Weathering/EPCC/LM5

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Weathering/EPCC/LM5

  1. 1. Chapter 6: WeatheringChapter 6: Weathering
  2. 2. WeatheringWeathering  WeatheringWeathering is the breaking down ofis the breaking down of Earth's rocks, soils and mineralsEarth's rocks, soils and minerals through direct contact with thethrough direct contact with the planet's atmosphere. Weatheringplanet's atmosphere. Weathering occursoccurs in situin situ, or "with no, or "with no movement", and thus should not bemovement", and thus should not be confused withconfused with erosionerosion, which, which involves the movement of rocks andinvolves the movement of rocks and minerals by agents such as water,minerals by agents such as water, ice, wind, and gravity.ice, wind, and gravity.
  3. 3. Introduction: Weathering—TheIntroduction: Weathering—The Breakdown of RockBreakdown of Rock  At the Earth’s surface, rocks areAt the Earth’s surface, rocks are exposed to the effects of weathering:exposed to the effects of weathering: the chemical alteration andthe chemical alteration and mechanical breakdown of rock, whenmechanical breakdown of rock, when exposed to air, moisture, and organicexposed to air, moisture, and organic matter.matter.  Weathering is an integral part of theWeathering is an integral part of the rock cycle.rock cycle.  There are two types of weathering:There are two types of weathering: Physical and ChemicalPhysical and Chemical
  4. 4. Figure 6.1
  5. 5. Physical WeatheringPhysical Weathering
  6. 6. Figure 6.5
  7. 7. JointsJoints  Joints occur as a widespread set orJoints occur as a widespread set or sets of parallel fractures.sets of parallel fractures.  When dikes, sills, lava flows, andWhen dikes, sills, lava flows, and welded tuffs cool they contract andwelded tuffs cool they contract and form columnar joints (joints that splitform columnar joints (joints that split igneous rocks into long prisms origneous rocks into long prisms or columns).columns).
  8. 8. Crystal GrowthCrystal Growth  Water moving slowly throughWater moving slowly through fractured rocks contains ions, whichfractured rocks contains ions, which may precipitate out of solution tomay precipitate out of solution to form salts.form salts.  The force exerted by salt crystalsThe force exerted by salt crystals growing can be very large and cangrowing can be very large and can result in the rupture orresult in the rupture or disaggregation of rocks.disaggregation of rocks.
  9. 9. Frost WedgingFrost Wedging  Wherever temperatures fluctuate about theWherever temperatures fluctuate about the freezing point, water in the groundfreezing point, water in the ground periodically freezes and thaws.periodically freezes and thaws.  This repeated freezing/thawing breaks theThis repeated freezing/thawing breaks the rockrock
  10. 10. Thermal Expansion and CoolingThermal Expansion and Cooling  Surface temperatures as high as 80Surface temperatures as high as 80oo C haveC have been measured on exposed desert rocks.been measured on exposed desert rocks.  Daily temperature variations of more thanDaily temperature variations of more than 4040oo have been recorded on rock surfaceshave been recorded on rock surfaces  Continued expansion/contraction causes theContinued expansion/contraction causes the rock to breakrock to break
  11. 11. Wedging by RootsWedging by Roots  When plants grow they extend theirWhen plants grow they extend their roots into the cracks in rock, whereroots into the cracks in rock, where their growth can force the rocktheir growth can force the rock apart.apart.
  12. 12. Organic ActivityOrganic Activity  You and meYou and me  Animals burrowAnimals burrow  PlantsPlants
  13. 13. Pressure ReleasePressure Release  Overlying materials (not necessarilyOverlying materials (not necessarily rocks) are removed (by erosion, orrocks) are removed (by erosion, or other processes), which causesother processes), which causes underlying rocks to expand andunderlying rocks to expand and fracture parallel to the surface.fracture parallel to the surface.  Example, a moving glacier.Example, a moving glacier.
  14. 14. Figure 6.11
  15. 15. Pressure ReleasePressure Release  Intrusive igneous rocks are formed deep beneathIntrusive igneous rocks are formed deep beneath the Earth's surface. They are under tremendousthe Earth's surface. They are under tremendous pressure because of the overlying rock material.pressure because of the overlying rock material. When erosion removes the overlying rockWhen erosion removes the overlying rock material, these intrusive rocks are exposed andmaterial, these intrusive rocks are exposed and the pressure on them is released. The outer partsthe pressure on them is released. The outer parts of the rocks then tend to expand. The expansionof the rocks then tend to expand. The expansion sets up stresses which cause fractures parallel tosets up stresses which cause fractures parallel to the rock surface to form. Over time, sheets ofthe rock surface to form. Over time, sheets of rock break away from the exposed rocks alongrock break away from the exposed rocks along the fracturesthe fractures
