1. Understanding Earth Chapter 21: GLACIERS The Work of Ice
Grotzinger • Jordan
Understanding Earth
Sixth Edition
Chapter 21:
GLACIERS
The Work of Ice
© 2011 by W. H. Freeman and Company

2. Chapter 21
Glaciers:
The Work of Ice

3. About Glaciers and the Work of Ice
Glacial erosion creates an enormous amount of sedimentary debris.
Ice covers only 10% of Earth now, but 21,000 years ago, the coverage was 30%.
The landscapes of vast areas of many continents have been sculpted by the flow of ice in glaciers.

4. Lecture Outline Ice as a rock 2. How glaciers form
3. How glaciers move
4. Glacial landscapes
5. Glacial cycles and climate change

5. ● Could be viewed as any kind of rock ● igneous – formed from a liquid
1. Ice as a Rock
● Could be viewed as any kind of
rock
● igneous – formed from a liquid
● sedimentary – formed in
layers
● metamorphic – transformed by
pressure and recrystallization

6. Ice as a Rock: Ice Under Microscope

7. ● Formation of glacial ice ● falls as snow and ice
1. Ice as a Rock
● Formation of glacial ice
● falls as snow and ice
● recrystallizes over time to ice
● with pressure of overlying ice,
ice under pressure starts to
flow like a very viscous fluid

8. ● Peculiar properties of the rock ice
1. Ice as a Rock
● Peculiar properties of the rock ice
● freezes at very low temperature
compared to other rocks
● ice is less dense than the liquid
it comes from (water)
● ice is much weaker than most
other rocks

9. ● valley glaciers (alpine glaciers)
1. Ice as a Rock
● Types of glaciers
● valley glaciers (alpine glaciers)
● continental glaciers (ice sheets)
● ice shelves and ice caps

10. Ice as a Rock: A Valley Glacier
near Juneau, Alaska

11. Ice as a Rock:
The Ice Cap
of Greenland

12. Ice as a Rock:
The Ice Cap of
Antarctica

13. ● Basic ingredients: cold and snow ● accumulation and ablation
2. How Glaciers Form
● Basic ingredients: cold and snow
● accumulation and ablation
● Glacial movement (glacial budget)
● advance or recession

14. How Glaciers
Form:
The Evolution
of Snow into
Glacial Ice

15. ● What causes glaciers to advance ● accumulation > ablation
2. How Glaciers Form
● What causes glaciers to advance
● accumulation > ablation
● What causes glaciers to recede
● ablation > accumulation

16. How Glaciers
Form:
The
Glacial
Budget

17. How Glaciers Form: Glacial Recession
Qori Kalis Glacier, Peru, 2004
Qori Kalis Glacier, Peru, 1978

18. ● Factors causing accumulation ● more snow falls than melts
2. How Glaciers Form
● Factors causing accumulation
● more snow falls than melts
● Factors causing ablation
● melting, iceberg calving,
sublimation, and wind erosion

19. How Glaciers Form: Iceberg Calving
St. Elias National Park, Alaska

20. ● How thick must ice be to flow? ● several 10s of meters
3. How Glaciers Move
● How thick must ice be to flow?
● several 10s of meters
● How does the glacier ice move?
● laminar flow; downslope

21. ● Mechanisms of glacial ice flow ● plastic flow ● basal slip
3. How Glaciers Move
● Mechanisms of glacial ice flow
● plastic flow
● basal slip

22. 3. How Glaciers Move
overall movement
of plastic flow

23. 3. How Glaciers Move
ice crystals
overall movement
of plastic flow

24. 3. How Glaciers Move
ice crystals
overall movement
of plastic flow
The flow of a glacier is accomplished by small slips along the microscopic planes of ice crystals.

25. 3. How Glaciers Move

26. In cold regions, the ice at the base of a glacier is frozen to the bedrock.

27. In cold regions, the ice at the base of a glacier is frozen to the bedrock.

28. In cold regions, the ice at the base of a glacier is frozen to the bedrock.
The rate of movement decreases toward the base.

29. If one drives a set of stakes deep into the glacier...

30. If one drives a set of stakes deep into the glacier...
…the stakes in the center will have moved farther. This indicates faster movement in the center and top of the glacier.

