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  • 1. Chapter 15: Preliminaries toErosion: Weathering andMass WeatheringMcKnight’s Physical Geography:A Landscape Appreciation,Tenth Edition, Hess
  • 2. Preliminaries to Erosion:Weathering and Mass Weathering• Denudation• The Impact of Weathering and Mass Wastingon the Landscape• Weathering and Rock Openings• Weathering Agents• Mass Wasting2© 2011 Pearson Education, Inc.
  • 3. Denudation• Overall effect ofdisintegration, wearingaway, and removal ofrock material• Three types of activities:– Weathering– Mass wasting– Erosion3© 2011 Pearson Education, Inc.Figure 15-1
  • 4. The Impact of Weathering andMass Wasting on the Landscape• Fragmentation of bedrock• Mass wasting can resultin open scar onlandscape; accumulationof debris• Grand Canyon is anexample4© 2011 Pearson Education, Inc.Figure 15-2
  • 5. Weathering and Rock Openings• Weathering destroysbedrock and fragments itinto smaller components• Any exposed bedrock isweathered• Openings in bedrocksurface allow weatheringto transfer deeper• Openings typicallymicroscopic5© 2011 Pearson Education, Inc.Figure 15-3
  • 6. Weathering and Rock Openings• Five types of openings– Microscopic: numerous, occur inspaces between rock crystals– Joints: cracks resulting fromstress that do not showappreciable displacement– Faults: breaks in bedrock withdisplacement– Lava vesicles: gas openings incooled lava– Solution cavities: holes createdby percolating water6© 2011 Pearson Education, Inc.Figure 15-4
  • 7. Weathering and Rock Openings• The importance of jointing– Almost all lithosphericbedrock is jointed– Block separation evidentsince weathering emphasizesfracture– Joint systems– Large joints that extendthrough large distances andthicknesses are called masterjoints7© 2011 Pearson Education, Inc.Figure 15-6
  • 8. Weathering Agents• Most are atmospheric• Oxygen, carbon dioxide, and water most important• Temperature changes• Water penetration in bedrock openings• Biotic agents– Burrowing and/or plant roots– Chemical alteration• Three principal categories– Mechanical– Chemical– Biotic8© 2011 Pearson Education, Inc.
  • 9. Weathering Agents• Mechanical Weathering– Physical disintegration ofrock without changes to itschemical composition• Frost wedging– Freeze-thaw action of water– Ice wedges downward inopenings– Ice melts and water fallsfarther into larger opening– Process repeats– Frost shattering9© 2011 Pearson Education, Inc.Figure 15-7
  • 10. Weathering Agents• Salt wedging– Salt left behind fromevaporated water collectsand pries apart rockopenings• Temperature changes– Diurnal and seasonaltemperature fluctuationsmodify volumes slightly– Fracturing of rock over longtime scales10© 2011 Pearson Education, Inc.Figure 15-8
  • 11. Weathering Agents• Exfoliation– Curved layers peel off ofbedrock– Exfoliation dome– Unloading through erosion– Hydration• Other mechanicalweathering processes– Chemical and biotic impactson mechanical weathering11© 2011 Pearson Education, Inc.Figure 15-14
  • 12. Weathering Agents• Chemical Weathering– Decomposition of rock materialthrough chemical alteration ofminerals– Greater surface area isdecomposed faster– Moisture required for mostprocesses• Oxidation– Oxygen combined with metallicelements in minerals to form newproducts– Iron oxide: rusting12© 2011 Pearson Education, Inc.Figure 15-15
  • 13. Weathering Agents• Hydrolysis– Union of water and another substance to produce a newsubstance– Igneous rock is particularly susceptible• Carbonation– Reaction between carbon dioxide and carbonate rocks• Less common processes exist as well• Chemically weathered rocks are less coherent andhave loose particles13© 2011 Pearson Education, Inc.
  • 14. Weathering Agents• Biological weathering– Plants and animals alter rockstructure– Impacts of lichens– Burrowing animals• Climate and weathering– High temperatures and abundantprecipitation increase chemicalweathering14© 2011 Pearson Education, Inc.Figure 15-17
  • 15. Mass Wasting• Process by which weatheredmaterial is moved shortdistances by gravity• Factors influencing masswasting– Angle of repose– Impact of water on lubrication ofrock material– Clay-water mixture very slick andmobile substance– Quick clays– Subarctic mass wasting15© 2011 Pearson Education, Inc.Figure 15-19
  • 16. Mass Wasting• Fall– Rockfall– Talus/Scree– Uniform accumulation of rockfallmaterial: talus apron– Material tends to collect in coneshaped heaps: talus cones– Talus cones grow up themountain– Slow talus flow in glaciers: rockglaciers16© 2011 Pearson Education, Inc.Figure 15-20
  • 17. Mass Wasting• Slide– Landslide, instantaneous massslope collapse with no fluid flowlubrication– Initiated from added weight fromrainfall or earthquakes– Rock avalanches– Lost material leaves land scar– Damming of valley streams– Rotation of sliding material:slump17© 2011 Pearson Education, Inc.Figure 15-23
  • 18. Mass Wasting• Flow– Wasting initiated or enhanced byaddition of water– Water primary force; clay canenhance motion as well– Earthflows: water saturated landmoved downhill– Mudflows: originate in aridbasins; muddy downslope flowsthat can accumulate large rockmaterial– Debris flows18© 2011 Pearson Education, Inc.Figure 15-25
  • 19. Mass Wasting• Creep– Unobtrusive downslope flowof soil and regolith– Freeze/thaw and wet/dryeffects on creep– Burrowing animals andplant root effects– Principle variables areslope angle, vegetativecover, and moisture supply– Terracettes– Solifluction: soil flowage19© 2011 Pearson Education, Inc.Figure 15-30
  • 20. Summary• Denudation is the overall process of rock weathering,disintegration, and mass wasting• Weathering and mass wasting will drastically alterlandscapes• Weathering breaks rocks into smaller fragments• Any exposed rock is weathered• There are five primary types of openings that areinvolved in weathering• There are numerous weathering agents, but they areclassified into three primary categories20© 2011 Pearson Education, Inc.
  • 21. Summary• Mass wasting is the process by which weatheredmaterial is moved by gravity• There are numerous factors which influence themagnitude of mass wasting that takes place• Falls involve the downward motion of rock materialwith no added water• Slides are instantaneous mass slope collapses, suchas landslides• Flows involve the displacement of weathered materialby water—earthflows and mudflows21© 2011 Pearson Education, Inc.
  • 22. Summary• Creep is a slow-moving displacement of weatheredmaterial22© 2011 Pearson Education, Inc.