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Laterite and Duricrust
Laterite formation
In past eras the climate in northern Australia was much wetter and warmer than now. Large quantities of warm rainwater soaking through rock broke down mineral grains dissolving the minerals and leaching them away. Some minerals, such as those containing calcium, magnesium, sodium, silicon and potassium were relatively easily leached through the soil and rock leaving a residue of the less soluble iron and aluminium minerals. This deeply-weathered residue became extremely hard when exposed to air and formed a hard, generally impermeable crust on the surface known as duricrust. A duricrust rich in iron is described as ferricrete.

Below the red-coloured duricrust are layers rich in pale-coloured clays with an intermediate mottled zone where leaching has been non-uniform. This sequence of ferricrete - mottled zone - pallid zone is described as 'laterite'. The term 'laterite' is also widely used to describe the red-coloured, iron-rich, sometimes gravelly soil rich in iron and aluminium oxides resulting from deep-weathering. Laterite also forms hard pea-shaped nodules or solid boulders.

The hard, impermeable duricrust layer protects the softer clays below it from further weathering and erosion so that over thousands of years, as surrounding strata have eroded, the duricrust layer and clays below it slowly emerge to form the elevated, flat-top, mesa formations known as 'breakaways' in some parts of Australia.

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