Weathering

Freeze-thaw. Based on © NERC Based on P551762.

What do we mean by weathering and erosion? Weathering is not the same as erosion, although they are sometimes confused. Weathering processes do not involve transportation.

What is weathering?

Weathering is the breaking up of rock into small pieces.

There are two types of weathering

Chemical weathering is the most important way that limestones are broken down and we are going to concentrate on this.

Mechanical weathering

Mechanical, or physical, weathering has taken place in limestone landscapes such as the Pennines of Yorkshire. An example is 'freeze–thaw' where water soaks into small fissures and cracks, expands when it freezes in the winter, and physically breaks the limestone.

Chemical weathering

Chemical weathering involves the decomposition of rocks due to chemical reactions between minerals such as calcite with water and gases in the atmosphere (e.g. carbon dioxide and sulphur dioxide). The solution of soluble minerals is particularly important in limestone landscapes.

Granite chemically weathers to form china clay

Rain water

Rain has a major impact on karst scenery through chemical weathering.

'Ordinary' rain is naturally acidic because it contains dissolved carbon dioxide that forms weak carbonic acid. When this weak acid comes into contact with calcite, the limestone begins to dissolve.

The video in Figure 1 shows how rain water weathers limestone:

  1. Droplets of rain water (H2O) in the clouds dissolve carbon dioxide (CO2) in the atmosphere.
  2. When combined, these form carbonic acid (H2CO3).
  3. The slightly acidic rain then falls onto the ground.
  4. The rain soaks into the soil or flows over the exposed limestone (CaCO3).
    (It may become even more acidic if it soaks into soil where there are naturally occurring acids from plant material or minerals such as pyrite. However, this is a complication that we will not go into here.)
  5. A chemical reaction takes place when the rain (i.e. carbonic acid) meets the limestone (i.e. calcite).
    The acid H2CO3 and the CaCO3 combine to form HCO3 + HCO3 + Ca.
    In other words, the calcite is converted to calcium bicarbonate, Ca(HCO3)2.
    This is soluble and is washed away by the rain.

Problems caused by acid rain

The problem of acid rain began with the Industrial Revolution (from about 1760 to 1840) and increased in the 20th century. This was the time when the majority of pollutants were created. They escaped into the atmosphere and were dissolved in rain water. The main pollutant is sulphur dioxide (SO2), but nitrogen oxides are also present.

In simple terms, acid rain is a weak sulphuric acid, and this is a significant cause of chemical weathering.

The video in Figure 2 shows how acid rain weathers limestone:

  1. Rain water (H2O) in the clouds dissolves some of the SO2, a pollutant from industrial manufacturing.
  2. This makes weak sulphuric acid (H2SO4).
  3. The acidic rain then falls on to the ground.
  4. The rain soaks into the soil and porous limestones and flows over massive limestone (CaCO3).
  5. A chemical reaction takes place. The acid rain (H2SO4) is added to the limestone (CaCO3). This causes the formation of H2O (water), CO2 (carbon dioxide) and soluble CaSO4 (calcium sulphate).
  6. The water is added to the rain and is lost when it soaks away or evaporates, the carbon dioxide is lost to the atmosphere and calcium sulphate is lost when it dissolves in water and is washed away.

Chemical weathering in limestone areas causes special topographical solution features to form, known as karst.

Karst features include limestone pavement, dolines (or sinkholes), and micro-solutional features called karren.

Limestone pavement
Sinkhole
Karren