Eventually the weight of snow exceeds the angle of repose and the ice begins to flow downhill as a glacier.

Two types of glaciers

• Valley (Alpine) glaciers -- figrue 12.2
  • Form in the mountains
  • Flows down a valley from an accumulation center at its head
• Continental glaciers -- Ice Sheets -- figure 12.3
  • Large scale than Valley glaciers -- covers large aras of land
  • There are two major Ice Sheets -- Greenland and Antarctica
  • Often called continental ice sheets
  • Ice flows out in all directions from one or more snow accumulation centers
  • associated with Ice shelves -- large, flat sheets of ice attached at some point to land
• Other types
  • Ice caps -- smaller features that cover the landscape
  • Outlet glaciers -- typically ice sheets and ice caps feed these, flow outward towards the sea
  • Piedmont glaciers -- broad sheets at the base of steep mountains where alpine glaciers emerge

What if the Ice Melted? -- figure 12.5

• Just over 2% of the world's water is tied up in glaciers
• Antarctic ice sheet
  • 80% of the world's ice
  • nearly 2/3 of the fresh water
  • Covers almost 1.5 times the area of the U. S.
  • Sea level would rise 60 to 70 meters ( ~197 to 230 feet)

Formation of a Glacier -- forms when more snow falss in winter then melts in the summer.

Processes in forming glacial ice: -- figure 12.6

• air infiltrates snow
• snowflakes become smaller, thicker, and more spherical
• air is forced out
• snow is recrystallized into much denser mass of small grains called firn
• once the thickness if the ice and snow exceeds 50 m, firn fuses into a solid mass of interlocking ice crystals -- glacial ice

Movements -- flow

• Types of Flow -- figure 12.7
  • Plastic
    • occurs within the ice
    • under pressure, the ice behaves plastically
  • Basal Slip
    • entire ice mass slipping along the ground
    • typicall for most glaciers
• Zone of Fracture
  • in the uppermost 50 m
  • tension causes crevasses or cracks to form in brittle ice -- figure 12.8
• Rates -- vary considerable from glacier to glacier
  • rates up to several meters per day
  • surges - rapid movement which be experienced by some glaciers -- p. 335

Budget of a glacier -- Snow piles up in the zone of accumulation and ablates or melts in the zone of melting. Meltwater flows from the glacier as it melts. When accumulation is faster than ablation, the glacier expands. When ablation is faster than accumulation, glaciers shrink. For the last 100 years most valley glaciers have been shrinking.

• Zone of accumulation
  • more snow falls than melts
  • elevation of snowline varies
• Zone of wastage -- where there is net loss to the glacier from:
  • melting
  • calving -- breaking off of large pieces of ice -- icebergs where it has reached the sea -- figure 12.10
• Balancing -- between accumulation at the upper end and loss at the lower end, which is referred to as the glacial budget -- figure
  • ablation -- accumulation exceeds loss then the glacial front advances
  • if ablation increses then the ice flont retreats

Erosion -- Glaciers erode very quickly. They pick up rocks and other debris and also they abrade and polish rock surfaces.

Occurs by two processes:

• Plucking -- lifting rocks
• Abrasion
  • Polishing - rocks within the ice sheet act like sandpaper and polish the surface below
  • Rock flour -- pulverized rocks -- crushed
  • Striations -- grooves in the bedrocks -- figure 12.12

• Glacial trough -- U-shaped valleys form when glaciers carve the walls and deposit till and outwash in the valley floor
• Truncated spurs -- triangular-shaped cliffs caused by the flows going around sharp corners.
• Hanging Valleys -- form where a main glacier carves a deeper valley than its tributaries.
• Pater noster lakes -- depressions filled with water created by plucking and abrasion.
• Cirques -- bowl-shaped hollows carved by glaciers where they start.
• Tarns -- small lake within a cirque basin.
• Col -- two glacial troughs intersect, creating a gap
• Fiords -- glacial valleys that have been flooded by the ocean.
• Aretes -- knife edged ridges between u-shaped valleys.
• Horns -- pyramid-like peaks -- figure 12.19.
• Roches Moutonnees -- aymmetrical knob of bedrock caused by abrasion on one side and plucking on the other -- figure 12.20.

Deposits

Glacial Drift

• Till -- unsorted debris dropped out of the ice of a glacier. It contains everything from boulders to clay. Typically poorly sorted, large grain size, and angular fragments.
• Stratified drift -- sediments laid down by glacial meltwater. Typically fine-grained and well sorted.

