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
