Glaciers Flashcards
Why study glaciers?
- Glacial ice represents ~84% of earths fresh water
- Glacial deposits account for large areas of fertile soil in the Northern US and southern Canada
What is a glacier?
A glacier is an accumulation of snow and ice thick enough to flow under its own weight
What are the 3 types of glaciers?
- Valley glacier
- Ice sheet
- Ice caps
Valley glaciers:
Are long, narrow glaciers confined to bedrock valleys that flow from high elevations to low elevations like a stream
Ice sheets:
Are unconfined glaciers larger than 50,000km squared. There are two ice sheets on earth in Greenland and the Antarctica
Ice caps:
Are unconfined glaciers that are smaller than ice sheets. Both ice sheets and ice caps exhibit radial flow
Where do glaciers form?
They form where snow persists year-round
- High altitudes where temperatures are cooler than the surrounding country side
- Snowline where the elevation above which snow persists throughout the year
- High altitudes where seasons are colder
How thick are valley glaciers?
They are commonly 50-300 meters thick
How thick is an ice sheet?
The Antarctic ice sheet is as thick as 4km and contains 29 million km cubes of glacial ice
Snow metamorphism
Snow is a mineral that metamorphoses at temperatures and pressures close to Earth’s surface
Stages of snow metamorphism
- Snowflakes turn to rounded ice grains and become more compact
- Rounded grains recrystallize as contacting grains transfer molecules from one to another, diminishing air space
- The result is interlocking crystals with a density of ~0.9gm/cm cubed
How long does it take for glacial ice to form?
In Alaska, hard ice has been found at 15m which are 3-5 years old
Areas such as Alaska have a ____ cycle
Freeze/thaw cycle
How do glaciers move?
They move faster in the middle and slower at the edges
Glaciers flow from the zone of ____ to the zone of ______ due to….
They flow from the zone of accumulation to the zone of wastage due to mass increase in the zone of accumulation
Which way do glaciers move?
They move downslope whether they are in advance or retreat at their toe
Glacial advance/retreat depends on…
The ratio of accumulation to wastage
The bottom temperature of a glacier is related to…
Sliding and sliding is related to erosion
Water lubricates which reduces…
Cohesion and friction
Water is pressurized by…
The weight of the overlying ice
Water pressure at the base of some glaciers is…
Nearly equal to the weight of the ice which substantially reduces friction by lifting the glacier off of the underlying rock
How do glaciers erode and transport sediment?
Think of glaciers as “rivers of ice” they share similarities to rivers
- They flow downhill, erode sediment and transport it
Where does erosion by glaciers happen?
Along the base, especially upslope where the ice is actively flowing downward, and along the valley walls
Glaciers transport sediment along…
the base, within the ice, and atop the glacier due to mass wasting along the valley walls and by sediment exposure by thawing
Where two glaciers meet…
their lateral moraines meet to form a medial moraine in the middle of the merged glacier
Till
Is sediment directly deposited by the glacier. Cobbles often have edges and striations
Outwash
Is sediment carried out by the meltwaters from a glacier. Cobbles are often stream-rounded
How do glaciers deposit sediment?
- Bottom sediment increases friction to the point where sediment sticks and bits are left behind
- Fragments lodged in the ice or on the surface get left behind
- Melting at the bottom of the glacier releases rock fragments frozen into the base of the flowing ice
Meltwater streams
Deposit sediment alongside of and underneath the glacial ice forming sinuous ridges in the ice tunnels and channels that are left behind as benches and ridges
Hoe do glaciers modify the landscape?
- Glaciers do more erosive work than streams
- In some places, glaciers are the dominant force of creating landscapes, especially in high, cold, mountainous regions
What are the 5 main glacial landscape features?
- Moraines
- U - shaped valleys
- Knife edge ridges and pointed peaks
- Overdeepened valleys forming glacial lakes
- Hanging valleys
Moraine
- Any unconsolidated material dropped off by the glacier
- Composed of glacial till that is unsorted
End moraine
- Till at the end of the glacier
- the lowest elevation
Medial Moraine
- Middle of glacier
- Joining of lateral moraines where glaciers combine
Lateral Moraine
- Along the sides of a glacier
- A ridge of unconsolidated rock debris along the valley side.
Crevasse
- A crack in the upper 30 meters (100 feet) of a glacier
- often perpendicular to flow (not always)
Arête
- Narrow sharp ridges separating two valleys
How do arêtes form?
From the erosion of two parallel valley glaciers OR erosion of two cirque glaciers headwards
Horn
- Meeting point of multiple cirques
- Forms a pyramidal peak
Glacial Trough
A glacially eroded valley
Hanging valleys:
Occur due to the more rapid erosion in the main glacier channel versus tributary channels causing a large vertical difference between the channels once the glacier melts
What are the 4 main landscape features associated with ice sheets?
- Large areas of scoured, plucked, and abraded rock surfaces
- Large regions thickly covered by till
- Streamlined ridges that parallel the direction of glacier movement
- Landscape of countless lakes, ranging from small ponds to the largest lakes on Earth formed by erosional and depositional processes
Drumlim:
- Low oval hill made of deposited till
- Upstream side is steep, downstream side is tapered
- 100-5000m long
Esker:
- Narrow ridge of coarse sand and gravel, formed by subglacial meltwater channel
- Sinuous but aligned parallel to glacial flow
- Not always continuous
Kettle:
- Steep sided hole
- Formed by gradual melting of large ice block left behind by glacier over period while sediment accumulated around it
Kame:
- Any stratified ice deposit
- Deposited in openings within or between ice blocks
What happens when glaciers reach the ocean?
Tide water glaciers
Tide water glacier:
occurs when a glacier descends into the sea and does not float. But when it does, it becomes an ice shelf
Icebergs
Are fragments that break off due to wave erosion oversteepening the face of the tidewater glacier
Ice Shelf
Is a segment of a tidewater glacier that is sufficiently submerged to buoy up and float
What must glacial ice be like in order to float on the ocean?
Glacial ice is nearly as dense as seawater, therefore it must be 9/10ths submerged before it can float
