Glacial Processes (AQA A Level Geography)

• Glaciers create distinct landforms through weathering, erosion, transportation and deposition
• 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 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 physical and chemical weathering that occurs 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 

Diagram showing the process of nivation and the formation of hollows

• Glaciers move very slowly, under the force of gravity 
• Movement in the upper zone of the glacier forms crevasses as the ice is relatively brittle and cracks
• The lower zone has a steady pressure which along with meltwater and frictional heat, leads to easier/faster movement

Types of glacial movement

Diagram showing basal sliding and internal deformation as ice flows

Diagram showing compressional, rotational and extensional ice flow

• After glaciers break down the rock through freeze-thaw action, erosion continues the process by plucking and abrasion
• Plucking:
  • As a glacier moves through a valley, pressure is exerted on the sides and bottom of the valley
  • This generates friction and heat, causing the edges of the glacier to melt a bit
  • This meltwater freezes around rocks and stones under the glacier
  • As the glacier moves forward, it 'plucks' this ice, pulling the rock 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 which 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

Main forms of glacial erosion

• Glaciers transport material such as clay, rock, and sand in the body, base and surface of the glacier over long distances
• The front of a glacier is called the 'snout' and acts as a bulldozer pushing loose rocks and debris (glacial till) downhill by the sheer force of the moving ice
• Overall, glaciers carry material in 3 ways:
  • Weathered material is carried on the top of the ice - supraglacial
  • Supraglacial material is covered by snowfall and is then carried within the glacier - englacial
  • Material dragged beneath the ice by the glacier - subglacial

Cross-sectional image showing three methods of glacial transportation

Deposition

• During the warmer summer months, glaciers begin to melt, and glacial till is deposited on the valley floor or sides of a moving glacier
• Till is unsorted, irregular debris ranging from clay to boulders of any size and shape 
• Meltwater will also flow out of the glacier's snout forming meltwater rivers
• These rivers carry large amounts of glacial till, which will undergo further erosion through attrition to become outwash 
• This finer till is sorted and when the energy of the river reduces, the outwash is deposited in layers further down the valley on the outwash plain

Glacial till                                                                                   Glacial outwash plain

Controlling factors

• The rate of ice flow is controlled by the following factors:

  • Gravity - the steeper the gradient the greater the pull of gravity
  • Friction - has to be overcome by the ice in order to move
  • Mass of the ice - the heavier the ice, the more potential it has to move
  • Meltwater - acts as a lubricant

  • Temperature - cold-based glaciers are frozen to the bedrock