London's Geology: Fieldwork
- Hampstead Heath
- St Pancras Gardens
- University College London
- City of London Cemetery
- Introduction and Access
- Gravestone Weathering
- Location One: Gravestones
- Location One: Exercises
- Location Two: Gravestones
- Location Two: Exercises
Unweathered granite contains the following minerals:
- Sodium Plagioclase feldspar (Na feldspar)
- Potassium feldspar (K feldspar)
- Accessory biotite, amphibole, or muscovite
What happens when granite is weathered?
- Na feldspar and K feldspar undergo hydrolysis to form kaolinite (clay) and Na + and K + ions.
- The quartz (and muscovite if present) remain as residual minerals due to their high weathering resistance.
- Biotite and/or amphibole undergo hydrolysis to form clay, and oxidation to form iron oxides.
- The weathered rock fragments become constituents of the soil.
What happens next?
- Quartz grains may then be eroded and become sediment. The quartz in granite is sand-sized and therefore forms quartz sand. This quartz sand will ultimately be transported to the sea (as bed load in a river) where it will accumulate to form beaches.
- Clays will ultimately be eroded and washed out to sea by rivers. Clay is fine-grained and remains as suspended load in the water column so it may be deposited in quiet water.
Dissolved ions will be transported by rivers to the sea in the dissolved load, and will become part of the salts in the sea.