Landforms made of Till -- figure 12.27

• Moraines -- layers or ridges of till -- till bulldozed into a pile by a glacier.
• Morianes produced by Alpine glaciers
  • Lateral moraine -- form at the sides -- figure 12.23.
  • Medial moraine -- form whe n two glaciers coelesce to form a single ice stream -- figure 12.24.
• Other types of moraines
  • End moraine - terminal or recessional -- form at the front of the glacier -- figure 12.25 and 12.26.
  • Ground moraine -- gently rolling layer of till deposited as the ice sheet recedes. Erratics are boulders of strange rocks, brought from other places by glaciers.
• Drumlins -- streamlined mounds of till that are parallel to the direction of ice movemen -- 12.28.
  • smooth, elongated, parallel hills
  • steep side faces the direction from which the ice advanced
  • occur in clusters call drumlin fields
  • formation is not fully understood

Landforms made by Stratified drift -- figure 12.27

• Outwash plains (with ice sheets) and Valley trains in a valley
  • Broad ramp-like surface deposted by meltwater leaving a glacier
  • located adjacent to the downstream edge of the end moraine
  • Kettles -- pockmarked depressions formed when chunks of ice included in till melt.
• Ice-contact deposits
  • By meltwater flowing over, within, and at the base of motionless ice.
  • Features:
    • Kames -- form of a mound or steep-sided hill
    • Kame terraces -- narrow masses laid down between the glacier and the valley by streams.
    • Eskers -- snakelike mounds formed by streams flowing in tunnels beneath a glacier.

Ice Age

• Four major stages recognized in North America
  • Nebraskan
  • Kansan
  • Illinoian
  • Wisconsinan
• 30% of land was covered by Ice -- figure 12.29
• Began between 2 and 3 million years ago
• Occurred mainly in the Pleistocene epoch

Indirect effects of Ice Age Glaciers

• forces migration of animals and plants
• changes the course of streams -- p. 354
• rebound effect in the crust -- figure 12.30
• worldwide change in sea level -- fig 12.31
• climatic changes -- e.g., pluvial lakes were created in arid and semi-arid regions -- figure 12.32.

Causes of ice ages

There is evidence of glaciation during other times in earth's history as recorded by tillite. Tillite is a sedimentary rock where till is lithified.

Theory:

• What causes the onset of glaciation?
• What caused the alteration of glacial and interglacial stages that been documented for the Plietocene epoch?

• Plate Tectonics
  • Change in postion of continents -- figure 12.33
  • Change in ocean circulation
• Variations in Earth's orbit -- Milankovitch hypothesis -- figure 12.34
  • Eccentricity of the orbit varies -- shape
  • Obliquity changes -- angle
  • Precession - axis wobbles
  • changes in climate are closely associated with variation in the geometry of the orbit.
• Other factors are probably out there as well.

Addtional Note:

The Pleistocene, the recent ice age (1.6 million year ago to 10,000 years ago) consisted of cycles of extensive glacier formation followed by warmer periods. We are in the middle of one of these warmer episodes and eventually, glaciers will probably return. The last major ice age began about 40,000 years ago and reached a maximum about 20,000 years ago. By about 10,000 years ago the glaciers had almost vanished. Starting in AD 1,350 to AD 1,700 a colder time, called the little ice age caused valley glaciers around the world to advance. There were famines in Europe, and the Viking settlements in Greenland died. At the height of the ice ages, glaciers covered most of Europe and North America. Sea level was about 130 m (about 400 ft) lower than our present-day level, and river valleys were cut deeper.

The climate and the volumes of glaciers are controlled by changes in climate. Currently one area of concern is the Greenhouse effect. As we burn coal and oil, we increase the carbon-dioxide in the atmosphere. CO2 traps heat in the atmosphere, and over the last 100 years the temperature increases as the CO2 increases.

Questions?

1. What happens if the ice melts?
2. What cycles are Glaciers part of?
3. What is a glacier?
4. What are the different types of glaciers?
5. How is glacial ice formed?
6. What types of flow do glaciers experience?
7. What is the zone of fracture?
8. What are the rates of glacial movement?
9. What is involved in the glacial budget?
10. How do glaciers erode the land?
11. What are the different features of glacial erosion?
12. What are the types of glacial sediments?
13. What are the different landforms made of till?
14. What are the different landforms made of stratified drift?
15. What are the four major stages recognized in the US for the Ice Age?
16. When do most of the glacial stage occur in Geologic time?
17. What are indirect effects of Ice Ages?
18. What is a long term cause of Ice Ages?
19. What is a short term cause of Ice Ages?

Vocabulary Terms

• Ablation
• Abrasion
• Alpine glacier
• Arete
• Basal slip
• Calving
• Cirque
• Col
• Crevasse
• Drumlin
• End moraine
• Esker
• Fiord
• Firn
• Glacial budget
• Glacial drift
• Glacial erratic
• Glacial striations
• Glacial trough
• Glacier
• Ground moraine
• Hanging valley
• Horn
• Ice cap
• Ice-contact deposit
• Ice sheet
• Kame
• Kame terrance
• Kettle
• Lateral moraine
• Medial moraine
• Outlet glacier
• Outwash plain
• Pater noster lakes
• Piedmont glacier
• Plastic flow
• Pleistocene epoch
• Plucking
• Pluvial lake
• Recessional moraine
• Roche moutonnee
• Rock flour
• Snowline
• Stratified drift
• Surge
• Tarn
• Terminal moraine
• Till
• Tillite
• Truncated spur
• Valley glacier
• Valley train
• Zone of accumulation
• Zone of fracture
• Zone of wastage

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