Geomorphic Processes

• Weathering

  When heat energy breaks down surface rock in situ to produce physically and chemically altered materials
• Three types of weathering

  • Physical - main type!
  • Chemical
  • Biological
• Physical weathering

  • Rock is broken down into small pieces
  • This increases the exposed surface area of a rock so more weathering can occur
• Chemical weathering

  Chemical reactions occur that breakdown rocks into chemical components or alter the chemical composition of a rock
• Biological weathering

  A combination of chemical and physical actions linked to plant growth

  Eg. Roots or the release of organic acids
• Types of physical weathering:

  • Freeze-thaw weathering
  • Frost shattering
  • Pressure release - dilation
• Freeze-thaw weathering

  • Water gets into cracks and weaknesses of a rock
  • When water freezes it expands by 10%
  • This puts pressure on the rock and enlarges the cracks
  • When it thaws the pressure is released
  • Eventually after many freeze-thaw cycles the crack gets so big that the rock breaks apart
  • This process can only happen in locations where temperature moves above and below PMP
• Thawing

  When ice becomes liquid or softer as a result from warming up
• Frost shattering

  • Water gets into the air pockets of porous rock
  • When it freezes it expands by 10%
  • This puts pressure on the rock from within
  • Eventually the rock cannot withstand the pressure from within and shatters
• Pressure release - dilation

  Fractures form on rocks in parallel sheets as it expands in response to a release of pressure
• Chemical weathering

  • Weathering through chemical reactions produces weaker residues of material from rock which is then easily removed by erosion
  • Rates of chemical reactions are faster in higher temperatures > faster chemical weathering
  • As CO2 levels increase > rain and ice becomes more acidic > increased chemical weathering
• Types of chemical weathering

  • Oxidation
  • Carbonation
  • Hydrolysis
  • Hydration
  • Solution
• carbonation

  carbonic acid is formed when rainwater combines with dissolved CO2 in the atmosphere
• Oxidation

  Minerals in rocks react with oxygen - O2 in water or air
• Solution


  Any process by which a mineral dissolves in water
• Hydrolysis

  Minerals in rocks can undergo a chemical reaction when in contact with water
• Hydration

  When water molecules are added to rock minerals they create minerals of a larger volume - some expand by 0.5% during chemical reactions because they absorb water
• Biological weathering - two types

  1. Tree roots
  2. Chelation - organic acid
• Tree roots

  • As tree roots grow into cracks or joints in rocks they exert pressure which forces the rock apart.
  • The roots of falling trees may also exert leverage on rocks and soil as they fall causing them to become exposed to the elements and therefore weathering
• Chelation

  As plants and animal litter decomposes, organic acids are released causing soil water to become more acidic. This can react with some minerals in a process called chelation which breaks rocks down
• Biological weathering processes are generally of low significance in glacial areas because plant and animal activity are limited by the low temperatures
• Mass movement

  The downslope movement of material by the force of gravity. There is no help from a moving medium such as water, ice or wind

  Eg. when gravity exceeds friction
• Types of mass movement

  1. Slides - most common
  2. Slumps - least common
• Slides

  • Occur when there is movement along a straight slip plane eg. A fault between two layers of rock
  • This usually occurs at a valley side
• Slump

  • Slumps involve a rotational movement along a curved slip plane
  • This is common in weaker rocks that have become saturated with water
• Processes of glacial erosion

  1. Plucking
  2. Abrasion
  3. Sub-glacial water erosion
• Plucking Process

  • Basal temperatures at or above PMP in a warm based glacier create basal meltwater
  • Basal meltwater enters cracks in bedrock
  • If temperatures drop below PMP this meltwater refreezes
  • The bedrock becomes attached to the base of the glacier
  • As the glacier continues to move downhill due to gravity it 'plucks' the rock out of the bedrock and transports it downhill within the ice - typically larger fragments of rock
• Factors affecting rates of plucking

  • Geology of bedrock - the presence of joints/ cracks
  • PMP - has to be reached for meltwater to be present
  • Weathering - increased weathering weakens the bedrock so increased erosion
• Abrasion process

  Sediment is transported by a glacier in one of three locations:

  • Supra-glacial = on top of the ice
  • Englacial = within the ice
  • Sub-glacial = the base of the glacier

  Englacial and subglacial debris scrapes against the valley sides and floor as the glacier moves.

  This erosion is most likely to take place in the sub glacial area and leaves behind grooves in the bedrock called striations
• Factors affecting the rate of abrasion

  • The presence of basal debris = more debris > more abrasion
  • Debris size and shape = larger, angular debris > more abrasion
  • Thickness of ice = thicker ice > increased GPE > faster movement > more abrasion
• Sub-glacial water erosion

  • Vast quantities of meltwater surge out from temperate glaciers in the summer
  • Glaciers have surface streams which plunge into deep crevasses to the valley floor
  • These streams carry weathered and glacial sediment
  • Beneath the glacier the streams erode the base rock
• Glaciers advance if...

  • More precipitation
  • More cloud coverage
  • Inputs > outputs
  • Less sun radiation
  • High altitude
• Glaciers retreat if...

  • Less precipitation
  • Warm temperatures
  • Outputs > inputs
  • More sun radiation
• Glacier budget

  Inputs = outputs
• Transportation

  Sediment is moved by a glacier:

  Supra-glacial = On top

  Englacial = inside

  Subglacial = at the base
• Deposition

  Glaciers deposit sediment - till

  • Ablation till: Deposited by melting ice from glaciers that are static or in retreat
  • Lodgement till: Deposited by advancing ice, which pressures sediment into the valley floor
• Plucking definition

  Meltwater seeps into joints in rocks on the valley floor and sides. It then freezes and becomes attached to the glacier, which then pulls pieces of rock away
• Abrasion definition

  Occurs as a glacier moves across a surface. The sediment embedded in its base and sides scours surface rocks, wearing them away
• What is nivation?


  Hollows formed under an emerging glacier as a result of freeze-thaw cycles