1. Glaciers and Glaciation
Chapter 14
Glaciers and Glaciation

2. Introduction
Pleistocene Glaciations: A series of "ice ages" and warmer intervals that occurred 2.6 million to 10,000 years ago.
The Little Ice Age was a time of colder winters and short, wet summers during which glaciers advanced and sea ice persisted.
It occurred from about 1500 to the middle to late 1800s, and had its greatest effect on Northern Europe and Iceland.
Geo-Insight 4, p. 342

3. Introduction
Glaciers are masses of ice which move over land by plastic flow and basal slip.
Glaciers presently contain 2.15% of all water on Earth, and cover about 10% of the land surface.
Fig. 14.2, p. 341

4. Introduction
Snowfields exist year round in some mountains, but because they do not move, they are not glaciers.
Icebergs are not glaciers because they float in water and not on land.
Fig. 14.2, p. 341

5. The Kinds of Glaciers Valley Glaciers
Confined within high mountain valleys
Long, narrow tongues of ice
Much smaller than continental glaciers
Flow from higher to lower elevations
Often have tributaries
Geo-Insight 5, p. 342

6. The Kinds of Glaciers Valley glaciers are also called alpine glaciers.
Tidewater glaciers are valley glaciers that flow into the sea.
Fig. 14.2, p. 341

7. The Kinds of Glaciers
Continental Glaciers or ice sheets are unconfined by topography and flow outward in all directions from a zone of accumulation.
Currently found in Antarctica and Greenland
They cover vast areas
Often develop large ice shelves where they flow outward into the sea
Fig. 14.3a, p. 341

8. The Kinds of Glaciers Ice Caps
Similar to continental glaciers, but much smaller
Some develop from valley glaciers when they grow out of their mountain valleys
Others develop in flat areas, such as in Iceland
Fig. 14.3b, p. 341

9. Glaciers - Moving Bodies of Ice on Land
Glaciers—Part of the Hydrologic Cycle
Glaciers are a reservoir in the hydrologic cycle where water is stored for long periods as it moves from the oceans to land and back to the oceans.
Besides melting, glaciers also sublimate, where ice converts directly to water vapor.
Fig. 14.1, p. 340

10. Glaciers - Moving Bodies of Ice on Land
How Do Glaciers Originate and Move? Glaciers form when winter snowfall exceeds summer melt and snow accumulates yearly.
Ice is a crystalline solid. Fresh snowflakes are about 80% air.
As the snow accumulates, it thaws and refreezes, becoming a granular type of ice called firn.
When firn is buried, it recrystallizes (metamorphoses) to glacial ice and will flow under its own weight.
Fig. 14.4a, p. 344

11. Snowflakes Glacial ice Granular snow Firn Stepped Art
Fig. 14-4, p. 344

12. Glaciers - Moving Bodies of Ice on Land
How Do Glaciers Originate and Move?
Glaciers move through Basal Slip and Plastic Flow.
If a slope is present, glaciers may slide over their underlying surface through basal slip. Meltwater helps to lubricate basal slip.
Most of their movement is plastic flow, a type of deformation that takes place in response to stress.
Fig. 14.5, p. 345

13. Glaciers - Moving Bodies of Ice on Land
How Do Glaciers Originate and Move?
The upper 40 m or so of a glacier is brittle and fractures.
Tension within the glacier forms large crevasses on the surface of the glacier.
Fig. 14.6a, p. 346

14. Crevasses
Fig. 14.6, p. 346

15. Glaciers - Moving Bodies of Ice on Land
Distribution of Glaciers
Glaciers exist only where there is:
Sufficient snow fall (excludes some polar deserts)
Summer temperatures are low enough to hinder melting
These conditions prevail in:
High mountains (some even at the equator)
High latitudes (such as in Alaska, the Canadian Arctic islands, Greenland, and Antarctica)

16. The Glacial Budget
Glacial budget - A glacier's behavior depends on the balance between accumulation and wastage (melting).
Fig. 14.7, p. 347

17. The Glacial Budget
The upper part of the glacier, where the snow cover is year-round, is the zone of accumulation.
The lower part, where losses from melting, sublimation, and calving of icebergs exceed gains, is the zone of wastage.
The line separating the two is the firn limit. It shifts at least somewhat each year.
Fig. 14.7, p. 347

18. The Glacial Budget
Glaciers having a balanced budget have a stationary terminus. The firn limit changes very little from year to year.
Positive and negative budgets result in advance and retreat of the terminus, respectively.
Fig. 14.7, p. 347

