Glacial Erosion

1. Glacial Erosion

3. How do Glaciers Move? • Internal deformation • Extending Flow • Compressive Flow • Basal Sliding • Regelation • Surging • Lateral Shearing • Creep • Rotational Flow

5. 1. Processes of Glacial Erosion 1. Abrasion 2. Plucking 3. Pressure Release (Dilitation) 4. Subglacial Water Erosion

6. 1. ABRASION With it's load of abrasive rock fragments, the base of the glacier acts like a belt sander, scraping across the rock, eroding it, producing characteristic erosional features, and creating a supply of material that leads eventually to the formation of depositional features as well. This scraping process is called Abrasion.

7. Factors affecting Abrasion • Hardness of particles and bedrock • Ice thickness • Basal Water Pressure • Sliding of Basal Ice • Movement of debris towards glacier base • Efficient removal of fine debris • Debris particle size and shape • Presence of debris in basal ice

8. STRIATIONS

10. Striations When a glacier moves across the underlying rock, the process of abrasion wears it away. It is the fragments of rock held in the ice that do the abrading, scraping across the rock surface like nails across a wooden desk top. Larger rock fragments leave deep scratch marks behind them. These scratch marks are straight parallel lines that reveal the direction of ice movement. Freshly exposed striations have a preferred orientation of rock grains. By lightly running a finger along the striation it is possible to discover that when moving one way along it, the rock feels smooth, but when moving the other way it feels more coarse. The moving ice leaves the rock grains aligned with the direction of movement, so when the striation feels smooth, your finger is moving in the direction of ice flow.

12. Striation Mt. Sirius, Antarctica, 1986 .

13. The Grooves on Kelley's Island have been the source of debate for over 100 years. Some say they were cut by glacier ice, others say by jets of subglacial water. Note the curved forms suggesting fluid flow.

14. Chattermarks: Mt. Sirius, Antarctica, 1986

15. Molded Wall, New Zealand • Lateral glacial abrasion has smoothed this metamorphic rock along the valley side. The flowlines indicate a downward direction.

16. 2. Plucking Occasionally, a moving glacier may become stuck on its bed. This occurs when for some reason a reduction in pressure causes liquid water to freeze, attaching the moving ice to the bedrock. As the ice continues to move an immense pulling force is applied to the attached rock which may then fracture and be plucked from its position. It involves the removal of much larger fragments of rock than abrasion.

17. The Growth of a Corrie • Snow falls on a north-facing slope and tends to stay (ie does not melt) as in the northern hemisphere it is colder here. Hence there is minimal (generally in summer) ablation and accumulation continues. Therefore, the Net Balance is positive - hence the glacier grows - and increases in size. • The snow eventually forms FIRN - and eventually a glacier is born (I.e. the accumulation of years of compressed snow). • At the ice-rock interface freeze-thaw starts to operate - attacking the bedrock and loosening rock particles, which break off and are used as tools of abrasion. These form striations, chattermarks and grooves in the bedrock - and SCOUR out the bedrock - hollowing it out. • The glacier “flows” downhill under gravity and ROTATIONAL FLOW exentuates this process

18. The Growth of a Corrie cont. • Over time, the arm-chair shaped hollow is formed - I.e the classical shape of a typical corrie (cirque, cwm). • Eventually, due to Milankovitch, the Ice Ages ebbed away and a corrie is left over - which has the typical shape of: • Upper cliff section • Scree slope • Armchair hollow (typically with a small lake left over) • Rock Lip (where the Ice Fall once would have been) • Example = Cwm Idwal, Nant Francon Valley, North Wales • Where 2 corries erode backwards an Arete is formed, and where 3 or more erode backwards a Pyramidal Peak is formed - eg Matterhorn, Switzerland

42. Plucking - eg Roche Mountonnees

43. Plucking

44. Roche Mountonnee

45. Stoss and Lee outcrop, Mount Desert Island, Maine • The various steps and controlling fractures are evident here. Notice that the general shape of the outcrop suggests ice diversion slightly upward.

46. Plucked Face, The Beehive, Mount Desert Island, Maine • Jointing patterns in the granite bedrock define the size and shape of blocks • that can be removed. Ice flow from left to right

47. 3. Pressure Release (Dilitation) • When a glacier erodes, the replacement of a certain volume of rock • by ICE (one-third of its density), causes dilitation and the separation of the rock along sheet joints. • For example after the Last Glacial Maximum in Europe, in the warmer interglacial period, the Laurentide Ice Sheet began to retreat. Consequently, a huge amount of ice melted from the land, releasing trillons of tons of pressure on the earths surface. As a result of this pressure release, rocks buckled along existing cracks and joints - making them vulnerable to processes of weathering and erosion in the Periglacial environment that followed.

48. 4. Sub-Glacial Water Erosion • Temperate glaciers, especially in summer, have many meltwater streams on their surface, which plunge down crevasses (called moulins) into the base of the glacier. Many such streams descend right to the valley floor, where they are another cause of erosion of the rock surface. This is particularly true of those which carry a large load of sediment, rock flour or material of morainic origin.

50. 2. Factors Affecting Erosion • Flow Types • Glacier Size • Temperature • Gradient • Weathering • Regime • Periglacial Processes • Rock Types - Geology