weathering and rocks

• weathering


  break down of rocks in-situ
• decomposition


  chemical weathering where rock substance is altered
• disintegration

  mechanical weathering produces smaller angular fragments of the same rock type
• biological weathering

  physical and chemical processes caused by plants and animals
• physical weathering:

  • freeze thaw
  • salt crystallisation
  • exfoliation (thermal fracture)
  • exfoliation (pressure release)
  • wetting/drying (slaking)
• chemical weathering:

  • carbonation-solution
  • hydrolysis
  • hydration
  • oxidation
• biological weathering:

  • chelation
  • microbial activity
  • weakening by roots and animals
• Freeze thaw - water repeatedly freezes and melts in a crack in a rock, which expands the rock over time

  1. water fills a crack
  2. water freezes over night and expands by 10%, which widens the crack
  3. water melts during the day
  4. process repeats

  When freeze thaw repeats, there is disintegration which produces scree and felsenmeer. It occurs due to temperature variation, so doesn't happen in areas of permanent frost.
• salt crystallisation - salt crystals expand as water evaporates, and continues to expand as temperatures rise, which outs pressure on the sides of cracks

  1. water fills a crack
  2. water evaporates
  3. salt forms and expands
  4. temperatures rise, so salt expands more
  5. same charges in the salt repel, widening the crack further
• exfoliation (thermal fracture) - top layer of rocks heats from the sun and expands, but the layer below is still cool, so the top layer separates from the rest of the rock
• exfoliation (pressure release) - deeply buried rocks expand when weight is removed from above, so rock separates into layers (pseudo bedding layers) as that pressure is released, because the rock expands at different speeds
• wetting/drying (slaking) - clay rich rocks expand when wet, and crack when dry, which makes cracks that are vulnerable to freeze thaw and salt crystallisation
• Water is the key medium in chemical weathering as it brings in acids and removes weathered products in solutions. It is most effective in the subsurface as percolating water has gained organic acids from soil and vegetation.
• carbonation/solution - water and carbon dioxide react to form carbonic acid, which reacts with calcium carbonate rocks e.g. limestone
• hydrolysis - chemical weathering of rocks by hydroxyl ions in water

  • e.g. granite is composed of feldspar, quartz and mica
  • feldspar reacts with water to make Kaolin (china clay), potassium hydroxyl and silicic acid, which washes away, leaving a weaker structure which can be broken down by other weathering processes
• hydration - rigid attachment of H+ and OH- ions to atoms and molecules, which increases volume and creates physical stress within the rock, but it is reversible
• oxidation - occurs when rock is exposed to oxygen from air or water, and causes iron in rocks to change from a ferrous state to a ferric state, turning a red colour (rusting)

  • e.g. FeO -> Fe2O3 which is only soluble in <3 pH acid so stays in rocks
• chelation - break down of certain minerals in rocks by organic substances (humid acids released by decomposition of organic matter in soil, and bacteria)
• Biological:

  • plant roots can exert stress on a rock, or bond weak rocks together
  • microbial activity breaks down rock to form soil e.g. lichen
  • burrowing animals break rock so it is more susceptible to weathering
  • soil insulates rock from extreme cold or heat, so freeze thaw is stopped
• equifinality - 2 different processes that produce the same landform
• gradient -> the steeper the slope is, the faster the weathering and material is moved away (rolls down) so new rock is exposed, and mass movement is likely
• aspect -> slopes that face south suffer increased freeze thaw weathering as they are warmed by the sun during the day, and freeze at night
• Granite Tor Formation (warm climate e.g. Maasai-mara, Kenya):

  1. acidic rainwater enters the subsurface
  2. breaks down rock by chemical processes e.g. hydrolysis
  3. areas with more cracks are weathered faster
  4. as the rest of the rock is removed or washed away, the tors are revealed
  5. (occurs in granite)
• Granite Tor Formation (cold climate e.g. Dartmoor, Devon):

  1. freeze thaw at the surface attacks cracks, and breaks off pieces of rock
  2. areas of rock with more cracks weather faster
  3. rocks with fewer cracks stick out further than the rest
• Spherical weathering:

  1. granite splits into cube-like blocks
  2. fractures in granite come into contact with water that causes chemical weathering, which breaks into sand (grus)
  3. erosion of granite sand exposes the boulders at the surface
  4. spherically-weathered boulders are "born-rounded" by subsurface weathering
• Limestone is an impermeable rock, but it contains many cracks, making it permeable. The rock has horizontal cracks from bedding planes, and vertical cracks (joints) from limestone drying and pressure release. Limestone is a calcite, so it reacts strongly with acids e.g. carbonation/solution
• factors influencing weathering:

  • climate -> determines which processes and the rates of weathering
  • rock structure -> joints bedding planes and cleavage allow water penetration and increase physical and chemical effects, which controls size and shape of fragments
  • rock texture -> coarse-grained rocks weather faster than fine-grained rocks, and sedimentary rocks weather faster than igneous rocks
  • human activity -> increased weathering by increased pollutants in the atmosphere, which causes acid rain, which increases hydrolysis and carbonation, and deforestation removes vegetation and soil
• factors influencing weathering (rock type):

  • sandstone - cement is attacked to produce granular disintegration, and joints and bedding planes provide lines of weakness for physical or chemical weathering
  • limestone - carbonate minerals are weathered by carbonation and solution, and clays are left as residue
  • granite - feldspar breaks foe by hydrolysis, and quartz and mica are more resistant to chemical decay
• factors influencing weathering (vegetation):

  • increased vegetation increases chemical weathering because it releases organic acids for chelation, and it releases CO2 which forms carbonic acid
  • increased vegetation widens cracks in rocks from root growth
  • soil acts as insulation which decreases freeze-thaw and thermal effects
• factors influencing weathering (relief):

  • relief influences climate, as rainfall and colder temperatures occur in up lands, increasing freeze thaw
  • slope processes (landslides) expose bare rock which is vulnerable
  • aspect
• CO2 + H2O -> H2CO3

  CaCO3 + H2CO3 -> Ca(HCO3)2
• hydration - water entering minerals in rocks which causes expansion and leads to rock disintegration
• hydrolysis - chemical process where water reacts with minerals in a rock leading to its decomposition
• pressure release - physical process where rock cracks in layers due to weight being removed from above