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GLGY201 Principles of Geoscience > Glaciers > Flashcards

Glaciers Flashcards

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
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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
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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)
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4
Q

3 conditions necessary to form a glacier:

A
  1. Cold local climate (polar latitudes or high elevation)
  2. Snow must be abundant; more snow falls than melts
  3. Snow must not be removed by avalanches or wind
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5
Q

Two Categories of Glaciers:

A
  1. Alpine (mountain)

2. Continental (ice sheets)

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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)
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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
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8
Q

How do glaciers move?

A
  • Basal sliding

- Ice deformation

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9
Q

Basal sliding

A

Significant quantities of meltwater forms at base of glacier, water decreases friction causing ice to slide along substrate

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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
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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
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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

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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)

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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
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15
Q

Glacial toe

A

leading edge of the glacier

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16
Q

If accumulation = ablation, the glacier toe:

A

stays in the same place

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17
Q

If accumulation > ablation, the glacial toe

A

advances

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18
Q

If accumulation < ablation, the glacial toe

A

will retreat upslope

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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
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20
Q

Tidewater glaciers

A

valley glaciers entering the sea

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21
Q

Ice shelves

A

continental glaciers entering the sea

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22
Q

Sea ice

A

non-glacial ice formed of frozen seawater

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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
Q

Glaciers Carve Deep Valleys Creating:

A

Polished granite domes and vertical cliffs

26
Q

Erosion by “plucking”

A
  • Ice freezes around bedrock fragments and plunks chunks as glacier advances
  • Forms a distinctive asymmetric hill: roche moutonée (rock sheep in French)
27
Q

Glacial abrasion:

A
  • a “sandpaper” effect on the substrate
    ○ The substrate is pulverised to fine “rock flour”
    ○ Sand in moving ice abrades and polishes
28
Q

Large rocks are dragged across bedrock gouge striations

A

Striations run parallel to the direction of ice movement

- like grooves/furrows on the surface of bedrock, prof said like scratches

29
Q

Erosional Features of Glaciated Valleys:

A
  • Cirques
  • Tarns
  • Aretes
  • Horns
  • U-shaped valleys
  • Hanging valleys
  • Fjords
30
Q

Cirques

A
  • 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
Q

Tarns

A

Proglacial (made by glacier) mountain lake formed in a cirque excavated by glacier

32
Q

Aretes

A
  • A “knife-edge” ridge (“ridge between two cirques”)

- Formed by two cirques that have eroded toward one another

33
Q

Horn

A
  • A pointed mountain peak

- Formed by 3 or more cirques that surround the peak

34
Q

U-shaped Valleys

A
  • Glacial erosion creates a distinctive trough

- Relative to V-shaped fluvial valleys

35
Q

Hanging Valleys

A
  • Intersection of a tributary glacier with trunk glacier
  • Trunk glacier incises deeper into bedrock
  • Troughs have different elevations
  • Often results in waterfalls
36
Q

Fjords

A
  • U-shaped glacial troughs flooded by the sea

- Accentuated by isostatic rebound

37
Q

Glaciers act as large-scale conveyors

A

○ 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
Q

Moraines

A

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
Q

Types of Glacial Sedimentary Deposits

A
  • 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
Q

Glacial outwash:

A 
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
Q

Glacial till:

A

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
Q

Erratics:

A

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
Q

Glacial marine:

A

sediments from an oceanic glacier
○ Calving icebergs (ice breaking off oceanic glacier) raft sediments
○ Melting icebergs deposit drop stones

44
Q

Glacial lake-bed sediment:

A
  • 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
Q

Loess:

A

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
Q

Glacial Consequences

A
  • 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
Q

Consequences of Continental Glaciation

A
  • 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
Q

Glacial sediments create distinctive landforms:

A 
○ End moraines and terminal moraines
○ Recessional moraines
○ Ground moraines
○ Drumlins
○ Kettle lakes
○ Eskers
49
Q

Depositional Moraine Landforms

A
  • 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
Q

Other Depositional Landforms

A
  • 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
Q

Pleistocene Ice Ages

A
  • Young (<2.6 Ma) glacial remnants are abundant
    ○ Northern North America
    ○ Scandinavia and Europe
    ○ Siberia
  • Landscapes in these regions are clearly glacial
52
Q

Pleistocene Life and Climate

A
  • 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
Q

Timing of the Pleistocene Ice Age

A
  • Multiple Pleistocene glacial advances are recognized. Youngest to oldest: Wisconsinan, Illinoian, Pre-Illinoian
  • Ice ages were separated by interglacial intervals
54
Q

Causes of Glaciation

A
  • 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
Q

The rate of flow is controlled by:

A

○ 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)

GLGY201 Principles of Geoscience (20 decks)

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