This chapter discusses glaciers and ice ages. It describes how glaciers form from compacted snow and move via gravity-driven flow. There are two main types of glaciers: mountain glaciers which form in mountainous areas and flow downhill, and continental glaciers which are large ice sheets that cover large land areas. Glaciers can advance or retreat depending on the balance between snow accumulation and melting. Glaciers erode land and transport sediment, leaving behind landforms like moraines and eskers when they melt. Periodic ice ages over Earth's history are caused by changes in factors like the Earth's orbit and atmospheric greenhouse gas levels.
2. Chapter
22
Outline
• Ice and glacial ice, glaciers and their formation.
• Glaciers
-Types: mountain and continental
-Thermal categories: temperate and polar
-Ice movement, glacial advance & retreat
-Ice in the sea
• Effects of glaciers:
-Erosion, sediment transport, deposition
-Depositional landforms
• Glaciation
-Theory of glaciation and ice ages
-Subsidence & rebound, sea level change, drainage modification.
-The Pleistocene Glaciation and earlier ones
-Causes
Chapter 22
3. Chapter
22
Ice: The Water Mineral
• Ice is solid water (H2O).
• Forms when water cools below freezing point.
• Natural ice is a mineral; grows in hexagonal forms.
4. Chapter
22
Formation of Glacial Ice
• Snow is transformed into ice.
• Flakes accumulate.
• Buried by later falls.
• Compression expels air.
• Pressure causes melting &
recrystallization.
• Snow turns into granular firn.
• Over time, firn melds into
interlocking crystals of ice.
5. Chapter
22
Glaciers & Their Formation
• Thick masses of recrystallized (glacial) ice.
• Last all year long, flow via gravity.
• 3 conditions necessary:
• Cold local climate
• Snow must be abundant; more snow must fall than melts
• Snow must not be removed by avalanches or wind
6. Chapter
22
Forming a Glacier
• Glacier-sustaining elevation is latitude controlled
• Polar regions -> glaciers form at sea level
• Equatorial regions -> glaciers form >5km elevation
• This elevation is marked by the “snow line”
7. Chapter
22
Outline
• Ice and glacial ice, glaciers and their formation.
• Glaciers
-Types: mountain and continental
-Thermal categories: temperate and polar
-Ice movement, glacial advance & retreat
-Ice in the sea
• Effects of glaciers:
-Erosion, sediment transport, deposition
-Depositional landforms
• Glaciation
-Theory of glaciation and ice ages
-Subsidence & rebound, sea level change, drainage modification.
-The Pleistocene Glaciation and earlier ones
-Causes
Chapter 22
9. Chapter
22
Mountain Glaciers
• Flow from high to low elevation in mountains
• Include a variety of types.
• Ice caps cover mountain peaks
• Cirque glaciers fill mountain top bowls
12. Chapter
22
Continental Glaciers
• Ice sheets covering large land areas.
• Flows outward from thickest portion
• 2 major ice sheets on Earth:
Greenland, Antarctica
Minor: Iceland
13. Chapter
22
Thermal Categories
• Used to classify glaciers; determined by climate.
• Temperate glaciers – ice at/near melting temperature
• Polar glaciers – ice well below melting temperature
14. Chapter
22
Glacial Ice Movement
• How do glaciers move?
• Wet-bottom glaciers – water flows along base
• Basal sliding – ice slips over a meltwater/sediment slurry
• Dry-bottom glaciers – cold base is frozen to substrate
• Movement is by internal plastic deformation of the ice
15. Chapter
22
Glacial Ice Movement
• 2 types of mechanical behavior.
• Brittle – Uppermost 60 m
• Tension initiates cracking
• Crevasses open and close with movement
• Plastic – Lower than 60 m
• Ductile flow occurs in deeper ice
• Ice flow heals cracks
16. Chapter
22
• Ice flows downhill via gravity.
• Gravity (g) can be resolved into 2 vectors
• Parallel to slope component (gs) drives flow
Glacial Ice Movement
17. Chapter
22
• Ice flows downhill via gravity.
