booklet 5 - processes

• subaerial processes


  - weathering

  - mass movement

  - run off
• weathering


  the weakening and breakdown of rocks in situ (in place)
• mass movement


  downward movement of material due to gravity
• run off


  overland flow where material is washed down a slope, potentially forming an input to the sediment cell
• mechanical weathering


  - freeze thaw

  - wetting and drying

  - pressure release

  - salt crystallisation

  - exfoliation
• biological weathering


  plants and animals
• chemical weathering


  - oxidation

  - hydrolysis

  - hydration

  - carbonation

  - solution

  - acid rain
• where / when might rate of chemical weathering be fastest?

  depends on rock type

  - limestone breaks down with acid rain and calcium carbonate on the coast

  - exposure to salt spray
• where / when might rate of mechanical weathering be fastest?

  - onion skin weathering in hot climates

  - freeze thaw weathering - temps above and below freezing, hard rock with lots of cracks / fault lines
• what is mass movement dependent on?

  - the cohesion of sediment

  - the height and angle of slope

  - grain size within the sediment
• types of mass movement

  - soil creep

  - mud flows

  - rockfall

  - rotational slumping

  - landslide
• soil creep


  - slow form of movement of individual soil particles downhill

  - involves particles rising towards ground surface due to wetting / freezing then returning to surface as soil dries / thaws

  - action implied by formation of shallow terracettes (build up of soil on upslopes)
• mud flows


  - earth and mud flowing downhill usually over unconsolidated / weak rock, often after heavy rainfall

  - water gets trapped, increasing pressure, forces rock particles apart

  - often sudden and fast flowing

  --- represent significant natural hazards
• rockfalls - AKA blockfalls

  - sudden collapse/ breaking away of individual rock fragments

  - commonlyh associated with steep or vertical cliffs in quite resistant rock

  - triggered by mechanical weathering or earthquake

  - rocks fall down slope to form a scree at foot of slope
• rotational slumping


  - found in unconsolidated sands + clays

  - slide surface is curved

  - when permeable rock overlies impermeable rock, causing build up of pore water pressure

  - multiple landslips result in terraced appearance on cliff face
• landslide


  - frequently triggered by earthquakes / very heavy rainfall

  - when slip becomes lubricated + frictions reduced

  - tend to be rapid + pose considerable threat to people and property