19. The Glacial Budget
A persistent negative budget leads to the disappearance of a glacier.
Glaciers are very sensitive to climatic changes.
Most glaciers are in retreat because of global warming.
In a few decades, the glaciers will probably be gone from Glacier National Park in Montana.
Fig. 14.7, p. 347

20. The Glacial Budget How Fast Do Glaciers Move?
The rate of glacial movement depends on the slope, discharge and season.
In general, valley glaciers move more rapidly than do continental glaciers.
Fig. 14.8a, p. 350

21. The Glacial Budget How Fast Do Glaciers Move?
The main valley glacier has more volume and flows faster than its tributaries.
Plastic flow occurs year round, but basal slip is more important in the summer.
Fig. 14.8a, p. 350

22. The Glacial Budget How Fast Do Glaciers Move?
Like streams, glacial ice flows faster away from walls and floors.
In general, glacial flow velocity increases down slope in the accumulation zone, but decreases in the zone of wastage.
Fig. 14.8a, p. 350

23. The Glacial Budget How Fast Do Glaciers Move?
Most continental glaciers flow at only a few cm to meters per day.
Because they are at high latitudes, continental glaciers are usually frozen to their underlying surfaces and basal slip is minimal.
Fig. 14.8a, p. 350

24. The Glacial Budget
Glacial Surges are short-lived episodes of accelerated flow.
Surges probably involve increased basal slip.
They are more common in valley glaciers.
Surges break the surfaces of glaciers into a maze of crevasses.
Glaciers may surge at tens of meters per day for weeks or months before slowing down to their normal rates.
Fig. 14.6b, p. 346

25. The Glacial Budget Hypotheses for the causes of surges:
1. Thickening of ice in the zone of accumulation and concurrent thinning in the zone of wastage increases the slope of the glacier and its velocity.
2. Pressure on soft sediment beneath the glacier squeezes fluids through the sediments and allows the overlying glacier to slide more effectively.

26. Erosion and Sediment Transport by Glaciers
Glaciers effectively erode, transport and deposit huge amounts of sediments because they are moving solids.
Glaciers deposit sediments of all grain sizes, from boulders the size of a house down to clay-sized rock flour.
Fig , p. 352

27. Erosion and Sediment Transport by Glaciers
Push or bulldoze loose materials in their paths
Erode by abrasion - that is, the movement of sediment-laden ice over rock surfaces
Erode by plucking when ice freezes in or around bedrock projections and pulls them loose
Fig. 14.9a, p. 351

28. Erosion and Sediment Transport by Glaciers
Abrasion and plucking can form a roche moutonnée.
Fig. 14.9, p. 351

29. Erosion and Sediment Transport by Glaciers
Rocks and sediment in glaciers…
abrade underlying rocks and grind them into a fine powder called rock flour
Abrasion also results in glacial striations – scratches made by rocks scraping against one another as the glacier moves.
Fig , p. 352

30. Erosion and Sediment Transport by Glaciers
Erosion by Valley Glaciers
Valley glaciers carve angular peaks and deep valleys
U-Shaped Glacial Troughs
When mountain valleys are eroded by glaciers, they deepen and widen so that they have a U- shaped profile with flat or gently rounded valley floors and near-vertical valley walls.
Fig b, p. 353

31. Erosion and Sediment Transport by Glaciers
Erosion by Valley Glaciers
Mountain river valleys usually have a V-shaped profile in contrast to the U- shaped valleys produced by valley glaciers.
A fiord forms when sea level rises and fills a U- shaped glacial valley with sea water.
Fig , p. 353

32. Erosion and Sediment Transport by Glaciers
Erosion by Valley Glaciers
Hanging Valleys: Create spectacular waterfalls
Form when the floor of a former glacial tributary is higher than the main valley
Fig c, p. 353

33. Erosion and Sediment Transport by Glaciers
Erosion by Valley Glaciers
Hanging Valleys
Fig , p. 354

34. Erosion and Sediment Transport by Glaciers
Erosion by Valley Glaciers
Cirques
At the upper end of the glacial trough, a scoop- shaped depression, or cirque, eroded into a mountain side marks the place where a glacier formed and moved out into a trough.
Fig c, p. 353

35. Erosion and Sediment Transport by Glaciers
Erosion by Valley Glaciers
Cirques
Fig b, p. 354

36. Erosion and Sediment Transport by Glaciers
Erosion by Valley Glaciers
Arêtes
An arête is a serrated ridge between U- shaped glacial troughs or between adjacent cirques.
Fig c, p. 353
Fig a, p. 354