• Ice flows from thickest part of continental glaciers
• Analogous to honey flowing…
Glacial Ice Movement
18. Chapter
22
• Variable flow rates (10 to 300 m/yr).
• Rarely, glaciers surge (20 to 110 m/day)
• Flow rate controlled by:
• Slope -> steeper = faster
• Basal water: wet-bottom = faster
• Location within glacier
• Greater velocity in ice center
• Friction slows ice at margins
Glacial Ice Movement
19. Chapter
22
• Glaciers behave like bank accounts
• Accumulation zone – Area of new snow addition
• Colder T prevent melting
• Snow remains even in summer
• Ablation zone – Area of net ice loss
• Zones abut at equilibrium line
Glacial Advance and Retreat
20. Chapter
22
Glacial Advance and Retreat
• Toe (leading edge) position:
• If accumulation = ablation the toe is stable
Note curved path of ice flow.
23. Chapter
22
Glacial Advance and Retreat
Glacial advance and retreat is determined by the balance between the
accumulation of snow and the removal of ice by sublimation, melting,
and calving (ablation). When the rate of ablation below the snowline
equals the rate of accumulation above it, the glacier is stationary, as in
View 1. During glacial retreat, View 2, the rate of ablation exceeds the
rate of accumulation, and the position of the toe retreats toward the
origin of the glacier. Glacial advance, View 3, occurs when the rate of
accumulation exceeds the rate of ablation. For all views, pay attention to
the motion of the stones. Note that in all cases, ice flows downhill. For
more information, see Section 22.2 Ice and the Nature of Glaciers
starting on p. 758 and Figure 22.14 on p. 768 in your textbook.
Glacial Advance and Retreat
24. Chapter
22
Ice in the Sea
• In polar regions, glaciers flow out over ocean water.
• Tidewater glaciers – valley glaciers entering sea
• Ice shelves – continental glaciers entering sea
• Sea ice – non-glacial ice formed on frozen seawater
29. Chapter
22
Glacial Erosion
2. Plucking – ice breaks off and removes bedrock fragments
• Ice melts by pressure against up-ice side of obstruction
• Enters cracks in bedrock, water re-freezes to the ice
• Glacial movement “plucks” away bedrock chunks
30. Chapter
22
Glacial Erosion
3. Abrasion – a “sandpaper” effect on substrate
• Rock fragments in moving ice abrades and polishes bedrock
• Leaves scratches called striations
32. Chapter
22
Glacial Erosion
• Cirque – bowl-shaped basin near mountain top
• Forms at uppermost portion of valley
• Erosion at glacier base over-steepens into bowl
• After ice melts, cirque is often filled with a lake (tarn)
Glacier, Nat’l Park, MT
36. Chapter
22
Glacial Erosion
• Hanging valleys.
• -Intersection of tributary glacier
• With trunk glacier
• -Trunk glacier incises deeper
• -Trough bases at different
• -Elevations
• -A waterfall results
37. Chapter
22
Glacial Sediment Transport
• Glaciers carry sediment of all sizes (lots of it!)
• Some falls onto from adjacent cliffs
• Some entrained from substrate erosion
• When ice melts, material is dropped
38. Chapter
22
Glacial Sediment Transport
• Moraines – unsorted debris carried/dumped by glaciers
• Lateral – along valley glacier flank
• Medial – mid-ice merging lateral moraines
39. Chapter
22
Glacial Sediment Transport
• Glaciers are large-scale conveyor belts
• Pick up, transport, deposit sediment.
• Transport is always downhill
• Debris at toe is an end moraine
40. Chapter
22
Glacial Deposition
• Many sediment types derive from glaciation.
• Called glacial drift, these include...
• Till.
• Erratics.
• Marine sediments.
• Outwash.
• Others (lake seds, loess)
• Stratified drift is water-
sorted; unstratified drift
isn’t.
41. Chapter
22
Glacial Deposition
• Glacial till – Sediment dropped by glacial ice.
