Week 10: Glaciers

What is a glacier?

A permanent body of ice, consisting largely of recrystallized snow

Shows evidence of slow downslope or outward movement due to it’s own weight

How does a glacier form?

More snow accumulates in the winter than the amount that melts, increasing snow depth gradually

Pressure recrystallizes deep snow into denser ice with less air space

Ice and snow thicken so that gravity causes the frozen mass to move

What are the 2 main glacier categories?

Mountain glaciers

Continental glaciers

Describe mountain glaciers?

Glaciers in mountainous regions that flow from high to low elevation

Describe continental glaciers?

Only found in Greenland and Antarctica today

Are over 50,000 km² in area

Extremely thick (up to 4km in Antarctica)

What happened in the Last Glacial Maximum?

The peak of the last ice age (21 ka) had extensive continental ice sheet, especially in Northern Hemisphere

Sea level was lower by 120 m

What are tidewater Glaciers?

Glaciers that meet the seaW

What are ice shelves?

Ice sheets extend over the sea and float on water

they are 250m-2.5 km thick

What are icebergs?

Free floating chunks of ice formed through calving of glaciers and ice shelves (breaking apart)

Can transport large sediment grains to open ocean (ice rafted debris/dropstones)

How does a glacier move?

When the gravitational force exceeds the strength of ice

In the top 60m of a glacier, is is too brittle to flow, so breaks to form large open crevasses

Below 60 m depth, glacial ice deforms by plastic deformation (when grains change shape and crystallize slowly)

Glaciers can also move with a wet base (basal sliding)

What is the glacial mass balance? Define the terms involved?

Glacial mass balance = accumulation - ablation

Accumulation: Addition of ice to the glacier from snowfall

Ablation: The loss of ice from the glacier (melting, sublimation, calving)

The equilibrium line separates areas of net accumulation from areas of net ablation

When does glacial advancement and retreat occur?

Advancement: When accumulation is greater than ablation (More ice is added than loss)

Retreat: When ablation is greater than accumulation (More snow is loss than added)

Describe characteristics of glacial erosion?

Rock fragments embedded in glacial ice abrade and polish the substrate, producing finely pulverized “rock flour”

The rock flour can act as suspended sediment in water, making the water appear as vibrant colors (Ie: Lake Louise, Banff, Canada)

Larger rocks gouge lines in bedrock (striations)

How does glacial erosion shape landscapes?

During glaciation, valleys fill with ice and are aggressively eroded and over steepened

Afterwards, U-shape valleys, hanging valleys, cirques, aretes, and horns are formed

What is a cirque?

A bowl-shaped basin formed at the uppermost portion of a glacial valley

After the ice melts, a cirque is often filled with a tarn lake

What is a horn?

A pointed mountain peak formed by three+ cirques that coalesce