Glacial Erosion
Sub-aerial weathering
- Freeze-thaw or frost shattering, happens when rocks contain cracks and where temperatures regularly dip below the freezing point
- Any water in the cracks will freeze as the temperature drops, which expands as it freezes, exerting pressure on the crack
- Repeated freezing and thawing of water will eventually break the rock apart and it will pile up as scree at the foot of the slope
- When trapped under the ice, the sharp, angular rocks are an effects abrasive tool
Carbonation
- Is an important process in cold environments and occurs in rocks with calcium carbonate, such as chalk and limestone
- Rainfall (pH of 5.6 ) combines with dissolved carbon dioxide or organic acid to form a weak carbonic acid solution
- Calcium carbonate (calcite) in rocks, reacts with the acidic water and forms calcium bicarbonate, which is soluble and removed in solution by meltwater
- The effectiveness of the solution is related to the pH of the water as carbon dioxide is more soluble at lower temperatures
Nivation
- A blanket term for active processes that occur at the edges of snow patches
- The processes include the physical and chemical weathering that occur underneath patches of snow
- Fluctuating temperatures and meltwater promote chemical weathering and freeze-thaw action
- Weathered material is transported with the summer meltwater
- Repeated cycles of melting, freezing, and transportation form nivation hollows
- Saturated debris (due to summer meltwater), destabilises the slope and slumping may occur
Glacial erosion
- After glaciers break down the rock through freeze-thaw action, erosion continues the process by plucking and abrasion
- Plucking:
- Movement of the ice mass generates friction and heat, causing the base of the glacier to slightly melt
- This meltwater freezes around rocks and stones under the glacier
- As the glacier moves forward, it 'plucks' this ice, pulling the rock away
- Quarrying:
- Similar to plucking in that pieces of bedrock are transported and eroded within the glacier
- As a glacier moves through a valley, pressure is exerted on the sides and bottom of the valley
- Friction causes melting, allowing meltwater to surround the rocks in the valley
- As the meltwater refreezes, it pulls on the ice and quarry's the sides of the valley away
- Abrasion:
- Abrasion occurs as bits of rocks, stones, and boulders stuck in the ice, grind against the rock below the glacier wearing it away and producing rock flour
- Striation (scratch) marks arise when rocks beneath the glacier are transported across the bedrock
- It is the weight of the ice in a glacier that forces it to advance downhill, eroding the landscape as it moves
- Ice advances in a circular motion called a rotational slip, which hollows and deepens the landscape
- Crushing
- This happens when pressure exerted by the ice mass and its debris, crushes the bedrock surface leaving chattermarks fractures as it moves over the bedrock
- Basal melting
- As pressure increases, the melting point of water decreases
- The thicker the glacier, the greater the pressure; the lower the temperature at which water melts
- As temperate glaciers move down the valley, friction melts the glacier's base
- This layer of meltwater acts as a lubricant and allows the glacier to 'float' allowing basal sliding and the glacier can move faster
- Mass movement
- Can occur quickly with the sudden movement of large ice masses, usually due to basal slipping - ice sheet calving is a good example of mass movement
- Processes do not work in isolation or at the same rate, there are continuous adjustments, particularly after glacial retreat; where landforms are not only shaped but reshaped by by the combined action of mass movement, weathering, erosion, and fluvial action