Tutorial:Weathering

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Weathering Processes
Weathering of rocks and sediments includes physical, chemical and biological processes. Not only do these processes affect the local geology, they can also degrade archaeological artefacts in the soil.

Physical
Physical or mechanical weathering processes result in the loosening of fragments from the underlying bedrock and a reduction in size of individual fragments, eventually producing silt sized particles. The mechanisms for physical weathering include thermal contraction and expansion, hydration shattering, frost weathering and wedging, salt crystallisation, and pressure changes as erosion brings rocks to the surface.

Each of these mechanisms results in pressure changes within rocks causing them to split and shatter, often following weakness in the rocks caused by mineral differences or the effects of chemical weathering.

Archaeological structures and artefacts can also be affected by these processes. For example salt crystallisation on pottery shards can cause shattering. Physical damage due to cultivation (plough damage) is also a significant factor in archaeological preservation.

Chemical
Chemical weathering involves chemical reactions and the formation of new minerals. Water is a vital component in many forms of chemical weathering.

Mechanisms of chemical weathering include solution, whereby minerals dissolve in water. Minerals such as halite (NaCl) are very soluble in water whilst quartz is relatively insoluble. Hydrolysis, when metal ions in the rock are replaced with hydrogen ions from water - hydrolysis is important in the formation of clays. Oxidation of elements such as iron, titanium and manganese to form oxides and hydroxides.

Archaeological artefacts are also subject to chemical weathering processes bone and shell for example are particularly sensitive to solution in acidic soils.

Biological
The action of biological organisms can result in both physical and chemical weathering. Roots for example can penetrate cracks and exert a physical pressure, whilst organisms such as limpets and lichens secrete organic acids that attack rock. Biological processes can also affect archaeological artefacts and deposits.

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