copy of glacial erosion
DESCRIPTION
Glacial ErosionTRANSCRIPT
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.