The processes of erosion - Abrasion

Question 1

When does glacial abrasion take place?

Answer 1

Glacial abrasion takes place when the base of the ice mass slides over the bedrock surface and basal debris grinds, scratches and polishes it, producing smooth surfaces and striations or striae.

Question 2

What is produced as a result of glacial abrasion?

Answer 2

• Smooth surfaces
• Striations or striae

Question 3

What are striations?

Answer 3

Striations are scratches or miniature grooves engraved into a hard rock surface by rock fragments transported by ice.

Question 4

Abrasion is confined to which type of ice sheets?

Answer 4

Abrasion is confined to warm-based ice sheets where meltwater and abundant debris facilitate the process.

Question 5

Which two things facilitate the process of abrasion?

Answer 5

• Meltwater
• Abundant debris

Question 6

Give three characteristics of glacial abrasion?

Answer 6

• Abrasion is negligible beneath cold-based glaciers owing to the lack of meltwater.
• Abrasion is controlled by ice velocity and the concentration, angularity and hardness of rock debris transported by the ice
• Abrasion is most effective when ice velocities are higher as more debris passes a given point per unit of time
• The process also requires that rock debris is present at the ice/rock interface and the rock particles are harder than the bedrock surface underneath.

Question 7

Abrasion grinds down rock material to produce what?

Answer 7

Abrasion grinds down rock material to produce rock flour (silt-sized particles of 0.1mm diameter or less).

Question 8

What impact does rock flour have on the streams and lakes on the margins of glaciers?

Answer 8

Rock flour gives the characteristic milky blue-green colour to streams and lakes on the margins of glaciers.

Question 9

How does meltwater aid the abrasion process?

Answer 9

Meltwater further aids the abrasion process by flushing away rock flour that might otherwise prevent direct contact between the abrading clasts and the underlying rock.

Question 10

Where are abrasion rates highest and where are they the lowest?

Answer 10

Abrasion rates are highest where glacier ice is warm-based, thick and fast and the bedrock is relatively weak. They are lowest wherever glaciers are cold-based and the bedrock is very resistant.