Weathering

A Soil consists of many fragments of rocks which were broken down or weathered. There are three main processes that an exposed rock may be weathered by, these are; chemical, physical and biological. A Rock may be weathered by one or more of the processes listed above. Lets take a closer look at each type:

Chemical

Chemical weathering occurs when any chemical reacts with the surface of a given rock. A good example of this is rainwater. Rain that falls from the sky is naturally acidic due to the fact that carbon dioxide gas can dissolve in water to form carbonic acid. This reaction can occur in the atmosphere while a rain cloud forms. When rain falls on an exposed rock such as limestone (very reactive with acid), a reaction occurs. The reaction will cause tiny pieces of the rock to become loose and they may then be easily removed by the physical process of erosion and deposited in a soil.

You can have a look at one of these reactions yourself by getting some vinegar (or acid) and a piece of limestone rock. Place a few drops of the vinegar onto the piece of limestone and you will see it appear to fizz (as demonstrated in the image above). This is because a gas is being released as the rock reacts with the acid. Rain water is not as acidic as vinegar so you will probably not see rocks fizzing like that when it rains!

The levels of rain acidity today are often increased as a result of human atmospheric carbon dioxide pollution such as that caused by cars. Therefore we should expect to see an increased level of chemical weathering of rocks in the future. Indeed, some statues and buildings in London, UK are being eroded at a quite fast rate.

Physical

Physical weathering occurs when energy is exerted on a rock. Such energy may be produced by the action of wind, waves in the sea or from a direct impact. If something hits a rock with a great enough force the rock will break into pieces. The most common forms of physical weathering are wind and waves. You might see the effect of the wind most when it passes over the sedimentary rock sandstone. There are many buildings made from this type of rock in the UK and if you look closely you will find that the action of wind passing over the surface of the rock can remove particles of sand.

Waves however cause one of the most interesting types of physical weathering as they cause chemical weathering as well. When a wave hits a cliff face it has the energy to force air into small cracks in the rock, this builds up pressure which blows pieces of rock away from the cliff (see the image below). At the same time the chemical properties of the sea water may be chemically weathering the rock. So a link between the two types of weathering can easily be observed.

Biological

Biological weathering occurs when a living organism breaks up a rock. A good example is the roots of a a plant which will often grow any crack that they can get into. A root would not normally be strong enough to break a rock, however over a long time the root will slowly grow in size pushing on the rock on an area that is already weak (hence the crack). Combined with the chemical and physical processes that will be occurring in relation to plant nutrient uptake the rock may eventually be broken down (see the image below).

It was mentioned above that once a rock has been weathered it will then be eroded, to find out more about this process please click here