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
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