• Consists of all grain sizes
• Unmodified by liquid water, hence:
• Unsorted
• Unstratified
• Accumulates…
• Beneath glacial ice
• At glacial toe
• Along glacial flanks
46. Chapter
22
Depositional Landforms
• End moraines form at stable glacier toe
• Terminal moraines form at farthest edge of flow
• Recessional moraines form as retreating ice stalls
49. Chapter
22
Outline
• Ice and glacial ice, glaciers and their formation.
• Glaciers
-Types: mountain and continental
-Thermal categories: temperate and polar
-Ice movement, glacial advance & retreat
-Ice in the sea
• Effects of glaciers:
-Erosion, sediment transport, deposition
-Depositional landforms
• Glaciation
-Theory of glaciation and ice ages
-Subsidence & rebound, sea level change, drainage modification.
-The Pleistocene Glaciation and earlier ones
-Causes
Chapter 22
50. Chapter
22
The Theory of Glaciation
• European farmers broke plows on large rocks.
• Buried in fine-grained soils, often of enormous size.
• Unlike local bedrock, they were from 100s of km away.
• Rocks became known as erratics
• Origins were a mystery…
51. Chapter
22
The Theory of Glaciation
• Louis Agassiz, a Swiss geologist, observed glaciers.
• Saw glaciers as landscape change agents
• They carried snad, mud, huge boulders long distances
• They dropped these materials, unsorted, upon melting
• Realized glaciers could explain the erratics
52. Chapter
22
The Theory of Glaciation
• Agassiz proposed an ice age had frozen Europe
• Ice sheets covered land
• Ice carried and dropped…
• Erratics and
• Unsorted soil (till)
53. Chapter
22
Ice Ages
• Glaciers presently cover ~10% of Earth
• During ice ages, coverage expands to ~30%
• Most recent ice age ended ~11 kyrs ago
• Covered New York, Montreal, London, Paris, Seattle
• Ice sheets were 100s to 1,000s of m thick.
55. Chapter
22
Glacial Consequences
• Sea level changes due to ice ages
• Water stored on land during ice age –> sea level fall
• Deglaciation returns water to oceans –> sea level rise
• Sea level was ~100 m lower during last glacial maximum
57. Chapter
22
The Pleistocene Glaciation
• Young (< 2 Ma) glacial
remnants are abundant.
• North America
• Scandanavia and Europe
• Siberia
• Distinctive glacial
landscapes.
58. Chapter
22
The Pleistocene Glaciation
• Glaciation chronology.
• Several Pleistocene glacial advances
• In North America, 4 recognized – youngest to oldest…
• Wisconsinan, Illinoian, Kansan, Nebraskan
• Ice ages are separated
by interglacials
59. Chapter
22
The Pleistocene Glaciation
• Glaciation chronology.
• Oxygen isotopes from plankton
suggest more ice ages
• They reveal 20 or more
glaciations
• Higher 18
O/16
O = colder.
• Lower 18
O/16
O = warmer.
• The “original 4” ice ages
may simply be the largest
60. Chapter
22
Earlier Glaciations
• Glaciation recurs across Earth history.
• Evidence? Fossil til (tillite), striated bedrock
• Tillite at equatorial latitiudes suggest an ice-covered world:
Snowball Earth
61. Chapter
22
Causes of Glaciation
• Long-term causes – set stage for ice ages
1. Plate tectonics – controls factors that influence glaciation
• Continents at high latitudes
• Sea level flux by mid-ocean ridge volume changes
• Oceanic currents
2. Atmospheric chemistry.
• Changes in greenhouse gas concentrations
• Carbon dioxide (CO2)
• Methane (CH4)
62. Chapter
22
Causes of Glaciation
• Short-term causes – govern advances and retreats
• Milankovitch cycles – climate variation over 10-100s kyrs
predicted by cyclic changes in orbital geometry
1. Earth’s orbit shape varies (~100,000 year cyclicity)
2. Earth’s axis tilt caries from 22.5degrees to 24.5degrees (~41,000
years)
3. Earth’s axis wobbles (23,000 years)
63. Chapter
22
Causes of Glaciation
• Milankovitch – cyclic changes in orbital geometry.
• Variations lead to excess warming/cooling.
• Ice ages result when cooling effects coincide.