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  1. 1. Chapter 19: GlacialModification of TerrainMcKnight’s Physical Geography:A Landscape Appreciation,Tenth Edition, Hess
  2. 2. Glacial Modification of Terrain• The Impact of Glaciers on the Landscape• Glaciations of the Past and Present• Types of Glaciers• Glacier Movement and Formation• The Effects of Glaciers• Continental Ice Sheets• Mountain Glaciers• The Periglacial Environment• Causes of the Pleistocene2© 2011 Pearson Education, Inc.
  3. 3. The Impact of Glaciers on theLandscape• Snowpack over years turns to ice• Ice mass motion under gravity grinds anything in itspath• Glaciation increases erosion rate on a mountain by atleast 10 times to an unglaciated mountain• Modifies flat landscapes as well3© 2011 Pearson Education, Inc.
  4. 4. Types of Glaciers• Continental ice sheets– Exist in nonmountainous areas– Antarctica and Greenland onlytwo– Outlet glaciers• Mountain glaciers– Highland icefields: ice sheetsthat submerge most underlyingtopography; valley and piedmontglaciers– Alpine glaciers: developindividually instead of part of icefield, cirque glaciers4© 2011 Pearson Education, Inc.Figure 19-2
  5. 5. Glaciations Past and Present• Glacial ice volume has varied considerably over last fewmillion years• Evidence left behind allows scientists to determine thechronology of past glaciations• Pleistocene glaciation– Began at least 2.59 million years ago– Last major ice retreat occurred only 9000 years ago– Dominant environmental characteristic was refrigeration ofhigh-latitude and high-elevation areas– Consistent alterations of glacial and interglacial periods– Wisconsin glacial stage marked end– At peak, 1/3 of total land covered in ice5© 2011 Pearson Education, Inc.
  6. 6. Glaciations Past and Present• Pleistocene glaciation (cont.)– Laurentide ice and the Driftless Area• Indirect effects of Pleistocene glaciation– Periglacial processes• Periglacial zone—zone where ice never existed but glacial factorsaffected the landscape such as erosion from ice melt, solifluction• Sea-level changes—buildup of ice on continents led to less drainwater oncontinents and brought about a lowering of sea levels• Crustal depression—the weight of the ice on the continents causedcontinents to sink, ice melt allowed for continental rebound• Pluvial developments—considerable runoff results in increased moisture,leading to increased precipitation and less evaporation. Developed manylakes, including the Great Salt Lake (formed from Lake Bonneville)6© 2011 Pearson Education, Inc.
  7. 7. Glaciations Past and Present• Maximum extent of the Pleistocene glaciation7© 2011 Pearson Education, Inc.Figure 19-5a
  8. 8. Glaciations Past and Present• Contemporary glaciation– Limited ice cover today (about10% of total land surface)– 96% of the total ice cover isGreenland and Antarctica– Antarctic ice cap• Consists of two unequal sectionsseparated by Transantarcticmountains• West Antarctica has a few “dryvalleys”– Greenland ice cap– North American glaciers8© 2011 Pearson Education, Inc.Figure 19-8
  9. 9. Glaciations Past and Present• Climate change related tocontemporary glaciation– Retreating of polar ice caps– Shrinking ice caps anindicator of a warmingclimate– Antarctic ice shelvesbreaking– Higher flow rates of outletglaciers9© 2011 Pearson Education, Inc.Figure 19-10
  10. 10. Glacier Formation and Movement• Require balance betweenaccumulation and ablation• Snow begins as crystallizedwater vapor• Compressed to granular form• More compression causesgranules to coalesce, névé/firn• Further compression results inglacial ice• Ablation and accumulationzones10© 2011 Pearson Education, Inc.Figure 19-11
  11. 11. Glacier Formation and Movement• Glacier “flow” is orderly slidingof ice molecules• Ice under extreme pressuredeforms instead of slipping• Meltwater contributes surfacefor glacier to slide on• Flow in response to overlyingweight• Plastic flow and basal slip• Glacier flow versus glacieradvance11© 2011 Pearson Education, Inc.Figure 19-12
  12. 12. The Effects of Glaciers• Erosion by glaciers– Volume and speed determinesuccess of glacial erosion– Erosive power of moving iceslightly larger than that of water– Glacial plucking—picking up ofrock material through refreezingof meltwater– Glacial abrasion—bedrockworn down by rock debrisembedded in glacier– Subglacial meltwater erosion12© 2011 Pearson Education, Inc.Figure 19-15
  13. 13. The Effects of Glaciers• Transportation by glaciers– Glaciers effective to movelarge rock pieces– Typically move glacial flour– Most rock materialtransported along baseof the ice– Remaining glacial ice freeof rock debris– Role of flowing water onmoving ice, melt streams– Cracks in ice in whichstreams run—moulins13© 2011 Pearson Education, Inc.Figure 19-16
  14. 14. The Effects of Glaciers• Deposition by glaciers– Glaciers move lithosphericmaterial from one region toanother in a vastly differentform– Material moved by glaciers—drift– Till—rock debris depositedby moving or melting ice– Large boulders that aredifferent from surroundinglocal bedrock, glacial erratics14© 2011 Pearson Education, Inc.Figure 19-18
  15. 15. The Effects of Glaciers• Deposition of meltwater– Large portion of debris carriedby glaciers deposited orredeposited by meltwater– Subglacial streams fromglaciers carry sedimentarymaterial– Glaciofluvial deposition15© 2011 Pearson Education, Inc.Figure 19-17
  16. 16. Continental Ice Sheets• Ice sheets third mostextensive feature on theplanet• Development and flow ofice sheets– Pleistocene ice sheetsoriginated in midlatitudesand subpolar regions– Ice flowed outward fromcenter of accumulation– Ice sheets ebbed and flowedwith changing climate16© 2011 Pearson Education, Inc.Figure 19-19
