Glaciers Flashcards
(55 cards)
1
Q
What is a glacier?
A
- Thick masses of recrystallized ice that flow via gravity and last all year long, can be mountain and continental
- Presently glacier coverage is ~10% of the Earth
2
Q
Ice age
A
- Expanded to ~30% coverage of the earth during ice ages, presently is ~10%.
- Most recent ice age was 11k ya, covered Montreal, New York, London, and Paris
- Ice sheets were 100s to 1000s of meters thick
3
Q
How do glaciers form?
A
- Snowfall accumulates and survives the following summer (think of snowflakes like sediments)
- Snow is then transformed into ice via: burial -> compression reduces volume -> burial pressure causes melting and recrystallization -> snow turns into granular firn (25% air) -> over time, firn becomes interlocking crystals of ice
- May occur rapidly (10s of years) or slowly (1ks of years)
4
Q
3 conditions necessary to form a glacier:
A
- Cold local climate (polar latitudes or high elevation)
- Snow must be abundant; more snow falls than melts
- Snow must not be removed by avalanches or wind
5
Q
Two Categories of Glaciers:
A
- Alpine (mountain)
2. Continental (ice sheets)
6
Q
Alpine Glaciers
A
- Flow from high to low elevation in mountain settings
- Include many types:
○ Cirque glaciers (fill mountain-top bowls, almost always on north side of mountain)
○ Valley glaciers (flow like rivers down valleys)
○ Ice caps (cover peaks and ridges)
○ Piedmont glaciers (spread out at the end of a valley)
7
Q
Continental Glaciers
A
- Vast ice sheets cover large land areas
- Ice flows outward from thickest part of the sheet
- Two major icesheets on Earth left: Greenland and Antarctica
8
Q
How do glaciers move?
A
- Basal sliding
- Ice deformation
9
Q
Basal sliding
A
Significant quantities of meltwater forms at base of glacier, water decreases friction causing ice to slide along substrate
10
Q
Ice deformation
A
- Occurs below about 60m in depth
- Grains of ice slowly change shape
- Crevasses form at surface, upper zone too brittle to flow
11
Q
Why do glaciers move?
A
- The pull of gravity is strong enough to make the ice flow
- Glaciers move downslope
- Ice base can flow up a local incline
12
Q
In continental glaciers, ice spreads away from the centre of accumulation
A
The ice sheet is always thicker in the middle, so it spreads towards the edges
13
Q
Movement of Glacial Ice
A
- Rates of flow vary widely (10 to 300m per year)
- Rarely, glaciers may surge (20 to 110m per day)
14
Q
Glacial Advance and Retreat
A
- Zone of accumulation: area of net snow addition
○ Colder temperatures prevent melting
○ Snow remains across summer months - Zone of ablation: area of net ice loss
- Zones meet at the equilibrium line
- Ice always flows downhill, even during retreat
15
Q
Glacial toe
A
leading edge of the glacier
16
Q
If accumulation = ablation, the glacier toe:
A
stays in the same place
17
Q
If accumulation > ablation, the glacial toe
A
advances
18
Q
If accumulation < ablation, the glacial toe
A
will retreat upslope
19
Q
Ice in the Sea
A
- In polar regions, glaciers flow out over ocean water
- Floating ice is normally 80% beneath the waterline
○ Iceberg: greater than 6m above the water
○ Ice shelves yield tabular bergs (iceberg with flattish top basically?) - Large areas of polar seas are covered with ice
- Global warming is causing a reduction in ice cover
20
Q
Tidewater glaciers
A
valley glaciers entering the sea
21
Q
Ice shelves
A
continental glaciers entering the sea
22
Q
Sea ice
A
non-glacial ice formed of frozen seawater
23
Q
Glaciers change the landscape via
A
- Erosion
- Transport
- Deposition
24
Q
Glacial Erosion and Its Products
A
- Carve deep valleys
- Can erode by “plucking”
- Glacial abrasion
- Large rocks are dragged across bedrock gouge striations
25
Glaciers Carve Deep Valleys Creating:
Polished granite domes and vertical cliffs
26
Erosion by "plucking"
- Ice freezes around bedrock fragments and plunks chunks as glacier advances
- Forms a distinctive asymmetric hill: roche moutonée (rock sheep in French)
27
Glacial abrasion:
- a "sandpaper" effect on the substrate
○ The substrate is pulverised to fine "rock flour"
○ Sand in moving ice abrades and polishes
28
Large rocks are dragged across bedrock gouge striations
Striations run parallel to the direction of ice movement
| - like grooves/furrows on the surface of bedrock, prof said like scratches
29
Erosional Features of Glaciated Valleys:
- Cirques
- Tarns
- Aretes
- Horns
- U-shaped valleys
- Hanging valleys
- Fjords
30
Cirques
- Bowl-shaped basins high on a mountain (as soon as ice formed there, it further excavated the low spot)
- Form at the uppermost portion of a glacial valley
- After the ice melts, cirque often supports a tarn (lake)
31
Tarns
Proglacial (made by glacier) mountain lake formed in a cirque excavated by glacier
32
Aretes
- A "knife-edge" ridge ("ridge between two cirques")
| - Formed by two cirques that have eroded toward one another
33
Horn
- A pointed mountain peak
| - Formed by 3 or more cirques that surround the peak
34
U-shaped Valleys
- Glacial erosion creates a distinctive trough
| - Relative to V-shaped fluvial valleys
35
Hanging Valleys
- Intersection of a tributary glacier with trunk glacier
- Trunk glacier incises deeper into bedrock
- Troughs have different elevations
- Often results in waterfalls
36
Fjords
- U-shaped glacial troughs flooded by the sea
| - Accentuated by isostatic rebound
37
Glaciers act as large-scale conveyors
