Weathering Process Rocks are classed as “dead”, hard and chemically inactive. Soils are considered to be “alive”, soft, porous and chemically active. They are inter-linked through bio–geo-chemical cycles.

Weathering Process Rocks are formed in the Earth’s crust under high pressures and temperatures. Over time they are exposed at the surface through a process known as uplift. Air, water and acidic gases all contribute towards the weathering process. The weathering of rocks involves 4 chemical processes and 1 physical.

Weathering Process Once exposed to the elements and there has been a release of pressure on the rocks, cracks / fissures develop and physical weathering can take place – this increased by the following: Cracks are increased by; Ice expansion Wetting & drying out Heating & cooling Flora, roots etc..

Weathering Process Chemical weathering can also take place. In doing so it forms other minerals, different character clays / clay minerals or soluble components may recombine with other compounds = new minerals Chemical weathering can be divided into 4 types; Simple dissolution Simple hydrolysis / complex hydrolysis Carbonation Reduction / oxidation reactions

Weathering Process Simple dissolution Rocks / minerals decompose by water or reactions with naturally occurring acids. NaCl (s) Na + (aq) + Cl - (aq)

Weathering Process Simple hydrolysis Hydrolysis – more extensive reaction occurs with water molecule, breaks the O-H bonds Hydrolytic weathering involves breaking the metal-oxygen bond.

Weathering Process Complex hydrolysis This is a more important as it produces clays / soils. It breaks down 3D silicates into 2D silicates. Example: Feldspar – KAlSi 3 O 8 is a 3D silicate, that is weathered to a clay by changing its composition and removing some of the metals, forming a 2D silicate. Further weathering, by loss of silica, results in the formation of oxides.

Weathering Process Carbonation involves the reaction of water and / or carbon dioxide with materials in the Earth’s crust CO 2 + H 2 O ↔ (H 2 CO 3 ) ↔ H + + HCO 3 - H + + CaCO 3 ↔ Ca 2+ + HCO 3 - The presence of the acidic H 2 O / CO 2 weathers out certain metals from rocks

Weathering Process Oxidation / reduction This is an important chemical process which occurs where cations / anions have >1 oxidation state Example: Fe (II)