Glacial Abrasion(create striations / chatter marks / polished surface)
Sub-glacial water erosion.
2. 3. Striations on Glacially Scoured Rock Source: http://www.bgrg.org/pages/education/alevel/coldenvirons/Lesson%2010.htm 4. Meltwater stream emerging from the snout of a glacier the milky appearance is due to rock flourSource: http://www.bgrg.org/pages/education/alevel/coldenvirons/Lesson%2010.htm 5. A boulder dislodged by the process of plucking Source: http://gemini.oscs.montana.edu/~geol445/hyperglac/eroproc1/plucking.htm 6. KEY: Factors related to presence / characteristics of debris 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 7. 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
8. Relationships between factors affecting glacial erosion:
Influence ofbasal water pressureonsliding of basal iceand therefore on rates of abrasion
Relationship betweenvelocity ,pressureand rates of abrasion
Relationship between debris supply / concentration, sliding velocity and rates of abrasion
9. Diagram showing theoretical relationship between rates of abrasion, ice velocity and pressure Source: http://www.bgrg.org/pages/education/alevel/coldenvirons/Lesson%2010.htm 10. Relationship between basal debris concentration, sliding velocity and abrasion rates. 11. 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)
Concentration of basal debris
12. Rates of Glacial Abrasion
These 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).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? 250 ma -1 100 m 36 mma -1 Glacier d Argentiere (France)15.4 ma -1 32 m 3.75 mma -1 Brei amerkerjokull 3 19.5 ma -1 15 m 3.4 mma -1 Brei amerkerjokull 2 9.6 ma -1 40 m 3 mma -1 Brei amerkerjokull 1 (Iceland) Velocity Ice thickness Abrasion rate Glacier 13.
Maximum erosion is likely to occur:
InPolythermal glaciersasabrasionandpluckingcan 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)
Wheretemperatures fluctuate around 0oCallowing frequent freeze-thaw
Inareas of jointed rock easily frost shattered (providing debris for erosion, and jointed bedrock will allow plucking to occur more easily)
Wheretwo tributary glaciers join / valley narrows gives increased depth of ice
Steep mountainous regionsin temperate latitudes where the velocity of the glacier is greatest.
Example of Essay
Distinguish between the glacial erosion processes ofabrasionandplucking(5)
(b) Assess the relative importance of the factors influencing rates of glacial erosion (20)
Total: 25 marks
Rememberplanyour essay carefully and consider the structure carefully a clear beginning, middle and end are needed.