○ Pick up, transport, and deposit a lot of sediment (both at the base and surface of the glacier, or some can be buried)
○ Sediment transport is always downhill
○ Debris at the toe of a glacier is called an end moraine
38
Moraines
unsorted debris deposited by a glacier
○ Lateral - forms along the flank (side) of a valley glacier (separates side of a mountain from a glacier)
○ Medial - mid-ice moraine from merging of lateral moraines
39
Types of Glacial Sedimentary Deposits
- Many types of sediment and structures are derived from glaciation
- Called glacial drift, includes:
○ Glacial till
○ Erratics
○ Glacial marine sediments
○ Glacial outwash
○ Loess
○ Glacial lake-bed sediment
- Stratified drift is water
- Stratified drift: sorted
- Unstratified drift: is not sorted
40
Glacial outwash:
```
sediment transported by meltwater (looks like glacial till minus clay and tilt)
○ Muds removed
○ Sizes graded and stratified
○ Grains abraded and rounded
○ Dominated by sand and gravel
```
41
Glacial till:
sediment dropped by glacial ice (incredibly porous and permeable, make great aquifers)
○ Consists of all grain sizes, boulders to clay
○ Unmodified by water: unsorted, unstratified
○ Accumulates: beneath glacial ice, at the toe of a glacier, along
glacial flanks
42
Erratics:
boulders dropped by glacial ice (looks both out of place in appearance and is geologically different from its surroundings i.e mineralogy)
○ Different from underlying bedrock
○ Often carried long distances
43
Glacial marine:
sediments from an oceanic glacier
○ Calving icebergs (ice breaking off oceanic glacier) raft sediments
○ Melting icebergs deposit drop stones
44
Glacial lake-bed sediment:
- Lakes are abundant in glaciated landscapes
- Fine rock flour settles out of suspension in deep lakes
- Muds display seasonal varve (thin layers of silt and clay) couplets: winter (finest silt and clay), summer (coarser silt and clay)
45
Loess:
wind-transported silt
○ Strong winds blow rock flour away
○ Sediment settles out near glaciated areas as loess
○ Deposits are unstratified and distinct in colour
46
Glacial Consequences
- Sea level: ice ages cause fluctuations
○ During an ice age, sea level falls
○ Deglaciation, sea level rises
○ Sea level was ~100m lower during the last ice age
○ If ice sheets melted, coastal regions would be flooded
- Gigantic proglacial lakes formed near the ice margin
○ Example - Glacial Lake Agassiz: covered a huge area, existed for 2,700 years, drained abruptly, and exposed mud-rich, extremely flat land
- Pluvial features: large lakes formed during an ice age
○ The American Southwest was much wetter, large lakes occupied today's deserts, Lake Bonneville (remnant is the Great Salt Lake)
- Periglacial (near-ice) environments are unique
○ Characterized by year-round frozen ground (permafrost)
○ Freeze-thaw cycles generated unusual patterned ground
47
Consequences of Continental Glaciation
- Ice loading and glacial rebound
○ Ice sheets depress the lithosphere
○ Slow crustal subsidence follows the flow of the asthenosphere
○ After the ice melts, the depressed lithosphere rebounds
○ Glacial rebound continues today (rapid is up to 4.1cm per year, according to wiki)
48
Glacial sediments create distinctive landforms:
```
○ End moraines and terminal moraines
○ Recessional moraines
○ Ground moraines
○ Drumlins
○ Kettle lakes
○ Eskers
```
49
Depositional Moraine Landforms
- End moraines: form at the stable toe of the glacier
- Terminal moraines: form at the farthest edge of flow (as far north or south as glacier expanded/advanced, caused by glacier pushing moraine forward as it advances)
- Recessional moraines: form as retreating ice stalls
50
Other Depositional Landforms
- Kettle lakes: form from stranded ice blocks ("terrestrial icebergs" that melted)
- Drumlins: long, aligned hills of moulded till (caused by resistant rock underneath glacier) (don't need stereoscope (from lab) to see them, "just cross your eyes" lmao)
○ Asymmetric form - steep up-ice; tapered down-ice
○ Commonly occur as swarms aligned parallel to flow direction
- Eskers: long, sinuous ridges of sand and gravel ("snake")
○ Form as meltwater channels within or below the ice
○ Channel sediment is released when the ice melts
51
Pleistocene Ice Ages
- Young (<2.6 Ma) glacial remnants are abundant
○ Northern North America
○ Scandinavia and Europe
○ Siberia
- Landscapes in these regions are clearly glacial
52
Pleistocene Life and Climate
- All climate and vegetation belts were shifted southward
○ The tundra limit was ~48ºN, today it is above 68ºN
○ Vegetation evidence is preserved as pollen
- Pleistocene fauna were well-adapted
- Mammals included now extinct giants: giant beaver, giant ground sloth, mammoths and mastodons
53
Timing of the Pleistocene Ice Age
- Multiple Pleistocene glacial advances are recognized. Youngest to oldest: Wisconsinan, Illinoian, Pre-Illinoian
- Ice ages were separated by interglacial intervals
54
Causes of Glaciation
- Short-term causes - govern advances and retreats
○ Milankovitch hypothesis - climate variation over 100 - 300 ka predicted by cyclic changes in orbital geometry
○ The shape of Earth's orbit varies (~100,000 year cyclicity)
○ The tilt of Earth's axis varies from 22.5º to 24.5º (~41,000 years)
○ Precession - Earth's axis wobbles like a top (23,000 years)
55
The rate of flow is controlled by:
○ The severity of the slope angle (steeper = faster)
○ Basal water (wet-bottom = faster)
○ Location within glacier (greater velocity in ice centre, friction slows ice at margins)
