GEOG 100--Lecture 15--Weathering and Mass Wasting


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GEOG 100--Lecture 15--Weathering and Mass Wasting

  1. 1. Weathering and MassWeathering and MassWastingWastingChapter 10
  2. 2. 22External vs. Internal ProcessesExternal vs. Internal Processes(the dynamic equilibrium model)(the dynamic equilibrium model)(the dynamic equilibrium model)(the dynamic equilibrium model)
  3. 3. 3DenudationDenudation—Large-scale removal of material—Large-scale removal of materialthat lowers the overall profile of thethat lowers the overall profile of thetopographytopography
  4. 4. 44Denudation ProcessesDenudation Processes• Weathering—Rock disintegrating in place• Mass wasting—Eroded rock moving downslope• Erosion—Extensive removal of rock material,generally transported long distances
  5. 5. 5The Grand CanyonThe Grand Canyon
  6. 6. 66WeatheringWeathering• The combined action of all atmospheric andbiologic processes that cause rock todisintegrate physically and decomposechemically because of exposure near Earth’ssurface– From bedrock to regolith
  7. 7. 7Weathering of BedrockWeathering of Bedrock► Wherever bedrock isWherever bedrock isexposed to the naturalexposed to the naturalelements, it weatherselements, it weathers► Any crack, joint, orAny crack, joint, orcavity in the rock willcavity in the rock willallow weatheringallow weatheringagents to penetrateagents to penetrateand break it apartand break it apart
  8. 8. As rocks weather, surface area increases,offering more surfaces to be weathered…
  9. 9. …producing this result.
  10. 10. 1010Mechanical WeatheringMechanical Weathering• Physical disintegration of rock as a resultof natural phenomena, without a changein its chemical composition– Pounding, pushing, cracking, breaking,wedging apart
  11. 11. Mechanical WeatheringProcesses• Frost wedging• Salt wedging• Unloading/pressure-release jointing• Thermal expansion and contraction• Biologic weathering
  12. 12. Frost Wedging• Repeated growth and melting of ice crystals in pore spaces ofrock fractures or joints• Expanding ice exerts pressure, breaking rocks apart• Most effective where there is repeated freeze and thaw (as inarctic or tundra environments)
  13. 13. Frost Wedging
  14. 14. Salt Wedging• Similar to frost wedging• Growth of salt crystals breaks rocks apart• Most effective in coastal environmentsand semi-arid environmentsHoneycomb (called“tafoni”), Salt Point,Sonoma Coast
  15. 15. 15Unloading or Pressure-release jointing• Rock brought near the surface relievesconfining pressure and allows the rock toexpand slightly, forming cracks
  16. 16. Thermal Expansion andContraction• Heated rock expands slightly, cooled rock contracts• Rapid expansion and contraction of the surface of the rockcauses cracks to form and propagate• Most effective in regions with large differences intemperature between daily highs and nightly lows
  17. 17. Thermal Expansion andContraction
  18. 18. 18Biologic Weathering• Growth of plant roots,burrowing animals loosenand break apart rocks
  19. 19. Chemical Weathering• Rocks forming at depth are stable underthose conditions• Once rocks are exposed to surfaceconditions many of the minerals becomeless stable and may undergo a slowchemical change that weakens theirinternal structures
  20. 20. Chemical Weathering• Decomposition of rock thorough thechemical alteration of its minerals– Air, soil water solutions, and groundwatersolutions contain dissolved oxygen, carbondioxide, or other reactive elements– Water is the greatest agent of chemicalweathering– Chemical weathering is most effective inwarm, moist climates
  21. 21. Chemical Weathering:The Influence of Temperature andPrecipitation
  22. 22. Oxidation• Oxygen dissolved in soil water orground water can bond with thechemical elements of the minerals toform new minerals• Causes expansion and exertspressure that breaks the rocks apart• Example: iron (Fe) turning to rust(Fe2O3) in the presence of oxygen andwater
  23. 23. Hydrolysis and Hydration• Hydrolysis--Water combines with someminerals, breaking down the rocks into newmineral compounds– granite: fedspar turns to clays + quartz sand– contributes to spheriodal weathering
  24. 24. Hydrolysis and Hydration• Hydration--Water molecules become part ofthe chemical composition of the rock (nobreakdown). Mineral alteration and expansionresults in grain-by-grain destruction of rocks.Formation of gypsum from anhydrouscalcium sulfate (the mineral anhydrite)which has absorbed water into itschemical structure
  25. 25. Carbonic AcidLimestone and marble are mostsusceptible to this type of weatheringCarbon dioxide dissolved inwater creates a weak acidcalled carbonic acid which candissolve some minerals,especially calcium carbonate
  26. 26. Acid Precipitation• In urban areas withpollution from sulfur andnitrogen oxides, thesegasses can mix withatmospheric water,forming acid precipitation• Dissolves limestone andmarble and other types ofbuilding stones
  27. 27. Organic Acids• Decaying vegetation mixes w/ water to formsoil water w/ complex organic acids that canreact to dissolve or chemically alterminerals
  28. 28. 2828Mass WastingMass Wasting• Spontaneous downslope movement of soiland eroded rock fragments under theinfluence of gravity, but without the actionof moving air, water or ice
  29. 29. Angle of Repose• The steepest angle that loose fragmentscan lie without movement if undisturbed
  30. 30. 30
  31. 31. Mass Wasting• The type of masswasting eventthat occurs willdepend uponspeed and thedegree ofsaturation
  32. 32. Mass Wasting(another view)
  33. 33. Types of Mass Wasting• Rock fall• Rock slide andTopple• Debris flow• Earth flow• Mudflow• Slump• Solifluction• Creep• Induced mass wasting
  34. 34. Rock FallTalus slopes—Regolith whichhas fallen down steep slopes,funneled into “blankets” of rockcalled talus conesFresh slopes are very unstable
  35. 35. Rock Fall
  36. 36. Rock Slide
  37. 37. Mudflow and Debris Flow• Mudflow—Rainwatermixed with soil flowingvery quickly downslopeas a river of mud• Debris flow—More rockfragment than mudflow,but similar in othercharacteristics
  38. 38. Earthflow• Water-saturated soil orrock material• Moves a limited distancedown slope as one largemass• Generally slower inmotion (over the courseof hours)• Common form of earthmovement causing roadclosures and propertydestruction during heavyrains
  39. 39. Near La Conchita Slide, alongHwy. 101 in Ventura CountyHwy. 101 in Ventura County
  40. 40. La Conchita Slide (Earthflow)Hwy. 101, Ventura County
  41. 41. La Conchita Slide (Earthflow)Hwy. 101, Ventura County
  42. 42. Slump—Slow, concave sliding
  43. 43. Slump
  44. 44. Slump
  45. 45. Solifluction• Continuous freeze andthaw cycles slowly moveweathered particlesdownslope• Over time, the entireslope moves downhill
  46. 46. Solifluction
  47. 47. Solifluction
  48. 48. Solifluction LobesEmphasized by Animal Trails
  49. 49. Soil Creep
  50. 50. Induced Mass WastingMass wasting caused by human activity• Moving weathered rock material downslope duringconstruction on steep hillsides– Carried away as debris flows or mudflows during heavy rains• Removal of material supporting the base of a slope• The wetting of weathered rock material and soil frompipe breakage, lawn watering, etc. causing slippage• Debris removal by heavy rains after fire may alsoremove stabilizing vegetation