Landforms of glacial erosion - Cirques

Question 1

What are cirques?

Answer 1

Cirques are armchair shaped hollows formed by the erosion of a nivation hollow by a glacier.

Question 2

Cirques are formed by the erosion of what?

Answer 2

Cirques are formed by the erosion of a nivation hollow by a glacier.

Question 3

Which 4 processes deepen the nivation hollow?

Answer 3

Nivation
Freezing and thawing
Chemical weathering
Removal of material by melting snow

Question 4

Cirques form in hollows where snow is able to do what?

Answer 4

Cirques form in hollows where snow can accumulate and, over time, form a glacier. The snow compacts into ice and this accumulates over many years.

Question 5

What is a cirques preferred orientation in the Northern Hemisphere?

Answer 5

In the Northern Hemisphere, this tends to be northeast-facing slopes which, because or their aspect, are protected from the sun: it is in these locations that accumulation is highest and ablation is lowest.

Question 6

Cirque development provides evidence of what type of feedback in the glacial system?

Answer 6

Positive feedback

Question 7

As a cirques capacity for storing and accumulating ice grows, so too does ice erosion, what impact does this have on the rate of cirque growth?

Answer 7

The rate of cirque growth accelerates further

Question 8

The back of the cirque is formed by what process?

Answer 8

Freeze-thaw weathering

Question 9

Explain how the processes of freeze thaw weathering and plucking create the steep headwall of the cirque?

Answer 9

Freeze thaw weathering loosens material before plucking removes it from the back of the hollow thereby creating a steep headwall.

Question 10

What is the landscape evidence for plucking?

Answer 10

The cirque headwall is characterised by a jagged edge.

Question 11

What impact does freeze thaw weathering that takes place above the hollow on exposed rocks have?

Answer 11

Freeze thaw weathering above the hollow on the exposed rocks shatters the rock and delivers shattered rock, known as scree, to the ice.

Question 12

Where is scree found in relation to the ice?

Answer 12

Scree is found on top of the ice, within it and under it.

Question 13

At a critical depth, what is the impact of the weight of the ice?

Answer 13

At a critical depth, the weight of the ice results in its rotational movement.

Question 14

What is the impact of the rotational movement of ice?

Answer 14

• The rotational movement of ice leads to the abrasion of the hollow and causes ice to thrust in an upward direction on the down-glacier side of the basin
• Plucked debris and rock that has been fractured by freeze-thaw weathering is therefore dragged across the base of the rock, deepening the floor of the hollow by abrasion and forming a rock basin

Question 15

In order for basal sliding to occur, what is needed?

Answer 15

Water

Question 16

What is a Bergschrund crevasse and how does it develop?

Answer 16

A bergschrund crevasse is a large crevasse that develops as the glacier pulls away from the cirque headwall.

Question 17

Which factor promotes basal sliding?

Answer 17

• Meltwater

Question 18

How does the Bergschrund crevasse increase the volume of basal sliding?

Answer 18

Water trickles down from the Bergschrund crevasse which leads to more basal sliding.

Question 19

How does the weight of the ice increase the rate of basal sliding and abrasion?

Answer 19

The weight of the ice may cause the pressure melting point to be surpassed. This generates meltwater at the base and leads to more basal sliding and abrasion.

Question 20

Which two processes steepen the headwall of the cirque and deepen the hollow?

Answer 20

The combined processes of weathering and erosion steepen the headwall of the cirque and deepen the hollow.

Question 21

What is the impact on the rate of erosion when the flow of the glacier changes from extending flow to compressional flow?

Answer 21

The rate of erosion decreases

Question 22

When is a rock lip formed?

Answer 22

A rock lip is formed when the rate of erosion decreases due to the flow of the glacier changing from extending flow to compressional flow. It is the result of reduced energy and is often heightened by the deposition of moraine.