Glaciers Flashcards
What is a glacier?
- 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
Ice age
- 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
How do glaciers form?
- 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)
3 conditions necessary to form a glacier:
- 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
Two Categories of Glaciers:
- Alpine (mountain)
2. Continental (ice sheets)
Alpine Glaciers
- 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)
Continental Glaciers
- 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
How do glaciers move?
- Basal sliding
- Ice deformation
Basal sliding
Significant quantities of meltwater forms at base of glacier, water decreases friction causing ice to slide along substrate
Ice deformation
- Occurs below about 60m in depth
- Grains of ice slowly change shape
- Crevasses form at surface, upper zone too brittle to flow
Why do glaciers move?
- The pull of gravity is strong enough to make the ice flow
- Glaciers move downslope
- Ice base can flow up a local incline
In continental glaciers, ice spreads away from the centre of accumulation
The ice sheet is always thicker in the middle, so it spreads towards the edges
Movement of Glacial Ice
- Rates of flow vary widely (10 to 300m per year)
- Rarely, glaciers may surge (20 to 110m per day)
Glacial Advance and Retreat
- 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
Glacial toe
leading edge of the glacier
If accumulation = ablation, the glacier toe:
stays in the same place
If accumulation > ablation, the glacial toe
advances
If accumulation < ablation, the glacial toe
will retreat upslope
Ice in the Sea
- 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
Tidewater glaciers
valley glaciers entering the sea
Ice shelves
continental glaciers entering the sea
Sea ice
non-glacial ice formed of frozen seawater
Glaciers change the landscape via
- Erosion
- Transport
- Deposition
Glacial Erosion and Its Products
- Carve deep valleys
- Can erode by “plucking”
- Glacial abrasion
- Large rocks are dragged across bedrock gouge striations
Glaciers Carve Deep Valleys Creating:
Polished granite domes and vertical cliffs
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)
Glacial abrasion:
- a “sandpaper” effect on the substrate
○ The substrate is pulverised to fine “rock flour”
○ Sand in moving ice abrades and polishes
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
Erosional Features of Glaciated Valleys:
- Cirques
- Tarns
- Aretes
- Horns
- U-shaped valleys
- Hanging valleys
- Fjords
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)
Tarns
Proglacial (made by glacier) mountain lake formed in a cirque excavated by glacier
Aretes
- A “knife-edge” ridge (“ridge between two cirques”)
- Formed by two cirques that have eroded toward one another
Horn
- A pointed mountain peak
- Formed by 3 or more cirques that surround the peak
U-shaped Valleys
- Glacial erosion creates a distinctive trough
- Relative to V-shaped fluvial valleys
Hanging Valleys
- Intersection of a tributary glacier with trunk glacier
- Trunk glacier incises deeper into bedrock
- Troughs have different elevations
- Often results in waterfalls
Fjords
- U-shaped glacial troughs flooded by the sea
- Accentuated by isostatic rebound
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
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
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
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
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
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
Glacial marine:
sediments from an oceanic glacier
○ Calving icebergs (ice breaking off oceanic glacier) raft sediments
○ Melting icebergs deposit drop stones
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)
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
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
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)
Glacial sediments create distinctive landforms:
○ End moraines and terminal moraines ○ Recessional moraines ○ Ground moraines ○ Drumlins ○ Kettle lakes ○ Eskers
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
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
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
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
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
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)
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)
