Page 16 - Curriculum Visions Dynamic Book

Iron colours in the landscape
Much of the colour in the world’s rocks and soils is produced by iron compounds. These colours allow geologists and soil scientists to tell much about the way the rocks and soils of the world have formed.
Deep red colours tell of iron that was oxidised in hot, tropical conditions; oranges and yellows tell of iron oxides formed in cooler climates; while greys, blues and greens are produced where iron compounds were short of oxygen, such as deep under the sea.
Soils formed from these rocks often have
a colour similar to that of the rock on which it has formed. However, when soils are affected by acid waters, iron is removed into solution, and the rock becomes dramatically paler in colour.
Iron and soils
Iron compounds are important constituents of
soils and responsible for many soil colours. In some parts of the world they are also responsible for poor drainage and a variety of other soil problems.
Soils are coloured by a combination of iron oxides and organic matter. Most organic matter occurs close to the surface. Its black colour makes the topsoil a darker colour than any other part of the soil.
In some cases, such as when a soil receives large amounts of rainfall, the organic matter reacts with the rainwater to produce acids that are strong enough to dissolve the iron compounds. When this happens, the iron compounds, which provide the brown, orange, red and yellow colours in a soil, are removed and the soil layer turns ashen grey. The ashen grey colour is the colour of the quartz sand and clay particles that remain.
In regions lower down in the soil, the acid conditions are less intense and the iron compounds are reprecipitated. Here the iron concentrates to produce a strong orange colour.
 Soils change colour through their profiles, in part because iron is carried by acid waters, as shown in this Podzol from New Zealand.
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