Glaciation Pack D

Question 1

How does ice move by basal sliding?

Answer 1

• Only in warm based glaciers as it requires meltwater at the base to act as a lubricant
• Reduces friction between the bedrock and debris at the glacier base

Enhanced basal creep:
- Basal ice deforms around irregularities on the underlying bedrock surface
- The pressure melting point is not reached so crystals deform around the large rock obstacle
- State of plastic flow

Regelation slip:
- Basal ice deforms under pressure when encountering small obstructions such as ‘rock steps’
- As glacier moves over it, the pressure on the basal ice increases
- In the area of high pressure, the ice melts and flows around the obstacle
- After it has flowed over, the pressure is lowered and the water refreezes

Question 2

How does ice move by subglacial bed deformation?

Answer 2

• Occurs locally when a glacier moves over relatively weak or unconsolidated rock
• The sediment deforms under the weight of the glacier which moves the ice on top along with it

Question 3

How does ice move by internal deformation?

Answer 3

• Ice deforms under its own weight
• Bottom of the ice crystal remains stationary and this forms a base for movement
• Gravity moves the top of the ice crystal downhill due to the shear stress placed on the crystal

Intergranular flow:
- Individual ice crystals deform under pressure and move in relation to each other

Laminar flow:
- Movement of individual layers within the glacier

Question 4

How does a glacier move by surging?

Answer 4

• A surge is period of rapid movement when a glacier advances 100x faster than normal
• A result of a change in the flow pattern of subglacial meltwater
• Cycle happens every 10 to 20 years
• Water builds up under the glacier during a phase of normal glacial flow and there is an increase in accumulation
• The weight of the ice accumulation prevents meltwater draining away and there is more due to the pressure melting point
• Meltwater separates the basal ice from the bedrock, lubricating it and allowing overlying ice to flow more freely
• The surge releases meltwater and glacier subsides onto bedrock, resuming normal flow

Question 5

How does ice move by compressional and extensional flow?

Answer 5

Extensional flow:
- Slope gradient increases
- Acceleration in ice movement
- Ice thins and surfaces of ice cracks, forming crevasses and bergschrunds

Compressional flow:
- Reduction in gradient causes ice to decelerate
- Ice thickens, crevasses close and thrust faults develop

Question 6

What factors affect the rate of movement of ice?

Answer 6

Size - greater mass has a greater potential velocity
Lithology - affects processes at the base and the level of friction
Mass balance - rate of accumulation and ablation which affects equilibrium and therefore advance or retreat
Altitude - affects temp and precipitation and therefore accumulation and meltwater
Gradient - steeper slops means faster movement