Sedimentary Rocks

• Weathering

  • breakdown of rocks 'in situ'
  • produce sedimentary clasts (grains / fragments) or ions in solution (dissolved minerals)
  • agents of weathering - precipitation
  • rate of dissolution is changed by temperature 


  1. physical / mechanical
  2. chemical
  3. biological
• Hydrological cycle

  1. water evaporated by heat energy from sun
  2. water vapour rises, cools and condenses forming clouds
  3. water falls back to earth as precipitation

  water cycle is driven by solar heating

  • solar heating drives weathering and erosion processes on earth surface
• Physical weathering

  1. freeze-thaw
  2. onion-skin / insolation

  freeze-thaw

  • water changes volume (9%) as it freezes
  • exerts great pressure on the rock to split it apart
  • continuous process

  onion-skin / insolation

  • thermal expansion of outer layer of rock
  • peeling off of large curved sheets or slabs of rock
  • incoming-solar-radiation
  • outer layers expand and contract under heat from sun so joints form between inner & outer layers (exfoliation)
• Chemical weathering

  1. hydrolosis
  2. carbonation
  3. solution
  4. oxidation

  hydrolosis

  • minerals break down by reacting with water
  • acidic conditions form clay minerals from feldspar

  carbonation

  • reaction of minerals with carbonic acid from solution of carbon dioxide and oxygen

  solution

  • dissolved in water and precipitate
  • evaporate or supersaturate in water

  oxidation

  • reaction of minerals with oxygen when exposed to the atmosphere
• Biological weathering

  • disintegration of rocks and minerals due to chemical and/or physical agents of organisms
  • physical or chemical changes
  • plants secrete chemicals to dissolve rocks
• Erosion

  • removal or transport of materials by natural agents
  • wind
  • water / ice (in glaciers)
  • gravity

  Processes of erosion

  1. abrasion - particles carried by water, wind or ice strike earths surface and slowly wear it away
  2. attrition - wearing down of sedimentary grains due to collisions with other grains during transport
• Transport

  1. solution
  2. suspension
  3. saltation
  4. traction

  solution - particles dissolved in water

  suspension - particles carried in the flow of water

  saltation - small grains bounced along the river bed

  traction - large grains rolled along the river bed
• Deposition

  • sediment deposited when energy levels drop to a point where the current can no longer transport sediment of a certain size
  • high energy levels - current speed increases so transports larger grains
  • low energy levels - current speed decreases so can only carry finer grains 


  sedimentary rocks reflect grains available and energy levels of the current
• Sedimentary Rocks

  1. colour
  2. mineralogy
  3. grain size
  4. grain shape
  5. sorting
  6. fossil content
• Grain size

  1. coarse grain > 2mm
  2. medium grain 1/16 - 2 mm
  3. fine grain < 1/16 mm
• Grain shape

  • sphericity - how like a sphere it is
  • roundness - extent originally angular edges and corners have been rounded off

  1. very angular
  2. angular
  3. sub-angular
  4. sub-rounded
  5. rounded
  6. well-rounded
• Sorting

  • range of grain sizes in a sediment

  well sorted - similar / small range grain sizes

  poorly sorted - different / large range grain sizes
• Coarse grain

  • breccia - angular fragments
  • conglomerates - rounded fragments
• Medium grain

  • orthoquartzite - quartz only
  • arkose - quartz and feldspar
  • greywacke - poorly sorted, tiny rock fragments
  • desert sandstone - red iron-oxide cement
• Fine grain

  • Mudstones
  • Shale