  17. 17. Continental Ice Sheets• Erosion by ice sheets– Principal topography from ice sheet is gently undulatingsurface– Valley bottoms created from moving ice– Roche mountonnée, stoss side versus lee side– Postglacial landscape has low relief but is not absolutely flat17© 2011 Pearson Education, Inc.Figure 19-20
  18. 18. Continental Ice Sheets• Deposition by Ice Sheets– Irregular, uneven surface ofdeposition, till plain– Moraines—land consistingprimarily of till– Three types of moraines• Terminal moraine—marksoutermost limit of glacial advance• Recessional moraine—positionswhere ice front is stabilized• Ground moraine—large quantitiesof till laid down from under aglacier instead of from its edge,kettles– Drumlins 18© 2011 Pearson Education, Inc.Figure 19-21Figure 19-24
  19. 19. Continental Ice Sheets• Glaciofluvial features– Deposition of debris by ice-sheetmeltwater produces features,composed of stratified drift– Composed of gravel, sand, siltsince meltwater is incapable ofmoving larger material– Outwash plains– Valley trains– Eskers– Kames– Lakes very common19© 2011 Pearson Education, Inc.Figure 19-26
  20. 20. Mountain Glaciers• Mountain glacierdevelopment and flow– Usually form in sheltereddepressions near heads ofstream valleys• Erosion by mountain glaciers– Basic landform in glaciatedmountains is the cirque– Marks the location where analpine glacier originated– Shifting equilibrium linegenerate quarrying action,bergschrund formation20© 2011 Pearson Education, Inc.Figure 19-29
  21. 21. Mountain Glaciers• Erosion by mountain glaciers(cont.)– Quarried fragments fromcirque carried away when iceflows out of cirque– Cirque ice melts away,depression that holds wateris a tarn– Several cirques cut back intointerfluve result in spine ofrock, an arête– Cols and horns21© 2011 Pearson Education, Inc.Figure 19-31
  22. 22. Mountain Glaciers• Erosion in the valleys– Some glaciers never leavecirques– Principle erosive work is todeepen, steepen, and widenvalley– U-shaped glacial troughs– Glacial steps result fromdifferences in rock resistance– Small cliffs and small lakes,paternoster lakes– Hanging valleys22© 2011 Pearson Education, Inc.Figure 19-38
  23. 23. Mountain Glaciers• Deposition by mountainglaciers– Continental ice sheetsmore responsible fordeposition than mountainglaciation– Moraines primarydeposition mechanism– Lateral moraines– Medial moraines23© 2011 Pearson Education, Inc.Figure 19-41
  24. 24. Mountain Glaciers• Distribution of moraines around a valley glacier24© 2011 Pearson Education, Inc.Figure 19-42
  25. 25. The Periglacial Environment• Periglacial—on theperimeter of glaciation• Permafrost presence• Frozen ground exists inAlaska, Canada, Russia• Extends to great depths• Patterned ground• Proglacial lakes25© 2011 Pearson Education, Inc.Figure 19-45
  26. 26. Causes of the PleistoceneGlaciations• What initiates ice ages?• Any plausible theory mustaccount for four maincharacteristics– Ice accumulation is in bothhemispheres but is non-uniform– Concurrent development ofpluvial conditions in drylandareas– Multiple ice advance andretreat cycles– Eventual total deglaciation26© 2011 Pearson Education, Inc.Figure 19-46
  27. 27. Causes of the PleistoceneGlaciations• Cold versus warm climate for glaciation• Role of Milankovitch cycles• Variations in solar output• Variations in carbon dioxide in atmosphere• Changes in continental positions• Atmospheric circulations• Tectonic upheaval• Are we still in an ice age?27© 2011 Pearson Education, Inc.Figure 19-47
  28. 28. Summary• Glaciers impact the landscape through ice mass motionand associated erosion• There are two primary well known eras for glaciation, thePleistocene and contemporary glaciation• During the Pleistocene, ice occupied a third of the totalland mass of the Earth• There were four indirect effects of the Pleistoceneglaciation• Antarctica and Greenland make up a large percentageof the contemporary glaciation28© 2011 Pearson Education, Inc.
  29. 29. Summary• There are two primary types of glaciers, continental icesheets and mountain glaciers• Glacier formation involves the process of convertingsnow to ice through intense pressure and snowaccumulation• Glaciers move via sliding along a land surface;meltwater helps enhance the ability of glaciers to move• Glaciers have two primary erosive effects• Glaciers are capable of transporting large rock materialas well as glacial flour29© 2011 Pearson Education, Inc.
  30. 30. Summary• Glaciers deposit material through their transport as wellas meltwater• Continental ice sheets have a unique set of erosive anddepositional characteristics• Moraines are glacier-deposited landforms that consistentirely or largely of till• Glaciofluvial features play an important role in thedistribution of deposited glacier material• Mountain glaciers have limited erosive and depositionalcharacteristics30© 2011 Pearson Education, Inc.
  31. 31. Summary• Valley effects of mountain glaciers can drastically alterthe landscape in these regions• The region surrounding a glacier that is modified by theglacier but not under it is called the periglacial• There are numerous unique characteristics of theperiglacial environment• The exact causes of ice ages, including the Pleistocene,are unknown, though many theories hypothesize aboutthe different effects that could have contributed• It is unknown if we are still in an ice age31© 2011 Pearson Education, Inc.