Copy Of Glacial Erosion


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Glacial Erosion

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Copy Of Glacial Erosion

  1. 1. <ul><li>PROCESSES OF GLACIAL EROSION: </li></ul><ul><li>Glacial Abrasion (create striations / chatter marks / polished surface) </li></ul><ul><li>Plucking </li></ul><ul><li>Sub-glacial water erosion. </li></ul>
  2. 3. Striations on Glacially Scoured Rock Source:
  3. 4. Meltwater stream emerging from the snout of a glacier – the milky appearance is due to rock flour Source:
  4. 5. A boulder dislodged by the process of plucking Source:
  5. 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
  6. 7. Factors affecting rates of Plucking <ul><li>Nature of the bedrock (presence of joints) </li></ul><ul><li>Weakening of bedrock by weathering making blocks more susceptible to erosion (dilation / frost shattering) </li></ul><ul><li>Pressure Melting Point </li></ul>
  7. 8. Relationships between factors affecting glacial erosion: <ul><li>Influence of basal water pressure on sliding of basal ice and therefore on rates of abrasion </li></ul><ul><li>Relationship between velocity , pressure and rates of abrasion </li></ul><ul><li>Relationship between debris supply / concentration, sliding velocity and rates of abrasion </li></ul>
  8. 9. Diagram showing theoretical relationship between rates of abrasion, ice velocity and pressure Source:
  9. 10. Relationship between basal debris concentration, sliding velocity and abrasion rates.
  10. 11. Spatial / Temporal Variations in factors affecting glacial erosion <ul><li>Spatial Variations ( from place to place) </li></ul><ul><li>Differences in bedrock </li></ul><ul><li>Variations in debris availability (linked to rates of weathering) </li></ul><ul><li>Basal thermal regime (cold based / warm based glaciers) </li></ul><ul><li>Temporal Variations ( over time) </li></ul><ul><li>Ice thickness (variations in glacial budget over the course of a year) </li></ul><ul><li>Meltwater availability </li></ul><ul><li>Concentration of basal debris </li></ul>
  11. 12. Rates of Glacial Abrasion <ul><li>These vary significantly: </li></ul>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: 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
  12. 13. <ul><li>Maximum erosion is likely to occur: </li></ul><ul><li>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) </li></ul><ul><li>Where temperatures fluctuate around 0oC allowing frequent freeze-thaw </li></ul><ul><li>In areas of jointed rock – easily frost shattered (providing debris for erosion, and jointed bedrock will allow plucking to occur more easily) </li></ul><ul><li>Where two tributary glaciers join / valley narrows – gives increased depth of ice </li></ul><ul><li>Steep mountainous regions in temperate latitudes where the velocity of the glacier is greatest. </li></ul>
  13. 14. <ul><li>Example of Essay </li></ul><ul><li>Distinguish between the glacial erosion processes of abrasion and plucking (5) </li></ul><ul><li>(b) Assess the relative importance of the factors influencing rates of glacial erosion (20) </li></ul><ul><li>Total: 25 marks </li></ul><ul><li>Remember plan your essay carefully and consider the structure carefully – a clear beginning, middle and end are needed. </li></ul>