Ch. 11 - Glaciers

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Ch. 11 - Glaciers

  1. 1. Glaciers Ch. 11
  2. 2. Glaciers Thick mass of ice - accumulates on land - flows downhill - caused by large snowfalls in winter and ____________
  3. 3. Types of Glaciers 1) Valley (alpine) - smallest glaciers - occupy valleys formerly carved by streams
  4. 4. Types of Glaciers 2) Piedmont “pied” = French for “foot” - valley glaciers coalesce at base of steep mountain front
  5. 5. Types of Glaciers 3) Ice caps - cover upland & plateau areas - not confined to valleys - smaller in extent than ice sheets Ex: Iceland
  6. 6. Types of Glaciers 4) Ice Sheets - largest glaciers - cover continental regions - today: Greenland & Antarctica
  7. 7. Ice sheets - several 1000’s feet thick - flow outward from central high spot - covers all but highest elevations
  8. 8. Ice shelves Occur where ice sheets flow into oceans - large flat masses of ice attached to land by one or more sides
  9. 9. Glaciers: Sediment Transport: High capacity – High competency –
  10. 10. Erosion Methods 1) Plucking – meltwater gets into cracks in bedrock & freezes - pieces incorporated into base of ice (worn flat on one side)
  11. 11. Striations Scratches in exposed bedrock - indicate direction of glacial movement
  12. 12. Erosion Methods 2) Abrasion - weight of moving rx & ice scrapes bedrock - occurs at point of contact
  13. 13. Erosional Landscapes Valley glaciers produce jagged mountain scenery Ex: Rocky Mountains Ice sheets smooth out landscape Ex: Peoria area
  14. 14. Erosional Landscapes 1) Glacial trough (U-shaped cross- section) Ice widens, deepens, & straightens former stream valleys
  15. 15. Erosional Landscapes 2) Hanging valley - smaller valley from a tributary glacier is left higher than main valley
  16. 16. Erosional Landscapes 3) Cirque (“circle”) - occurs at head of glacial trough - steep walls around 3 sides but opens on 4th side
  17. 17. Erosional Landscapes 4) Horn Sharp pyramid-shaped peak
  18. 18. Erosional Landscapes 5) Arete Knife-like ridge separating adjacent glacial troughs
  19. 19. Erosional Landscapes 6) Tarn – lake occupying a cirque after glaciers melt 7) Paternoster lakes - series of lakes that sit in glacial trough
  20. 20. Erosional Landscapes 8) Fiord Drowned glacial trough after last Ice Age passed Occur where mtns. are adjacent to oceans at high latitudes
  21. 21. Depositional Landscapes Glacial drift = all sediment deposited by glaciers Two types of drift: a) Till = sediment deposited directly by glaciers (unsorted)
  22. 22. Two types of drift: b) Stratified drift = sediments deposited by glacial meltwater - sorted by size
  23. 23. Landforms composed of till 1) Erratic - boulders that differ from underlying bedrock - source area is outside region where they’re found
  24. 24. Landforms composed of till 2) Moraine – ridge of till Several types: a) Lateral moraine – found only w/ valley glaciers - parallels sides of valley
  25. 25. Lateral Moraine Material comes from: a) ice scraping valley walls b) rx from cliffs above
  26. 26. Types of Moraines b) Medial moraine - lateral moraines from joining glaciers merge
  27. 27. Types of Moraines c) End moraine - associated w/ stationary glaciers
  28. 28. Types of Moraines d) Recessional moraines - end moraines created as a receding glacier occasionally stabilized
  29. 29. Types of Moraines e) Terminal moraine - outermost end moraine that marks limit of glacial advance
  30. 30. Types of Moraines f) Ground moraine - associated w/ receding glaciers - forms behind end moraines - acts to level the land
  31. 31. Landforms composed of stratified drift Deposited by glacier’s melt water - flows through moraines & picks up sediment - braided stream channels common
  32. 32. Landforms composed of stratified drift 1) Outwash plains - broad ramp-like surface built in front of end moraines - associated w/ ice sheets (called “valley trains” w/valley glaciers)
  33. 33. Landforms composed of stratified drift 2) Kettle/kettle lakes - large blocks of stagnant ice buried in sediment - ice melts - creates a depression
  34. 34. Depositional Landforms Loess - windblown silt deposits
  35. 35. Loess - associated with meltwater drainages from glaciation - thickest near drainages and thins away from valleys
  36. 36. Movement of a glacier Two methods: 1) Plastic flow = internal flow - occurs in ice below 50 meters - occurs fastest in center of glacier (less friction)
  37. 37. Zone of Fracture Upper 50 meters of glacier - brittle - breaks into cracks called “crevasses”
  38. 38. Movement of a glacier 2) Basal slip Melt water at base of glacier - entire ice mass slips over surface
  39. 39. Glacial Budgets Zone of Accumulation = area where snow accumulates & ice forms Zone of Wastage = net loss as glacier advances into warmer climates (lower elevations)
  40. 40. Zone of Wastage Loss of snow & ice is called ablation Ablation is due to: - melting - calving
  41. 41. Glacial Budgets Snowline = boundary between zone of accumulation & zone of wastage
  42. 42. Glacial Budgets 1) Advancing glacier – more accumulation than ablation - glacial front advances - snowline drops in elevation
  43. 43. Glacial Budgets 2) Retreating glacier - ablation exceeds accumulation - snowline rises in elevation - caused by warming trend or decrease in snowfall
  44. 44. Glacial Budgets 3) Stationary Budget - accumulation equals ablation - total area of glacier is not changing NOTE: Ice is always moving downhill!
  45. 45. Evidence for past glacial periods (“Ice Ages”) Periodically, northern Europe & N. America covered by great ice sheets Last one melted from Canada <10,000 years ago
  46. 46. Evidence for past glacial periods (“Ice Ages”) 1) Glacial erosion Ice sheets: polished bedrock in northern regions, striations, recessional moraines Ex: Central Park, New York City
  47. 47. Evidence for past glacial periods (“Ice Ages”) Valley glaciers: - Yosemite National Park - Yellowstone National Park
  48. 48. Yosemite
  49. 49. Evidence for past glacial periods (“Ice Ages”) 2) Pluvial lakes - form during cooler times w/ moderate rainfall - occurred at lower latitudes where ice did not advance
  50. 50. Pluvial lakes Most evident in Basin & Range region in western U.S. Ex: Lake Manley, Death Valley Ex: Lake Bonneville, Utah (Great Salt Lake is a remnant)
  51. 51. Evidence for past glacial periods (“Ice Ages”) 3) Decreasing sea level Water is locked onto land by ice Estimated maximum 100 meters lower than present sea level
  52. 52. Decreasing sea level Evidence: Submerged stream channels on continental shelves
  53. 53. Evidence for past glacial periods (“Ice Ages”) 4) Crustal Rebound Land readjusts upward after ice sheets melt Hudson Bay region has uplifted 300 meters since end of last ice age
  54. 54. Crustal rebound
  55. 55. Evidence for past glacial periods (“Ice Ages”) 5) Lake Missoula Ice dam blocked melt water - formed huge lake that flooded western Montana (half the size of Lake Michigan)
  56. 56. Lake Missoula Map
  57. 57. Lake Missoula Melting weakened ice dam - lake emptied in 1-2 days - discharge ~ 386 million cfs (Amazon discharge = 6 million cfs)
  58. 58. Lake Missoula Evidence: a) channeled scablands in western Oregon & southern Idaho b) giant ripples of coarse gravel 30’ high, 300’ apart, 2 miles long

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