PROCESSES OF GLACIAL EROSION:

(iii) Glacial Abrasion (create striations / chatter marks / polished surface)

(iv) Plucking

(v) Sub-glacial water erosion.

Striations on Glacially Scoured RockSource: http://www.bgrg.org/pages/education/alevel/coldenvirons/Lesson%2010.htm

Meltwater stream emerging from the snout of a glacier – the milky appearance is due to rock flour Source: http://www.bgrg.org/pages/education/alevel/coldenvirons/Lesson%2010.htm

A boulder dislodged by the process of plucking

Source: http://gemini.oscs.montana.edu/~geol445/hyperglac/eropr

oc1/plucking.htm

Factors related to presence /

characteristics of debris

KEY:Factors related to the ice

FACTORS AFFECTING RATES OF GLACIAL ABRASION

GLACIAL ABRASION

Relative hardness of particles and bedrock

Ice thickness

Basal Water Pressure Movement of debris to the base

Sliding of Basal Ice

Removal of fine debris

Debris size and shape

Presence of Basal Debris

Concentration of Basal Debris

Velocity

Other factors

Factors affecting rates of Plucking

• Nature of the bedrock (presence of joints)

• Weakening of bedrock by weathering making blocks more

susceptible to erosion (dilation / frost shattering)

• Pressure Melting Point

Relationships between factors affecting glacial erosion:

• Influence of basal water pressure on sliding of basal

ice and therefore on rates of abrasion

• Relationship between velocity, pressure and rates of

abrasion

• Relationship between debris supply / concentration,

sliding velocity and rates of abrasion

Diagram showing theoretical relationship between rates of abrasion, ice velocity and pressure

Source: http://www.bgrg.org/pages/education/alevel/coldenvirons/Lesson%2010.htm

Relationship between basal debris concentration, sliding velocity and abrasion rates.

Spatial / Temporal Variations in factors affecting glacial erosion

Spatial Variations (from place to place)• Differences in bedrock• Variations in debris availability (linked to rates of

weathering)• Basal thermal regime (cold based / warm based glaciers)

Temporal Variations (over time)• Ice thickness (variations in glacial budget over the course

of a year)• Meltwater availability• Concentration of basal debris

Rates of Glacial AbrasionThese vary significantly:

During the 1970s Geoffrey Boulton, a leading glacial geomorphologist, fixed rock plates (composed of

marble) in front of moving ice at the base of two glaciers. After the glacier had overridden the plates

they were removed and inspected (Table 1).

250 ma-1100 m36 mma-1Glacier d Argentiere (France)

15.4 ma-1 32 m3.75 mma-1Breiamerkerjokull 3

19.5 ma-115 m3.4 mma-1Breiamerkerjokull 2

9.6 ma-140 m3 mma-1 Breiamerkerjokull 1 (Iceland)

VelocityIce thicknessAbrasion rateGlacier

Table 1. Rates of glacial abrasion. Source Boulton 1974.

Source: http://www.bgrg.org/pages/education/alevel/coldenvirons/Lesson%2010.htm

Look at the figures in the table above for the Breidamerkurjokull glacier – which of the factors appears to be most important? Ice thickness or velocity?

Maximum erosion is likely to occur:

• In Polythermal glaciers as abrasion and plucking can occur (cold

(subfreezing) ice at the surface, margins, and terminus of the glacier

surrounds a core zone where ice at the glacier bed is at the pressure melting

point and basal melting occurs)

• Where temperatures fluctuate around 0oC allowing frequent freeze-thaw

• In areas of jointed rock – easily frost shattered (providing debris for erosion,

and jointed bedrock will allow plucking to occur more easily)

• Where two tributary glaciers join / valley narrows – gives increased depth

of ice

• Steep mountainous regions in temperate latitudes where the velocity of the

glacier is greatest.

Example of Essay

(c) Distinguish between the glacial erosion processes of abrasion and plucking (5)

(b) Assess the relative importance of the factors influencing rates of glacial erosion (20)

Total: 25 marks

Remember plan your essay carefully and consider the structure carefully – a clear beginning, middle and end

are needed.