same phenomenon occurs within pore spaces of rocks. The ice accumulations grow larger as they attract liquid water from the surrounding pores. The ice crystal growth weakens the rocks which, in time, break up.[4] It is caused by the approximately 10% (9.87) expansion of ice when water freezes, which can place considerable stress on anything containing the water as it freezes.
Freeze induced weathering action occurs mainly in environments where there is a lot of moisture, and temperatures frequently fluctuate above and below freezing point, especially in alpine and periglacial areas. An example of rocks susceptible to frost action is chalk, which has many pore spaces for the growth of ice crystals. This process can be seen in Dartmoor where it results in the formation of tors. When water that has entered the joints freezes, the ice formed strains the walls of the joints and causes the joints to deepen and widen. When the ice thaws, water can flow further into the rock. Repeated freeze-thaw cycles weaken the rocks which, over time, break up along the joints into angular pieces. The angular rock fragments gather at the foot of the slope to form a talus slope (or scree slope). The splitting of rocks along the joints into blocks is called block disintegration. The blocks of rocks that are detached are of various shapes depending on rock structure.
Ocean waves
Coastal geography is formed by the weathering of wave actions over geological times or can happen more abruptly through the process of salt weathering.
Pressure release
In pressure release, also known as unloading, overlying materials (not necessarily rocks) are removed (by erosion, or other processes), which causes underlying rocks to expand and fracture parallel to the surface.
Intrusive igneous rocks (e.g. granite) are formed deep beneath the Earth's surface. They are under tremendous pressure because of the overlying rock material. When erosion removes the overlying rock material, these intrusive rocks are exposed and the pressure on them is released. The outer parts of the rocks then tend to expand. The expansion sets up stresses which cause fractures parallel to the rock surface to form. Over time, sheets of rock break away from the exposed rocks along the fractures, a process known as exfoliation. Exfoliation due to pressure release is also known as "sheeting".
Retreat of an overlying glacier can also lead to exfoliation due to pressure release.
Salt-crystal growth
Salt crystallization, otherwise known as haloclasty, causes disintegration of rocks when saline solutions seep into cracks and joints in the rocks and evaporate, leaving salt crystals behind. These salt crystals expand as they are heated up, exerting pressure on the confining rock.
Salt crystallization may also take place when solutions decompose rocks (for example, limestone and chalk) to form salt solutions of sodium sulfate or sodium carbonate, of which the moisture evaporates to form their respective salt crystals.
The salts which have proved most effective in disintegrating rocks are sodium sulfate, magnesium sulfate, and calcium chloride. Some of these salts can expand up to three times or even more.
It is normally associated with arid climates where strong heating causes strong evaporation and therefore salt crystallization. It is also common along coasts. An example of salt weathering can be seen in the honeycombed stones in sea wall. Honeycomb is a type of tafoni, a class of cavernous rock weathering structures, which likely develop in large part by chemical and physical salt weathering processes.
Biological effects on mechanical weathering
Living organisms may contribute to mechanical weathering (as well as chemical weathering, see 'biological' weathering below). Lichens and mosses grow on essentially bare rock surfaces and create a more humid chemical microenvironment. The attachment of these organisms to the rock surface enhances physical as well as chemical breakdown of the surface microlayer of the rock. On a larger scale, seedlings sprouting in a crevice and plant roots exert physical pressure as well as providing a pathway for water and chemical infiltration.
Building weathering
Buildings made of any stone, brick or concrete are susceptible to the same weathering agents as any exposed rock surface. Also statues, monuments and ornamental stonework can be badly damaged by natural weathering processes. This is accelerated in areas severely affected by acid rain.
Properties of well-weathered soils
Three groups of minerals often remain in well-weathered soils: silicate clays, very resistant end products including iron and aluminium oxide clays, and very resistant primary minerals such as quartz. In highly weathered soils of humid tropical and subtropical regions, the oxides of iron and aluminium, and certain silicate clays with low Si/Al ratios, predominate because most other constituents have been broken down and removed.
Gallery
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Weathering effect of acid rain on statues
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Weathering effect on a sandstone statues in Dresden, Germany
See also
References
- ^ Hall, K. The role of thermal stress fatigue in the breakdown of rock in cold regions, Geomorphology, 1999.
- ^ Paradise, T. R. (2005). "Special Paper 390: Stone Decay in the Architectural Environment". 390: 39. doi:10.1130/0-8137-2390-6.39. ISBN 0-8137-2390-6.
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(help) - ^ Taber, Stephen (1930). "The mechanics of frost heaving" (PDF). Journal of Geology. 38 (4): 303–315. Bibcode:1930JG.....38..303T. doi:10.1086/623720Template:Inconsistent citations
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: CS1 maint: postscript (link) - ^ Goudie, A.S.; Viles H. (2008). "5: Weathering Processes and Forms". In Burt T.P.; Chorley R.J.; Brunsden D.; Cox N.J.; Goudie A.S. (eds.). Quaternary and Recent Processes and Forms. Landforms or the Development of Gemorphology. Vol. 4. Geological Society. pp. 129–164. ISBN 1-86239-249-8.
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