37. Erosion and Sediment Transport by Glaciers
Erosion by Valley Glaciers
Horns
A horn is a pyramid-shaped peak left when headword erosion takes place by at least three glaciers in the same peak.
Fig , p. 355
Fig c, p. 353

38. Erosion and Sediment Transport by Glaciers
Continental Glaciers and Erosional Landforms
Areas eroded by continental glaciers
Are smooth and rounded, ice-scoured plains
Create deranged drainages with swamps and lakes
Exhibit large areas of polished and striated bedrock
Fig , p. 355

39. Glacial Deposits
Glacial Drift – a general term for all glacial deposits
Erratics – huge boulders derived from distant source areas, transported to their current location by glaciers
Fig a,b, p. 356

40. Glacial Deposits
Glacial Drift – a general term for all glacial deposits
Two types of drift
1. Till – sediments deposited directly by glacial ice; poorly sorted
2. Stratified drift – sediments deposited by running meltwater, usually in braided streams; well-sorted

41. Glacial Deposits Landforms Composed of Till
End moraines – Crescent shaped deposits of till that form near the terminus of the glacier. That is, they form as a pile of rubble at the front of the glacier.
Fig , p. 357

42. Glacial Deposits Landforms Composed of Till
Ground moraines – deposits of till left by a retreating glacier
Ground moraine has irregular, rolling topography, whereas end moraines are elongated ridges.

43. Glacial Deposits Landforms Composed of Till Recessional moraine.
Suppose that a glacier reaches its maximum extent and has a balanced budget.
Accordingly, it deposits a terminal moraine.
If it then has a negative budget,
Its terminus retreats, deposits ground moraine and perhaps becomes stabilized once again if its budget is balanced.
In this case another end moraine is deposited but it is called a recessional moraine.

44. Glacial Deposits
Recessional and Terminal Moraines
Fig , p. 356

45. Glacial Deposits Landforms Composed of Till
Lateral and Medial Moraines – Ridge shaped deposits of till that form within the glacier
Created by plucking rock from the valley walls
Lateral moraines form along the sides of the glacier
Medial moraines form where two lateral moraines meet
Fig , p. 357

46. Glacial Deposits Landforms Composed of Till Drumlins
Streamlined hills of till shaped by continental glaciers or by glacial meltwater floods. Drumlin fields may contain hundreds of them.
Fig b, p. 356

47. Glacial Deposits Landforms Composed of Stratified Drift
Sediments deposited by glacial meltwater; well sorted
Outwash Plains – vast blankets of sediment, usually sand and gravel, that form in front of the glacier as it melts
Valley Trains – deposits of braided streams that form long, narrow deposits of stratified drift
Fig , p. 356

48. Glacial Deposits Landforms Composed of Stratified Drift
Kames – conical hills created when a stream deposits sediment in a depression on the glacier’s surface
Fig , p. 356
Fig a, p. 358

49. Glacial Deposits Landforms Composed of Stratified Drift
Eskers – snake-like deposits from sub-glacial streams
Fig , p. 356
Fig b, p. 358

50. Glacial Deposits Deposits in Glacial Lakes
The most distinctive deposits in glacial lakes are varves.
Varves consist of couplets of dark and light, laminated, fine-grained sediment.
The dark layers form during the winter when small particles of clay and organic matter are deposited.
The light layers are made up of silt and clay that form during the warmer months.
Fig a, p. 359

51. Glacial Deposits Deposits in Glacial Lakes
The age and history of a glacial lake may be determined by counting and studying the layers.
Fig a, p. 359

52. What Causes Ice Ages? The Milankovitch Theory
An explanation for the onset of the glacial episodes
Milankovitch claimed that irregularities in Earth’s rotation and orbit bring about complex climatic changes that provide the triggering mechanism for glacial episodes.
Fig , p. 360

53. What Causes Ice Ages? The Milankovitch Theory
The 3 primary factors are:
orbital eccentricity
changes in axial tilt
precession of the equinoxes
Fig , p. 360

54. What Causes Ice Ages? Short-Term Climatic Events
Milankovitch cycles can be measured in 10’s of thousands of years. They are too long to explain events like the Little Ice Age that lasted just a few hundreds of years.
Several hypotheses have been proposed:
Variations in solar energy due to solar flares or interstellar dust
Volcanic eruptions are known to cause short term climate change. A series of large eruptions could produce a prolonged event.

55. End of Chapter 14