  16. 16. Chemical WeatheringChemical Weathering
  17. 17. Chemical WeatheringChemical Weathering  In chemical weathering, chemicalIn chemical weathering, chemical reactions transform rocks andreactions transform rocks and minerals into new chemicalminerals into new chemical combinations.combinations.  There are four different chemicalThere are four different chemical pathways by which chemicalpathways by which chemical weathering proceeds:weathering proceeds: – Dissolution.Dissolution. – Hydrolysis.Hydrolysis. – Oxidation.Oxidation. – CarbonationCarbonation
  18. 18. DissolutionDissolution  The easiest reaction pathway toThe easiest reaction pathway to comprehend is dissolution; thiscomprehend is dissolution; this means that chemicals in rocks aremeans that chemicals in rocks are dissolved in water.dissolved in water.  Halite (NaCI) is a mineral that can beHalite (NaCI) is a mineral that can be removed completely from a rock byremoved completely from a rock by dissolution.dissolution.
  19. 19. HydrolysisHydrolysis  Any reaction involving water thatAny reaction involving water that leads to the decomposition of aleads to the decomposition of a compound is a hydrolysis reaction.compound is a hydrolysis reaction. – Potassium feldspar, for instance,Potassium feldspar, for instance, decomposes in the clay mineraldecomposes in the clay mineral kaolinite.kaolinite.  Hydrolysis is one of the chiefHydrolysis is one of the chief processes involved in the chemicalprocesses involved in the chemical breakdown of common rocks.breakdown of common rocks.
  20. 20. OxidationOxidation  Oxidation is a process by which anOxidation is a process by which an ion loses an electron.ion loses an electron. – The oxidation state of the ion is said toThe oxidation state of the ion is said to increase.increase. – Most often Fe and OMost often Fe and O – Better known as “rusting”Better known as “rusting” – Examples are limonite and hematiteExamples are limonite and hematite
  21. 21. CarbonationCarbonation  Carbonation takes place when carbonCarbonation takes place when carbon dioxide reacts with certain types ofdioxide reacts with certain types of rocks forming a solution, that canrocks forming a solution, that can easily be carried away by watereasily be carried away by water  Example: Limestone and waterExample: Limestone and water
  22. 22. Biological ProcessesBiological Processes  Plants and animals may createPlants and animals may create chemical weathering through releasechemical weathering through release of acidic compounds, i.e. moss onof acidic compounds, i.e. moss on roofs is classed as weathering.roofs is classed as weathering.
  23. 23. Surface AreaSurface Area  The effectiveness of chemicalThe effectiveness of chemical weathering increases as the surfaceweathering increases as the surface area exposed to weatheringarea exposed to weathering increases.increases.  Surface area increases simply fromSurface area increases simply from the subdivision of large blocks intothe subdivision of large blocks into smaller blocks.smaller blocks.  Chemical weathering therefore leadsChemical weathering therefore leads to a dramatic increase in the surfaceto a dramatic increase in the surface area.area.
  24. 24. Figure 6.12
  25. 25. Factors Influencing WeatheringFactors Influencing Weathering  Mineralogy.Mineralogy. Bowen’s Reaction Series upside down!Bowen’s Reaction Series upside down!
  26. 26. Factors Influencing WeatheringFactors Influencing Weathering – Most stable chemical compositions:Most stable chemical compositions: Quartz.Quartz. Muscovite.Muscovite. Potassium feldspar.Potassium feldspar. Biotite.Biotite. Sodium feldspar (albite-rich plagioclase).Sodium feldspar (albite-rich plagioclase). Amphibole.Amphibole.
  27. 27. Factors Influencing WeatheringFactors Influencing Weathering – Least stable chemical compositions:Least stable chemical compositions: Pyroxene.Pyroxene. Calcium feldspar (anorthite-richCalcium feldspar (anorthite-rich plagioclase).plagioclase). Olivine.Olivine. Calcite.Calcite.  Rock type and structure.Rock type and structure. – Differences in the composition andDifferences in the composition and structure of adjacent rock units can leadstructure of adjacent rock units can lead to contrasting rates of weathering andto contrasting rates of weathering and to landscapes that reflect suchto landscapes that reflect such differential weathering.differential weathering.
  28. 28. Factors Influencing WeatheringFactors Influencing Weathering  Climate.Climate. – Moisture and heat promote chemicalMoisture and heat promote chemical reactions.reactions. Therefore, weathering is more intense andTherefore, weathering is more intense and generally extends to greater depths in agenerally extends to greater depths in a warm, moist climate than in a cold, dry one.warm, moist climate than in a cold, dry one. In moist tropical lands, like Central AmericaIn moist tropical lands, like Central America and Southeast Asia, obvious effects ofand Southeast Asia, obvious effects of chemical weathering can be seen at depthschemical weathering can be seen at depths of 100 m or more.of 100 m or more.

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