31. In temperate regions, the pressure of the overlying ice melts water at the glacier’s base.
liquid
water

32. In temperate regions, the pressure of the overlying ice melts water at the glacier’s base.
liquid
water

33. The layer of water acts as a lubricant.
In temperate regions, the pressure of the overlying ice melts water at the glacier’s base.
liquid
water
The layer of water acts as a lubricant.

34. 3. How Glaciers Move

35. 3. How Glaciers Move
In continental glaciers, the ice moves down and out from the thickest section.

36. ● Flow in valley glaciers ● plastic (laminar) flow –
3. How Glaciers Move
● Flow in valley glaciers
● plastic (laminar) flow –
central part of the glacier moves faster
● basal slip – uniform
movement or surges
● crevasses form on surface

37. 3. How Glaciers Move
formation of crevasses

38. Crevasses form where a glacier moves over steps in bedrock topography…

39. Crevasses form where a glacier moves over steps in bedrock topography…
…and where it curves around topography.

40. ● Earth’s ice cap (Antarctica) ● continental glaciers flowing
3. How Glaciers Move
● Earth’s ice cap (Antarctica)
● continental glaciers flowing
away from center of land mass
● plastic flow > basal slip
● highest flow in center of ice

41. How Glaciers Move: Flow Lines
Lambert Glacier, Antarctica

42. How Glaciers Move:
Flow Lines on Glacier
Lambert Glacier, Antarctica

43. ● Earth’s ice cap (Antarctica) ● ice streams flow within ice cap
3. How Glaciers Move
● Earth’s ice cap (Antarctica)
● ice streams flow within ice cap
● ice streams have basal slip
● ice cap leads to ice shelf
● icebergs snap off ice cap and
float away from ice shelf

44. How Glaciers
Move:
Collapse of
the Larson
Ice Shelf,
Antarctica,
March 2002

45. How Glaciers Move: Icebergs Floating

47. Thought questions for this chapter
One of the dangers of exploring glaciers is the possibility of falling into a crevasse. What topographic features of a valley glacier or its surroundings would you use to infer that you were on a part of the glacier that was badly crevassed?
The density of ice (0.92 g/cm3) is less than that of water (1.0 g/cm3), which is why icebergs float. Using the principle of isostasy, compute the fraction of an iceberg’s mass that floats above the sea surface?

48. Thought questions for this chapter
Evidence from bore holes drilled into the ice shows that there is liquid water at the base of some glaciers. What kinds of glaciers might have liquid water at the base? What factors might be responsible for the melting of ice at the bottom of these glaciers?

49. ● Glacial erosion and its landforms ● glacial sediment: poorly sorted
4. Glacial Landscapes
● Glacial erosion and its landforms
● glacial sediment: poorly sorted
mix of boulders to clay (rock
flour)
● scratches and grooves on
bedrock (striations)

50. Glacial
Landscapes:
Glacial Polish
and Striations
on Bedrock,
Glacier Bay
National Park,
Alaska

51. ● Glacial erosion and its landforms ● smooth bedrock hills (roches
4. Glacial Landscapes
● Glacial erosion and its landforms
● smooth bedrock hills (roches
moutonées)
● amphitheater-shaped bowls on
mountain peaks (cirques)
● knife-like ridges between
glacial valleys (arêtes)

52. ● Glacial erosion and its landforms ● U-shaped (glacial) valley
4. Glacial Landscapes
● Glacial erosion and its landforms
● U-shaped (glacial) valley
● U-shaped tributary valleys
(hanging valleys)
● submerged U-shaped valleys
(fjords)
● sculpted peaks (horns)

53. Glacial
Landscapes:
Formation
of the
Roche
Moutonée

54. Glacial Landscapes: Roche Moutonée
The Beehive, Sand Beach, Acadia National Park,
Maine

55. Formation of Glacial Erosion Features
4. Glacial Landscapes:
Formation of Glacial Erosion Features

62. Glacial
Landscapes:
A Horn
The Matterhorn, Switzerland

63. ● Glacial sedimentation and landforms
4. Glacial Landscapes
● Glacial sedimentation and landforms
● glacial drift (all glacial sediment)
● glacial till (from melting ice)
● erratics – glacial boulders in till
● outwash  glacial sediments
redeposited by glacial meltwater

