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Weathering Processes

One important influence on the formation of soils is weathering. There are two main categories of weathering, both having different effects:

Physical Weathering

Freezing and Thawing - here, the expansive force of water pushes the soil structures apart. Water expands considerably when frozen and this expansion literally pushes the soil apart, breaking it down. When the ice thaws the soil can slump back again. The overall process is rather like a very slow 'churning'. Freeze-thawing can literally grind mountains down over time!

Heating and Cooling - here soils subjected to extremes of temperature are affected as they expand and contract. The effect is less pronounced that that of freezing and thawing but over time this can become significant.

Wetting and Drying - soils that are wetted up may be prone to swelling. Clay minerals in particular exhibit this property. The soils that have thus expanded then shrink when the soils dry out. These seasonal effects are termed shrinkage and swelling. Many household claims for subsidence are based upon such shrinkage and swelling of clays under foundations.

Grinding or Rubbing - most obvious on the beach, grinding of particles against each other leads to particle disintegration. This is why beach pebbles become smooth. Abrasion similarly breaks down the soil particles.

Organisms - the effect of organisms, plants and animals, living in the soil cannot be overstated. Soil is home for a wide range of organisms. If plants can push through concrete - soil presents little obstacle! Worms churn their way through soil, mixing and aerating it all through their lives and there can be thousands of worms in a field.

Unloading - when pressure is placed upon soil it becomes compressed. Never mind tractors, imagine the weight of a glacier! When ice melts a huge weight is lifted and the soils may react accordingly by uplifting and expanding.

Chemical Weathering

Solution - certain solid components in the soil can be dissolved in soil water. In this way underground caverns can form in limestone karst landscapes. The name Karst comes from the Krass plateau in Slovenia where there are some of the most magnificent cave systems anywhere in the world - valley sized caves!

Hydrolysis - certain compounds in the soil can react to elements in the water.

Carbonation - soil compounds can react with carbonic acid.

Hydration - water in the soil can act to change the chemical structure of the soil components.

Oxidation - oxygen in the soil can act to change the chemical structure of the soil components.

Reduction - a lack of oxygen in the soil can act to change the chemical structure of the soil components