Glaciers Flashcards
Glaciers
- Thick masses of recrystallized ice (from snow) that lasts all year long, flow via gravity, and can be in the form of mountains or spread continentally
- Presently cover about 10% of the Earth, expands to 30% during ice ages (most recent ice age ended 11ka)
How a glacier forms
- Snowfall accumulates and survives the following summer
- Snow is transformed into ice
- May occur rapidly (10s of years) or slowly (1000s of years)
Three conditions necessary to form a glacier
- Cold local climate
- Snow must be abundant; more snow must fall than melt
- Snow must not be removed by avalanches or wind
Two types of glaciers
Alpine (Mountains) and Continental (Ice Sheets)
Alpine glaciers
From high to low elevation in mountain settings
Many types:
- Cirque glaciers that fill mountain-top bowls
- Valley glaciers
- Ice caps covering peaks and ridges
- Piedmonts glaciers spreading out at the end of valleys
Continental Glaciers (Ice Sheets)
- Vast ice sheets covering large land areas
- Ice flows outward from the thickest part of the sheet
Where are the two major ice sheets that still remain on earth?
Greenland and Antarctica
How do glaciers move?
- Basal sliding - significant quantities of meltwater form at the base of the glacier, water decreases friction, and ice slides along the substrate)
- Plastic deformation - occurs below 60 m depth, grains of ice change shape slowly, crevasses form at the surface because the upper layer is too brittle to flow)
Why do glaciers move?
The pull of gravity is strong enough to make ice flow, which can cause the glacier to move down a slope or the ice at the base of the glacier can flow up a local incline
Ice sheets are also thicker in the middle, so they slowly spread toward the edges
Factors affecting the rate of movement of glacial ice
- rates vary from 10 to 300 m. per year, sometimes (rarely) 20 to 110 m per day
- Steeper slopes = faster movement
- Basal water (wet bottom) = faster movement
- Location within the glacier (Greater velocity in ice center, friction slows ice at margins
Zone of accumulation
Area of net snow addition (the area that is gaining mass)
- Colder temperatures prevent melting
- Snow remains across the summer months
Zone of ablation
Area of net ice loss (area that is losing mass)
Where do zone of ablation and zone of accumulation meet?
At the equilibrium line
Toe of the glacier and how it relates to the zone of ablation and zone of accumulation
The leading edge of a glacier
If accumulation = ablation, the toe stays in the same place
If accumulation > ablation, the toe advances (spreads down)
If accumulation < ablation, the toe will retreat upslope
Tidewater glaciers
Valley glaciers entering the sea
Ice shelves
Continental glaciers entering the sea
Sea ice
Nonglacial ice formed of frozen seawater
Iceberg
- Usually fourth-fifths beneath the waterline
- Icebergs are always greater than 6 m above the water
- Ice shelves yield tabular bergs
How do glaciers change landscapes?
Erosion, transport, deposition
Glacial erosion and its products
Glaciers carve deep valleys, such as Yosemite Valley
- Polished granite domes and vertical cliffs are the results of glacial erosion
Glacial abrasion - a “sandpaper” effect on substrate
- Substrate is pulverized to fine “rock flour”
- Sand in moving ice abrades and polishes
- Large rocks dragged across bedrock gouge striations
- Run parallel to the direction of ice movement
Erosional features of glaciated valleys (7)
-Cirques
- tarns
- aretes
- horns
- u-shaped valleys
- hanging valleys
- fjords
Cirques
Bowl-shaped basins high on a mountain
- Form at the uppermost portion of a glacial valley
- After ice melts, cirque often supports a tarn (lake)
Arete
A “knife-edge” ridge
- formed by two cirques that have eroded toward one another
Horn
A pointed mountain peak
- formed by three or more cirques that surround the peak
U-shaped valleys
Glacial erosion creates a distinctive trough
- Relative to V-shaped fluvial valleys
- Constant erosion everywhere
Hanging valleys
- Intersection of the tributary glacier with trunk glacier
- Trunk glacier incises deeper into bedrock
- Throughs have different elevations
- Often results in waterfalls
Fjords
- U-shaped glacial troughs flooded by the sea
- Accentuated by isostatic rebound
Roche Mountonee
Asymmetric ice hill
- Glaciers can erode by plucking
- Ice freezes around bedrock fragments and plucks chunks as the glacier advances
End moraine
Debris at toe of a glacier
Moraines
Unsorted debris deposited by a glacier
Lateral moraines
Form along the flank of a valley glacier
Medial moraines
Mid-ice moraine from merging of lateral moraines
Types of glacial sedimentary deposits from glacial drifts (6)
- Glacial till
- erratic
- glacial marine sediments
- glacial outwash
- loess
- glacial lake-bed sediment
Stratified vs Unstratified drift
Stratified drift is water sorted, unstratified is not sorted
Glacial till
Sediment dropped by glacial ice
- Consists of all grain sizes - boulders to clay
- Unmodified by water: Unsorted and Unstratified
- Accumulates: beneath glacial ice, at the toe of a glacier, along glacial flanks
Erratics
Boulders dropped by glacial ice and carried through long distances; rocks existing where they don’t “belong:
Glacial marine deposits (Carving vs Melting Icebergs)
Sediments from an oceanic glacier
- Calving iceberg rafts sediments
- Melting iceberg deposits drop stones
Glacial outwash
Sediments transported by meltwater
- Mud is removed
- Sizes graded and stratified
- Grains abraded and rounded
- Dominated by sand and gravel
Glacial lake-bed sediment
Fine rock flour settles out of suspension in deep lakes
Muds display seasonal varve couplets (seasonal variations in depositional sequences)
- Finest silt and clay: winter
- Coarser silt and sand: summer
Loess
Wind-transported silt
Glacial depositional landforms (7)
- End moraines
- terminal moraines
- recessional moraines
- ground moraines
- drumlins
- kettle lakes
- eskers
Where / how do end, terminal, and recessional moraines form?
End - the stable toe of the glacier
Terminal - farthest edge of the flow
Recessional - form as retreating ice stalls
How do kettle lakes form?
They form from stranded ice blocks
Drumlin
Long, aligned hills of molded till
- Asymmetric form
- Commonly occur as swarm aligned parallel to flow direction
Eskers
Long, sinuous ridges of sand and gravel that form as meltwater channels within or below ice; channel sediment is released when the ice melts
Consequences of continental glaciation in relation to earth’s mechanical layers
Ice loading and glacial rebound
- Ice sheets depress the lithosphere
- Slow crustal subsidence follows the flow of asthenosphere
- After ice melts, depressed lithosphere rebounds
- Glacial rebound continues today
What happens to sea level during ice age and deglaciation?
Ice age - sea level falls
Deglaciation - sea level rises
If ice sheets melted, coastal regions would be flooded
Pluvial features
Large lakes formed during ice age
Periglacial environments
(near ice)
Characterized by year-round frozen ground (permafrost)
Freeze-thaw cycles generate unusual patterned ground
Pleistocene Ice Ages
All climate and vegetation belts were shifted southward
Giant beavers, sloths, and mammoths were the existing animals