64. Glacial
Landscapes:
Formation
of Till and
Outwash
Deposits

65. Glacial
Landscapes:
Formation
of Various
Moraine
Features

66. ● Glacial sedimentation and landforms ● ice-laid deposits ● morraine
4. Glacial Landscapes
● Glacial sedimentation and landforms
● ice-laid deposits
● morraine
● drumlins
● water-laid deposits
● kames, eskers, and kettles
● lake varves

67. Glacial
Landscapes:
Moraine
(Till)
Sierra Nevada Mountains, California

68. Glacial Landscapes: Till with Kettles
Coteau des Prairies, South Dakota

69. 4. Glacial Landscapes

70. ● Permafrost - perennially frozen soil ● soil and sediment
4. Glacial Landscapes
● Permafrost - perennially frozen soil
● soil and sediment
● aggregates of ice crystals
● layers
● wedges
● irregular masses

71. Glacial
Landscapes:
Permafrost
of the
World

72. Thought questions for this chapter
Some parts of a glacier contain much sediment, others very little. What accounts for the difference?
Contrast the kinds of till that you would expect to find in two glaciated areas: one a terrain of granitic and metamorphic rocks; the other a terrain of soft shales and loosely cemented sands?
What geological evidence would you search for if you wanted to know the direction of ancient glacial movements across the Canadian Shield?

73. Thought questions for this chapter
You are walking over a winding ridge of glacial drift. What evidence will you look for to discover whether you are on an esker or an end moraine?

74. ● The Wisconsin glaciation ● most recent glaciation of the
5. Glacial Cycles and Climate Change
● The Wisconsin glaciation
● most recent glaciation of the
Pleistocene ice ages
● reached its maximum about
21,000 to 18,000 years ago
● a global event with profound
implications

75. Glaciations
and the
Climate
System:
The
Pleistocene
Glacial
World, as it
Was 18,000
Years Ago

76. ● Wisconsin sea-level change ● lower sea level due to ice
5. Glacial Cycles and Climate Change
● Wisconsin sea-level change
● lower sea level due to ice
covering the land
● more continental dry land
● related climate changes

77. ● Geological record of Pleistocene glaciations
5. Glacial Cycles and Climate Change
● Geological record of Pleistocene
glaciations
● isostasy and sea-level change
● climate changes
● Milankovitch cycles

78. ● Geological record older ice ages ● Permian-Pennsylvanian tillies
5. Glacial Cycles and Climate Change
● Geological record older ice ages
● Permian-Pennsylvanian tillies
● Ordovician tillites
● Precambrian tillites (2,400 and
million years old)

79. Glacial Cycles and Climate change:
Permian-Pennsylvanian glaciation of the southern supercontinent
PACIFIC
OCEAN
Africa
TETHYS
OCEAN
South
America
India
Australia
Antarctica

80. Glacial Cycles and Climate Change:
Permian-Pennsylvanian tillites and their distribution today
ATLANTIC
OCEAN
Africa
India
PACIFIC
OCEAN
South
America
INDIAN
OCEAN
Australia

81. Glacial Cycles and Climate Change: Permian Tills on Bedrock with
Striations
South Africa
glacial till
glacial striations on bedrock

82. Glacial Cycles and Climate Change: Permian Glacial Lake Sediments with
Dropstones
lake sediments
South Africa
glacial dropstones

83. ● Time of the “snowball” Earth ● global record of tillites 750 to
5. Glacial Cycles and Climate Change
● Time of the “snowball” Earth
● global record of tillites 750 to
600 million years old
● ice may have covered Earth
from each pole to the equator
● implications for life on Earth

84. Glacial Cycles
and
Climate
Change:
Progressive
Freezing
of the
“Snowball”
Earth

85. Thought questions for this chapter
You live in New Orleans, not far from the mouth of the Mississippi River. What might be your first indication that the world is entering a new glacial age?
Some geologists think that one result of continued global warming could be the shrinkage and collapse of the West Antarctic ice sheet. How might this affect the populations of North America and Europe?

86. Key terms and concepts Accumulation Basal slip Cirque
Ablation
Accumulation
Basal slip
Cirque
Continental glacier
Crevasse
Drift
Drumlin
Esker
Fjord
Glacier
Hanging valley
Iceberg calving
Ice shelf
Ice stream

87. Key terms and concepts Moraine Outwash Permafrost Plastic flow
Iceberg calving
Kettle
Moraine
Outwash
Permafrost
Plastic flow
Striation
Surge
Till
Tillite
U-shaped valley
Valley glacier
Varve