Rock and weathering

What is weathering?

Weathering is the breakdown of rocks in situ (in the place where they are). Rocks are susceptible to weathering. The composition of the rock and the climate determines the type of weathering that is most effective. There are three main types of weathering. These are mechanical, chemical and biological weathering.

Mechanical Weathering
Mechanical weathering involves the breakdown of rock by physical processes. The chemical composition of the rock is not changed in any way. There are two types of mechanical weathering. These are freeze-thaw and exfoliation.

Freeze-thaw weathering
Most rocks are very hard. However, a very small amount of water can cause them to break. When water seeps into cracks and freezes, it then expands. This powerful force can increase the size of cracks. Over time, the repeated freeze-thaw action of water can break rocks apart. Eventually, pieces of rock break off, creating scree.

Exfoliation or Onion Skin weathering 
This type of erosion is common in warm areas. As the sun shines on rocks during the day, it causes them to expand. During the night, the rock contracts due to the colder temperature. Over time, this continued process causes small pieces of surface rock to flake off.

Chemical Weathering
Chemical weathering causes an alteration to the chemical composition of rock due to a reaction. Slightly acidic water can dissolve rock. An example of this would be slightly acidic rain changing the chemical composition of limestone to form a limestone pavement. This occurs on the surface and along the joints and bedding planes of limestone. You can also see evidence of this in buildings made from limestone.

The image below shows limestone that has been chemically weathered. The resulting formation is known as a limestone pavement. You can find out more about limestone scenery here.

Biological Weathering
Biological weathering is the effect of living things. For example, as the roots of a tree extend into the ground, they can prise rocks apart. Ivy growing up in a building can cause bricks to loosen. It also occurs on a much smaller scale through lichen and moss.