Rock weathering

Rock weathering

1. Physical - mechanical breakdown of rocks

   • Wetting & drying cracking of some rocks such as shale or mud stone as a result of expanding and contracting of clays when its wetted and dried repeatedly
   • Salt Cystalisation - slightly porous rocks when exposed to salt water and repeatedly dried the cystalisation of salt can loosen or crush rock minerals
   • Frost weathering - water in cracks freeze and expand can force open and widen rock cracks
   • Thermal expansion - alternating between hot and cold temperatures without freezing can strip layers off boulders
   • Insolation - intense sun exposure can expand the surface layers while the internal layers remain cool. only occurs sunlit side of rock

2. Chemical

   • Oxidation - many metallic ores are only stable in absence free oxygen. when exposed to air oxidise to crumbly forms
   • Hydrolysis - Hydrogen ions in waterwith CO2 create a weak acid which dissolves rock minerals

3. Biological

   • Lichens excrete organic acids and chelates which free minerals from the rocks
   • break down of Organic matter releases humic acids which dissolves minerals from rocks
   • Plant roots break rocks apart through growth and excreating Hydorgen ions

4. rates of weathering

   • Bowen's Reaction Series - the higher the temperature where the rock mineral was formed the more unstable it is at the earths surface
     • resistant weathering rocks e.g quatz and potassium fieldspar
     • easily weathering minerals - pyroxene and olivine
   • Rainfall - low = slow, moderate = intermediate, high = fast
   • Temperature hot = fast, warm = intermediate, cold = slow
   • vegetation - sparse = slow, moderate = intermediate, dense = fast
   • slope - steep = slow, moderate = intermediate, gentle = fast

5. chemical index of Alteration - measurement of chemical weathering done

   • removal of potassium, Calicum and sodium